Polycyclic aromatic hydrocarbons (PAHs), formed through incomplete combustion process, have adverse health effects. To investigate spatial distribution and sources of PAHs in North China, PAHs with ...passive sampling in 90 gridded sites during June to September in 2011 were analyzed. The average concentration of the sum of fifteen PAHs in North China is 220±14ng/m3, with the highest in Shanxi, followed by Shandong and Hebei, and then the Beijing-Tianjin area. Major sources of PAHs are identified for each region of North China, coke process for Shanxi, biomass burning for Hebei and Shandong, and coal combustion for Beijing-Tianjin area, respectively. Emission inventory is combined with back trajectory analysis to study the influence of emissions from surrounding areas at receptor sites. Shanxi and Beijing-Tianjin areas are more influenced by sources nearby while regional sources have more impact on Hebei and Shandong areas. Results from this study suggest the areas where local emission should be the major target for control and areas where both local and regional sources should be considered for PAH abatement in North China.
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•Influence of regional PAH sources is investigated by multi-methods in North China.•PAH sources are better identified by considering both local emission and transported PAHs.•PAHs in Hebei and Shandong are more influenced by regional sources.
•178 nitrate modeling studies in China published during 2007–2021 were reviewed.•Model performance on nitrate and possible reasons for model-observation biases were summarized.•The contribution of ...N2O5 heterogeneous chemistry to nitrate varies from 21.0% to 51.6% among studies.•Decreased SO2 emission, enhanced AOC, and weakened deposition account for increasing PM2.5 nitrate fraction.•Multiple-pollutant control strategies involving NH3, NOx, and VOCs are needed to mitigate nitrate pollution.
Particulate nitrate (pNO3) is now becoming the principal component of PM2.5 during severe winter haze episodes in many cities of China. To gain a comprehensive understanding of the key factors controlling pNO3 formation and driving its trends, we reviewed the recent pNO3 modeling studies which mainly focused on the formation mechanism and recent trends of pNO3 as well as its responses to emission controls in China. The results indicate that although recent chemical transport models (CTMs) can reasonably capture the spatial–temporal variations of pNO3, model-observation biases still exist due to large uncertainties in the parameterization of dinitrogen pentoxide (N2O5) uptake and ammonia (NH3) emissions, insufficient heterogeneous reaction mechanism, and the predicted low sulfate concentrations in current CTMs. The heterogeneous hydrolysis of N2O5 dominates nocturnal pNO3 formation, however, the contribution to total pNO3 varies among studies, ranging from 21.0% to 51.6%. Moreover, the continuously increasing PM2.5 pNO3 fraction in recent years is mainly due to the decreased sulfur dioxide emissions, the enhanced atmospheric oxidation capacity (AOC), and the weakened nitrate deposition. Reducing NH3 emissions is found to be the most effective control strategy for mitigating pNO3 pollution in China. This review suggests that more field measurements are needed to constrain the parameterization of heterogeneous N2O5 and nitrogen dioxide (NO2) uptake. Future studies are also needed to quantify the relationships of pNO3 to AOC, O3, NOx, and volatile organic compounds (VOCs) in different regions of China under different meteorological conditions. Research on multiple-pollutant control strategies involving NH3, NOX, and VOCs is required to mitigate pNO3 pollution, especially during severe winter haze events.
In recent years, fine particulate matter (PM) pollution and visibility degradation have become severe air quality issues in China. In this study, PM2.5 pollution over the Pearl River Delta (PRD) ...region during January, April, August, and November 2009 was simulated using the Community Multiscale Air Quality (CMAQ) model. An in-depth diagnostic analysis, focused on November 2009, was also conducted to reveal the patterns of sulfate and nitrate distribution, and to identify the main factors that influence the formation of sulfate and nitrate under typical meteorological conditions. The CMAQ model reasonably reproduced the observed concentrations, but showed better performance for January and November than it did for April and August, for which there was light-moderate underestimation of SO2, NOx, O3, PM10, and PM2.5 concentrations, and slight overestimation of daily 8-h maximum concentrations of O3. Utilizing a sulfate tracking technique, it was found that on nearly 20 days in November 2009, characterized by northeasterly winds, cross-boundary transport contributed to >75% of the total sulfate budget, while local gas phase oxidation and primary emissions averaged 10% and 8%, respectively. Aqueous sulfate typically contributed less than 1% of the total sulfate budget, except when the winds were directed from the sea and high humidity favored aqueous oxidation, and the percentage contribution reached up to 46%. NH3 was generally sufficient to fully neutralize H2SO4; however, the formation of nitrate over the PRD was limited by the availability of NH3.
•The capability of CMAQ to reproduce PM2.5 pollution in the Pearl River Delta (PRD) was assessed.•Influences of different formation pathways of sulfate in PRD were quantified.•Trans-boundary transport was the major source of sulfate over the PRD in the fall.•The formation of nitrate over the PRD was limited by the availability of NH3.
During the COVID-19 lockdown period (from January 23 to February 29, 2020), ambient PM2.5 concentrations in the Yangtze River Delta (YRD) region were observed to be much lower, while the maximum ...daily 8 h average (MDA8) O3 concentrations became much higher compared to those before the lockdown (from January 1 to 22, 2020). Here, we show that emission reduction is the major driving force for the PM2.5 change, contributing to a PM2.5 decrease by 37% to 55% in the four YRD major cities (i.e., Shanghai, Hangzhou, Nanjing, and Hefei), but the MDA8 O3 increase is driven by both emission reduction (29%–52%) and variation in meteorological conditions (17%– 49%). Among all pollutants, reduction in emissions mainly of primary PM contributes to a PM2.5 decrease by 28% to 46%, and NOx emission reduction contributes 7% to 10%. Although NOx emission reduction dominates the MDA8 O3 increase (38%–59%), volatile organic compounds (VOCs) emission reduction lead to a 5% to 9% MDA8 O3 decrease. Increased O3 promotes secondary aerosol formation and partially offsets the decrease of PM2.5 caused by the primary PM emission reductions. The results demonstrate that more coordinated air pollution control strategies are needed in YRD.
Ozone (O3) has become a significant air pollutant in China in recent years. O3 abatement is challenging due to the nonlinear response of O3 to precursors nitrogen oxides (NOx) and volatile organic ...compounds (VOCs). Photochemical indicators are widely used to estimate the O3–NOx–VOC sensitivity, and this has important policy implications. However, the effectiveness of the indicators has seldom been evaluated. This study examined the applications of four indicators that include the ratio of the production rates of H2O2 and HNO3 (PH2O2/PHNO3), HCHO/NO2, HCHO/NOy, and reactive nitrogen (NOy) in the Yangtze River Delta (YRD) with localized thresholds. The overall accuracy was high (>92%) for all indicators and not significantly reduced with different simulation periods or in different locations of the region. By comparing with the O3 isopleths, it was found that HCHO/NO2 and HCHO/NOy showed the most consistency, whereas PH2O2/PHNO3 (NOy) tended to underestimate (overestimate) the positive response of O3 to NOx. Additionally,PH2O2/PHNO3 was less likely to attribute the O3 formation to mixed sensitivity than the other indicators, and this demonstrated a preference for a single-pollutant control strategy. This study also revealed that the details in the methodology used to derive the threshold values impacted the results, and this may produce uncertainties in the application of photochemical indicators.
Air quality in China has been undergoing significant changes due to the implementation of extensive emission control measures since 2013. Many observational and modeling studies investigated the ...formation mechanisms of fine particulate matter (PM
2.5
) and ozone (O
3
) pollution in the major regions of China. To improve understanding of the driving forces for the changes in PM
2.5
and O
3
in China, a nationwide air quality modeling study was conducted from 2013 to 2019 using the Weather Research and Forecasting/Community Multiscale Air Quality (WRF/CMAQ) modeling system. In this study, the model predictions were evaluated using the observation data for the key pollutants including O
3
, sulfur dioxide (SO
2
), nitrogen dioxide (NO
2
), and PM
2.5
and its major components. The evaluation mainly focused on five major regions, that is , the North China Plain (NCP), the Yangtze River Delta (YRD), the Pearl River Delta (PRD), the Chengyu Basin (CY), and the Fenwei Plain (FW). The CMAQ model successfully reproduced the air pollutants in all the regions with model performance indices meeting the suggested benchmarks. However, over-prediction of PM
2.5
was noted in CY. NO
2
, O
3,
and PM
2.5
were well simulated in the north compared to the south. Nitrate (NO
3
−
) and ammonium (NH
4
+
) were the most important PM
2.5
components in heavily polluted regions. For the performance on different pollution levels, the model generally over-predicted the clean days but underpredicted the polluted days. O
3
was found increasing each year, while other pollutants gradually reduced during 2013–2019 across the five regions. In all of the regions except PRD (all seasons) and YRD (spring and summer), the correlations between PM
2.5
and O
3
were negative during all four seasons. Low-to-medium correlations were noted between the simulated PM
2.5
and NO
2
, while strong and positive correlations were established between PM
2.5
and SO
2
during all four seasons across the five regions. This study validates the ability of the CMAQ model in simulating air pollution in China over a long period and provides insights for designing effective emission control strategies across China.
We investigated the spatial distribution and trend of double high pollution (DHP), in which the daily average concentration of fine particulate matter (PM2.5) was above 75 μg/m3 and the daily maximum ...8-hour average ozone (MDA8 O3) concentration was above 160 μg/m3, in the Yangtze River Delta (YRD) region during 2015–2019, along with the meteorological and chemical characteristics during DHP and differences compared to high O3 pollution (HOP) and high PM2.5 pollution (HPP). In the YRD, Shanghai had the highest frequency of DHP at 7.6%, while Anhui had the least (2.1%). DHP mostly occurred in the northwest and along the Yangtze River in the east of the YRD, especially in spring (April) and autumn (October). MDA8 O3 level was relatively higher during DHP than HOP, while PM2.5 level was relatively higher during HPP than DHP. In 2015–2019, the total number of DHP events decreased in the YRD, but the changes in PM2.5 and O3 concentrations showed great spatial variations. DHP was often associated with a weak pressure field, under meteorological conditions with east winds, temperatures of 18.7–26.1 °C, relative humidity of 65.7–77.1%, sea level pressure of 1008.2–1019 hPa, wind speed of 1.4–2.4 m/s, and visibility of 3.1–7.5 km. Water-soluble ions (NO3−, NH4+, and SO42−) were the dominant components of PM2.5 during DHP at Nanjing and Changzhou City in 2019. Although the fraction of those ions during DHP and HPP were similar, the secondary conversion of NO2 and SO2 was stronger in HPP. The concentrations of those ions were lowest in HOP, with a higher fraction of sulfate than the other two types of pollution. The conversion of SO2 to sulfate was easier to occur than that of NO2 to nitrate under all the polluted conditions in the two cities.
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•DHP mostly occurred in the northwest and along the Yangtze River in the east of the YRD.•The number of DHP events showed an overall downward trend during 2015–2019.•DHP events were usually associated with a weak pressure field.
A regional ozone (O3) pollution event occurred in the Yangtze River Delta region during August 17–23, 2020 (except on August 21). This study aims to understand the causes of O3 pollution during the ...event using an emission-based model (i.e., the Community Multiscale Air Quality (CMAQ) model) and an observation-based model (OBM). The OBM was used to investigate O3 sensitivity to its precursors during the O3 pollution, concluding that O3 formation was limited by volatile organic compounds (VOCs) on August 19, but was co-limited by VOCs and nitrogen oxides (NOx) on other polluted days. Aromatics and alkenes were the two main VOC groups contributing to the O3 formation, with trans-2-butene and m/p-xylene as the key species among the VOCs measured at the Nanjing urban site. The source apportionment results estimated using the source-oriented CMAQ model suggest that the transportation and industry sources dominated the non-background O3 production in Nanjing, which were responsible for 52% and 24.7%, respectively. The O3 concentration attributed to NOx (~70%) was significantly higher than that attributed to VOCs (approximately 30%). The process analysis revealed that vertical mixing increased the O3 concentrations in the early morning, and photochemical reactions promoted O3 formation and accumulation during the daytime within the planetary boundary layer. At night, outflow from horizontal transport and nocturnal chemistry jointly resulted the O3 depletion. The contributions of inter-city transport during the O3 pollution period in Nanjing were also estimated. The predicted O3 concentration was largely recorded from long-distance regions, reaching 46%, followed by local sources (38%) and surrounding cities (16%). The results indicate that both NOx and VOCs contributed significantly to O3 pollution during this event, and the emissions controls of NOx and the key VOC species of aromatics and alkenes from a cooperative regional perspective should be considered to mitigate O3 pollution.
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•O3 formation was limited by VOCs on 1 day, but co-limited by VOCs and NOx on other O3 pollution days.•trans-2-Butene and m/p-xylene as the two VOCs species with the largest RIR values in Nanjing.•Transportation and industry sources dominated the non-background O3 production in Nanjing.•Vertical mixing and photochemical reactions dominate O3 accumulation during the daytime.•Local emissions and transport accounted for 38% and 62% of the non-background O3, respectively.
The atmospheric oxidizing capacity (AOC), reflecting the self-cleansing capacity of the atmosphere, plays an important role in the chemical evolution of secondary fine particulate matter (PM2.5) and ...ozone (O3). In this work, the AOC and its relationships with PM2.5 and O3 were investigated with a chemical transport model (CTM) in the Yangtze River Delta (YRD) region during the four seasons of 2017. The region-wide average AOC is ~4.5×10−4 min−1 in summer and ~ 6.4×10−5 min−1 in winter. Hydroxyl (OH) radicals oxidation contributes 55–69% to the total AOC, followed by nitrate (NO3) radicals and O3 (accounting for 19–34% and < 10%, respectively). The AOC attains a strong positive correlation with the O3 level in all seasons. However, it is weakly or insignificantly correlated with PM2.5 except in summer. Additionally, AOC×(SO2 + NO2 + volatile organic compound (VOC)) is well correlated with the PM2.5 level, and high levels of precursors counteract lower AOC values in cold seasons. Collectively, the results indicate that the abundance of precursors could drive secondary aerosol formation in winter, and aqueous or heterogeneous reactions (not considered in the AOC estimates) are likely of importance at high aerosol loadings in the YRD. The relationship between the daily PM2.5 and O3 levels is affected by the AOC magnitude. PM2.5 and O3 are strongly correlated when the AOC is relatively high, but PM2.5 is independent of O3 under low-AOC (<6.6×10−5 min−1, typically in winter) conditions. This work reveals the dependence of PM2.5-O3 relationships on the AOC, suggesting that joint PM2.5 and O3 reduction could be realized at moderate to high AOC levels, especially in spring and autumn when the cooccurrence of high O3 and PM2.5 events is frequently observed.
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•The AOC was dominated by OH in the daytime and NO3 at night in the urban YRD.•High AOC occurs with elevated O3, but is not observed in PM events in cold seasons.•High PM2.5 in cold seasons is associated with the abundance of precursors and the level of AOC.•PM2.5 and O3 are highly correlated at moderate to high AOC levels.
The Yangtze River Delta (YRD) urban agglomeration is one of the most developed regions in China. During recent decades, this region has experienced severe regional haze and photochemical smog ...pollution problems. In this study, we used a source-oriented chemical transport model to quantitatively estimate the effects of inter-city transport on fine particulate matter (PM2.5) and ozone (O3) among the 41 cities in the YRD urban agglomeration during the EXPLORE-YRD (EXPeriment on the eLucidation of the atmospheric Oxidation capacity and aerosol foRmation, and their Effects in the Yangtze River Delta) campaign (May 17 to June 17, 2018). The results show that inter-city transport is very significant in the YRD region. On average, the emissions from the local city, the other YRD cities, and the regions outside of the YRD contribute 25.3%, 49.9%, and 24.8% to the PM2.5, respectively, and they contribute 33.7%, 46.8%, and 19.5% of the non-background O3, respectively. On PM2.5 or O3 pollution days, the transport contribution from the non-local YRD cities becomes much more important, while the local emissions and the transport from non-YRD emissions become less important. The results also suggest that the cities within a distance of 184 km and 94 km contribute 60% of the PM2.5 and O3, respectively. Therefore, we recommend that regional cooperative control programs in the YRD consider emission controls over cities within these ranges. The range for primary PM2.5 (92 km) is very different from that for secondary PM2.5 (515 km). Cooperative emission controls of SO2 and NOx on a much larger regional scale are required to reduce the secondary PM2.5 in the YRD.
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•Inter-city transport among 41 YRD cities is conducted for O3 and PM2.5 during the EXPLORE-YRD campaign.•Inter-city transport has the largest contribution to PM2.5 and O3 in most cities.•Transport from nearby cities becomes more important on pollution days.•The cities within a distance of 184 km and 94 km contribute 60% of the PM2.5 and O3, respectively.•Reducing secondary inorganic PM2.5 in YRD requires cooperative emission controls on a much larger regional scale.
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