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Published in final edited form as: Chemosphere. 2020 Jul 10;262:127595. doi: 10.1016/j.chemosphere.2020.127595

Effect of Bromine and Iodine Chemistry on Tropospheric Ozone over Asia-Pacific Using the CMAQ Model

Yeqi Huang 1, Xingcheng Lu 1, Jimmy C H Fung 1,2, Golam Sarwar 3, Zhenning Li 4, Qinyi Li 5, Alfonso Saiz-Lopez 5, Alexis K H Lau 1,6
PMCID: PMC7658052  NIHMSID: NIHMS1622620  PMID: 32784061

Abstract

Recent studies have focused on the chemistry of tropospheric halogen species which are able to deplete tropospheric ozone (O3). In this study, the effect of bromine and iodine chemistry on tropospheric O3 within the annual cycle in Asia-Pacific is investigated using the CMAQ model with the newly embedded bromine and iodine chemistry and a blended and customized emission inventory considering marine halogen emission. Results indicate that the vertical profiles of bromine and iodine species show distinct features over land/ocean and daytime/nighttime, related to natural and anthropogenic emission distributions and photochemical reactions. The halogen-mediated O3 loss has a strong seasonal cycle, and reaches a maximum of −15.9 ppbv (−44.3%) over the ocean and −13.4 ppbv (−38.9%) over continental Asia among the four seasons. Changes in solar radiation, dominant wind direction, and nearshore chlorophyll-a accumulation all contribute to these seasonal differences. Based on the distances to the nearest coastline, the onshore and offshore features of tropospheric O3 loss caused by bromine and iodine chemistry are studied. Across a coastline-centric 400-km-wide belt from onshore to offshore, averaged maximum gradient of O3 loss reaches 1.1 ppbv/100 km at surface level, while planetary boundary layer (PBL) column mean of O3 loss is more moderate, being approximately 0.7 ppbv/100 km. Relative high halogen can be found over Tibetan Plateau (TP) and the largest O3 loss (approximately 4–5 ppbv) in the PBL can be found between the western boundary of the domain and the TP. Halogens originating from marine sources can potentially affect O3 concentration transported from the stratosphere over the TP region. As part of efforts to improve our understanding of the effect of bromine and iodine chemistry on tropospheric O3, we call for more models and monitoring studies on halogen chemistry and be considered further in air pollution prevention and control policy.

Keywords: bromine and iodine chemistry, CMAQ, tropospheric O3, onshore/offshore features, vertical structure

1. Introduction

Since the late 1980s, stratospheric ozone (O3) depletion events caused by reactive halogen species have attracted widespread attention from the research community and the public (Solomon et al. 1999), due to the important biosphere-protecting ultra-violet-absorbing role played by the stratospheric O3 layer. In contrast, tropospheric O3 is a major ambient photochemical pollutant that damages human health and reduces crop yields (Tai and Martin 2017, Maji et al. 2019). Thus, many recent studies have focused on the role of halogen chemistry in depleting O3 concentration in the troposphere (Saiz-Lopez and von Glasow 2012, Simpson et al. 2015, Wang et al., 2019).

The catalytic depletion of tropospheric O3 by halogen involves the general reaction X (Br=bromine, I=iodine) + O3 → XO (halogen oxide) + O2 (oxygen) (Platt and Hönninger 2003, von Glasow et al. 2004, Saiz-Lopez et al., 2007, Read et al. 2008). O3 is partially responsible for the atmospheric oxidation level and is the precursor for several atmospheric oxidizers. Therefore, halogen can indirectly affect atmospheric oxidative capacity and particle formation by altering the budgets of atmospheric oxidizers, e.g., HOx (OH=hydroxyl radical, and HO2=hydroperoxy radical) and NOx (oxides of nitrogen) (Parrella et al. 2012, Schmidt et al. 2016, Sherwen et al. 2016, Hoffmann et al. 2018, Li et al. 2019). Halogens can also potentially affect the climate because as both methane (CH4) and O3, two of the most potent greenhouse gases, are affected by halogen chemistry. Hossaini et al. (2016) estimated that around 2.5–2.7% of the total global CH4 oxidation is due to chlorine (Cl) atoms in the troposphere, and Saiz-Lopez A, et al. (2012) found that halogen-induced O3 loss accounts for approximately −0.10 Wm−2 to the radiative flux at the tropical tropopause. Moreover, reactive halogen species also oxidize the atmosphere and provide a sink for mercury (Holmes et al. 2009, Hynes et al. 2009, Horowitz et al. 2017).

In global and hemispheric chemical transport models, systematic overestimation of surface O3 can be found at regional coastal or island-based sites without consideration of halogen chemistry (Han et al. 2008, Reidmiller et al. 2009). In recent years, a number of modeling studies have attempted to include marine halogen emission sources, and studied the effect of halogen chemistry to better represent air quality simulation in marine boundary layers. The Models-3/Community Multiscale Air Quality (CMAQ) model, which incorporates the halogen chemistry and related emission and deposition mechanisms, is wildly used. Sarwar et al. (2014) developed the chlorine chemistry in the CMAQ and evaluated the impact over the northern hemisphere. Liu et al. (2018) employed CMAQ with chloride chemistry and anthropogenic chloride emissions to study the impact on air quality in China. Embedded with the bromine and iodine chemistry, Sarwar et al. (2015) studied the influence on surface O3 over the northern hemisphere. In the study by Sarwar et al. (2019), the mechanism of bromine/iodine was further improved.

Asia is bounded on the east by the world’s largest ocean, the Pacific, and on the south by the warmest, the Indian Ocean. Nonetheless, to our knowledge, only a few regional modeling studies have embedded halogen chemistry to investigate the influence on O3 over this region (Li. et al. 2020), especially from the viewpoint of vertical distribution. Meanwhile, the long coastline of Asia provides a unique environment with complicated physical and chemical conditions. Specifically, the land-sea breeze micro-scale circulation, the shallow boundary layer near the coast, and the interaction between polluted urban air and natural marine emissions can affect concentrations of pollutants and their precursors (e.g., O3, PM2.5, etc.; Angevine et al. 2004, Huang et al. 2010, Loughner et al. 2014). Furthermore, the Tibetan Plateau (TP), the highest and largest plateau region in the world, exerts considerable influence on the regional and even global atmosphere. The TP region is also a region of frequent stratosphere-to-troposphere transport (Lin et al. 2016; Luo et al. 2019).

In this study, the CMAQ modeling system version 5.2 (www.epa.gov/cmaq) incorporating bromine and iodine chemistry (Sarwar et al. 2015, Sarwar et al. 2019) was applied to quantify the effect of bromine and iodine chemistry on O3 concentrations and the atmospheric oxidation capacity over Asia.

2. Data and Method

2.1. Emission inventories

The Emissions Database for Global Atmospheric Research (EDGAR) version 4.3.2 (Crippa et al. 2018; https://edgar.jrc.ec.europa.eu/overview.php?v=432_AP) with a 0.1° × 0.1° gridded resolution was used for anthropogenic emissions (except for ship emission). The database includes 26 aggregated emission sectors covering power, different types of industries, various transport, agriculture, and so on. 10 emission species include six gaseous air pollutants: carbon monoxide (CO), NOx, sulfur dioxide (SO2), CH4, total non-methane volatile organic compounds (NMVOC), and ammonia (NH3), and four aerosols: PM10, PM2.5, black carbon (BC), and organic carbon (OC) have been calculated. Specific profiles for different sectors were allocated based on the emission sector categories in EDGAR. Annually total emission amount has been allocated into hourly emission rate by monthly, weekly, and hourly allocation ratios (shown in Figures S1, S2, and S3). Time zone adjustment was implemented in the research domain. The mapping relationship of emission species between EDGAR and CMAQ-ready input can be found in Table S1. Considering great effort of emission control strategies in China, corresponding emission reduction were implemented in our model from the base year 2012 to the simulated year 2016. In addition, the data were re-allocated into a 27-km spatial resolution and an appropriate vertical structure, in accordance with the CMAQ system. As this research focused on halogen species originating from the marine region, the results are very sensitive to ship emissions. Therefore, an up-to-date ship emission developed by Zhang et al. (2019) was selected to replace the corresponding emissions from EDGAR.

Biogenic emissions used in the simulations were produced from the Model of Emissions of Gases and Aerosols from Nature version 3 (MEGANv3; Guenther et al. 2018), using the leaf-area index data from 2016 (http://globalchange.bnu.edu.cn/research/lai#download) as the input.

Two categories of marine bromine and iodine species are considered in CMAQ: halocarbons, and inorganic iodine. The former include five bromocarbons and four iodocarbons, and the latter consist of two inorganic iodine species, respectively. These halogenated species are generated by different mechanisms, the detailed information for which can be found in the studies by Sarwar et al. (2015) and Sarwar et al. (2019). Halocarbons were calculated using the monthly climatological chlorophyll value from the Moderate Resolution Imaging Spectroradiometer (MODIS), and inorganic iodine emissions were calculated using the parameterization of McDonald et al. (2014). The monthly mean climatological chlorophyll distribution and the sea surface temperature (SST) distribution, which are the important determinant conditions of halogen emissions, can be seen in Figures S4 and S5. Finally, no additional anthropogenic sources of halogen emissions were included.

2.2. Model configuration

The Weather Research and Forecasting (WRF) model v3.9 was used to generate meteorological fields, and the 6-hourly National Centers for Environmental Prediction Final (NCEP FNL) operational global analysis data (http://rda.ucar.edu/datasets/ds083.2/) on 1°× 1° grids were used to drive the WRF model. The specific WRF parameterization schemes are summarized in Table S2. The WRF outputs were further processed by the Meteorology Chemistry Interface Processor version 4.3 (MCIP v4.3, Otte and Pleim 2010) to drive the CMAQ model v5.2 (Mathur et al. 2017) in this study. The study area was modeled as a 27-km horizontal resolution domain covering most of Asia, including the Indian subcontinent, the entirety of China, the Asian continental shelf sea, and part of the tropical western Pacific (as Figure S6 blue shaded areas). The 27-km spatial resolution is limited by the resolution of sea spray emission inventory. It is coarse to distinguish the detailed ozone distribution in urban regions, which limits deeper analysis in the current study. Nevertheless, the study focused on the Asia-Pacific region, in which the 27-km resolution can provide meaningful signals for identifying large-scaled features. WRF uses 39 vertical layers from the surface to 50-hPa altitude. The CMAQ extracted 25 vertical layers in WRF output and the vertical extension ranges from surface to a height of 50-hPa. To refine the boundary layer simulation, 19 model layers were allocated from the surface to 1500-m altitude. Detailed CMAQ vertical layer configuration can be found in Table S3.

The study was completed for the whole month simulation covering January, April, July, and October in 2016, respectively, representing the four seasons in annual cycle. The spin-up period of 10 days was excluded from the final results. Two scenarios were modeled and compared to isolate the effect of bromine and iodine chemistry: (1) base simulation without bromine and iodine chemistry (BASE); and (2) sensitive simulation with bromine and iodine chemistry (HAL). The difference between HAL and BASE simulations was used to demonstrate the impact of bromine and iodine chemistry. In the simulation, both BASE and HAL include chlorine chemistry. Anthropogenic emissions make a large contribution to the total chlorine emissions, but were unavailable in our study. Therefore, the chlorine chemistry is identical in BASE and HAL provided by CB6, and its effects were not analyzed. According to Liu et al. (2018), the anthropogenic chlorine emissions can increase monthly mean daily maximum 8 h and 1 h O3 concentration by up to 2.0 ppbv (4.1 %) and 7.7 ppbv in China. The boundary conditions, with and without bromine and iodine chemistry, were generated from the hemispheric CMAQ simulations (Mathur et al. 2017) corresponding to the same two scenarios of CMAQ.

Detailed descriptions of gas-phase bromine (Yang et al. 2005; Ordóñez et al. 2012; Sommariva et al. 2012) and iodine chemistry (Saiz-Lopez et al. 2014) have been established in previous model studies, and Sarwar et al. (2015, 2019) also elaborated the halogen chemistry in CMAQ model. In this study, 38 gas-phase and 8 heterogeneous chemical reactions for bromine, and 44 gas-phase and 20 heterogeneous chemical reactions for iodine were combined with the CB6 gas-phase chemistry scheme and AE6 aerosol mechanism used both for the BASE and HAL simulations (Sarwar et al., 2019). Sea spray emissions in CMAQ (Gantt et al., 2015) are chemically speciated into chloride (Cl−), sodium (Na+), sulfate (SO42-), calcium (Ca2+), magnesium (Mg2+), and potassium (K+). To facilitate the implementation of heterogeneous reactions, sea spray emissions were further speciated to include bromide (Br−) (Sarwar et al., 2019). The halogen chemistry also includes several aqueous-phase reactions of bromine species (Sarwar et al., 2019).

3. Results and Discussion

3.1. Model evaluation

Because of the importance of meteorological condition for regional transport, simulated horizontal wind in vertical profile was evaluated by atmospheric radiosonde data. 8 representative sounding stations were selected (shown in blue triangles in Figure S6). From the monthly mean perspective, the WRF model is capable to capture the observed wind profiles across the 8 representative sounding stations (see figure S7).

The model performance of simulated O3 and NO2 with two different scenarios against observation is shown in Table S4. The model can basically simulate the temporal variation of hourly surface O3, and the daily maximum 8-hour averaged (MDA8) surface O3, with hourly Index of Agreement (IOA_h) of 0.57–0.68 in both BASE and HAL, and IOA_8 of 0.50–0.60 (0.49–0.61) in BASE (HAL). Both BASE and HAL simulations overestimate the MDA8 O3 in January and October but underestimates the MDA8 O3 in April and July. The 8-hour normalized mean bias (NMB_8) for O3 is 14.1% in January with BASE which is reduced to 7.8% with HAL. The spatial distribution of MDA8 O3 in Figure S8 shows that CMAQ can generally reproduce the O3 pattern in four seasons. However, the model overestimates O3 concentration in central China, while underestimates in eastern China.

The model can reproduce NO2 concentrations in July and October, but underestimate the magnitude in January and April. The model has difficulty in capturing the temporal variations of NO2, with IOA_h ranging from 0.46 to 0.58 among all cases. The 27-km horizontal grid resolution is relatively coarse for accurately simulating NO2 since it is a short-lived species. Results indicate that bromine and iodine chemistry can change model performance slightly. It also demonstrates that the CMAQ model can be used to investigate the effect of halogen chemistry on air quality over the region of our interests.

3.2. Spatial features of bromine and iodine species

To investigate the spatial characteristics of bromine and iodine species, we divided the results into four categories: inland daytime, marine daytime, inland nighttime, and marine nighttime. Inland/marine can be separated by land-sea mask directly, and the daytime and nighttime are defined as 7:00 to 17:00 and 21:00 to 5:00, respectively, considering the seasonal variation in the length of day and night.

As shown in Figures 1 and 2, the CMAQ simulated vertical distribution of bromine and iodine species display remarkable differences in four scenarios. In this study, the ocean is the only source of bromine and iodine species, and the concentrations are much higher near the marine surface. Moreover, the abundant bromine and iodine species over the upper air and inland regions indicate the importance of vertical and horizontal transport. Bromine and iodine concentration decrease with altitude from the surface, while in some cases, the maximum occurs at 800–1000 meters height (near the top of the boundary layer), which is more frequent in marine regions. A relatively higher bromine and iodine concentration can be found on the upper air inland, which suggests the free tropospheric transport is a significant source of inland halogen.

Figure 1.

Figure 1

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Vertical profiles of model-simulated bromine species (a–d) over land during daytime (7:00–17:00), (e–h) over the ocean during daytime (7:00–17:00). (i–l) Same as (a–h) but during nighttime (21:00−5:00) in four seasons. The vertical axes are the altitude (unit: m) and horizontal axes are pollutant concentration (unit: ppbv).

Figure 2.

Figure 2

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Same as Figure 1 but for iodine species (pptv).

Hypobromous acid (HOBr) is the dominant daytime bromine species in the model simulation and is primarily produced by the reaction of BrO (bromine monoxide) with HO2. The maximum monthly averaged HOBr concentrations are 0.2–0.3 pptv and 1.6–2.0 pptv over land and ocean during four seasons, respectively. HOBr concentration reaches to very low levels (< 0.1 pptv) at night since the production of HOBr from the reaction of BrO with HO2 becomes negligible due to low nighttime HO2 levels and the consumption of HOBr by the heterogeneous reaction of “HOBr + Cl− (chloride) → BrCl (bromine chloride)” becomes important. BrCl is the dominant nighttime bromine reservoir (0.3–0.4 pptv and 1.6–2.4 pptv over land and ocean, respectively). However, BrCl concentrations reach to very low levels during daytime since it is rapidly photolyzed into Br and Cl atoms after sunrise. Hydrobromic acid (HBr) is produced by the reactions of Br with HO2, formaldehyde, and acetaldehyde and is the second most abundant inorganic bromine species in the daytime (~0.2 pptv and 1.0–1.4 pptv over land and ocean, respectively). However, HBr decreases in the nighttime since the production from the reaction of Br with HO2 is lower. Bromine nitrate (BrNO3) is produced from the reaction of BrO and NO2 and is the third highest inorganic bromine species in the daytime (~0.2 pptv and ~1.0 pptv over land and ocean, respectively). Molecular bromine (Br2) is formed from the reactions of BrO + BrO, Br + BrNO3, and the heterogeneous reaction of “HOBr + Br− → Br2”. Br2 concentration in the daytime is low since it undergoes photolysis and also reacts with OH. However, it becomes the second most contributor to inorganic bromine species at night (0.1–0.2 pptv and 0.6–1.1 pptv over land and ocean, respectively). Formyl bromide (FMBr) is formed from the reactions of Br with isoprene and terminal alkene and has a relatively high concentration over land (~0.3 pptv and 0.1–0.2 pptv in the daytime and nighttime, respectively). In contrast, concentrations of FMBr over ocean are negligible.

Hypoiodous acid (HOI) can be produced by both primary emissions and chemical reactions of I2 (molecular iodine) + OH and IO (iodine monoxide) + HO2. As the dominant iodine species, the maximum HOI concentrations reach 0.3–0.5 pptv and 3.5–5.4 pptv over land and ocean in the daytime, while 0.2–0.4 pptv and 1.4–2.2 pptv at night. A relatively higher concentration of HOI is found in the upper troposphere. Similar to BrCl, iodine chloride (ICl) becomes the second abundant inorganic iodine species at night (0.3–0.5 pptv and 2.2–3.5 pptv over land and ocean, respectively) and is produced from heterogeneous reactions of Cl− with HOI, iodine nitrate (INO3), and iodine nitrite (INO2). ICl concentrations reach to very low levels during daytime since it undergoes photolysis in the morning. IO is the second-highest abundant inorganic iodine species in the daytime and is formed from the reactions of I + O3, HOI + OH, I + BrO, OIO + NO, and INO3. INO3 is the third most abundant inorganic iodine species in the daytime (0.2–0.4 pptv and ~1.0 pptv over land and ocean, respectively) and is formed from the reaction of IO + NO2. Methyl iodide (CH3I) emitted from phytoplankton and macroalgae also has a high concentration due to its significant emission and long atmospheric lifetime.

The surface spatial distribution of critical radicals-Br, BrO, I, and IO are displayed in Figure 3. The maximum monthly mean of surface concentrations can reach to 0.08–0.14 pptv, 1.0–2.04 pptv, 0.36–1.29 pptv, and 1.35–3.38 pptv for Br, BrO, I and IO during four seasons, respectively. Because solar radiation influences the formation of halogens, the significant seasonal cycle is found in the spatial distribution of halogens, in accordance with the solar radiation seasonal cycle. Coastal areas have higher halogen concentration than inland regions, but still much lower compared with that on the marine surface. Moreover, it can be seen that over the TP, the highest and largest plateau region in the world, has relatively higher halogen concentration than other hinterlands such as northern and central China, Mongolia, etc.

Figure 3.

Figure 3

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(a–d) Shaded areas show monthly mean of surface Br mixing ratios (pptv) in four representative months for the HAL simulation. (e–h) same as (a–d) but for BrO. (i–l) same as (a–d) but for I. (m–p) same as (a–d) but for IO.

Although a number of studies on halogen observation were conducted hitherto, there are still relatively rare studies in Asia. Koenig et al. (2017) reported several observed BrO profiles over the western Pacific in Jan 2014. One around Koror showed the BrO concentrations in the lowest 500m were 1.7±1.2 pptv. Mahajan et al. (2019a) reported observations of daily averaged IO mixing ratio peaked at 0.47±0.29 pptv in the equatorial Indian Ocean (~8.5°N), during a cruise in December 2015 along a track from India to Mauritius. Other studies also provided evidence that a tropospheric background BrO and IO concentration of the order of 1 pptv (Peters et al. 2005, Sinnhuber et al. 2005, Read et al. 2008, Mahajan et al. 2010, Wang et al. 2015), which is comparable in the current simulations. The simulated results of previous model studies can also support the current simulation results. Li et al. (2020) employed WRF-Chem with state-of-the-art halogen chemistry in China, and showed similar spatial distribution pattern in the current research, with a seasonal average of daily-maximum BrO ranges from 0.5 to 2 pptv and IO from 0.75 to 2.5 pptv over the South China Sea. Zhu et al. (2019) presented a new mechanism of SSA debromination in the GEOS-Chem with a detailed representation of halogen chemistry. They reported a level of 1–3 pptv of BrO over the Western Pacific Ocean and the Northern Indian Ocean, and a slightly higher BrO concentration (< 1 pptv) over the TP.

3.3. Effect of bromine and iodine chemistry on surface O3

Figures 4(a–d) show the monthly mean values of the MDA8 surface O3 mixing ratios in the four representative months in the HAL scenario. Cyclic seasonal differences among the four months are evident. Areas of high O3 mixing ratio are mainly distributed over continental China in April and July and over the subcontinental India in January, April, and October. Intense anthropogenic emissions from the two regions promote the formation of this secondary pollutant. Summer is the only season during which India does not exhibit a high O3 mixing ratio, mainly due to the cloudy and rainy weather predominating during the summer monsoon season. High O3 mixing ratios are evident over the TP throughout all four months, despite the lack of local O3 sources. The effects of the TP will be further discussed in the next section. Figures 4(e–h) present the spatial distributions of surface absolute O3 loss while Figure 4(i–l) present the spatial distributions of surface percentage O3 loss due to the bromine and iodine chemistry. The halogen-mediated pathways greatly reduce the O3 concentrations over marine and coastal areas compared with those over continental regions. The extent of this reduction corresponds to the spatial distribution of bromine and iodine species (see Figure 3). The maximum O3 loss throughout the four seasons is 11–16 ppbv over the ocean and 9–13 ppbv over the land. The O3 loss percentages (Figure 4(i–l)) reach 41–44% and 27–39% over the ocean and land, respectively. The seasonal cycle is evident in both scenarios. Specifically, the dearth of solar radiation and the dominant northerly wind in January leads to relatively low production of bromine and iodine over the northern marine regions, and there is little transport of these species onshore (see Figure 3). Therefore, the largest O3 loss occurs over the remote southern marine regions. Bromine and iodine is mainly distributed from tropical to subtropical ocean regions in April and October, leading to the maximum of O3 depletion instead occurring at 20°N. Owing to the high O3 mixing ratio in India, it also experiences a relatively large loss of O3, especially near the coastal regions, the Arabian Sea, and the Bay of Bengal. In summer, the largest decreased O3 loss occurs over the northern Arabian Sea and the marine area around Japan. On the TP surface, there is also a relative high value of O3 depletion compare to other hinterlands.

Figure 4.

Figure 4

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(a–d) Shaded areas show monthly mean values of the daily maximum 8-h average (MDA8) surface O3 mixing ratios (ppbv) in four representative months, with the HAL simulation. (e–h) Absolute difference of MDA8 between the two simulations (HAL-BASE). (i–l) Percent difference of MDA8 between the two simulations [(HAL − BASE)/BASE × 100%].

3.4. Onshore/offshore features and vertical structure

To reveal the essential effects of the bromine and iodine chemistry on coastal regions explicitly, the distance from each grid point in the simulated region to the nearest coast was first calculated (see Figure S9, in which distances >0 indicate that the target grid point is offshore, and distances <0 indicate that it is onshore). Next, the distances were statistically classified by splitting the grid points into multiple bins. Figure 5 shows the gridded monthly mean of MDA8 O3 loss for the difference between the two simulations (HAL − BASE) as a function of the distance to the nearest coast. This method clearly demonstrates the onshore/offshore features caused by the involvement of bromine and iodine chemistry.

Figure 5.

Figure 5

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(a) Blue (red) curve shows the gridded mean difference of surface (PBL column mean) MDA8 O3 mixing ratio (HAL − BASE, unit: ppbv) for January as a function of the distance to the nearest coast. Distances > 0 indicate the target grid point is at sea, whereas distances < 0 indicate it is on land. The black dashed line denotes the coast. The shaded area denotes the ±1 standard deviation range of the gridded O3 mixing ratio. The PBL column mean is calculated from the surface to approximately 1500 m above the surface (the 19th model layer). (b–d) Same as (a) but for April, July, and October.

The overall features are similar among the four seasons both at the surface and for the mean of the planetary boundary layer (PBL) column. For example, the O3 mixing ratio shows a larger loss in offshore grid points than onshore grid points (Figure 5). The mean difference between onshore and offshore O3 loss is approximately 3–4 ppbv, while the sharpest gradient can be found on both sides of the coast. These features are significant in both the surface and the PBL column mean. Meanwhile, the surface and PBL column mean have distinct O3 loss trends, both onshore and offshore. Specifically, over the four seasons, surface O3 concentrations decrease more than the PBL column mean O3 concentration decrease offshore, while the PBL column means O3 concentration decrease is greater than the surface O3 concentration decrease onshore. In general, considering a coastline-centric 400-km-wide belt, average O3 loss increases with a gradient of 1.1 ppbv/100 km at the surface level from onshore to offshore. At the same time, the mean O3 loss is more moderate in the PBL column, being approximate with a gradient of 0.7 ppbv/100 km. This is intuitively understandable, as the halogen originate from the ocean surface, and are transported onshore by lifted high-altitude atmospheric circulation. Thus, the surface loss of O3 offshore is larger than the PBL column mean loss offshore, whereas the PBL column-mean loss is larger compared to the surface loss onshore, and the transition point is very close to the coast.

The monthly mean O3 losses in Figure 5 show that there are obvious seasonal differences. The O3 loss is smallest in summer both onshore and offshore, it should be associated with the relative lower halogen concentration in summer (see Figure 1 and 2) and the high-O3 region mostly limited to the continent (see Figure 4). The other three seasons show greater O3 loss via bromine and iodine chemistry on both sides of the coast. Another interesting feature is that the largest offshore O3 loss occurs approximately 600–1000 km away from the coast in October. Similar patterns of maximum offshore O3 loss also occur in other seasons, although not as significant as in October. The position of the maximum offshore O3 loss far out at sea is due to the higher sea surface temperature over the far tropical Pacific Ocean (see Figure S5) which results in lager halogen emission, in turn accelerating the O3 consumption in this area (Mahajan et al. 2019b).

To further clarify the differences between the surface and column mean results, Figure 6 demonstrates the detailed vertical distribution of O3 loss for HAL − BASE as a function of height and the distance to the nearest coast. Throughout the PBL, the coastline is an evident dividing line for O3 loss: i.e., on the sea-side of the coastline, the O3 loss near the top of the PBL is less than that at the surface, which may be related to the free tropospheric offshore wind from the continent. The greatest O3 loss in the lower PBL on the sea-side occurs approximately 600–1000 km from the coast, while in summer, the maximum loss is only 200 km from the coast, mainly due to the different spatial distribution of the high-O3 region in summer. Meanwhile, over landside, a high O3 loss is found near the top of the PBL, approximately 1500 km from the coast, with a strong seasonal cycle in which the peak loss occurs in January.

Figure 6.

Figure 6

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(a) Shaded areas denote the gridded monthly mean difference of MDA8 O3 mixing ratio (ppbv) (HAL − BASE) for January as a function of height and the distance to the nearest coast. (b–d) Same as (a) but for April, July, and October.

Figure 7(a) illustrates the cross-section (cross-sectional position shown as the blue curve in Figure S9) of January’s monthly mean MDA8 O3 mixing ratio along the west-to-east grid row that crosses the TP. The PBL O3 loading is evidently much higher on the western foothills and at the top of the TP. The O3 losses due to the bromine and iodine chemistry (HAL - BASE) are presented in Figure 7(b). The largest O3 loss (approximately 4–5 ppbv) in the PBL can be found between the western boundary of the region and the TP, associated with the O3 loss maximum at 1500 km inland in Figure 6(a). It is evident that O3 loss occurs in the higher troposphere, which implies that the involvement of halogen derived from marine sources leads to a systematic reduction of O3 in the upper atmosphere. The horizontal advection of free tropospheric air from the nearshore western boundary contributes to the O3 loss in the PBL over the TP. Taking BrO as an example, Figure 7(c) shows that there is a facile passage of BrO over the oceanic region to the high troposphere which is corresponding to the updraft over the marine areas, leading to a reduction in the O3 concentration there. Meanwhile, the relatively high BrO concentration near the western TP is related to bromine/iodine advection from the ocean surface and contributes to the in situ O3 loss over the TP. Strong downdraft can also been seen over the western part of TP, where the maximum O3 loss occurs. The TP is the highest and largest plateau region in the world and can exert considerable influence on the regional and even global atmosphere. Previous studies found that effect of lightning NOx can enhance the ozone concentration over TP due to its high elevations (Murray 2016, Lu et al. 2019). The TP is also a region of frequent stratosphere–troposphere exchange (STE) (Lin et al. 2016; Luo et al. 2019) that brings air containing high concentrations of O3 from the stratosphere (Tian et al. 2008, Chen et al. 2013). Lu et al. (2019) stated that effect of lightning NOx and STE can contribute more than 12 and 20 pptv to mean surface MDA8 O3 on the TP, respectively. Halogens originating from marine sources can potentially affect O3 transported from the stratosphere over the TP region. Future studies can be designed to explore the role of halogen chemistry over the TP.

Figure 7.

Figure 7

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(a) Cross-section plot of January monthly mean of MDA8 O3 mixing ratios (ppbv) along the blue curve in Figure S9. (b) Same as (a), but for the difference in O3 mixing ratio (ppbv; HAL − BASE). (c) Same as (a), but for BrO (ppbv) as an example, and overlaid vectors (unit: m/s) are for monthly mean vertical velocity filed.

4. Summary

The effect of bromine and iodine chemistry on tropospheric O3 concentrations in Asia within the annual cycle was investigated using the CMAQ modeling system with the bromine and iodine chemistry. Results indicate that the WRF model can generally capture the wind field and CMAQ model can reasonably simulate the spatial and temporal variation of O3 and NO2. The vertical profiles of bromine and iodine species show distinct features over land/ocean and daytime/nighttime, related to emission distributions and photochemical reactions. The spatial distribution of halogens shows a significant seasonal cycle as the production of some halogen species is highly dependent on solar radiation.

The bromine and iodine chemistry can cause the maximum of O3 loss around 11–16 ppbv (41–44 %) over the ocean and 9–13 ppbv (27–39 %) over the continental Asia with the strong seasonal cycle. This seasonality in O3 loss is linked to the cycles of solar radiation and the dominant wind direction. The mean difference between onshore and offshore O3 loss was approximately 3–4 ppbv, while the sharpest gradient occurred on both sides of the coast. In a coastline-centric 400-km-wide belt from onshore to offshore, the average O3 loss increases with the gradient of 1.1 ppbv/100 km at surface level, while the column mean O3 loss was more moderate in the PBL, being approximately 0.7 ppbv/100 km. Vertical structure analysis showed that the O3 loss on the sea-side of the coastline was lower at the top of the PBL than at the surface, which may be related to free tropospheric offshore wind from the continent. Relative high halogen originating from marine sources can be found over the TP. The updraft over the marine areas and strong downdraft over the western part of TP implies the transport path of marine sourced bromine and iodine due to the large-scale overturning circulation. Accordingly, the largest O3 loss (approximately 4–5 ppbv) in the PBL occurs between the western boundary of the domain and the TP.

Supplementary Material

Supplement1

Acknowledgment

This work was supported by the Air Pollution Control Program of the National Key Research & Development Plan (2018YFC0213902), RGC Research Impact Fund (R6011‐18), Special Fund Project for Science and Technology Innovation Strategy of Guangdong Province (Grant No.2019B121205004), and the European Research Council Executive Agency under the European Union’s Horizon 2020 Research and Innovation Programme (Project ERC-2016- COG 726349 CLIMAHAL).

Footnotes

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