Urban air pollution control policies and strategies: a systematic review

Abstract

A wide range of policies, strategies, and interventions have been implemented to improve air quality all over the world. This systematic review comprehensively appraises the policies and strategies on air pollutants controls enacted in different countries, worldwide. Three databases, Web of Science, PubMed and Scopus, were used for the search. After screening, a total of 114 eligible manuscripts were selected from 2219 documents for further analysis. Selected articles were divided into two categories: (1) articles focusing on introducing the policies and strategies enacted for controlling air pollution in different countries, and (2) articles which focused on different policies and strategies to control one or more specific pollutants. In the former one, urban air pollution control strategies and policies were divided into four categories, namely, general strategies and policies, transportation, energy, and industry. In case of latter category, policies and strategies focused on controlling six pollutants (PM, SO₂, NO₂, VOC_S, O₃ and photochemical smog). The results indicated that, the most common policies and strategies enacted in most countries are pertinent to the transportation sector. Changing energy sources, in particular elimination or limited use of solid fuels, was reported as an effective action by governments to reduce air pollution. Overall, most policies enacted by governments can be divided into three general categories: (a) incentive policies such as implementing a free public transportation program to use fewer private cars, (b) supportive policies such as paying subsidies to change household fuels, and (c) punitive policies such as collecting tolls for cars to enter the congestion charging areas. Depending on the circumstances, these policies are implemented alone or jointly. In addition to the acceptance of international agreements to reduce air pollution by governments, greater use of renewable energy, clean fuels, and low-pollution or no-pollution vehicles such as electric vehicles play an important role in reducing air pollution.

Introduction

Nowadays, air pollution is known to be closely related to economic growth, population and energy consumption. There is a chain relation among the maintenance of economic development and improvement the living standards, population growth, increased demand for energy, and increasing the production and emission of air pollutants. Air pollution is considered as a serious issue; it causes different concern including climate changes, biodiversity reduction, affects crops and agricultural production and soil acidification [1, 2]. According to WHO reports (2016), 4.2 million premature deaths were attributed to air pollution in both developed and developing countries; 91% premature deaths are observed in low- and middle-income countries including South-East Asia and Western Pacific regions [3]. The studies indicate that people living in low-level social-economic regions encountered with high level of air pollutants concentrations in ambient air [4–10]. A variety of diseases including respiratory diseases (asthma, bronchitis, pneumonia), different types of allergies, disturbances of the central nervous system, and circulatory problems are attributed to air pollution [11–17].

Road transportation are known as the main source for air pollution. In addition, domestic, commercial, and industrial activities may also cause air pollution. It is reported that more than 70–80% of air pollution in large cities in developing countries are attributed to greenhouse gas emissions from a large number of worn-out vehicles combined with poor vehicle maintenance, poor road structure, and poor fuel quality [18–24]. According to WHO report (2016), 91% of the world's population experienced the air with quality lower than the standard values [3]. The key air pollutants are: particulate matters (PM_2.5 and PM₁₀), nitrogen dioxide (NO₂), sulfur dioxide (SO₂), carbon monoxide (CO), and (O₃) [25]. The major sources of air pollution, their corresponding health effects and standard values recommended are summarized in Table 1.

Table 1.

Summary of the sources and limits or guidelines of the major air pollutants [26–29]

Pollutant	Sources	Health effects	Limits/guidelines
			WHO (WHO, 2006)	U.S. (U.S. EPA, 2012)
PM	Transport (including exhaust fumes and tire and brake wear), combustion, industrial processes, construction and demolition, wind erosion	breathing problems, the increase of cardiopulmonary and lung cancer mortality, reduced lung functions in children and adults leading to asthmatic bronchitis and chronic obstructive pulmonary disease (COPD), cardiovascular and cardiopulmonary mortality	PM2.5(μg m−3): 24-h mean = 25 Annual mean = 10	PM2.5(μg m−3): 24-h mean = 35 Annual mean = 15
			PM10(μg m−3): 24-h mean = 50 Annual mean = 20	PM10(μg m−3): 24-h mean = 150 Annual mean = -
NO2	Combustion processes (heating, power generation, and engines in vehicles and ships)	Coughing, wheezing, eyes, nose or throat irritations, headache, dyspnea, chest pain, diaphoresis, fever, bronchospasm, and pulmonary edema	NO2(μg m−3): 1 h mean = 200 Annual mean = 40	NO2(ppb): 1 h mean = 100 Annual mean = 53
SO2	Use of sulfur-containing fossil fuels for domestic use, power generation and vehicles	Problems in the respiratory system and the functions of the lungs, irritation of the eyes, inflammation of the respiratory tract leading to cough, mucus secretion, aggravation of asthma and chronic bronchitis	Unit:μg m−3 24-h mean = 20 10-min mean = 500	Unit: ppb 1 h mean = 75 3-h mean = 500 24-h mean = 140 Annual mean = 30
CO	Transport (especially petrol-based), combustion and industry	Headache, dizziness, weakness, nausea, vomiting, loss of consciousness	Unit:ppb 8-h mean = 9000 1 h mean = 35,000	–
O3	Photochemical reaction of NOx from industrial sources and vehicles with VOCs emitted by automobiles, industrial sources, and solvents	Breathing problems, trigger asthma, reduction of lung functions, lung diseases	Unit: μg m−3 8-h mean = 100	Unit: ppb 8-h mean = 75 1 h mean = 120

The basic and inherent advantages of establishment the legislations, strategies, and policies in the fields of air pollution are preventive measurement and requirement to control the air pollution in emission sources, improving the air quality and avoiding negative health outcomes. Air pollution control efforts dates back to more than a century. The Chicago legislation (1881), smoke abatement in Boston in the States of America (1910–1912) and Clean Air Act (1925) following “Great London Smog” in the UK are the earliest strategies for air pollution control [26–28]. However, afterwards, a series of legislations and programs have been enacted at the international level and some at the national and local levels. International actions such as the Paris Agreement (2015) recommend to the countries; it recommends governmental authorities to lower the increasing global warming and temperature below 2 °C and to in particular prevent dangerous impacts of climate change [29, 30]. At the continental level, European Union (EU) Directives enacted the values guidelines for air pollutants and the convention on long-range trans-boundary air pollution (CLRTAP) and protocols related to emission reductions for specific air pollutants [31, 32]. In case of local preventive air pollution actions, Prohibition of solid fuel use in homes, low emission zones (LEZs) and the introduction of vehicle exhaust catalysts (VECs) are the most common approaches [27, 33, 34].

The purpose of this study was to comprehensively cover the strategies and policies that governments have enacted to improve air quality or urban air pollution control. To the best of our knowledge, this is the first global systematic review on urban air pollution control policies and strategies with this number of countries; it can contribute to advance the knowledge in this field.

Methods

Search strategy

The present systematic review was performed according to standard methods the Preferred Reporting Items for Systematic Review and Meta-Analysis (PRISMA) [35]. A comprehensive screening approach to find the urban air pollution control strategies and policies published in peer-reviewed journal was performed up to 19 January 2021 in three scientific databases: PubMed, Web of Science, and Scopus databases. The selected keywords included “air pollut*” OR “atmospheric pollut*” OR “air contaminant*” OR “polluted air” OR “air quality” OR “outdoor air pollut*” OR “ambient air pollut*” OR “atmospheric pollut*”AND “Control policy” OR “Control management” OR “Control strateg*” OR “intervention plan*”. Based on the exclusion and inclusion criteria, the most relevant articles were selected. The search was limited to the English language. We also searched the references lists of qualified articles and grey literature databases to find additional pertinent studies.

Study selection, eligibility criteria, and data extraction

Briefly, four sequential steps were performed to select the eligible manuscripts (Fig. 1). First, the selected manuscripts were entered into the EndNote X8. Second step was removing the duplicated studies. Third, the eligibility of articles were screened by the titles and abstracts of the remaining articles were reviewed for suitability. Fourth, the full texts of the potentially eligible manuscripts were evaluated to meet the aim of present systematic review. Finally, the selected study was throughoutly screened to meet the inclusion criteria mentioned in Table 2 The extracted data from the selected articles cover the following items: urban air pollution control policies and strategies, country, geographical location, start time or period time covered, authors' names, publication year, title, type of pollutant.

Fig. 1.

The screening process of articles

Table 2.

Inclusion and exclusion criteria for selection the eligible articles

Inclusion	Exclusion
All articles written in English language	Non-English, even with English abstract
Scientific peer-reviewed articles, including conference articles	Papers on indoor air pollution control interventions
Published No restrictions in the city or country	Published articles that contain only comments or suggestions or scenarios written by researchers
Articles published before 19 January 2021
Papers focusing on ambient air pollution

Results and discussion

Characteristics of literature

After removing the duplicates, 2219 documents remained for review based on titles and abstracts (Fig. 1). The full texts of 515 relevant documents were retrieved. Finally, 114 documents were selected according to inclusion criteria. The articles were divided into two categories. The first category articles include urban air pollution control strategies and policies based on geographical location (Table 3), the second category articles include urban air pollution control strategies and policies based on the type of pollutant (Table 4). Out of 114 final articles selected, 81 articles were placed in the first category and 33 articles in the second category.

Table 3.

Strategies and policies for urban air pollution control, categorized based on geographical locations

Country	Location		Time Period Covered	Strategy/policy	Study & publication year Reference
China	Beijing		Since the mid-1990s-2009	Outflow control on in-use and modern vehicles; Improving the quality of fuel consumption; Use of alternative and upgraded fuel in cars; Economic programs; Public transportation; Traffic control in high traffic times, a summary of emission standards for new cars adopted in Beijing light-duty gasoline vehicles (LDGVs) Euro1:1999.1.1 Euro2: 2003.1.1, Euro3:2005.12.30 (OBD 2006.12.1), Euro4:2008.3.1, Heavy-duty gasoline vehicle (HDGV) Euro1:2002.7.1 Euro2: 2003.9.1, Euro3:2009.7.1, Heavy-duty diesel vehicle (HDDV) Euro1:2000.1.1 Euro2: 2003.1.1, Euro3:2005.12.30, Euro4:2008.7.1 Motorcycle (MC) Euro1:2001.1.1 Euro2: 2004.1.1, Euro3:20085.7.1	Wu et al. (2011) [36] Zhou et al. (2007) [37]
			1999–2000	Retrofit carbureted LDGVs (light-duty gasoline vehicles) of almost all favorite models with three-way catalysts; downtown travel limitation rules; labeling based on pollutant emissions	Zhou et al. (2007) [37]
			2005	Using cleaner energy; industrial structure enhancement; Modification of transportation mode and control of vehicle exhaust emissions	Hao et al. (2005) [38]
			2005	Brisk growth in rail transit infrastructure	Guo et al. (2019) [39]
	the Beijing-Tianjin-Hebei (BTH) region, China		2014	Vehicles that meet China II and prior emission standards have obsoleted. Use of China V gasoline and diesel standards in Beijing, China IV gasoline and China III diesel standards in Tianjin and Hebei	Yang et al. (2018) [40]
			2014	Reducing local primary PM emissions; Diminishing local emission sources of pollutant gases (i.e., SO2, NOx, and VOCs); Control measures for NH3	Xing et al. (2019) [41]
			2019	Adjusting the economy to a balanced development, industrialization, and urbanization	Xie et al. (2019) [42]
	Chengdu-Chongqing		1999–2015	Energy replacement; elimination of substandard vehicles; elimination of motorcycles; raising fuel standards	Song et al. (2019) [43]
	Shanxi		2012	SO2 taxation; clean technical progress (CTP) and energy-efficient (EE) improvement	Weiet al. (2018) [44]
	Xi'an		2013	Emission reduction policies; Fuel standard advancements; Obsoleting of “yellow-marked” vehicles; More use of less polluting fuels (CNG, LNG); Installing DPF on older vehicles	Qiu et al. (2016) [45]
			2020	optimize the traffic control strategies to mitigate the vehicular emissions	Chen et al. (2020) [46]
	Hangzhou		2014	limitations on the utilization of private vehicles and the (re)location of industrial facilities	Ahlers et al. (2018) [47]
	National level Xi’an Beijing		2013–2019	“Clean Air Action” issued; “Comprehensive Action” issued; coal-fired emissions; adjustment and optimization of the energy structure; utilization of clean coal; increasing clean energy alternatives such as coal-to-gas or coal-to-electricity; increasing clean energy sources such as hydropower, wind power, and solar energy; renovation or elimination of coal-fired boilers; improvement of energy efficiency; elimination of civil bulk coal consumption Industrial emissions: Process improvement in industries, Modification or closure of high-pollution industries, diminish VOCs emission, Use of updated technologies Vehicle emissions: Production of new cars with strict emission standards; obsoleting of "yellow-marked" vehicles; progressing fuel quality Dust emissions: Improving the quality of the road cleaning process, closing down excess pollutants in concrete production, building forests	Li et al. (2020) [48] Cui et al. (2020) [49] Jin et al. (2016) [50]
		National level	2005	Expanding clean energy sources, uses of more high-quality coal, more control of vehicle emissions	Wang and Hao (2012) [51]
			2013	The Air Pollution Control Action Plan (APCAP)	Zhan et al. (2020) [52]
			2016–2020	According to13th Five-Year Plan (13th FYP) the emission of SO2 and NO2 should decrease 15% compared to its amount in 2015, and the ratio of heavy pollution days in 2020 should reduce 25% compared to that in 2015
			2015	Reduction of carbon emissions by 60%-65% in 2030 coal control in electricity generation sector and local pollutant control	Yang and Teng (2018) [53]
			2018–2020	The Blue-Sky Defense War: Adjustment and optimization of the industrial structure to achieve green industrial development; Creating conditions for clean energy production; adjustment of the transportation structure and build a green transportation system; “Environmental Protection Tax Law”	Jiang et al. (2021) [54]
Taiwan	National level		Since 1975	Air Pollution Control Act (APCA) issued on May 1975 and amended on May 2006); the PCDD/PCDF emission standard for municipal solid waste (MSW) incinerators (August 1997); upgrading or improvement of the performance of air pollution control devices Upgrading or improvement of the performance of air pollution control equipment ; Subsidizing local governments to accomplish air quality improvement programs (e.g., clean air zone, green vehicles promotion)	Tsai (2016) [55]
			in early 1999	The Air Pollution Control Act: tax-allowance subsidy; emission fee; cap-and-trade	Shaw and Hung (2001) [56]
			1995	The air emission fee program: Implementation stricter standards for air pollution control in industries; improvement in fuel quality; Production of new cars with strict emission standards	Kuo et al. (2009) [57]
Hong Kong	National level		1992	Expanding the existing “Smoky Vehicle Program” to detect and cite vehicles emitting excessive smoke; using unleaded petrol; Production of new cars with strict emission standards	Rusco and Walls (1995) [58]
			2003–2015	Implement strict standards for vehicle emissions; Apply new car fuel standards; Changing the fuel of diesel cars to liquefied petroleum gas (LPG); Reinforcement of vehicle pollution control equipment; tightening Euro 3 standard to Euro 5 standard from 2012	Cui et al. (2018) [59]
			2013–2014	Development of a policy for the use of converters of liquefied petroleum gas (LPG) fueled vehicles	Yao et al. (2019) [60]
Bhutan	National level/Thimphu		1999	Non-use of leaded gasoline, ban on the import of second-hand cars, implementation of car emission standards	Phuntsho and Kanitpong (2010) [61]
India	National level/ Delhi		Since 1981	Current and planned policies & programs: General: National Ambient Air Quality Standards (NAAQS), National Air Quality Index (AQI), Air Prevention and Control of Pollution Act (1981), Air Quality legislation/programs Power: advancement of renewables and energy efficiency; Use of air pollution control devices in coal-fired power plants Industry: Implementation of industrial pollution control laws, increase investment in renewable energy, use of air pollution control equipment Transport: Implementation the restrictions on the emission of car pollutants; implementation programs to make strides in public transportation; change the fuel of city buses from diesel to CNG in Delhi; Banning the import of second-hand cars Open burning of waste (outdoor): Implementing the law of non-incineration of waste Open burning of biomass (indoor): cleaner cooking fuels (i. e. LPG, electricity) and cleaner cookstoves	Amann et al. (2017) [62] Dholakia et al. (2013) [63] Majumdar et al. (2020) [64]
	National level		2019	National Clean Air Program (NCAP): increase the capacity of renewable energy; clean air policies and low-carbon energy targets and clean coal policies	Peng et al. (2020) [65]
	Delhi		In late 2001	Use of CNG as a cleaner fuel in vehicles	Chelani and Devotta (2007) [66]
			*	Complete removal of leaded petrol (01.09.1998); Gradual removal of old vehicles (30.11.1998); expansion of CNG supply network (31.03.2000); desulfurization of petrol (31.05.2000); 15–18 years old 2-wheelers to be prohibited (31.03.2001); 4-stroke 2-wheelers fitted with catalytic converters to be registered in Delhi (01.10.2000)	Kathuria (2002) [67]
			2010–2017	Cleaning the streets with a vacuum cleaner and sprinkler machine; limitation on construction activities; limitation on entry of truck traffic; limitation on the use of DG (diesel generator) sets; Fuel change in gasoline and diesel vehicles; Implementing the odd–even plan for personal vehicles	Goyal (2019) [68]
Japan	National level		Since 1968	Road Vehicles Act (1968): Reducing emissions of vehicles; improve road conditions; road pricing	Nishioka (1989) [69]
			*	Law or action (Brief Description) Establishment of the Smoke and Soot Law (1962) (Introduction of control of dust and smoke emission from factories in designated area) Establishment of the Basic Environmental Pollution Control Law (1967) (Clarification of pollutants emission responsibility and introduction of environmental quality standards) Establishment of the Air Pollution Control Act (1968) (Amendment of the Smoke and Soot Regulation Law) Revision of the Air Pollution Control Act (1970) (Introduction of nationwide uniform emission control and direct penalty) Establishment of the Environmental Agency (1971) Revision of the Air Pollution Control Act (1978) (Introduction of control of automobile exhaust gas) Establishment of the Basic Environmental Act (1993) (Introduction of the concept of environmental pollution Prevention) Establishment of the Ministry of the Environment & Establishment of the Automotive NOx/PM Law (2001) (Upgrade of the Agency & Amendment of the Automotive NOx Law) Revision of the Air Pollution Control Act (2006) (Introduction of control of VOC emission) Establishment of AQS for PM2.5 in addition to the standard for PM (2009) (Introduction of Air quality standard for PM2.5) Revision of the Air Pollution Control Act (2018) (Introduction of BAT control of mercury emission)	Botta and Yamasaki (2020) [70]
			1989	Setting emission standards for diesel vehicles regarding particulate pollutants; retiring older diesel trucks and buses in cities	Rutherford and Ortolano (2008) [71]
	Tokyo		1999	‘‘Operation No Diesel”; The installation of either diesel particulate filters (DPF) or diesel oxidation catalysts (DOC) on all diesel-powered trucks, buses, and special purpose vehicles in operation for more than seven years and used within city limits
Iran	Tehran		*	Clean Air Act (1995): Four-stroke engines must meet the ECE-40- 01 standard ( 2004); the third economy, social and cultural development plan of the country (2000); Removal of lead from petrol; establishing restricted area at the center of the city; Launching smart traffic signals on important streets; Establishment of the subway, the integrated master plan of air pollution control in Tehran (2000);Construction of parking meters in the city center; Installation of catalytic converters in cars(by 2005), imported cars must comply to the mentioned standard (2002)	Asadollah-Fardi (2008) [72]
Ten Asian countries	National level		Since 2007	The common target of Asian countries was to introduce EURO 2 by 2008 and EURO4 by 2012, to meet the EURO 2 standard level, it is important to install catalytic converters to meet the emission regulation levels (CO, HC, NOx, etc.), Prohibition of leaded gasoline in Asian countries (2008): Bio-fuel vehicle:(China, Indonesia, Philippines, Singapore, Thailand, India, Japan, Malaysia) Natural gas::(China, Indonesia, Philippines, Singapore, Thailand, India, Japan, Korea, Malaysia) DME(Japan) Hybrid vehicle:(China, Korea, Philippines, Singapore, Thailand, India, Japan, Malaysia) Electric vehicle::(China, Philippines, Singapore, Thailand, India, Vietnam, Japan) LPG::(China, Indonesia, Philippines, Korea, Thailand, India, Vietnam, Japan, Malaysia)	Hirota (2010) [73]
New Zealand	National level		1996	Land Transport Pricing Study, the government's Vehicle Fleet Emissions Control Strategy (VFECS)	Irving and Moncrieff (2004) [74]
Australia	Launceston, Tasmania		1994	Distribution of educational leaflets	Johnston, Fay et al. (2013) [75]
			July 2001 to June 2004	Wood heater replacement program
United States	National level/Los Angeles		1974	The Clean Air Act Amendments of 1970, Collecting emission taxes, improving fuel quality standards; Use of air pollution reduction equipment	Lareau (1981) [76] Judd (1977) [77]
	National level /Chicago		1970–1975	‘London Law’: restriction on the use of coal by residential and commercial sources in Chicago	Norco and Cohen (1973) [78]
	National level		1975	Constitutional limitations on emission quotas	Mandelker and Taub (1979) [79]
			1983	A comprehensive urban air quality control strategy: Reduction of air pollution by plants; Design of green belts as one of the components of reducing emissions and land use planning	Guldmann (1983) [80]
			1989	A new dramatic approach to constructing clean air legislation using alternative fuels as the important factor to reduce O3 and carbon monoxide concentrations	Fraas and McGartland (1990) [81]
	Kentucky		1970	Changes in the price of electricity. Methods available in Kentucky to control the emission of NOx and SO2 from fixed emission sources are: flue gas desulfurization, duct sorbent injection, limestone injection, multistage burners, physical coal cleaning, selective catalytic reduction, low NO, combustion, fluidized bed combustion, fuel switching and blending, and least-emissions load dispatching and production distribution	Scott et al. (1988) [82]
	Boston		1973	Dynamic emission control (DEC) systems such as fuel and load switching	Gaut et al. (1974) [83]
	Los Angeles		*	A Clean Fuels Program (January 1988): Program to reduce NOx emissions; the use of solar energy Air Quality Management Plan (1989): the use of electric vehicles that run on batteries or fuel cells; the use of clean fuels (such as methanol); advanced combustion modifications	Lloyd et al. (1989) [84]
	California		1991	Air Quality Management Plan: application of fuel cell technology	Lloyd (1992) [85]
	Houston		2004–2009	Clean Air Act (CAA)1990, Phase 1: Expanding local enforcement capabilities (January 2004–August 2005). Phase 2: Moving beyond enforcement authority (September 2005–July 2006), Phase 3: Alternate compliance and enforcement strategies (August 2006–2008)	Bruhl et al. (2013) [86]
Mexico	National level		1990	Driving bans; development of public transportation; utilize of alternative fuels; closure of polluting factories, industrial relocations	Collins and Scott (1993) [87]
	Mexico City		1989	With the implementation of the “Hoy No Circular" program, vehicle owners will not be able to use the car one day a week based on the latest number plate	Davis (2008) [88]
			1993	Conversion of trucks fuel to LP gas and installation of catalytic converters; Control of pollutants from boilers by installing control devices, removal of worn-out vehicles; prohibition of outdoor incineration; installation of vapor recovery systems in gasoline distribution; Improvement and expanding the electric trolley transport	Hardie et al. (1995) [33]
			1993 and following	Using catalytic converters on cars, Construct line 8 of the Subway; production of the gasoline based on international standards; prohibition of open burning; installation of control equipment on commercial boilers	Streit and Guzman (1996) [89]
Brazil	São Paulo		1986	Control of Air Pollution Emissions by Motor Vehicles (PROCONVE) (Use of fuels such as compressed natural gas and ethanol derived from sugarcane)	Carvalho et al. (2015) [90]
			2012	The use of Diesel (S50) with low sulfur content (50 ppm)
			2013	Use of diesel (S10–10 ppm of sulfur); Implement a policy to control pollution from motorcycles
ITALY	National level		1970	Controlling of domestic heating; regulation of industrial emissions; Reducing the amount of sulfur in fuels; changing the fuel; increasing the height of the chimney or use simple devices to reduce pollutants	Dente and Lewanski (1982) [91]
	Rome		2001	Establishment of two low-emission zones	Cesaroni et al. (2012) [92]
Germany	National level		1978	Measures related to licensing procedures to prevent air pollution in industrial plants; Implementation the air quality control programs in large polluting areas	Weber (1981) [93]
				Determination the emission restricts; limiting the sulfur content of gas-oil and diesel–fuel; determine installed gas-fired capacity (all types of gases)	Beck (1984) [94]
	Many German cities		Since 2008	Low emission zones (LEZ) have been implemented in many German cities, Munich since (2009)	Jiang et al. (2017) [95] Qadir et al. (2013) [96]
United Kingdom	National level		Since 1993	Production of All new gasoline-engine automobiles with three-way vehicle exhaust catalytic converters (VECs) containing platinum, palladium, and rhodium	Hutchinson and Pearson (2004) [97]
			*	Clean Air Act introduced in response to the notorious sulfurous ‘smog’ episodes in London and elsewhere (1956); air quality management (1997); Examples of the regulations include emissions from combustion plants, machinery, road and off-road vehicles, and industrial solvent usage	Carnell et al. (2019) [98]
	London		Beginning in February 2008	Setting a minimum Euro 3 standard for vehicles operating in the LEZ and a gross vehicle weight of more than 12 tons	Ellison et al. (2013) [99]
North Wales	National level		1996–2000	By-pass construction in the congested area	Burr et al. (2004) [100]
Netherlands	Amsterdam		Since 9/1/2009	implementation of LEZ	Panteliadis et al. (2014) [101]
Turkey	National level		Since 1983	Environmental protection law in 1983 and the regulation for the protection of air quality (1986); Fuel change; Use of air pollutant reduction methods	Plinke et al. (1992) [102]
	Istanbul		*	Natural Gas agreement with the former USSR in 1984; establishment of Istanbul Gas Distribution Industry and Commerce Corporation (IGDAŞ) in 1986 and began working in 1989, distribute natural gas on January 1992 Banning of poor-quality lignite usage in late 1993	Atimtay and Incecik (2004) [103]
Poland	National level		1990	Compliance with pollution standards for large combustion sources (installations greater than 200 kW); Significant increase in fines compared to the past	Toman et al. (1994) [104]
	Krakow		1990	Economic restructuring, tax on a single fuel, such as coal, tradable permits, banning the coal usage, emission tax	Adamson et al. (1996) [105]
Norway	Oslo		2011	Increase tolls for entering the busy areas of the city; the establishment of LEZs; allowing for temporary free public transport, implementation of even and odd plans, determining special routes for cars with less pollution, receiving more fees for parking	Sousa Santos et al. (2020) [34]
Hungary	National level		1994	A specific energy-saving program from National Energy Efficiency Improvement and Energy Conservation Programs (NEEIECP)	Aunan et al. (1998) [106]
Ireland	Dublin		from 1 September 1990	Establishment of the marketing, sale, and distribution of bituminous coal bans by the Irish government in the city of Dublin, the ban was extended to Cork in 1995 and ten other communities in 1998 and 2000	Clancy (2002) [107] Dockery et al. (2013) [108]
Greece	Athena		1983	Environmental optimization of traffic signal timings; pollution-sensitive traffic re-routing; presentation of “clean” vehicles	Stathopoulos and Argyrakos (1993) [109]
	Thessaloniki		2003	Effective car maintenance; full elimination of low technology private car	Mavrotas et al. (2006) [110]
South Africa	National level		*	The Atmospheric Pollution Prevention Act (APPA) 1965 (Legislation); the constitution of the Republic of South Africa 1996 (Legislation); the National Environmental Management Act (NEMA) 1998(Legislation); Air Quality Act 2004/2005 (Legislation); the national framework for air quality management in South Africa 2007 (policy)	Naiker et al. (2012) [111]
Chile	National level		1993	Levy a tax on wood consumption, and establishment of subsidies for more efficient combustion technologies	Chávez et al. (2011) [112]
	Santiago de		2006	A large transformation in the public transportation system	Gramsch et al. (2013) [113]
the United Kingdom, New Zealand, Australia, United States of America, Canada, and The Netherlands	United Kingdom		*	A 70% subsidy for the cost of coal heater replacement with a clean-burning; designing various types of solid smokeless fuels in 1956; formulating a standard method BS 3841in1972 to deal with made solid smokeless fuels for application in residential open fires	Quraishi (1988) [27]
	New Zealand		*	Enacting the Clean Air Act in 1972 (coming into force in 1973); The establishment of the Clean Air Council (1973); enacting the law on the use of residential solid fuels
	Australia		*	Standards for air pollutants (1972), amended (1979); enactment of the Clean Air Act (1961); The strategy adopted in this Act combines the two principles of “best practical means” and “air quality management.” (1974); Amendment Act; (1986) increase chimney height
	States of America		*	In response to the requirements of the Clean Air Act (1970), Major sources of particulate matter and other pollutants (mostly industry and automobiles) must take appropriate control measures to reduce pollutants
	Canada		from March 1985	Installation of heaters and furnaces under the Canada Oil Substitution Program use of heaters and furnaces under the Canada Oil Substitution policy
	The Netherlands		1982	A foundat ion “Kachel en Schoorsteen**” was formed

*The time period is cited in the Policies and Strategies column

**Stove and Chimney

Table 4.

Strategies and policies for urban air pollution control, categorized based on the type of pollutant

pollutant	Country Geographical	Time Period Covered	Strategy/policy	Study & Publish Time
PM	China National level	(2011–2015)	The 12th Five Year Plan with the aim of 10% and 8% reduction in the emission of NOx and SO2	Zhao et al. (2013) [132]
	Iran Tehran	*	Vehicle emission regulations and policies (2012); Trucks and LDV must have a minimum standard of Euro IV and motorcycles standard of Euro III; Temporary and immediate strategies to reduce air pollution in emergencies, such as shutting down construction activities, enforcing traffic restrictions, and closing schools and government offices	Daneshpajooh et al. (2020) [133]
PM10	United Kingdom London	2005	Measures related to reducing pollutant emissions; changing vehicles fuels; taking measures related to transportation and traffic; implementation of LEZ	Mediavilla and ApSimon (2006) [134]
PM2.5	China National level	2011–2018	Control of open biomass burning and fossil-fuel combustion	Ding et al. (2019) [135]
		2013	Use of electric public vehicles; Improvement of coal-fired power plants; the 13th Five Year Plan (FYP)	Zhang et al. (2020) [115]
		2013	The air pollution prevention and control action plan (APPCAP); Installation of control equipment for VOCs emissions, change of domestic fuel consumption with natural gas or electricity	Gao et al. (2020) [136]
	Brazil Sao Paulo	2018	An updated National Ambient Air Quality Standards (AQS) policy; establishing intermediary progressive levels for each pollutant; establishing two major emission control programs in São Paulo: Program de Control de Emissions de Fontes Estacionárias (PREFE)**and Plano de Control da Poluição Vehicular(PCPV)***	Albuquerque et al. (2019) [137]
fine and coarse particles	India Delhi	Since 2016	Applying the odd–even car trial scheme to reduce car traffic between 8.00 and 20.00 h daily	Kumar et al. (2017) [138]
PM 2.5 -bound nickel (Ni)	China Xi'an	2004–2013	Use of less coal, modify energy structure, enforce strict rules to reduce pollutant emissions	Xu et al. (2017) [139]
Particulate and lead	Egypt Cairo	1994	Law for the Environment; formulation of the general policy and preparing the necessary plans for the protection and promotion of the environment; controls for particulate; reducing emissions from industries, implementing programs aimed at improving and reducing fuel consumption of cars, changing the fuel of city buses to CNG	Raufer (1997) [140]
PM10 and diesel soot	German many of cities	since 2010	Implementation of LEZ	Cyrys and Josef (2014) [141]
Fine Dust	Korea National level	2018	The comprehensive action plan on fine dust: Power Generation: Reduction of share of coal-fired power generation by shutting down old coal-fired plants; establishing the “8th National Electricity Supply Plan” from an eco-friendly perspective; increasing the share of renewable energy Industry: Extending the scope of targets of emissions cap regulation; implementing the emissions cap regulation to reduce total suspended particles (TSP); introducing emission charges for NOx Transportation: Reducing emissions from old, diesel vehicles, and expand the targets for driving restriction; increasing the use of eco-friendly vehicles including electric cars and hydrogen cars; implementing the “bonus-malus system (BMS)”; strengthening control over fine dust emissions from vessels and construction machines, Surroundings: intensive controlling over blind spots in management such as construction sites and illegal incinerations; distribution of road-cleaning vehicles and extend urban forests	Trnka (2020) [142]
Primary Particulate Matter, So2 and NOx	United States Los Angeles	Since 1987	Particle control measures: use of electrostatic precipitators; road improvement; speed reduction to 15 km/h on unpaved roads; use of new technologies for diesel vehicles Measures to control SO2: reduction of vehicle fuel sulfur, use of electric vehicles Measures to control NOx: use of alternative fuels such as natural gas for domestic use; Use of pollution control equipment such as selective catalyst reduction and combustion modification in industries	Kleeman and Cass (1999) [143]
SO2	Japan–China National level	since the early 1960s	end-of-pipe treatment (desulfurization); pretreatment (fuel desulfurization) and substitution with low-sulfur alternatives; industrial structural change; production process efficiency; Environment Law (or “Basic Anti-Pollution Law”) was enacted (1996 in Japan)	LI et al. (1999) [2] Kanada et al. (2013) [144]
	China National level	1995	Changing to lower sulfur content fuel; introducing desulfurization devices for emission sources; promoting efficient energy use	Ikeda et al. (2001) [145]
		1997	Measures to reduce local air pollutants; sulfur tax; improvement of energy efficiency	Xu and Masui (2009) [146]
	China the Pearl River Delta region	since 2005	Increasing energy efficiency (2005–2008); closing small and high-emitting power plants; installing flue-gas desulfurization (FGD) equipment in all the power plants; encouraging the use of fuel with lower sulfur contents	Wang et al. (2013) [147]
	United States National level	1970	Use of low-sulfur coal; Use of control equipment to remove SO2; Applying Intermittent Control Systems	MacDonald (1975) [148]
		1970	Applying supplementary control systems (SCS)	Ruane et al. (1976) [149]
NOx	China Guangzhou	Since 2000	Vehicle emission standards, inspection and maintenance program; scrappaging and retrofitting old vehicles; gasoline quality improvement; alternative-fuel vehicles	Shao et al. (2001) [150]
NO2	Korea National level	since June 2000	Replacing diesel fuel buses with the natural gas buses; use of diesel particulate filter (DPF) or diesel oxidation catalyst (DOC); cash incentives or tax reductions for the vehicles of low pollutant emissions and hybrid fuel	Shon and Kim (2011) [151]
VOCs	Hong Kong National level	*	Changing fuel of city buses from diesel to liquefied petroleum gas (LPG) (2000); implementation of the catalytic converter replacement program (CCRP) (2013)	Cui et al. (2021) [152]
	Korea Seoul	2005	Special measures for metropolitan air quality improvement; reducing the use of solvents; reducing the VOCs of vehicle exhaust	Shin et al. (2013) [153]
Ozone	United States California's South Coast Air Basin	Since the 1960s	Progressive and more stringent controls on emissions of NOx and VOCs	Fujita et al. (2013) [154]
	United States National level	*	Establishment of National Ambient Air Quality Standards (NAAQS) by the US Clean Air Act (CAA) for various types of air pollutants (1990); enacting a law to reduce ozone (1997); Clean Air Act baseline (2007): Ozone control measures from production sources:: electric utility point sources; nonutility (industrial) point sources; highway vehicles; non-road mobile sources (1997)	Streets et al. (2001) [155]
		Since 1977	According to the Clean Air Act Amendments (1977), Twenty-four states have improved State implementation plans (SIP) to reduce ozone; VOCs emission control programs: vapor control in gasoline marketing, low solvent; vehicle inspection and maintenance programs	Pratapas and Calcagni (1983) [156]
	United States Connecticut	1990	Increasing tax on gasoline for VOCs reduction; assigning of cleaner emission standards on newer cars according to the Clean Air Act	Heninger and Shah (1998) [157]
	United States Macon, Georgia	2004	Assigning Air Quality Standards for ozone (U.S. EPA 2004b); expanding control plans in mid-sized metropolitan regions; compulsion of Macon for attaining ozone standards by 2009	Cohan et al. (2006) [158]
	China Pearl River Delta	2006	limiting Heavy trucks from driving within urban areas during the daytime in major PRD cities	Yu et al. (2014) [159]
	Spain Southwestern	Since 2008	Determination of ozone threshold value by the European Union Directive 2008/50/EC; reducing both NOx and VOCs anthropogenic emissions; Implementation of the even–odd plan, implementation of traffic limitation in some areas of the city; closure of some polluting industries	Castell et al. (2011) [160]
PM2.5 and O3	China Beijing	2014	Implementing a series of emergency pollution control measures at the Asia–Pacific Economic Cooperation (APEC) meeting; Implementation of the even–odd plan; suspending or reducing the operations of power plants and factories; implementation of the couple and individual plan, closure of construction activities in case of emergency	Tong et al. (2020) [161]
photochemical smog	United States Los Angeles	*	Adopting emission standards (1959); creating a Motor Vehicle Pollution Control Board by the State of California (1960); The Federal Clean Air Act (1963), its amendments (1965); the Air Quality Acts (1967, 1970); ambient air standards; emission standards; applying appropriate control techniques	Haagen Smit (1972) [162]

*The time period is cited in the policies and strategies column

**Stationary sources emission control program

***Vehicular pollution control plan

Characteristics of the retained studies

Table 3 summarizes the urban air pollution control strategies and policies based on geographical location. Among the 81 articles, the summarized number of studies performed for each country was as follows: China (n = 19), USA (n = 11), India (n = 7), Germany and Mexico (n = 4), the United Kingdom, Japan, Taiwan, and Hong Kong (n = 3), Italy, Turkey, Poland, Ireland, Egypt, and Chile (n = 2), Iran, Bhutan, New Zealand, Australia, Brazil, Wales, Norway, the Netherlands, Hungary, South Africa (n = 1), an article covered ten Asian countries and an article focused on the United Kingdom, New Zealand, Australia, States of America, Canada and the Netherlands.

Among 33 articles classified in second category, the summarized number of articles based on the type of pollutant in was as follows (see Table 4): PM: China and Iran (n = 1); PM₁₀: United Kingdom (n = 1); PM_2.5: China (n = 3), and Brazil (n = 1); fine and coarse particles: India (n = 1); PM_2.5-bound nickel (Ni): China (n = 1); [Particulate and lead: Egypt (n = 1); PM₁₀ and diesel soot: German (n = 1); Fine Dust: Korea (n = 1); Primary Particulate Matter, SO₂, ROG and NO₂: the United States (n = 1); SO₂: China (n = 3), United States (n = 2), Japan (n = 1), and Japan–China (n = 1); NO₂: China (n = 1); NO₂: Korea (n = 1); VOCs: Hong Kong and Korea (1 each); O₃: United States (n = 5), China and Spain (1 each); PM_2.5 and O₃: China (n = 1); and photochemical smog: United States (n = 1).

A summary of planned urban air pollution control strategies and policies

Figure 2 shows the classification and categories of different air pollution control policies enacted in different countries. In general, the selected studies were classified in two categories: (1) studies focused on introduction the policies and strategies to control air pollution in different countries of the world, and (2) articles focused on different policies and strategies to control one or more specific pollutants.

Fig. 2.

Segmentation of urban air pollution control policies and strategies

Strategies and policies for urban air pollution control, categorized based on geographical locations

In this section, urban air pollution control strategies and policies were classified into four categories, namely, general strategies and policies, transportation, energy, and industry.

General strategies and policies

The most important regulations, general strategies and policies to air pollution control enacted in different countries are as follows: (See Table 3).

Clean air action [48–50], clean air act (CAA) [27, 72, 86, 98, 114], air pollution control act (APCA) [55], Environmental Protection Law [102], the atmospheric pollution prevention act (APPA), Air Quality Act and the National Framework for Air Quality Management in South Africa [111], The Third Development Plan of Economy, Social and Cultural of the Country and Integrated Master Plan of Air Pollution Control [72], the Air Pollution Control Action Plan (APCAP) and 13th Five-Year Plan (13th FYP) [52], the Blue-Sky Defense War and Environmental Protection Tax Law [54], Comprehensive Action issued and Improve environmental law [48–50], Comprehensive Urban Air Quality Control Strategy [81], Air Quality Management Plan [84], Road Vehicles Act [69], Smoke and Soot Law, Establishment of the Basic Environmental Pollution Control and Establishment of the Air Pollution Control Act [70], the PCDD/PCDF Emission Standard for Municipal Solid Waste (MSW) Incinerators and Subsidize Local Governments to Accomplish Air Quality Improvement Programs [55], Expanding the Existing Smoky Vehicle Program [58], National Ambient Air Quality Standards (NAAQS) and Air Prevention and Control of Pollution Act [62–64], National Clean Air Program [65], the Comprehensive Survey on Air Pollution [61], the Clean Air Act Amendments [76, 77], Clean Fuels Program [84], the low for open burning of waste (outdoor) [62–64].

Economic policies [36, 48–50], economic incentive policy [69], reduce carbon emissions [53], reforestation program [33], restriction on use of DG (diesel generator) sets [68], distribution of educational leaflets [75], emissions tax [76, 77, 105], subsidy for expenditure on abatement capital [76, 77], joint prevention and control of regional air pollution [51], reducing regional emission sources of gaseous pollutants [41], adjusting the economy to balanced development and industrialization and urbanization [42], strengthen government governance responsibilities and implement environmental supervision actions [54], SO₂ taxation [44], Prohibit all open burning [38, 89], prohibition in “backyard” refuse burning [27].

Air pollution control regulations and policies have been enacted in many countries around the world; most cases had positive consequences. For example, in a study focused on the impact of the Blue-Sky Defense War law in China, a 14.49 and 23.43 μg m⁻³ reduction in the monthly average concentration of PM_2.5 and PM₁₀ were obtained [54]. Zhang et al. (year) showed that over the 40-year period in the UK, PM_2.5 and NO₂- attributed mortalities were reduced by 56% and 44% respectively. While, the O₃- attributable mortalities experienced an increase of 17% over the same period [115]. It has also been shown that the most effective way to control air pollution, especially greenhouse gases, is to accept and implement international agreements such as the Paris Agreement [116].

A preliminary analysis in the United States (1975) indicated that emissions tax and an increase in the gasoline tax were more effective than emission standards [77]. China in 2005 performed a comprehensive control policy in order to control the multiple pollutants (SO₂, NO₂, VOC and PM) emissions and emission sources in local and regional levels [51]. The results of study Streit in Mexico City showed that the prohibition of all open burning, implemented in 1993, significantly reduced CO, HCs, and NO₂ emission compared to other pollutants [89].

Strategies and policies related to transport

The most common measurements for air pollution control with special focus on transportation are divided into four categories: improving infrastructure and transportation structure, control measures on vehicles, fuel quality improvements and alternative fuels, and traffic restrictions.

Improving infrastructure and transportation structure.

The most important measures related to the improvement of infrastructure and transportation structure to control air pollution are as follows (See Table 3):

Improvement of road structure [69], adjustment the transportation structure and building a green transportation system [54], transportation mode modification [27], expansion of public transportation [87, 113], public transport, improvement and promotion the public transport [36, 62–64], allowing for temporary free public transport [34], phasing out of worn-out vehicles [33, 67], Phasing out of worn-out Two-wheeled engines [67], retirement of older diesel trucks and buses in urban areas [71], elimination of motorcycles and substandard vehicles [43], retire old vehicles [48–50], elimination of “yellow-brand” vehicles [45, 48–50], obsoleting vehicles with earlier emission standards [40], optimize the traffic control strategies [46], install enough numbers of parking meter [72], imposing higher parking fees [34], advanced vehicles [36], public awareness of traffic [72], make strict standards for new vehicles [48–50], temporal traffic control measure [36, 48–50], control of imported cars [72], banning of imported used cars [61–64], electric vehicle incentive program [62–64], growth in rail transit infrastructure [39], construction of underground lines [72], expansion of the Subway [89].

Congestion is considered as one of the most important concern in large cities. Traffic congestion causes high costs, delays, and increased fuel consumption; it has unpleasant social and environmental consequences [117]. One of the most important policies and strategies related to transportation is recommendation for less use of private cars and more use of public transportation and bicycles. A study conducted by Rojas-Rueda, et al. in Barcelona (Spain) revealed that 40% decrease in car trips would prevent 10.03 deaths, due to a 0.64% reduction in exposure to PM_2.5 [118]. In San Francisco, some incentives are considered for the employers to less use of private cars, public transport use and bicycle uses; the people are given some subsides or pre-tax deductions of transport costs [119]. The establishment of subways in cities as a means of public transportation has a major impact on reducing air pollution. For instance, since 2005, Beijing has experienced a rapid growth in rail transportation infrastructure and improved air quality. The results showed that the operation of the rail transit system has a great effect on reducing most of the air pollutants concentrations (PM_2.5, PM₁₀, SO₂, NO₂, and CO), however, it has little effect on the reduction of O3 pollution [39]. Incentive programs such as free public transport, and restrictive programs such as increasing parking fees can reduce air pollution in cities. In Oslo, increases in parking fees were the most effective measurements to control the air pollution in Norway (2011) [34].

Control measures on vehicles.

The most important control measures on vehicles to control air pollution listed in Table 3, are as follows:

Emission control on in-use and new vehicles [36], tightening vehicle emission standards [51, 57–59, 61, 62, 72, 76, 94], the installation of either diesel particulate filters (DPF) or diesel oxidation catalysts (DOC) on all diesel-powered trucks, buses [71], convert delivery trucks to LP gas and install catalytic converters [53], use of catalytic converters [33, 67, 72], use of three-way vehicle exhaust catalytic converters (VECs) [97], retrofit in-use vehicles [48–50], retrofitting emission control devices [59], motorcycle (and Similar) Emission Control Program [90], retrofit carbureted LDGVs [37].

Focusing on vehicle emission restrictions is more important than on urban air quality restrictions. Therefore, stricter standards are mainly considered for new cars [67]. Standards are a series of driving cycle emission regulations enacted for control the air pollutants emitted from the light- and heavy-duty vehicles and motorcycles [36]. Although vehicle emission control strategies can be easily proposed, however, they are hard to implement due to their dependency on the public acceptance and support [61]. Euro emission standards are one of the most important measures that governments plan and implement to control vehicle emissions. In Beijing, for example, from 1999 to 2008, Euro 1 to 4 standards were planned and implemented for variety of vehicles [36]. Vehicle Exhaust Catalysts (VECs) was enacted to control or reduce the many air pollutants (NO_x, VOCs and CO) emitted from the petrol-fueled vehicles. The results of study conducted by Hutchinson in the UK in 2004 reported that VECs significantly reduce the concentration of pollutants: NO₂ (20%), PM10 (10%), VOCs (30%) and CO (70%) [97]. In diesel vehicles, the traditional Diesel Catalyst Oxidation (DOC) has been employed to decrease the concentration of THC and CO. In addition, Diesel Particle Filters (DPF) and Selective Catalyst Reduction (SCR) systems have been also employed to reduce the particle and NOx emissions, respectively [120, 121]. In a study focused on investigation the contribution of a new generation diesel light-duty vehicles (Euro 6) on the urban air quality, the results showed a 60% decrease in NO₂ emissions [122].

Fuel quality improvements and alternative fuels.

The most important measurements related to fuel quality improvements and the use of alternative fuels to control air pollution are listed in Table 3:

Fuel quality improvements or raising fuel standards or updating vehicle fuel standards [36, 40, 43, 45, 48–50, 57–59, 62, 73, 89, 94], complete removal of leaded petrol [67, 72], diesel with low Sulfur content [90], the use of alternative fuels [36, 81, 83, 87, 91, 102], the use of ethanol derived from sugar cane [90], switching diesel vehicles to liquefied petroleum gas (LPG) vehicles [59], increased use of alternative fuels (CNG, LNG) [45, 48–50], the use of CNG as a cleaner fuel in vehicles [65, 67, 73, 90], the use of electric vehicles powered by batteries or fuel cells and use of cleaner-burning fuels (such as methanol) [84], shifting the public transport buses from diesel to CNG [62], bio-fuel vehicle, Hybrid vehicle, Electric vehicle, LPG [73], buses to switch over to CNG or other clean fuel [67, 72] promotion of biofuels [62–64].

Several studies have confirmed the relationship between fuel quality and air pollution [123–125]. One of the preconditions for adopting emission standards is to improve the fuel quality. As an example, the removal of Lead (Pb) from gasoline is the main prerequisite for accepting the Euro 1 standard [36]. Using alternative fuels instead of fossil fuels is an important step in reducing urban air pollution. Brazil uses fuels such as sugar cane-derived ethanol and compressed natural gas on a large scale as gasoline alternatives. More than 50% of the fuel used in Brazilian light vehicles is ethanol [90]. In 1999, buses running on CNG were introduced into the Beijing bus fleet [36]. Hybrid and electric vehicles are a good alternative to fossil fuel vehicles and are effective in reducing air pollution. Many of the biggest automobile producers have been producing hybrid and electric vehicles for years, and very countries like China, Korea, Philippines, Singapore, Thailand, India, Japan, Malaysia use them [73, 126].

Traffic restrictions.

The most important measurements related to Traffic restrictions to control air pollution listed in Table 3, are as follows:

Establishing restricted area in the center of the city [72], implementation of LEZ [34, 87, 92, 99, 101], increasing the tolls that give access to the inner parts of the city [34], by-pass construction in the congested area [100], environmental optimization of traffic signal timings [109], other vehicle restrictions [88], restrictions on the use of private cars [47], restriction on entry of truck traffic [68], enforcement of odd–even scheme for private cars [34, 68], defining priority lanes for low emission vehicles [34].

One of the control measurements that has attracted a lot of attention in many industrialized countries is LEZ. The use of vehicles is restricted in these areas, mostly in the city centers and crowded places of the city. In some cities, LEZ is considered for diesel-burning heavy-duty vehicles. However, some of these restrictions are also being applied to other types of vehicles, such as very old and polluting vehicles. In some countries such as England, Germany, and the Netherlands, vehicles are assigned a sticker (red, yellow, or green) according to the tax class and Euro emission standard. In some countries, only vehicles with certain emission standards and with the relevant label are allowed to enter to the special zone [95, 127]. Numerous studies have been investigated to analyze the influence of LEZ on air quality. Most of them declared the LEZ to be effective in reducing the concentration of air pollutants [99, 101, 128, 129]. The results of study conducted by Cesaroni in 2012 Rome reported that implementation the LEZ approach have been reduced the NO₂ and PM₁₀ concentrations by 23 and 10%, respectively. However, the reductions were mostly in the intervention area not the whole city [92]. Congestion charging areas are defined as areas of the city that have special rules for entering them, such as paying tolls. There is evidence that congestion charging improves the air quality via behavior changes including using public transportation instead of private car [130]. In Oslo, Norway, since 2017, in the congestion charging zones, vehicles pay different prices depending on the type of fuel, the EURO standard, type of car (heavy, light), and time of day (rush hour, not rush hour). Of course, Electric vehicles can pass free in this area [34]. The odd–even scheme for private vehicles based on license plate numbers is one of the traffic restrictions which is usually applied in the case of emergency air pollution and temporarily for some big cities in a larger geographical area than other traffic restrictions [34, 68].

Strategies and policies related to energy

The most important measurements related to energy consumption for control air pollution listed in Table 3, are as follows:

Cleaner energy use [38, 51, 54], energy efficient (EE) improvement [44, 48, 106], energy saving [106], energy replacement [43], use of solar energy [84], clean technologies such as the fuel cell [85], utilizing the clean coal [48–51, 56], coal control in electricity generation sector [40], increasing clean energy alternatives such as coal-to-gas or coal-to-electricity, elimination of civil bulk coal consumption and adjusting and optimizing the energy structure [48–50], the use of renewables and nuclear energy [62–64], cleaner cooking fuels (i. e. LPG, electricity) and cleaner cook stoves [62–64], wood heater replacement program [75], restriction on the use of coal by residential, and commercial sources [78], changes in the price of electricity and shifts in coal usage [82], control of domestic heating [91], tax on a single fuel, such as coal and ban on coal use [105], banning the marketing, sale, and distribution of bituminous coal [107, 108], signing the natural gas agreement with the former USSR and banning the use of poor quality lignite [103], restriction on use of coal/wood-based tandoor in restaurants and street eateries [68], taxes on wood consumption for heating and cooking and subsidies for more efficient combustion technologies [112], a 70% subsidy for the cost of coal heater replacement with a clean-burning, solid fuel replacement, using only a certain type of fuel and formation of foundation [27].

Energy is an integral part of human life. Energy sources have a lot of changes due to the advancement of science and technology. Coal is one of the energy sources used for many years in both industry and home heating. The residential coal-burning was known as the most probable source of pollutants for the London Smog. This disaster caused 4000 excess deaths in one week in the London area in1952 [27]. Two of the most important actions in the UK government after this catastrophe were approval of the Clean Air Act in 1956 and allocating 70% subsidies to replace coal fuel heaters with clean fuel heaters. The contribution of the federal government for controlling the air pollution was calculated to be 40%, while the contribution of local government were the remaining 30% [131]. Other countries also take good measures to remove coal, especially from home use. The Turkish government signed a natural gas agreement with the former USSR in 1984. Distribution of the natural gas started in Istanbul in January 1992 and the use of poor-quality lignite was banned in late 1993 [103]. Wood stoves became increasingly the popular for home heating in Tasmania, Australia in the late 1980s and early 1990s. In this regard, some studies were conducted from 1991 to 1993, and results indicated that the main source of air pollution was solid fuel. The government improved air quality by allocating funds to replace electricity instead of solid fuel [75]. In 2013, it was reported that coal in China is still the main source of energy, unlike other developed countries in the world. Approximately 52% of coal in China is consumed in the electricity generation sector [53]. The results of a study in Dublin, Ireland reported that after banning coal sales, the average black smoke concentrations, adjusted non-trauma death rates, respiratory deaths and cardiovascular deaths declined by 70%, 5.7%, 5.15%, 10.3%, respectively [107]. Clean technical advances (CTP), energy efficient (EE) improvement, and energy savings, in addition to socioeconomic impacts, can reduce emissions of SO₂, CO₂, and PM_2.5 [44].

Strategies and policies related to the industry

The most important measurements related to the industry for control air pollution listed in Table 3, are as follows:

Industrial structure improvement [38, 48–50, 54, 91], adjustment of industrial structure [48–50], installation of control equipment on industrial and commercial boilers [33], required use of abatement equipment [76, 77], measures in connection with the licensing procedures for industrial plants [93], the substantial elevation of emissions fees and fines [104], the (re)location of industrial facilities [47, 87], elimination or upgrading industries with excessive, backward, and polluting industries, reduce volatile organic compounds (VOCs) emission, promote cleaner production (CP) and accelerate the technological transformation and improve innovation capability [48–50], tightening exhaust emission standards for industry [57], use of higher chimney [27, 91], stopping some factories in a pollution emergency [48–50, 87], use of high efficiency electrostatic precipitator technology in large coal-based power plants and emission regulations for industries [62–64].

The presence of industries near the cities and the emission of various pollutants have made governments to enact strict laws and policies in order to control air pollution caused by industries. Due to the antiquity of some industries and the advancement of scientific knowledge, improving the industrial structure is considered the most important and effective measurement for reduction the urban air pollution [38, 48–50, 91]. Implementation the industrial structure improvement and regulation policies in Jinan and China, reduced SO₂ and NO₂ emissions by 37.51% and 7.47%, respectively [49]. Licensing procedures for industrial plants, the substantial elevation of pollutants emission fines, and stopping some factories in a pollution emergency are other useful measures to significantly reduce air pollution [48–50, 87, 93, 104]. Of course, every industry should have control measures according to the type of environmental pollutants [48], which is not discussed in this article.

Strategies and policies for urban air pollution control, categorized based on the type of pollutant

The urban air pollution control strategies and policies with focus on six categories based on the types of pollutants such as PM, SO₂, NO₂, VOC_S, O₃, and photochemical smog are listed in Table 4.

The most important measures to PM control

Reduction the nitrogen oxides (NO_x) and sulfur dioxide (SO₂) [132], fuel quality improvements [133, 140], school and governmental sector closure, shutdowns of construction works, and traffic limitation (at the time of air pollution) [133], low emission zones (LEZ) [134, 141], traffic management and improved public transport [134, 137], control of open biomass burning and fossil- fuel combustion [135], electrifying public transportation, upgrading coal-fired power plants [115], eliminating small coal-fired boilers, phasing out small high-emitting factories [136], the odd–even car scheme [138], coal consumption reduction, energy structure reconstruction tighter emission rules, improvement of the industrial and motor vehicle waste control techniques [139], industrial source controls, targeted mobile source controls, street sweeping, road paving, general mobile source controls, general fugitive control strategies [140], restrict speed, treat unpaved access roads, required use of abatement equipment in industry [143], reduce emissions from old, diesel vehicles, increase the use of eco-friendly vehicles including electric cars and hydrogen cars [142].

A variety of air pollutant, in particular, PM adversely affect the quality of Urban air. Among major contaminants, fine particulates due to adverse health effects has drawn much attention [163, 164]. In the case of air pollution, governments often use both short-term and long-term strategies. For example in Iran, government have made such effort to control the via implementing the temporary urgent control strategies including as shutdowns of construction works and traffic restrictions and school and governmental sector closure, [133]. Control measures in transportation can reduce concentration of particulate matter. The results of a study on the low emission zones in Germany showed that after implementation of LEZ in Cologne, Berlin, and Munich in a short time, the PM₁₀ concentration considerably decreased. A significant reduction in PM_2.5 related to traffic-related particle fraction was observed in Munich [141]. Implementation of the odd–even car trial scheme in India in 2016 showed that the average hourly rates of PM_2.5 and PM₁₀ decreased by 74% compared to the corresponding hours during the previous year [138]. The results of a study conducted in eastern China indicated that the most important factors influencing PM_2.5 concentration are controlling the open biomass burning and fossil-fuel combustion [135]. Depending on the origin of the particulate matter, controlling other pollutants can also reduce its concentration. For instance, research on air pollution control measures in Los Angeles showed that controls of SO_x applied to the base emissions inventory decreased the average PM_2.5 concentrations by 1.2 µg m⁻³. The ammonia control measures reduced daily PM_2.5 concentrations by 10.9 µg m⁻³ in comparison with its base concentrations [143].

The most important measures to SO₂ control

Regional Clean Air Incentives Market, electric vehicles [143] reducing the sulfur content of the fuel [143, 145, 147], fuel desulfurization, substitution with low-sulfur alternatives, industrial structural change, production process efficiency [144], promoting efficient energy use [145–147], sulfur tax [146], closing small and high-emitting power plants [147], installing flue-gas desulfurization (FGD) equipment in all the power plants [147, 148], burning low sulfur western coal [148], fuel switching, load shifting, environmental dispatch or intermittent controls [149]. Most of the control measures for reducing the SO₂ emission are related to the improvement and modification of the energy efficiency. Changing the energy structure with a significant reduction in coal consumption and the use of alternative fuels such as natural gas and electricity has been the most important measure to reduce SO₂ emissions [48]. Increasing energy efficiency from 2005 to 2008 and measures such as closing small and high-emitting power plants, installation of flue gas desulfurization (FGD) equipment in all power plants, emission control from boilers and kilns, promotion of cleaner production, and using fuel with lower sulfur contents in the Pearl River Delta (PRD) region of China reduced SO₂ concentration by 39% between 2006 and 2009 [147]. Improving and changing the vehicle fuel and the use of electric vehicles are other important measures to reduce SO₂ emissions [143].

The most important measures to NO₂ control

Vehicle emission standards, inspection and maintenance (I/M) program, scrappaging and old vehicle retrofitting practices, gasoline quality improvement, controlling non-road engines, alternative-fuel vehicles [150], the natural gas vehicle supply (NGVS) program, use of diesel particulate filter (DPF) or diesel oxidation catalyst (DOC), cash incentives or tax reductions for the vehicles of low pollutant emissions and hybrid fuel [151]. Electrify stationary no generating internal combustion engines and turbines, eliminate planned fires, selective catalytic reduction in petroleum refinery, radiant burners; alternative fuels in small boilers and process heaters [143].

Vehicles (especially privatized cars and motorcycles) contribute between 30 and 75% of the total NO_x emissions in cities, therefore, most measures to reduce NO₂ emissions in urban areas are related to transportation [165–167]. A study focused on NO₂ trend in the long-term period alongside 16 urban roadside locations in 7 major Korean cities over 11-year period (1998–2008). The use of low emission diesel engines and natural gas-burning vehicles program were considered as key emission control strategies since June 2000. The results showed that NO₂ levels decreased over 11 years due to the combined effects of control strategies [151]. Energy-related control measures comprise the use of natural gas for conservation of commercial and residential energy. Important measures taken in the industry to reduce NO₂ emissions include the use of selective catalytic converters, the elimination of radiant burners, the improvement of combustion, and the use of alternative fuels instead of highly polluting fossil fuels [143].

The most important measures to VOCS control

Change from diesel to liquefied petroleum gas (LPG), use of catalytic converter replacement [152], reduce the use of solvents, reduce the VOCs of vehicle exhaust [153].

The most important sources of VOCs emission are biological resources, vehicles, and industries. After industrial revolution in the eighteenth century, the spread of VOCs gradually increased in cities. [168]. In the light of industrial revolution and controlling measurement for VOCs, most public light buses and taxies in Hong Kong were modified from the diesel-burning buses to liquefied petroleum gas (LPG) fuel type in the early 2000s. The government carried out an LPG catalytic converter replacement program (CCRP) from October 2013 to April 2014 which reduced VOCs by 36.7% [152]. Reducing solvent use and vehicle exhaust VOCs have been implemented in the Korean metropolis of Seoul since 2005 as appropriate strategies for reducing VOCs [153]. While the local reduction of nitrous oxides was the main controlling strategy for ozone in the Macon, Georgia in 2004 [158].

The most important measures to O₃ control

Progressive and more stringent controls on emissions of oxides of nitrogen (NO_x) and volatile organic compounds (VOCs) [154], ozone control of four source categories including electric utility point sources, nonutility (industrial) point sources, highway vehicles, and non-road mobile sources [155], vapor control in gasoline marketing [156], cleaner emission standards on newer cars according to the Clean Air Act [157], achieve ozone standards [158], restricting heavy-duty diesel trucks from driving within urban areas during the daytime in major cities [159], reduction of both NO_x and VOC anthropogenic emissions, circulation on alternate days of vehicles with odd and even number plates, traffic restrictions on some roads, closure of industries, reduction in the operation regime of some industries [160].

Ozone in the Earth's atmosphere is produced by the photochemical reactions of NO_x and VOCs in the presence of sunlight. The relationship between ozone formation and concentration of its precursors is nonlinear [169–171]. The rate of ozone formation depends on the NO_x concentration and the VOC/NO_x ratio [154]. To achieve the national standard for ambient air quality for ozone, the US government implemented different preventive measurements: uses of low solvent technologies and/or add-on control equipment for surface coating operations, vapor control in gasoline marketing, maintenance programs for proper VOCs control as the procedures for ozone, and vehicle inspection [156]. Implementation the strict limitation for driving the heavy-duty diesel trucks in urban areas during the daytime in several major Chinese cities in 2006 reduced ozone by about 25 and 20 ppb at night and day, respectively [159].

The most important measures to photochemical smog control

Ambient air standards, emission standards, and applying appropriate control techniques [162].

Photochemical smog was first detected in Los Angeles. The main influencing factor for this phenomenon was emission of air pollutants from different plants (power plants, smelters, foundries, open dumps, incinerators), and the release of sulfur oxides from different refineries. Adoption of emission standards regarding the control of vehicles and industry was one of the most important measures taken by the government to solve the problem of photochemical smog [162].

Conclusions

A systematic review was conducted to identify policies and strategies to control urban air pollution without restrictions in countries and time. In this study, the selected articles were divided into two categories: (1) that introduce policies and strategies to control air pollution in different countries of the world, and (2) articles that introduce different policies and strategies to control one or more specific pollutants. In the first category, urban air pollution control strategies and policies were classified into four categories, namely focusing on general strategies and policies, transportation, energy and Industry. In the second category, policies and strategies focused on controlling six pollutants (PM, SO₂, NO₂, VOCS, Ozone and photochemical smog).

Legislations and policy interventions to control urban air pollution are commonly enacted and implemented in most of the large countries in the world. Policies used by governments to control air pollution can be encouraging, supportive, or punitive. Depending on the circumstances, these policies are implemented alone or jointly.

The largest share of air pollution, especially in large cities, is related to transportation [172]. According to our investigations, the number of articles covering transportation control strategies and policies is higher than other topics. Important policies and strategies related to transportation include the use of technology to improve vehicle efficiency, improve transportation structure, use of emission standards, fuel quality improvements and alternative fuels, traffic restrictions, and use of less polluting vehicles such as hybrid vehicles and electric. Preliminary studies on energy-related air pollution control policies and strategies have focused on eliminating solid fuels, especially coal, and shifting fuel from coal to cleaner energy sources such as electricity and natural gas. This fuel shift has taken place in many countries for domestic use. But some governments in recent years have defined policies and strategies to remove coal from industry, especially power plants, which show that coal is still an important fuel in those countries. In recent years the approach of some countries is to use cleaner energies such as solar and wind energies. Installation of air pollution control systems, use of clean energy, use of up-to-date technologies in production, and, if necessary, the relocation of industrial facilities are the most important measures to control urban air pollution related to the industry.

This study may enhance knowledge about air pollution control interventions. We provided a complete set of implemented policies and strategies by governments to reduce air pollution or improve air quality in various fields to be used by planners and decision makers in this area.

Declarations

Conflicts of interest

The authors formally declare no conflict(s) of interest.