Typhoon-boosted biogenic emission aggravates cross-regional ozone pollution in China

Enhanced anthropogenic and biogenic interaction by approaching typhoons deteriorates cross-regional ozone pollution in China.

Abstract

Ozone pollution that threatens human health and the ecosystem is a global environmental challenge. In megacities, ozone pollution has long been mainly attributed to anthropogenic sources. However, the processes and mechanisms of cross-regional transport of ozone and its precursors under interactions between mixed sources remain unclear. Here, we show that Northwest Pacific typhoons could intensify the chemical interactions between anthropogenic and biogenic emissions, resulting in extreme ozone pollution in two main city clusters in China. By integrating field and satellite observations together with model simulations, we show that biogenic emission and cross-regional ozone transport are greatly enhanced by approaching typhoons, with the increments reaching up to 78.0 and 22.5%, respectively. Ozone formation efficiency has more than doubled because of abundant precursors and active photochemistry. This study highlights the importance of natural emissions in areas with intensive human activity, which needs to be considered in future air pollution control in China.

INTRODUCTION

Ground-level ozone (O₃) is an air pollutant that poses detrimental impacts on human health and crop productivity (1–4). O₃ also drives the production of the hydroxyl radical, which controls the photochemical lifetime of many atmospheric pollutants and reactive greenhouse gases (5–9). As a secondary pollutant, tropospheric O₃ is predominantly formed in situ by photochemical reactions involving nitrogen oxides (NO_x) and volatile organic compounds (VOCs) (7, 10–12). Thus, quantitatively understanding the sources and formation process of O₃ is of great importance for the mitigation policy of air pollution.

Emissions of NO_x are dominated by anthropogenic combustion processes, whereas VOCs originate from both human activities and biogenic emissions especially from forest ecosystems (5, 13–16). Severe O₃ pollution and its deterioration have long been attributed to intensive anthropogenic activities (6, 17, 18). However, despite that strict measures have been widely implemented to cut down anthropogenic emissions in the past decades, increases in ground-level O₃ pollution have been witnessed in many places of the Northern Hemisphere (19, 20). The highest O₃ concentrations and largest increment predominantly concentrate in the mid-latitude (21), particularly evident in China. Specifically, O₃ pollution days in China are 93 to 575% more than other industrialized countries, jeopardizing the health of the world’s largest population (22).

Meteorological parameters including incident sunlight and air temperature modulate the O₃ formation, accumulation, and transport (13, 23, 24). The extreme O₃ pollution generally outbreaks in summer and has been proven to be closely linked to high air temperature and strong solar radiation together with stagnant weather conditions (24–28). Recent observations have identified O₃ mixing ratios in cities of eastern China well in excess of present air quality standards under the influence of Northwest Pacific typhoon, a frequent weather activity in warm seasons (29–33). Because of the fact that the typhoon weather is highly variable and the precursor emissions in this region are very complex, quantitatively understanding O₃ pollution under this circumstance is particularly challenging. Here, using in situ and satellite observations and chemical transport model for 6 years from 2013 to 2018, we provide a quantitative assessment of how the Northwest Pacific typhoon leads to these extreme O₃ pollution events in eastern China and also disentangle the key factors that control O₃ production and its transport under this unique environment.

RESULTS

Aggravating O₃ pollution in city clusters associated with approaching typhoons

China’s O₃ features highest levels in summer with 90th percentile of maximum daily 8-hour average (MDA8) of O₃ concentration reaching up to approximately 90 parts per billion (ppb) (5, 34). Summer and autumn are also the seasons when China is subject to the most frequent typhoon activities in the Western Pacific. Long-term measurements from 2013 to 2018 in Fig. 1A and fig. S1 indicate that, under the typhoon-associated weather systems, O₃ pollution would get even worse, especially in two most densely populated city clusters—Yangtze River Delta (YRD) region and Pearl River Delta (PRD) region. Quantitatively, MDA8 O₃ concentrations in these two regions rise by 28.3 and 53.8%, respectively, under the influence of typhoons, comparing to the summertime climatology. Shanghai and Guangzhou, the two largest megacities located in YRD and PRD, respectively, have witnessed 30.3- and 39.1-ppb high anomalies as typhoons approach. Such an O₃ increment is particularly remarkable when typhoon tracks are nearby the island of Taiwan. Even more noteworthily, O₃ soars in succession with YRD polluted a few days earlier (Fig. 1, C and D).

Fig. 1. Typhoon tracks and its associated abnormally high O₃ pollution in city clusters YRD and PRD.

(A) MD8A O₃ concentration anomaly during typhoon-associated period compared with the summer average (dotted) from 2013 to 2018 and summer averaged isoprene (ISOP) emission calculated by MEGAN model. (B) Distribution of annual anthropogenic NO_x emissions (contoured with blue), averaged 500-hPa pressure (contoured with red), and wind based on ERA5 [ECMWF (European Centre for Medium-Range Weather Forecasts) Reanalysis v5] reanalysis data during the phase of YRD O₃ pollution. The solid red line highlights the position of WPSH, and the green lines show the tracks of historic typhoons crossing Taiwan. (C) Ninetieth percentile of MDA8 O₃ levels in YRD corresponding to the position of typhoon. (D) Same as (C) but for PRD.

The coastal eastern and southern China, including both YRD and PRD, is characterized by a very typical subtropical monsoon climate and western pacific subtropical high (WPSH) in summer (35–37). The intervention of a typhoon tends to weaken and stretch westward WPSH (Fig. 1B). The change in large-scale circulation and synoptic weather is highly sensitive to the specific position of typhoon (38). To understand the typhoon tracks in the Western Pacific, we applied an objective method to classify typhoons in the past decade. It is found that typhoons passing by Taiwan are the most conducive ones, bringing about O₃ pollution episodes in both YRD and PRD city clusters (Fig. 1, C and D, and figs. S2 and S3). This kind of typhoon notably weakens the subtropical high, retracting the WPSH around 30°N (Fig. 1, C and D, and fig. S4) and greatly modifying the weather system in both regions. As the typhoons reshape the circulation, O₃ pollution starts to cover YRD and PRD in succession. Statistically, the typhoon-induced O₃ pollution features two distinct phases, that is, O₃ pollution tends to first deteriorate in YRD (the YRD phase) and then followed by PRD (the PRD phase) (Fig. 1, C and D, and fig. S5). Spatially, it is found that the YRD region is the first to suffer from severe O₃ pollution when the typhoon center is located around 15°N to 20°N. In the subsequent phase of PRD pollution, typhoon center ranges from 20°N to 25°N (fig. S5).

Typhoon-boosted biogenic emissions and cross-regional O₃ transport

The summertime extreme O₃ pollution in eastern China has been long attributed to fast photochemical production of intensive anthropogenic NO_x and VOC emissions under intense solar radiation (13, 20, 27). Moreover, southern China settles a large area of forest, just between YRD and PRD, which releases tremendous biogenic VOCs (BVOCs) as additional O₃ precursors (Fig. 1A) (35). The spatial heterogeneity of these O₃ precursors and nonlinearity of O₃ chemistry make the regional transport and mixing processes substantially modulate O₃ pollution. When a typhoon approaches, the circulation in eastern China shows great shift from the typical summer monsoon to prevailing northerly winds (Fig. 2, A and B), corresponded to the aforementioned YRD and PRD phases.

Fig. 2. Typhoon-modulated circulations and meteorological conditions at different ozone pollution phases.

Averaged wind field and sea-level pressures showing the position of typhoon at the phase of YRD pollution (A) and PRD pollution (B), respectively. (C to E) Difference of 2-m temperature, relative humidity (RH), and incident ultraviolet radiation b (UVB) at 14:00 local time between typhoon-associated weather system and the summertime climatology in PRD and YRD, respectively.

During the YRD phase, the mainland China prevails southeast wind, which is mainly affected by the summer monsoon system (Fig. 2A). The air flow is relatively clean for PRD as it comes from the South China Sea. Quite the reverse, the low-pressure system of the cyclone makes the WPSH move northward to the mid-latitude and then cover the YRD region, leading to a stagnant condition there. As the cyclonic center moves northwest, the external air flow of the typhoon sweeps the southern China, the mainland prevails northerly winds (Fig. 2B), and subsequently, PRD steps into pollution. The strong atmospheric subsidence of a typhoon periphery favors an intense solar radiation and stagnant condition. By comparing the meteorological conditions under the influence of typhoon with the summer average, it is clear that the noontime air temperature and incident solar radiation are increased by 4.6 K and 9.1 W/m², together with the noticeable drop of relative humidity in both YRD and PRD (Fig. 2, C to E). These parameters are all conducive to vigorous photochemistry and the resultant O₃ formation.

In addition, biogenic emission itself is also highly dependent on the meteorology (15, 39). The broad area of forest in southern China does undergo marked variations under typhoon-associated weather system based on the meteorology-driven calculation using the Model of Emissions of Gases and Aerosols from Nature (MEGAN) (see Materials and Methods). Take the Typhoon Nesat in 2017 as an example, isoprene, the most abundant BVOC species (15, 40), witnesses a remarkable rise of 94% over the forest area in southern China. Regarding the spatial distribution, isoprene emission enhancement is found in central China and the northern part of PRD during the YRD phase and in most areas of PRD and its adjoining northern areas during the PRD phase (Fig. 3, A and B). The significant increase in BVOC emissions correlates well with the rising air temperature, indicating the predominant impact of weather conditions (16, 38). Statistically, the averaged isoprene emission of historical typhoon events from 2013 to 2018 is 78.0% greater than the summer climatology, providing solid evidence that BVOC emissions are prominently intensified by the typhoon-associated weather systems (Fig. 3D). To identify the impact of the enhanced BVOCs on O₃ pollution in city clusters, we further adopt the backward Lagrangian particle dispersion modeling (LPDM) to simulate the potential source region of air masses for YRD and PRD. The 48-hour backward potential source regions of YRD and PRD point to the high vegetable coverage area in the southern China (Fig. 3C). Considering the typical circulation of typhoon periphery, it is indicated that the intensified BVOCs are prone to be transported to city clusters with dense anthropogenic emission and contribute to O₃ pollution there.

Fig. 3. Typhoon-boosted biogenic emissions and cross-regional O₃ transport in eastern China during Typhoon Nesat in 2017.

(A) Spatial distribution of BVOC emission enhancement (contour) and 2-m air temperature anomaly (isolines) in the phase of YRD pollution while the Typhoon Nesat is approaching China. (B) Same as (A) but in the phase of PRD pollution. The dashed lines mark the track of the Typhoon Nesat, and the circles show the typhoon position at the corresponding phases. (C) Forty-eight–hour retroplume (footprint residence time) showing transport pathways of air masses at YRD and PRD during the two pollution phrases, respectively. (D) Comparisons of isoprene emissions during the historic typhoon periods from 2013 to 2018 and the summer average in YRD and PRD

Accelerated O₃ production by the interaction of anthropogenic and biogenic sources

In addition to meteorological variations, O₃ formation sensitivity regime features great regional disparities over the eastern China. As clearly demonstrated in fig. S6, in megacities with high NO_x environments like YRD and PRD, the formation of O₃ is typically VOC-limited, while the O₃ production over the forest area is highly sensitive to the ambient NO_x availability (41). In normal days without intense cross-regional transport in warm seasons, the urban emissions or biogenic emissions alone are insufficient to build up O₃ pollution as the production efficiency maintains at a relatively low level. Conversely, the typhoon-associated weather tends to intensely mix anthropogenic and biogenic emissions by strong rotational winds. To quantitatively understand the interactions therein, we conduct parallel simulations using chemical transport model Weather Research and Forecasting (WRF) coupled Community Multiscale Air Quality (CMAQ) (WRF-CMAQ) (see Materials and Methods). The modeling results reveal that typhoon-boosted BVOC emissions enhance photochemical reactions in city clusters with contributions to O₃ formations by more than 30 ppb in both regions. Because both the city clusters are in the VOC-limited or transitional regime, the intervention of BVOCs plays an “adding fuel” role in urban photochemical pollution. Specifically, O₃ pollution is enhanced by anthropogenic and biogenic interactions via two pathways. One is the reaction with primary biogenic emissions, especially isoprene near the ground surface. It is worth noting that the reactivity of BVOCs often doubles or even triples that of anthropogenic VOCs (42, 43). Once released, isoprene reacts with atmospheric oxidants and subsequently generates O₃. This contribution is drastic near the ground surface, with the maxima contribution more than 30 ppb (Fig. 4, A and C). The other is via the reaction with the transported biogenic intermediate products, including methyl vinyl ketone, methacrolein, methyl glyoxal, etc., in the upper planetary boundary layer (PBL) even during nighttime (44). In general, isoprene itself predominately involves in local photochemistry due to its high reactivity; however, its intermediate product can be long-rangely transported hundreds of kilometers. The model results demonstrate that a huge amount of biogenic intermediate products inburst to downwind city clusters following typhoon circulation (fig. S7) reacts with local oxidants, such as OH radical and O₃ and NO₃ radicals, and then generates RO₂ radical, thereby enhancing O₃ by almost 10 ppb. Consequently, influenced by the intensified BVOC emissions, the downwind urban features an increase in O₃ formation efficiency by 126%, suffering from severe O₃ pollution.

Fig. 4. Chemical interactions between anthropogenic emission and natural BVOCs accelerate O₃ formation under the influence of typhoon.

(A) Cross section of wind field and BVOCs’ impact on O₃ in YRD along the blue cross-regional transport pathway marked in Fig. 3C. The red contour shows the chemical contribution by BVOC emissions to O₃ formation. The blue contour shows isoprene concentrations by BVOC emissions. The green contour shows BVOC intermediate product (ISPD; lumped methacrolein, methyl vinyl ketone, methyl glyoxal, etc.) concentrations. (B) Source apportionment of O₃ formation in YRD. The inner pie shows the summer average, and the outer pie shows the typhoon-affected period. To be noted that the local, regional, PRD/YRD all refer to anthropogenic emissions. (C) Same as (A) but for PRD. (D) Same as (B) but for PRD.

Besides the vigorous interactions of BVOCs and anthropogenic emissions, cross-regional transport also matters because of the typical circulation of typhoon periphery. As shown in fig. S8, during the first phase (YRD pollution), intensive anthropogenic emission in PRD could substantially affect the downwind areas. The YRD region, even further away, is still influenced with O₃ contribution more than 10 ppb. As a secondary pollutant with relatively long lifetime, O₃ could be either directly transported or locally produced by precursors from upwind areas (18, 45). Our modeling results indicate that photochemically produced O₃ from PRD emissions is mainly distributed locally, and hardly can the precursors from PRD be transported to YRD. Instead, it is the direct advection of O₃ in the upper PBL and daytime convective mixing that make O₃ enhancement in PRD greatly contribute to O₃ pollution in YRD (fig. S8, A and B). In the PRD phase, the outflow of typhoon dominates the southern China, and the prevailing wind becomes from YRD to PRD. The accumulated O₃ in YRD is, in turn, horizontally transported back and lastly influences PRD with a contribution more than 20 ppb (fig. S8C). By analyzing O₃ diurnal patterns along the transport pathway from tens of monitoring stations, the O₃ peaks depict a “lagging effect” from the upwind area to the downwind regions, which further supports the importance of cross-regional interaction based on direct in situ observations (fig. S9).

To identify the contribution of typhoon-enhanced O₃ pollution statistically, we then carried out the exactly same simulations for all the historical events from 2013 to 2018, as shown in fig. S10. By integrating the findings, we propose a conceptual scheme illustrating the enhancement of cross-regional O₃ pollution by typhoon-intensified anthropogenic and biogenic interaction and quantify the contributions from key factors in Fig. 5. As shown, NO_x and other pollutants first emitted in PRD pass the BVOC-concentrated forest area, and then the mixed anthropogenic and biogenic plume results in O₃ pollution when arriving YRD. In the second phase, pollutants accumulated in YRD are then swept back under the influence of typhoon outflow, passing the BVOC-concentrated forest area again, and give rise to PRD O₃ pollution. Comparisons of O₃ source apportionment between typhoon events and the summer average are analyzed using WRF-CMAQ (Fig. 4, B and D). In YRD, the contributions of BVOC emissions are significantly increased from 16.9 to 33.6%. The cross-regional transport by PRD anthropogenic emissions also increases from 4.8 to 8.2%. In terms of PRD pollution, the regional transport dominates O₃ formation with an increase from 31 to 53.5%, among which the YRD contribution increases by approximately five times from 1.7 to 9.9%. The contribution of BVOC emission also gets more vital, increasing from 16.9 to 26.6%. Because that it is the anthropogenic and biogenic interaction that greatly enhances O₃ pollution, we further explored the potential benefits of a precedent control of anthropogenic emission before typhoon in pollution mitigation by using the model tools (fig. S11). Promisingly, with the help of meteorology and air quality forecast, regional coordinated emission control in short term could help us to alleviate the O₃ enhancement due to approaching typhoons.

Fig. 5. Conceptual scheme illustrating how typhoon aggravates cross-regional ozone pollution in eastern China and quantitative assessment of key factors therein.

DISCUSSION

O₃ pollution features the nonlinear response to emission intensity and involves multiple precursors and is also greatly modulated by synoptic weather. Southern China, characterized by two emission-intensive city clusters and a broad vegetation coverage, provides a unique lens to dig into the interaction of biogenic and anthropogenic emissions, particularly under typhoon when O₃ pollution peaks. Our study shows that a typhoon-affected weather system is the trigger to intensify the natural emissions of BVOCs and creates favorable meteorological conditions for photochemical reactions. Furthermore, the circulation of typhoon periphery enhances cross-regional transport and interactions of anthropogenic and biogenic emissions, facilitating O₃ pollution in urban areas. This O₃ enhancement is expected in other regions featuring both mixed emissions and frequent typhoon or hurricane activity, like the southeastern United States and northern India (46, 47). Our results emphasize the importance of natural sources on tropospheric O₃ pollution even in areas with dense anthropogenic activity and replenish the understanding on typhoon-intensified interactions of anthropogenic and biogenic emissions on tropospheric O₃ pollution. Even in highly polluted regions like eastern China, more consideration ought to be given to the coordinated cross-regional joint emission control. Moreover, the typhoon activities are strongly linked with climate variability and climate change (48, 49), and biogenic emission could be also enhanced because of the global warming or carbon neutral–induced greening (50). Therefore, these interactions between biogenic and anthropogenic activities may be further intensified and play a more vital role in O₃ pollution in the future.

MATERIALS AND METHODS

Clustering of typhoon tracks with O₃ levels

Clustering analysis is introduced to distinguish historical typhoon tracks. Because the number of clusters is already known, K-means clustering method is used. This method classifies n points of data X into initializing k clusters

$$J={\displaystyle \sum _{k=1}^K}{\displaystyle \sum _{i=1}^n}{\Vert x_i-u_i\Vert }^2$$

where u is the center points of an individual cluster and J is the minimum distance from every point to its center point. This classification is repeated until each cluster gets stable. This algorithm has been used in many aspects (28, 51).

In this study, we collect 11-year historical typhoons (2008 to 2018) affecting China with a total number of 88 tracks. The clustering analysis lastly provides four types of tracks. O₃ network data obtained from China National Environmental Monitoring Centre (CNEMC) are integrated corresponding to the position of the typhoon. There are 42 monitoring sites distributed in YRD and 40 in PRD, respectively. The 90th percentile of MDA8 O₃ is used to represent the degree of O₃ pollution in a specific region. Because the O₃ network data are only available after 2013, we then integrate O₃ data with the classified tracks from 2013 to 2018.

Lagrangian particle dispersion modeling

LPDM was adopted with the aim to investigate the potential source region affecting air masses in YRD and PRD, respectively. Global Data Assimilation System (GDAS) data are used as the meteorological input, and the model is conducted using hybrid single-particle Lagrangian-integrated trajectory (HYSPLITY). In this study, we released 3000 particulates at 100 m over the receptor region, and the model was traced backward for 48 hours with a time resolution of 1 hour. After a calculation of mean wind and a turbulence transport component, the position of each particulate was identified in spatial. The footprint “retroplume,” representing the distribution of the surface probability or the residence time of the simulated air mass, was used to identify the contribution from potential source regions.

MEGAN model

BVOC emissions by terrestrial ecosystems are estimated with the aid of MEGAN model (version 2.10). As a biogenic emission model, MEGAN can estimate global and regional BVOC emissions, with horizontal resolution ranging from 1 × 1 km to around 100 × 100 km. The model can estimate up to 146 BVOC species, for example, isoprene, monoterpene, sesquiterpene, and so on, and lump them according to a specific gas-phase chemistry mechanism. In this study, WRF-simulated results provide meteorological conditions including temperature, solar radiation, and relative humidity for MEGAN. Leaf area index and plant functional types retrieved from moderate resolution imaging spectroradiometer (MODIS) products are used to provide basic vegetation information. All the calculated BVOC species are mapped to Carbon Bond 05 (CB05) gas-phase mechanism. Major BVOC emissions, i.e., isoprene emissions, are identified and compared between typhoon-associated events and the summer averages.

Diagnosing O₃-NO_x-VOC sensitivity by satellite retrievals

To illustrate the responses of O₃ to its precursors, the O₃-NO_x-VOC sensitivities are explored by using the ratios of satellite-retrieved formaldehyde (HCHO) and NO₂. Daily HCHO and NO₂ column densities are obtained from the resolution of the GOME-2 retrievals. The data control is based on the principle by previous studies (52). In this study, the area with HCHO/NO₂ ratio below 1 is considered as NO_x-saturated (VOC-limited) regime, the area with HCHO/NO₂ ratio above 2 is identified as the NO_x-limited regime, and the HCHO/NO₂ ratio between 1 and 2 indicates the mixed-limited regime (53).

WRF-CMAQ simulation

WRF (version 3.9.1) coupled CMAQ (version 5.0.2) model is applied to simulate O₃ and its precursors in southern China. The WRF model is driven by Final (FNL) Operational Global Analysis data. To better represent the meteorological conditions, the National Centers for Environmental Prediction (NCEP) Automated Data Processing Global Upper Air Observational Weather Data have been assimilated. The modeling system consists of two domains, with a grid resolution of 36 × 36 km and 12 × 12 km, respectively. The outer domain covers most part of East Asia; the inner domain mainly focuses on the eastern China. In vertical, there are 39 levels for all domains with the top at 100 hpa and ~20 levels within the boundary layer. The chemical boundary and initial conditions for WRF-CMAQ model are derived from a global chemical model [MOZART-4 (Model for Ozone and Related chemical Tracers, version 4)]. Anthropogenic emission inventories are obtained from the Multi-resolution Inventory for China, which has a grid resolution of 0.25° and covers anthropogenic emissions from human activities of power plant, transportation, industry, agriculture, and residential. Natural emissions including BVOCs and soil-emitted NO are calculated using the MEGAN (version 2.10). The WRF-CMAQ spins up for 10 days before starting a simulation.

The key configurations of WRF-CMAQ include the Rapid Radiative Transfer Model (RRTM) for short- and long-wave radiation scheme, the Noah Land Surface Model for land-atmospheric interactions, the Yonsei University boundary layer scheme, the Lin microphysics scheme, the Kain-Fritsch scheme for cumulus parameterization, and the CB05 combined with AERO6 (aerosol 6 module) for gas-phase and aerosol chemistry, respectively.

The model performance is then validated by the comparisons with available observations. Statistical metrics are introduced, including the averages (Obs_mean and Sim_mean), mean bias (MB; Obs_mean − Sim_mean), normalized MB (NMB;$\text{NMB}={\displaystyle {\sum }_{i=1}^n}[(\text{Sim}(i)-\text{Obs}(i))/\text{Obs}(\mathrm{i})]$), root mean square error (RMSE; $\text{RMSE}=\sqrt{\frac{1}{n}{\displaystyle {\sum }_{i=1}^n}{(\text{Sim}(i)-\text{Obs}(i))}^2}$), and the index of agreement (IOA; $\text{IOA}=1-\frac{{\sum }_{i=1}^n{(\text{Sim}(i)-\text{Obs}(i))}^2}{{\sum }_{i=1}^n{(\mid \text{Sim}(i)-\overline{\text{Obs}}\mid +\mid \text{Obs}(i)-\overline{\text{Obs}}\mid )}^2}$). The statistical results are summarized in table S1. In general, the evaluated results show that the WRF-CMAQ model can well reproduce the historical O₃ pollution events.

The integrated process rate (IPR) module in CMAQ is used to calculate the contributions from each individual physical and chemical process to O₃ formation. The considered atmospheric processes include the photochemical process (the net O₃ formation rate via gas-phase reactions), the horizontal transport (including horizontal convection and diffusion), the vertical a transport (including vertical convection and diffusion), the emissions, the deposition (including dry and wet deposition), and the cloud processes with the aqueous chemistry. The individual contribution is calculated at each time step during the simulation. In this work, we mainly use the technique to investigate the key photochemical species including O₃, CO, NO_x, anthropogenic VOCs, BVOCs, and the intermediate BVOC product.

To quantify all the individual impact of anthropogenic emissions and BVOC emissions to O₃ pollution, all the historical cases with typhoon crossing the Taiwan from 2013 to 2018 are simulated with WRF-CMAQ model. Parallel simulations are conducted under different emission scenarios. The scenarios include considering/excluding BVOC emissions, anthropogenic emissions of PRD, anthropogenic emissions of YRD, and anthropogenic emissions out of YRD and PRD. For example, to identify the impact from PRD anthropogenic emissions on YRD, parallel simulations are conducted with and without PRD anthropogenic emissions. Then, the difference in O₃ between the two simulations is identified as the impact from PRD anthropogenic emissions. The influence from the emissions within the region itself is considered as the local contribution; otherwise, the one out of the border is regarded as the regional transport. By using this method, O₃ source apportionment is analyzed during historical typhoon events and the summer season. Moreover, by combining with the IPR technique, the changes of net O₃ formation rate due to gas-phase reactions (including photochemically production and consumption processes) and atmospheric physical process (i.e., advective transport) driven by a given set of emissions are recorded at each time step. Accordingly, the individual impact of anthropogenic emissions and BVOCs on O₃ formation can be identified.

Supplementary Materials

This PDF file includes:

Figs. S1 to S11

Table S1