No Evidence for a Significant Impact of Heterogeneous Chemistry on Radical Concentrations in the North China Plain in Summer 2014

Deciphering the seasonal dynamics of multifaceted aerosol-ozone interplay: Implications for air quality management in Eastern China

We employed an enhanced WRF-Chem to investigate the discrete mechanisms of aerosol-radiation-feedback (ARF), extinction-photochemistry (AEP), and heterogeneous-reactions (AHR) across different seasons in eastern China, aiming to assess the synergistic effects arising from the simultaneous operation of multiple processes on O3 and PM2.5. Our findings demonstrated that ARF fostered the accumulation of pollutants and moisture, initiating two distinct feedback mechanisms concerning O3. The elevation in the NO/NO2 ratio amplified O3 consumption. Increased near-surface moisture diminished upper-level cloud formation, thereby enhancing photolysis rates and O3 photochemical production. The pronounced impact of heightened NO/NO2 on O3 led to a decrease of 0.1-2.7 ppb. When decoupled from ARF, AEP led to a more significant reduction in photolysis rates, resulting in declines in both O3 and PM2.5, except for an anomalous increase observed in summer, with O3 increasing by 1.6 ppb and PM2.5 by 2.5 μg m-3. The heterogeneous absorption of hydroxides in spring, autumn, and winter predominantly governed the AHR-induced variation of O3, leading to a decrease in O3 by 0.7-1 ppb. Conversely, O3 variations in summer were primarily dictated by O3-sensitive chemistry, with heterogeneous absorption of NOy catalyzing a decrease of 2.4 ppb in O3. Furthermore, AHR accentuated PM2.5 by facilitating the formation of fine sulfates and ammonium while impeding nitrate formation. In summer, the collective impact of ARF, AEP, and AHR (ALL) led to a substantial reduction of 6.2 ppb in O3, alleviating the secondary oxidation of PM2.5 and leading to a decrease of 0.3 μg m-3 in PM2.5. Conversely, albeit aerosol substantially depleted O3 by 0.4-4 ppb through their interactions except for summer, aerosol feedback on PM2.5 was more pronounced, resulting in a significant increase of 1.7-6.1 μg m-3 in PM2.5. Our study underscored the seasonal disparities in the ramifications of multifaceted aerosol-ozone interplay on air quality.

Multi-scale analysis of the chemical and physical pollution evolution process from pre-co-pollution day to PM2.5 and O3 co-pollution day

PM2.5 and O3 are two of the main air pollutants that have adverse impacts on climate and human health. The evolution process of PM2.5 and O3 co-pollution are of concern because of the increased frequency of PM2.5 and O3 co-pollution days. Here, we examined the chemical coupling and revealed the driving factors of the PM2.5 and O3 co-pollution evolution process from cleaning day, PM2.5 pollution day, or O3 pollution day, applied by theoretical analysis and model calculation methods. The results demonstrate that PM2.5 and O3 co-pollution day frequently occurred with high concentrations of gaseous precursors and higher sulfur oxidation ratio (SOR) and nitrogen oxidation ratio (NOR), which we attribute to the enhancement of atmospheric oxidation capacity (AOC). The AOC is positively correlated with O3 and weakly correlated with PM2.5. In addition, we found that the correlation coefficients of PM2.5-NO2 (0.62) were higher than that of PM2.5-SO2 (0.32), highlighting the priority of NOx controlling to mitigate PM2.5 pollution. Overall, our discovery can provide scientific evidence to design feasible solutions for the controlling PM2.5 and O3 co-pollution process.

Peroxy radical chemistry during ozone photochemical pollution season at a suburban site in the boundary of Jiangsu-Anhui-Shandong-Henan region, China

Ambient peroxy radical (RO2⁎ = HO2 + RO2) concentrations were measured at a suburban site in a major prefecture-level city (Huaibei) in the boundary of Jiangsu-Anhui-Shandong-Henan region, which is the connecting belt of air pollution in the Beijing-Tianjin-Hebei region and the Yangtze River Delta. Measurements were carried out during the period of September to October 2021 to elucidate the formation mechanism of O3 pollution. The observed maximum concentration of peroxy radicals was 73.8 pptv. A zero-dimensional box model (Framework for 0-Dimensional Atmospheric Modeling, F0AM) based on Master Chemical Mechanism (MCM3.3.1) was used to predict radical concentrations for comparison with observations. The model reproduced the daily variation of peroxy radicals well, but discrepancies still appear in the morning hours. As in previous field campaigns, systematic discrepancies between modelled and measured RO2⁎ concentrations are observed in the morning for NO mixing ratios higher than 1 ppbv. Between 6:00 and 9:00 am, the model significantly underpredicts RO2⁎ by a mean factor of 7.2. This underprediction can be explained by a missing RO2⁎ source of 1.2 ppbv h-1 which originated from the photochemical conversion of an alkene-like chemical species. From the model results it shows that the main sources of ROx (= OH + HO2 + RO2) are the photolysis of oxygenated volatile organic compounds (OVOCs, 33 %), O3 and HONO (25 %), and HCHO (24 %). And the major sinks of ROx transitioned from a predominant reaction of radicals with NOx in the morning to a predominant peroxy self- and cross-reaction in the late afternoon. The introduction of an alkene-like species increased RO2 radical concentration and resulted in 14 % increase in net daily integrated ozone production, indicating the possible significance of the mechanism of alkene-like species oxidation to peroxy radicals. This study provides important information for subsequent ozone pollution control policies in Jiangsu-Anhui-Shandong-Henan region.

Strategies for the coordinated control of particulate matter and carbon dioxide under multiple combined pollution conditions

Air pollutants represented by fine particulate matter (PM2.5) and the greenhouse effect caused by carbon dioxide (CO2), are both urgent threats to public health. Tackling the synergistic reduction of PM2.5 and CO2 is critical to achieving improvements in clean air worldwide. A persistent issue is the identification of their common sources and integrated impacts under different environmental conditions. In this study, we investigated the characteristics of the pollution types captured by combined analysis through a comprehensive observational dataset for 2017-2020, and applied machine learning algorithms to quantify the effects of drivers on air pollutants and CO2 formation. More importantly, detailed conclusions were drawn for the joint control of PM2.5-CO2 in multiple pollution types by using ensemble traceability technique. We demonstrated that reducing coal combustion emissions was an effective measure to maximize the benefits of PM2.5-CO2 in weather with low CO2 levels and no PM2.5 pollution. Correspondingly, on days with severe PM2.5 episodes, prioritizing control of vehicle emissions can simultaneously mitigate PM2.5 and CO2. Similar conclusions were found at high CO2 levels, accompanied by a more extensive role of vehicle emissions. Furthermore, a comparison of the differences in source impacts between PM2.5-CO2 and individual species suggests that focusing only on the sources that contribute significantly to one species may result in an underestimation or overestimation of PM2.5-CO2 source impacts. One such implication, as evidenced by our findings, is that synergistic controlling common sources of pollutants should be efficient. Thereby, common source management targeting PM2.5-CO2 under multiple pollution types is a more workable solution to alleviate environmental pollution.

Drivers of Increasing Ozone during the Two Phases of Clean Air Actions in China 2013-2020

In response to the severe air pollution issue, the Chinese government implemented two phases (Phase I, 2013-2017; Phase II, 2018-2020) of clean air actions since 2013, resulting in a significant decline in fine particles (PM_2.5) during 2013-2020, while the warm-season (April-September) mean maximum daily 8 h average ozone (MDA8 O₃) increased by 2.6 μg m^-3 yr^-1 in China during the same period. Here, we derived the drivers behind the rising O₃ concentrations during the two phases of clean air actions by using a bottom-up emission inventory, a regional chemical transport model, and a multiple linear regression model. We found that both meteorological variations (3.6 μg m^-3) and anthropogenic emissions (6.7 μg m^-3) contributed to the growth of MDA8 O₃ from 2013 to 2020, with the changes in anthropogenic emissions playing a more important role. The anthropogenic contributions to the O₃ rise during 2017-2020 (1.2 μg m^-3) were much lower than that in 2013-2017 (5.2 μg m^-3). The lack of volatile organic compound (VOC) control and the decline in nitrogen oxides (NO_x) emissions were responsible for the O₃ increase in 2013-2017 due to VOC-limited regimes in most urban areas, while the synergistic control of VOC and NO_x in Phase II initially worked to mitigate O₃ pollution during 2018-2020, although its effectiveness was offset by the penalty of PM_2.5 decline. Future mitigation efforts should pay more attention to the simultaneous control of VOC and NO_x to improve O₃ air quality.

Impacts of synoptic circulation on surface ozone pollution in a coastal eco-city in Southeastern China during 2014-2019

The coastal eco-city of Fuzhou in Southeastern China has experienced severe ozone (O₃) episodes at times in recent years. In this study, three typical synoptic circulations types (CTs) that influenced more than 80% of O₃ polluted days in Fuzhou during 2014-2019 were identified using a subjective approach. The characteristics of meteorological conditions linked to photochemical formation and transport of O₃ under the three CTs were summarized. Comprehensive Air Quality Model with extensions was applied to simulate O₃ episodes and to quantify O₃ sources from different regions in Fuzhou. When Fuzhou was located to the west of a high-pressure system (classified as "East-ridge"), more warm southwesterly currents flowed to Fuzhou, and the effects of cross-regional transport from Guangdong province and high local production promoted the occurrence of O₃ episodes. Under a uniform pressure field with a low-pressure system occurring to the east of Fuzhou (defined as "East-low"), stagnant weather conditions caused the strongest local production of O₃ in the atmospheric boundary layer. Controlled by high-pressure systems over the mainland (categorized as "Inland-high"), northerly airflows enhanced the contribution of cross-regional transport to O₃ in Fuzhou. The abnormal increases of the "East-ridge" and "Inland-high" were closely related to O₃ pollution in Fuzhou in April and May 2018, resulting in the annual maximum number of O₃ polluted days during recent years. Furthermore, the rising number of autumn O₃ episodes in 2017-2019 was mainly related to the "Inland-high", indicating the aggravation of cross-regional transport and highlighting the necessity of enhanced regional collaboration and efforts in combating O₃ pollution.

Atmospheric oxidizing capacity in autumn Beijing: Analysis of the O3 and PM2.5 episodes based on observation-based model

Atmospheric oxidizing capacity (AOC) is the fundamental driving factors of chemistry process (e.g., the formation of ozone (O₃) and secondary organic aerosols (SOA)) in the troposphere. However, accurate quantification of AOC still remains uncertainty. In this study, a comprehensive field campaign was conducted during autumn 2019 in downtown of Beijing, where O₃ and PM_2.5 episodes had been experienced successively. The observation-based model (OBM) is used to quantify the AOC at O₃ and PM_2.5 episodes. The strong intensity of AOC is found at O₃ and PM_2.5 episodes, and hydroxyl radical (OH) is the dominating daytime oxidant for both episodes. The photolysis of O₃ is main source of OH at O₃ episode; the photolysis of nitrous acid (HONO) and formaldehyde (HCHO) plays important role in OH formation at PM_2.5 episode. The radicals loss routines vary according to precursor pollutants, resulting in different types of air pollution. O₃ budgets and sensitivity analysis indicates that O₃ production is transition regime (both VOC and NO_x-limited) at O₃ episode. The heterogeneous reaction of hydroperoxy radicals (HO₂) on aerosol surfaces has significant influence on OH and O₃ production rates. The HO₂ uptake coefficient (γHO₂) is the determining factor and required accurate measurement in real atmospheric environment. Our findings could provide the important bases for coordinated control of PM_2.5 and O₃ pollution.

Variability of PM2.5 and O3 concentrations and their driving forces over Chinese megacities during 2018-2020

Recently, air pollution especially fine particulate matters (PM_2.5) and ozone (O₃) has become a severe issue in China. In this study, we first characterized the temporal trends of PM_2.5 and O₃ for Beijing, Guangzhou, Shanghai, and Wuhan respectively during 2018-2020. The annual mean PM_2.5 has decreased by 7.82%-33.92%, while O₃ concentration showed insignificant variations by -6.77%-4.65% during 2018-2020. The generalized additive models (GAMs) were implemented to quantify the contribution of individual meteorological factors and their gas precursors on PM_2.5 and O₃. On a short-term perspective, GAMs modeling shows that the daily variability of PM_2.5 concentration is largely related to the variation of precursor gases (R = 0.67-0.90), while meteorological conditions mainly affect the daily variability of O₃ concentration (R = 0.65-0.80) during 2018-2020. The impact of COVID-19 lockdown on PM_2.5 and O₃ concentrations were also quantified by using GAMs. During the 2020 lockdown, PM_2.5 decreased significantly for these megacities, yet the ozone concentration showed an increasing trend compared to 2019. The GAMs analysis indicated that the contribution of precursor gases to PM_2.5 and O₃ changes is 3-8 times higher than that of meteorological factors. In general, GAMs modeling on air quality is helpful to the understanding and control of PM_2.5 and O₃ pollution in China.

Multiple Impacts of Aerosols on O3 Production Are Largely Compensated: A Case Study Shenzhen, China

Tropospheric ozone (O₃) is a harmful gas compound to humans and vegetation, and it also serves as a climate change forcer. O₃ is formed in the reactions of nitrogen oxides and volatile organic compounds (VOCs) with light. In this study, an O₃ pollution episode encountered in Shenzhen, South China in 2018 was investigated to illustrate the influence of aerosols on local O₃ production. We used a box model with comprehensive heterogeneous mechanisms and empirical prediction of photolysis rates to reproduce the O₃ episode. Results demonstrate that the aerosol light extinction and NO₂ heterogeneous reactions showed comparable influence but opposite signs on the O₃ production. Hence, the influence of aerosols from different processes is largely counteracted. Sensitivity tests suggest that O₃ production increases with further reduction in aerosols in this study, while the continued NO_x reduction finally shifts O₃ production to an NO_x-limited regime with respect to traditional O₃-NO_x-VOC sensitivity. Our results shed light on the role of NO_x reduction on O₃ production and highlight further mitigation in NO_x not only limiting the production of O₃ but also helping to ease particulate nitrate, as a path for cocontrol of O₃ and fine particle pollution.

Evidence for Reducing Volatile Organic Compounds to Improve Air Quality from Concurrent Observations and In Situ Simulations at 10 Stations in Eastern China

Ground-level ozone (O₃) has been an emerging air pollution in China and interacts with fine particulate matters (PM_2.5). We synthesized observations of O₃ and its precursors in two summer months of 2020 at 10 sites in the Zhejiang province, East China and simulated the in situ photochemistry. O₃ pollution in the northeastern Zhejiang province was more serious than that in the southwest. The site-average daytime O₃ increment correlated well (R² = 0.73) with the total reactivity of volatile organic compounds (VOCs) and carbon monoxide toward the hydroxyl radical (OH) in urban areas. Model simulation revealed that the main function of nitrogen oxides (NO_x) at the rural sites where isoprene accounted for >85% of OH reactivity of VOCs was to facilitate the radical cycling. With NO_x reduction from 0 to 90%, the self-reactions between peroxy radicals (Self-Rxns), a proven pathway for secondary organic aerosol formation, were intensified by up to 23-fold in a NO_x-rich environment. In contrast, reducing VOCs could weaken the Self-Rxns while reducing O₃ production rate and atmospheric oxidation capacity. This study observes and simulates O₃ chemistry based on extensive measurements in typical Chinese cities, highlighting the necessity of reducing VOCs for co-benefit of O₃ and PM_2.5.

Potential Factors Contributing to Ozone Production in AQUAS-Kyoto Campaign in Summer 2020: Natural Source-Related Missing OH Reactivity and Heterogeneous HO2/RO2 Loss

This study presents total OH reactivity, ancillary trace species, HO₂ reactivity, and complex isoprene-derived RO₂ reactivity due to ambient aerosols measured during the air quality study (AQUAS)-Kyoto campaign in September, 2020. Observations were conducted during the coronavirus disease (COVID-19) pandemic (associated with reduced anthropogenic emissions). The spatial distribution of missing OH reactivity highlights that the origin of volatile organic compounds (VOCs) may be from natural-emission areas. For the first time, the real-time loss rates of HO₂ and RO₂ onto ambient aerosols were measured continuously and alternately. Ozone production sensitivity was investigated considering unknown trace species and heterogeneous loss effects of XO₂ (≡HO₂ + RO₂) radicals. Missing OH reactivity enhanced the ozone production potential by a factor of 2.5 on average. Heterogeneous loss of radicals could markedly suppress ozone production under low NO/NO_x conditions with slow gas-phase reactions of radicals and change the ozone regime from VOC- to NO_x-sensitive conditions. This study quantifies the relationship of missing OH reactivity and aerosol uptake of radicals with ozone production in Kyoto, a low-emission suburban area. The result has implications for future NO_x-reduction policies. Further studies may benefit from the combination of chemical transport models and inverse modeling over a wide spatiotemporal range.

Elucidating Contributions of Anthropogenic Volatile Organic Compounds and Particulate Matter to Ozone Trends over China

In China, emissions of ozone (O₃)-producing pollutants have been targeted for mitigation to reduce O₃ pollution. However, the observed O₃ decrease is slower than/opposite to expectations affecting the health of millions of people. For a better understanding of this failure and its connection with anthropogenic emissions, we quantify the summer O₃ trends that would have occurred had the weather stayed constant by applying a numerical tool that "de-weathers" observations across 31 urban regions (123 cities and 392 sites) over 8 years. O₃ trends are significant (p < 0.05) over 234 sites after de-weathering, contrary to the directly observed trends (only 39 significant due to high meteorology-induced variability). The de-weathered data allow categorizing cities in China into four different groups regarding O₃ mitigation, with group 1 exhibiting steady O₃ reductions, while group 4 showing significant (p < 0.05) O₃ increases. Analysis of the relationships between de-weathered odd oxygen and nitrogen oxides illustrates how the changes in NO_x, in anthropogenic volatile organic compounds (VOCs), and reductions in fine particulate matter (PM_2.5) affect the O₃ trends differently in these groups. While this analysis suggests that VOC reductions are the main driver of O₃ decreases in group 1, groups 3 and 4 are primarily affected by decreasing PM_2.5, which results in enhanced O₃ formation. Our analysis demonstrates both the importance of and possibility for isolating emission-driven changes from climate and weather for interpreting short-term air quality observations.

Interannual variations, sources, and health impacts of the springtime ozone in Shanghai

In spring, ozone (O₃) pollution frequently occurrs in eastern China, but key drivers remain uncertain. In this study, interannual variations in springtime ozone in Shanghai, China, from 2013 to 2021, were investigated to assess the health impacts and the effectiveness of recent air pollution control measures. A combination of ground-level measurements of regulated air pollutants, lidar observations, and backward trajectories of air masses was used to identify the key drivers for enhancing springtime O₃. The results show that external imports of O₃ driven by atmospheric circulation are notable sources of springtime surface O₃. For example, the downward transport from the free troposphere could contribute to over 50% of surface O₃ in the morning. The surface O₃ mixing ratios in spring exhibited an upward trend of 0.93 ppb yr^-1 (p < 0.05) from 2013 to 2021. The change in meteorological variables, particularly the increase in air temperature, could explain nearly 87% of the springtime O₃ upward trend. The change in anthropogenic emissions of precursors only contributed to a small fraction (<13%) of the increase in springtime O₃. The cumulative exposure of urban residents to O₃ in spring also exhibited a significant upward trend (111 ppb yr^-1, p < 0.05). With the rapid increase in surface O₃, premature respiratory mortality attributable to O₃ exposure has fluctuated at approximately 2933 deaths per year since 2016, even though the total deaths from respiratory diseases have significantly declined. Long-term exposure to high O₃ concentrations is a significant contributor to premature respiratory mortality.

Long-Term Variations of Meteorological and Precursor Influences on Ground Ozone Concentrations in Jinan, North China Plain, from 2010 to 2020

Ground-level ozone (O3) pollution in the North China Plain has become a serious environmental problem over the last few decades. The influence of anthropogenic emissions and meteorological conditions on ozone trends have become the focus of widespread research. We studied the long-term ozone trends at urban and suburban sites in a typical city in North China and quantified the contributions of anthropogenic and meteorological factors. The results show that urban O3 increased and suburban O3 decreased from 2010 to 2020. The annual 90th percentile of the maximum daily 8-h average of ozone in urban areas increased by 3.01 μgm−3year−1 and, in suburban areas, it decreased by 3.74 μgm−3year−1. In contrast to the meteorological contributions, anthropogenic impacts are the decisive reason for the different ozone trends in urban and suburban areas. The rapid decline in nitrogen oxides (NOX) in urban and suburban areas has had various effects. In urban areas, this leads to a weaker titration of NOX and enhanced O3 formation, while in suburban areas, this weakens the photochemical production of O3. Sensitivity analysis shows that the O3 formation regime is in a transition state in both the urban and suburban areas. However, this tends to be limited to volatile organic compounds (VOCs) in urban areas and to NOX in suburban areas. One reasonable approach to controlling ozone pollution should be to reduce nitrogen oxide emissions while strengthening the control of VOCs.

Effects of hydroperoxy radical heterogeneous loss on the summertime ozone formation in the North China Plain

Hydroperoxy radical (HO₂) is a crucial oxidant participating in the oxidation of nitrogen oxide to nitrogen dioxide which constitutes one of the most important pathways for the ozone (O₃) photochemical formation in the troposphere. Laboratory experiments and field observations have revealed efficient HO₂ heterogeneous uptake on wet aerosols, but its impact on the O₃ formation remains controversial. A severe and persistent O₃ pollution episode has been simulated using the WRF-Chem model to evaluate the impacts of the HO₂ heterogeneous loss on the O₃ formation in the North China Plain (NCP) during the summertime of 2018. Comparisons between experimental simulations with the HO₂ effective uptake coefficient of 0.2 and 0.0 shows that the HO₂ heterogeneous loss decreases the daytime HO₂ and maximum daily average 8-hour (MDA8) O₃ concentrations by about 5% and 1% in the NCP, respectively. Emission mitigation from 2013 to 2018 is found to contribute a 2.1 μg m^-3 (5%) increase in the MDA8 O₃ concentration due to decreased aerosol sink for the HO₂ heterogeneous loss in the NCP. Our results reveal that decreased HO₂ heterogeneous uptake does not constitute an important factor driving the O₃ trend since 2013 in the NCP.

Historically understanding the spatial distributions of particle surface area concentrations over China estimated using a non-parametric machine learning method

A non-parametric ensemble model was proposed to estimate the long-term (2015-2019) particle surface area concentrations (S_A) over China for the first time on basis of a vilification dataset of measured particle number size distribution. This ensemble model showed excellent cross-validation R² value (CV R² = 0.83) as well as a relatively low root-mean-square error (RMSE = 195.0 μm²/cm³). No matter in which year, considerable spatial heterogeneity of S_A was found over China with higher S_A in Beijing-Tianjin-Hebei (BTH), Yangtze River Delta (YRD), and Middle Lower Reaches of Yangtze River (MLYR). From 2015 to 2019, S_A significantly decreased in representative city clusters. The reduction rates were 140.1 μm²·cm^-3·a^-1 in BTH, 110.7 μm²·cm^-3·a^-1 in Pearl River Delta (PRD), 105.2 μm²·cm^-3·a^-1 in YRD, and 92.4 μm²·cm^-3·a^-1 in Sichuan Basin (SCB), respectively. Even though such quick reduction, high S_A (ranged from ~800 μm²/cm³ to ~1750 μm²/cm³) during the heavy pollution period (PM_2.5 > 75 μg/m³) still existed in the above-mentioned city clusters and may provide rich reaction vessels for multiphase chemistry. A dichotomy of enhanced annual 4th maximum daily 8-h average O₃ concentrations (4MDA8 O₃) and decreased S_A during summertime was found in Shanghai, a representative city of YRD. In Chengdu (SCB), increased 4MDA8 O₃ concentration was associated with a synchronous increase of S_A from 2017 to 2019. Differently, 4MDA8 O₃ concentrations enhanced in Beijing (BTH) and Guangzhou (PRD), while not significant for S_A before 2018. This work will greatly deepen our understanding of the historical variation and spatial distributions of S_A over China.

Drivers of 2013-2020 ozone trends in the Sichuan Basin, China: Impacts of meteorology and precursor emission changes

The Sichuan Basin (SCB) of China is known for excessive ozone (O₃) pollution owing to high anthropogenic emissions combined with terrain-induced poor ventilation and weak wind fields against the surrounding mountains. While O₃ pollution has emerged as a prominent concern in southwestern China yet variations in O₃ levels during 2013-2020 are still unclear and the dominant factor in explaining the long-term O₃ trend throughout the SCB remains elusive due to uncertainties in emission inventory and variability associated with meteorological conditions. Here, we use extensive basin-wide ambient measurements to examine the spatial pattern and trend of O₃ and leverage OMI and TROPOMI satellites in conjunction with MEIC emission inventory to track emission changes. Sensitivity simulations are conducted by using WRF-CMAQ model to investigate the impacts of meteorological variability and emission changes on O₃ changes over 2013-2020. O₃ concentrations exhibit obvious interannual increases during 2013-2019 and a slight decrease in 2020. Both decreases in the MEIC emission inventory (-2.9% yr^-1) and OMI NO₂ column density (-3.1% yr^-1) reflects the declining trend in NO_x emissions over 2013-2020, while anthropogenic VOCs were not adequately regulated during 2013-2017, which explained the majority of deteriorated O₃ pollution from 2013 to 2017. Furthermore, attribution analysis based on CMAQ simulations indicate that the unexpected aggravated O₃ levels in 2019 is not only modulated by disproportional reductions in VOCs and NO_x emissions, but also associated with unfavorable meteorological conditions featured by profound heatwaves and frequent stagnant conditions. In 2020, the abnormal meteorological conditions in May leads to substantial increase of O₃ by 26.8 μg m^-3 as compared to May 2019, while the considerable enhancement was fully offset by low O₃ levels over the whole period which attributes to substantial emission reductions. This study reveals the long-term trend of O₃ levels and precursor emissions and highlights the effects of meteorological variability and emission changes on O₃ pollution over the SCB, with strong implications for designing effective O₃ control measures.

Revealing the driving effect of emissions and meteorology on PM2.5 and O3 trends through a new algorithmic model

Quantifying the driving effect of each factor on atmospheric secondary pollutants is crucial for pollution prevention. We aim to establish a simple and accessible method to identify ozone (O₃) and particulate matter (PM_2.5) concentration trends induced by emissions and meteorology. The method comprises five main steps, which involve matrix construction and mutual calculations, and the whole process is demonstrated and verified by employing long-term monitoring data. With regard to the case study, O₃ and PM_2.5 concentration variance between the target and base year are respectively -4.74 and 0.20 μg/m³ under same meteorological conditions, among which the contribution of the emissions driver and meteorological driver are respectively -5.81 and 1.07 μg/m³ for O₃ and respectively 0.55 and -0.35 μg/m³ for PM_2.5. Additionally, 84.45% of O₃ variance is attributable to the emissions driver in terms of relative importance, which is 52.88% for PM_2.5. The meteorological driver is further separated into atmospheric secondary reaction and regional transport. The results reveal that ongoing prevention policy for O₃ is effective; however, it needs to be further optimized for PM_2.5.

Impact of the Levels of COVID-19 Pandemic Prevention and Control Measures on Air Quality: A Case Study of Jiangsu Province, China

In order to control the spread of the COVID-19 pandemic, the prevention and control measures of public health emergencies were initiated in all provinces of China in early 2020, which had a certain impact on air quality. In this study, taking Jiangsu Province in China as an example, the air pollution levels in different regions under different levels of pandemic prevention and control (PPC) measures are evaluated. The implementation of the prevention and control policies of COVID-19 pandemic directly affected the concentration of air pollutants. No matter what level of PPC measures was implemented, the air quality index (AQI) and pollutant concentrations of NO2, CO, PM10 and PM2.5 were all reduced by varied degrees. The higher the level of PPC measures, the greater the reduction was in air pollutant concentrations. Specifically, NO2 was the most sensitive to PPC policies. The concentrations of CO and atmospheric particulate matter (PM10 and PM2.5) decreased most obviously under the first and second level of PPC. The response speed of air quality to different levels of PPC measures varied greatly among different cities. Southern Jiangsu, which has a higher level of economic development and is dominated by secondary and tertiary industries, had a faster response speed and a stronger responsiveness. The results of this study reflect the economic vitality of different cities in economically advanced regions (i.e., Jiangsu Province) in China. Furthermore, the results can provide references for the formulation of PPC policies and help the government make more scientific and reasonable strategies for air pollution prevention and control.

Rural vehicle emission as an important driver for the variations of summertime tropospheric ozone in the Beijing-Tianjin-Hebei region during 2014-2019

Tropospheric ozone (O₃) pollution is increasing in the Beijing-Tianjin-Hebei (BTH) region despite a significant decline in atmospheric fine aerosol particles (PM_2.5) in recent years. However, the intrinsic reason for the elevation of the regional O₃ is still unclear. In this study, we analyzed the spatio-temporal variations of tropospheric O₃ and relevant pollutants (PM_2.5, NO₂, and CO) in the BTH region based on monitoring data from the China Ministry of Ecology and Environment during the period of 2014-2019. The results showed that summertime O₃ concentrations were constant in Beijing (BJ, 0.06 µg/(m³•year)) but increased significantly in Tianjin (TJ, 9.09 µg/(m³•year)) and Hebei (HB, 6.06 µg/(m³•year)). Distinct O₃ trends between Beijing and other cities in BTH could not be attributed to the significant decrease in PM_2.5 (from -5.08 to -6.32 µg/(m³•year)) and CO (from -0.053 to -0.090 mg/(m³•year)) because their decreasing rates were approximately the same in all the cities. The relatively stable O₃ concentrations during the investigating period in BJ may be attributed to a faster decreasing rate of NO₂ (BJ: -2.55 µg/(m³•year); TJ: -1.16 µg/(m³•year); HB: -1.34 µg/(m³•year)), indicating that the continued reduction of NO_x will be an effective mitigation strategy for reducing regional O₃ pollution. Significant positive correlations were found between daily maximum 8 hr average (MDA8) O₃ concentrations and vehicle population and highway freight transportation in HB. Therefore, we speculate that the increase in rural NO_x emissions due to the increase in vehicle emissions in the vast rural areas around HB greatly accelerates regional O₃ formation, accounting for the significant increasing trends of O₃ in HB.

Observation and simulation of HOx radicals in an urban area in Shanghai, China

A continuous wintertime observation of ambient OH and HO₂ radicals was first carried out in Shanghai, in 2019. This effort coincided with the second China International Import Expo (CIIE), during which strict emission controls were implemented in Shanghai, resulting in an average PM_2.5 concentration of less than 35 μg/m³. The self-developed instrument based on the laser-induced fluorescence (LIF) technique reported that the average OH radical concentration at noontime (11:00-13:00) was 2.7 × 10⁶ cm^-3, while the HO₂ concentration was 0.8 × 10⁸ cm^-3. A chemical box model utilizing the Regional Atmospheric Chemical Mechanism 2 (RACM2), which is used to simulate pollutant reactions and other processes in the troposphere and which incorporates the Leuven isoprene mechanism (LIM1), reproduced the OH concentrations on most days. The HO₂ concentration was underestimated, and the observed-to-modelled ratio demonstrated poor performance by the model, especially during the elevated photochemistry period. Missing primary peroxy radical sources or unknown behaviors of RO₂ for high-NOx regimes are possible reasons for the discrepancy. The daytime ROx production was controlled by various sources. HONO photolysis accounted for more than one half (0.83 ppb/h), and the contribution from formaldehyde, OVOCs and ozone photolysis was relatively similar. Active oxidation paths accelerated the rapid ozone increase in winter. The average ozone production rate was 15.1 ppb/h, which is comparable to that of a Beijing suburb (10 ppb/h for the 'BEST-ONE') but much lower than that of Beijing's center (39 ppb/h in 'PKU' and 71 ppb/h in 'APHH') in wintertime. Cumulative local ozone based on observed peroxy radicals was five times higher than the value simulated by the current model due to the underprediction of HO₂ and RO₂ under the high-NOx regime. This analysis provides crucial information for subsequent pollution control policies in Shanghai.

Temporal and spatial variations of air pollution across China from 2015 to 2018

This study investigated concentrations of PM_2.5, PM₁₀, SO₂, NO₂, CO and O₃, and air quality index (AQI) values across 368 cities in mainland China during 2015-2018. The study further examined relationships of air pollution status with local industrial capacities and vehicle possessions. Strong correlations were found between industrial capacities (coal, pig iron, crude steel and rolled steel) and air pollution levels. Although statistical and significant reductions of PM_2.5, PM₁₀, SO₂, NO₂, CO and AQI values were observed in response to various laws and regulations in industrial sectors, both particle and gaseous pollutants still had annual average concentrations above recommended limits. In order to further reduce air pollution, more efforts can be done to control traffic emissions caused by minicars and heavy trucks, which was revealed after investigating 16 vehicle types. This was also consistent with the apparent air quality improvement during the COVID-19 lockdown period in China in 2020, despite industrial operations being still active at full capacities.

Characterizing nitrate radical budget trends in Beijing during 2013-2019

Nitrate (NO₃) radical is an important oxidant in the atmosphere as it regulates the NO_x budget and impacts secondary pollutant formation. Here, a long-term observational dataset of NO₃-related species at an urban site in Beijing was used to investigate changes in the NO₃ budget and their atmospheric impacts during 2013-2019, in this period the Clean Air Actions Plan was carried out in China. We found that (1) changes in NO₃ precursors (NO₂ and O₃) led to a significant increase in NO₃ formation in the surface layer in winter but a decrease in summer; (2) a reduction in NO_x promoted thermal equilibrium, favoring the formation of NO₃ rather than dinitrogen pentoxide (N₂O₅). The simultaneous decrease in PM_2.5, during these years, further weakened the N₂O₅ heterogeneous uptake; (3) a box model simulation revealed that both the reactions of NO₃ with volatile organic compounds (VOC) and N₂O₅ uptake were weakened in summer, implying that the policy actions implemented help to moderate secondary aerosol formation caused by NO₃ and N₂O₅ chemistry in summer; and (4) during winter, both NO₃ + VOC and N₂O₅ uptake were enhanced. Specifically, for the N₂O₅ uptake, the rapid increase in NO₃ production, or to some extent, NO₃ oxidation capacity, far outweighed the negative shift effect, leading to a net enhancement of N₂O₅ uptake in winter, which indicates that the action policy implemented led to an adverse effect on particulate nitrate formation via N₂O₅ uptake in winter. This may explain the persistent winter particulate nitrate pollution in recent years. Our results highlight the systematic changes in the NO₃ budget between 2013 and 2019 in Beijing, which subsequently affect secondary aerosol formation in different seasons.

Chemical Production of Oxygenated Volatile Organic Compounds Strongly Enhances Boundary-Layer Oxidation Chemistry and Ozone Production

Photolysis of oxygenated volatile organic compounds (OVOCs) produces a primary source of free radicals, including OH and inorganic and organic peroxy radicals (HO₂ and RO₂), consequently increasing photochemical ozone production. The amplification of radical cycling through OVOC photolysis provides an important positive feedback mechanism to accelerate ozone production. The large production of OVOCs near the surface helps promote photochemistry in the whole boundary layer. This amplifier effect is most significant in regions with high nitrogen oxides (NO_x) and VOC concentrations such as Wangdu, China. Using a 1-D model with comprehensive observations at Wangdu and the Master Chemical Mechanism (MCM), we find that OVOC photolysis is the largest free-radical source in the boundary layer (46%). The condensed chemistry mechanism we used severely underestimates the OVOC amplifier effect in the boundary layer, resulting in a lower ozone production rate sensitivity to NO_x emissions. Due to this underestimation, the model-simulated threshold NO_x emission value, below which ozone production decreases with NO_x emission decrease, is biased low by 24%. The underestimated OVOC amplifier effect in a condensed mechanism implies a low bias in the current 3-D model-estimated efficacy of NO_x emission reduction on controlling ozone in polluted urban and suburban regions of China.

Quantifying the role of PM2.5 dropping in variations of ground-level ozone: Inter-comparison between Beijing and Los Angeles

In recent decade the ambient fine particle (PM_2.5) levels have shown a trend of distinct dropping in China, while ground-level ozone concentrations have been increasing in Beijing and many other Chinese mega-cities. The variation pattern in Los Angeles was markedly different, with PM_2.5 and ozone decreasing together over past decades. In this study, we utilize observation-based methods to establish the parametric relationship between PM_2.5 concentration and key aerosol physical properties (including aerosol optical depth and aerosol surface concentration), and an observation-based 1-D photochemical model to quantify the response of PM_2.5 decline in enhancing ground-level ozone pollution over a large PM_2.5 concentration range (10-120 μg m^-3). We find that the significance of ozone enhancement due to PM_2.5 dropping depends on both the PM_2.5 levels and optical properties of particles. Ozone formation increased by 37% in 2006-2016 due to PM_2.5 dropping in Beijing, while it becomes less important (7%) as PM_2.5 reaches below 40 μg/m³, similar to Los Angeles since 1980s. Therefore, the two cities show the convergence of air pollutant characteristics. Hence a control strategy prioritizing reactive volatile organic compound abatement is projected to yield simultaneous ozone and PM_2.5 reductions in Beijing, as experienced in Los Angeles.

Increased ozone pollution alongside reduced nitrogen dioxide concentrations during Vienna's first COVID-19 lockdown: Significance for air quality management

BACKGROUND

Lockdowns amid the COVID-19 pandemic have offered a real-world opportunity to better understand air quality responses to previously unseen anthropogenic emission reductions.

METHODS AND MAIN OBJECTIVE

This work examines the impact of Vienna's first lockdown on ground-level concentrations of nitrogen dioxide (NO₂), ozone (O₃) and total oxidant (O_x). The analysis runs over January to September 2020 and considers business as usual scenarios created with machine learning models to provide a baseline for robustly diagnosing lockdown-related air quality changes. Models were also developed to normalise the air pollutant time series, enabling facilitated intervention assessment.

CORE FINDINGS

NO₂ concentrations were on average -20.1% [13.7-30.4%] lower during the lockdown. However, this benefit was offset by amplified O₃ pollution of +8.5% [3.7-11.0%] in the same period. The consistency in the direction of change indicates that the NO₂ reductions and O₃ increases were ubiquitous over Vienna. O_x concentrations increased slightly by +4.3% [1.8-6.4%], suggesting that a significant part of the drops in NO₂ was compensated by gains in O₃. Accordingly, 82% of lockdown days with lowered NO₂ were accompanied by 81% of days with amplified O₃. The recovery shapes of the pollutant concentrations were depicted and discussed. The business as usual-related outcomes were broadly consistent with the patterns outlined by the normalised time series. These findings allowed to argue further that the detected changes in air quality were of anthropogenic and not of meteorological reason. Pollutant changes on the machine learning baseline revealed that the impact of the lockdown on urban air quality were lower than the raw measurements show. Besides, measured traffic drops in major Austrian roads were more significant for light-duty than for heavy-duty vehicles. It was also noted that the use of mobility reports based on cell phone movement as activity data can overestimate the reduction of emissions for the road transport sector, particularly for heavy-duty vehicles. As heavy-duty vehicles can make up a large fraction of the fleet emissions of nitrogen oxides, the change in the volume of these vehicles on the roads may be the main driver to explain the change in NO₂ concentrations.

INTERPRETATION AND IMPLICATIONS

A probable future with emissions of volatile organic compounds (VOCs) dropping slower than emissions of nitrogen oxides could risk worsened urban O₃ pollution under a VOC-limited photochemical regime. More holistic policies will be needed to achieve improved air quality levels across different regions and criteria pollutants.

Observations and modeling of OH and HO2 radicals in Chengdu, China in summer 2019

This study reports on the first continuous measurements of ambient OH and HO₂ radicals at a suburban site in Chengdu, Southwest China, which were collected during 2019 as part of a comprehensive field campaign 'CompreHensive field experiment to explOre the photochemical Ozone formation mechaniSm in summEr - 2019 (CHOOSE-2019)'. The mean concentrations (11:00-15:00) of the observed OH and HO₂ radicals were 9.5 × 10⁶ and 9.0 × 10⁸ cm^-3, respectively. To investigate the state-of-the-art chemical mechanism of radical, closure experiments were conducted with a box model, in which the RACM2 mechanism updated with the latest isoprene chemistry (RACM2-LIM1) was used. In the base run, OH radicals were underestimated by the model for the low-NO regime, which was likely due to the missing OH recycling. However, good agreement between the observed and modeled OH concentrations was achieved when an additional species X (equivalent to 0.25 ppb of NO mixing ratio) from one new OH regeneration cycle (RO₂ + X → HO₂, HO₂ + X → OH) was added into the model. Additionally, in the base run, the model could reproduce the observed HO₂ concentrations. Discrepancies in the observed and modeled HO₂ concentrations were found in the sensitivity runs with HO₂ heterogeneous uptake, indicating that the impact of the uptake may be less significant in Chengdu because of the relatively low aerosol concentrations. The ROx (= OH + HO₂ + RO₂) primary source was dominated by photolysis reactions, in which HONO, O₃, and HCHO photolysis accounted for 34%, 19%, and 23% during the daytime, respectively. The efficiency of radical cycling was quantified by the radical chain length, which was determined by the NO to NO₂ ratio successfully. The parameterization of the radical chain length may be very useful for the further determinations of radical recycling.

Secondary Organic Aerosol Formation Potential from Ambient Air in Beijing: Effects of Atmospheric Oxidation Capacity at Different Pollution Levels

Secondary organic aerosol (SOA) plays a critical role in sustained haze pollution in megacities. Traditional observation of atmospheric aerosols usually analyzes the ambient organic aerosol (OA) but neglects the SOA formation potential (SOAFP) of precursors remaining in ambient air. Knowledge on SOAFP is still limited, especially in megacities suffering from frequent haze. In this study, the SOAFP of ambient air in urban Beijing was characterized at different pollution levels based on a two-year field observation using an oxidation flow reactor (OFR) system. Both OA and SOAFP increased as a function of ambient pollution level, in which increasing concentrations of precursor volatile organic compounds (VOCs) and decreasing atmospheric oxidation capacity were found to be the two main influencing factors. To address the role of the atmospheric oxidation capacity in SOAFP, a relative OA enhancement ratio (ER_OA = 1 + SOAFP/OA) and the elemental composition of the OA were investigated in this study. The results indicated that the atmospheric oxidation capacity was weakened and resulted in higher SOAFP on more polluted days. The relationship found between SOAFP and the atmospheric oxidation capacity could be helpful in understanding changes in SOA pollution with improving air quality in the megacities of developing countries.

Transport and boundary layer interaction contribution to extremely high surface ozone levels in eastern China

Vertical measurements of ozone (O₃) within the 3000-m lower troposphere were obtained using an O₃ lidar to investigate the contribution of the interactions between the transport and boundary layer processes to the surface O₃ levels in urban Shanghai, China during July 23-28, 2017. An extremely severe pollution episode with a maximum hourly O₃ mixing ratio of 160.4 ppb was observed. In addition to enhanced local photochemical production, both downward and advection transport in the lower troposphere may have played important roles in forming the pollution episode. The O₃-rich air masses in the lower free troposphere primarily originated from central China and the northern Yangtze River Delta (YRD) region. The downward transport of O₃ from the lower free troposphere may have an average contribution of up to 49.1% to the daytime (09:00-16:00 local time) surface O₃ in urban Shanghai during the pollution episode (July 23-26, 2017). As for the advection transport, large amounts of O₃ were transported outward from Shanghai in the planetary boundary layer under the influence of southeasterly winds during the field study. In this condition, the boundary-layer O₃ that was transported downward from the free troposphere in Shanghai could be transported back to the northern YRD region and accumulated therein, leading to the occurrence of severe O₃ pollution events over the whole YRD region. Our results indicate that effective regional emission control measures are urgently required to mitigate O₃ pollution in the YRD region.

Avoiding high ozone pollution in Delhi, India

Quantify the influence of aerosol light extinction on surface ozone photochemistry, highlight controlling VOC for improving air quality in Delhi.