Ozone Pollution Episodes Research Articles

Formation and impacts of nitryl chloride in Pearl River Delta

Abstract. Here we present a field measurement of ClNO2 (nitryl chloride) and N2O5 (dinitrogen pentoxide) by a time-of-flight chemical ionization mass spectrometer (ToF-CIMS) with the Filter Inlet for Gas and AEROsols (FIGAERO) at a regional site in the Pearl River Delta during a photochemical pollution season from 26 September to 17 November 2019. Three patterns of air masses are sampled during this campaign, including the dominating air masses from the north and northeast urban regions (Type A), the southeast coast (Type B), and the South China Sea (Type C). The concentration of ClNO2 and N2O5 was observed to be much higher in Type A and B than in Type C, indicating that the urban nighttime chemistry is more active than the background marine regions. The N2O5 uptake coefficient and ClNO2 production yield were estimated based on the field measurement, and the performance of the previously derived parameterizations was assessed. The nighttime ClNO2 correlated with particulate chloride and the mass concentration of fine particles (most likely due to aerosol surface area) suggested that the ClNO2 formation was limited by the N2O5 uptake at this site. By examining the relationship between particulate chloride and other species, we implied that anthropogenic emissions (e.g., biomass burning) rather than sea salt particles dominate the origin of particulate chloride, although the site was only about 100 km away from the ocean. A box model with detailed chlorine chemistry is used to investigate the impacts of ClNO2 chemistry on atmospheric oxidation. Model simulations showed that the chlorine radical liberated by ClNO2 photolysis during the next day had a slight increase in concentrations of OH, HO2, and RO2 radicals, as well as minor contributions to RO2 radical and O3 formation (< 5 %, on daytime average), in all the three types of air masses. Relatively high contributions were observed in Type A and B. The overall low contributions of ClNO2 to atmospheric oxidation are consistent with those reported recently from wintertime observations in China (including Shanghai, Beijing, Wangdu, and Mt. Tai). This may be attributed to the following: (1) relatively low particle mass concentration limited ClNO2 formation; (2) other reactions channels, like nitrous acid (HONO), oxygenated volatile organic compounds (OVOCs, including formaldehyde), and ozone photolysis had a more significant radical formation rate during the ozone pollution episodes and weakened the ClNO2 contribution indirectly. The results provided scientific insights into the role of nighttime chemistry in photochemical pollution under various scenarios in coastal areas.

Understanding Regional Background Ozone by Multiple Methods: A Case Study in the Shandong Region, China, 2018–2020

AbstractUprising ground‐level ozone (O3) and its regional pollution in Northern China are attracting more attention. Besides local precursor emissions and photochemistry, background ozone and long‐range transport also contribute significantly to O3 concentrations. To quantify the regional background O3 concentrations and their temporal and spatial variations, multiple methods, including the principal component analysis (PCA) and the Texas Commission on Environmental Quality (TCEQ) method, were applied as a case study in Shandong province in Northern China, where serious O3 pollution occurred frequently yet the background contributions have not been well quantified. We used four methods to quantify the regional background O3: Method 1 is PCA analysis with only ambient O3 data as input; Method 2 is PCA analysis considering O3 and meteorological parameters; Method 3 combines multiple linear regression and the traditional PCA method; and Method 4 is based on TCEQ and consists of the lowest MAD8 O3 measured to represent regional background O3 concentrations. Results derived from multiple methods show an overall consistent trend with 2018–2020 averaged regional background O3 (MDA8) of 41.5 ppb, accounting for 79.4% of the total O3 in the region. From 2018 to 2020, the changes in regional MDA8 O3 estimated by Methods 1–4 are −1.8, 0.7, −2.4, and 0.4 ppb, respectively. Clear seasonal variations in the regional background O3 are observed, showing a pattern of summer > spring > autumn > winter. In addition, the regional ozone contribution at coastal cities was larger than that for inland cities with local O3 contribution gradually increasing from coastal areas to inland areas. The 3‐year average sea‐land breeze contribution to summertime O3 in the eastern coastal cities was estimated to be around 2.1%, while the local photochemistry to O3 in the inland cities was about 29.7% during ozone pollution episodes, with maximum contribution estimated up to 55.8%. Overall, our study provides insights into the regional background ozone and local photochemistry in Northern China.

Ozone pollution in the plate and logistics capital of China: Insight into the formation, source apportionment, and regional transport

As the logistics and plate capital of China, the sources and regional transport of O3 in Linyi are different from those in other cities because of the significant differences in industrial structure and geographical location. Twenty-five ozone pollution episodes (OPEs, 52 days) were identified in 2021, with a daily maximum 8-h moving average O3 concentration (O3-MDA8) of 184.5 ± 22.5 μg/m3. Oxygenated volatile organic compounds (OVOCs) and aromatics were the dominant contributors to ozone formation potential (OFP), with contributions of approximately 23.5–52.7% and 20.0–40.8%, respectively, followed by alkenes, alkanes, and alkynes. Formaldehyde, an OVOC with high concentrations emitted from the plate industry and vehicles, contributed the most to OFP (22.7 ± 5.5%), although formaldehyde concentrations only accounted for 9.4 ± 2.7% of the total non-methane hydrocarbon (NMHC) concentrations. The source apportionment results indicated that the plate industry was the dominant O3 contributor (27.0%), followed by other sources (21.6%), vehicle-related sources (18.0%), solvent use (16.9%), liquefied petroleum gas (LPG)/natural gas (NG) (8.8%), and combustion sources (7.7%). Therefore, there is an urgent need to control the plating industry in Linyi to mitigate O3 pollution. The backward trajectory, potential source contribution function (PSCF), and concentration weighted trajectory (CWT) models were used to identify the air mass pathways and potential source areas of air pollutants during the OPEs. O3 pollution was predominantly affected by air masses that originated from eastern and local regions, while trajectories from the south contained the highest O3 concentrations (207.0 μg/m3). The potential source area was from east and south Linyi during the OPEs. Therefore, it is critical to implement regional joint prevention and control measures to lower O3 concentrations.

Impact of a subtropical high and a typhoon on a severe ozone pollution episode in the Pearl River Delta, China

Abstract. A record-breaking severe ozone (O3) pollution episode occurred in the Pearl River Delta (PRD) in early autumn 2019 when the PRD was under the influence of a Pacific subtropical high followed by Typhoon Mitag. In this study, we analyzed the effects of meteorological and photochemical processes on the O3 concentration in the PRD during this episode by carrying out the Weather Research Forecast–Community Multiscale Air Quality (WRF-CMAQ) model simulations. Results showed that low relative humidity, high boundary layer height, weak northerly surface wind, and strong downdrafts were the main meteorological factors contributing to O3 pollution. Moreover, delayed sea breezes that lasted into the night would transport O3 from the sea back to the land and resulted in secondary O3 maxima at night. In addition, O3 and its precursors stored in the residual layer above the surface layer at night can be mixed down to the surface in the next morning, further enhancing the daytime ground-level O3 concentration on the following day. Photochemical production of O3, with a daytime average production rate of about 7.2 ppb h−1 (parts per billion), is found to be the predominate positive contributor to the O3 budget of the boundary layer (0–1260 m) during the entire O3 episode, while the horizontal and vertical transport fluxes are the dominant negative contributors. This O3 episode accounted for 10 out of the yearly total of 51 d when the maximum daily 8 h average (MDA8) O3 concentration exceeded the national standard of 75 ppb in the PRD in 2019. Based on these results, we propose that the enhanced photochemical production of O3 during the episode is a major cause of the most severe O3 pollution year since the official O3 observation started in the PRD in 2006. Moreover, since this O3 episode is a synoptic-scale phenomenon covering the entire eastern China, we also suggest that the enhanced photochemical production of O3 in this O3 episode is a major cause of the extraordinarily high O3 concentrations observed in eastern China in 2019.

Contribution of local and surrounding area anthropogenic emissions to a high ozone episode in Zhengzhou, China

This study analyzed an ozone pollution episode that occurred in the summer of 2020 in Zhengzhou, the provincial capital of Henan, China, and quantified the contribution of local and surrounding area anthropogenic emissions to this episode based on the Weather Research and Forecasting with Chemistry (WRF/Chem) model. Simulation results showed that the WRF/Chem model is well suited to simulate the ozone concentrations in this area. In addition, four simulation scenarios (removing the emissions from the northern Zhengzhou, southwestern Zhengzhou, Zhengzhou local and southeastern Zhengzhou) were conducted to explore the specific contributions of local emissions and emissions from surrounding areas within Henan to this ozone pollution episode. We found that contributions from the northern, local, southwestern, and southeastern regions were 6.1%, 5.9%, 1.7%, and 1.5%, respectively. The northern and local emissions of Zhengzhou (only emissions from Zhengzhou) were prominent contributors within the simulation areas. In other words, during this episode, most of the ozone pollution in Zhengzhou appeared to be transported in from regions outside Henan Province.

Local production, downward and regional transport aggravated surface ozone pollution during the historical orange-alert large-scale ozone episode in eastern China

Increasing severe and persistent ozone pollution in China has resulted in serious harm to human health in recent years, yet the precise pollution sources are poorly known because there is few knowledge on large-scale extreme ozone episodes. Here, we studied the formation of the historical orange-alert regional ozone episode in eastern China on 6 June, 2021, by combining process analysis, integrated source apportionment modelling, and chemical and meteorological data. Results show that during the pollution episode, 94% of cities in eastern China suffered ozone pollution, and 39% had daily maximum 8-h average ozone concentrations higher than 100 ppb. This is explained by favorable local ozone formation and transports provided by the prevailing northwestern winds in the upper air, and by sinking atmospheric motions favoring the persistence of high surface ozone concentrations. During daytime, local photochemical production induced an ozone increase of 0.3–28.4 ppb h−1 and vertical transport induced an ozone increase of 0.4–56.1 ppb h−1. As a consequence, vertical downward transport of ozone generated in the upper air by photochemical reactions aggravated surface ozone pollution. Surface ozone concentrations include 25.8–53.9% of ozone from local provincial emissions, 0–42.6% of ozone from inter-regional transports from neighboring regions, 4.6–23.1% of ozone from outer-regional transport, and 13.6–52.9% of ozone from boundary conditions in the selected cities. Overall, our findings show that favorable meteorological conditions promoted the chemical productions of ozone on the surface and at high altitudes, thus resulting in this heavy ozone pollution. In addition, regional and vertical downward transports of aloft ozone further aggravated the surface ozone pollution, leading to the large-scale extreme ozone pollution episode.

Unexpected fast radical production emerges in cool seasons: implications for ozone pollution control

Ozone is a crucial air pollutant that damages human health and vegetation. As it is related to the photo-oxidation of the nitrogen oxides and volatile organic compounds, the summertime reduction of these precursors is the primary focus of current ozone mitigation strategies. During ozone pollution episodes in eastern China, an observed accumulation of daily total oxidants (O<italic>_x</italic>=NO₂+O₃) in cool seasons (spring and autumn: 60 ppb and winter 40 ppb) is comparable to that in summer (60 ppb), indicating fast photochemical production of secondary pollutants including ozone over the year. Unrecognized fast radical primary productions are found to counteract the increased termination of hydroxyl radical and unfavorable meteorological conditions to maintain the rapid total oxidant formations in cool seasons. Elucidating and regulating the primary radical sources may be critical for the secondary air pollution control in cool seasons.

The impact of peripheral circulation characteristics of typhoon on sustained ozone episodes over the Pearl River Delta region, China

Abstract. The peripheral circulation of typhoon forms sustained ozone episodes. However, how it impacts the day-to-day ozone pollution levels during the episodes has not been clearly studied, which is crucial for better prediction of the daily ozone variation. In this study, the analysis of ground observation, wind profile data, and model simulation is integrated. By analysing the wind profile radar observations, we found a weak wind deepening (WWD; vertical depth of the weak winds increased), more correlated with the ground-level ozone variation than surface weak wind. Long-term statistical analyses showed that the WWD is a common weather phenomenon in the peripheral subsidence region of typhoons and is generally accompanied by ozone pollution episodes. The Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) with process analysis simulation showed that the peripheral subsidence chemical formation (CHEM) and vertical mixing (VMIX) effects are two major contributors to the enhancement of ozone levels to form the episode, while the advection (ADV) showed negative values. However, the day-to-day variation of the daytime ozone levels during the episode is not determined by the daily variation of daytime CHEM and VMIX but is dominated by the ADV terms. Therefore, the ozone and its precursors accumulation, including the enhancement during the night-time, contribute to the daytime ozone increase in the following day. A detail day-to-day process analysis showed that in additional to decrease of negative ADV values (e.g. the weakened advection outflow or dispersion) on the ground, the integrated effect of the daily variation of the accumulative CHEM and ADV above the ground throughout the planetary boundary layer (PBL) together determined the overall day-to-day daytime ozone variation on the ground through the VMIX process. The results indicate that the peripheral characteristics of approaching typhoon not only form the ozone episode by the enhanced photochemical reactions, but also could increase the day-to-day daytime ozone levels via pollution accumulation throughout the PBL due to the WWD up to 3–5 km. These results illustrate the important role of the WWD in the lower troposphere for the formation of sustained ozone episodes due to the peripheral circulation of the typhoon, which helps to better predict the daily changes of daytime ozone levels.

Global Perspective of Drought Impacts on Ozone Pollution Episodes.

Ozone (O3) pollution threatens global public health and damages ecosystem productivity. Droughts modulate surface O3 through meteorological processes and vegetation feedbacks. Unraveling these influences is difficult with traditional chemical transport models. Here, using an atmospheric chemistry-vegetation coupled model in combination with a suite of existing measurements, we investigate the drought impacts on global surface O3 and explore the main driving processes. Relative to the mean state, accelerated photochemical rates dominate the surface O3 enhancement during droughts except for eastern U.S. and western Europe, where reduced stomatal uptakes make comparable contributions. During 1990-2012, the simulated frequency of O3 pollution episodes in western Europe decreases greatly with a negative trend of -5.5 ± 6.6 days per decade following the reductions in anthropogenic emissions if meteorology is fixed. However, such decreased trend is weakened to -2.1 ± 3.8 days per decade, which is closer to the observed trend of -2.9 ± 1.1 days per decade when year-to-year meteorology is applied because increased droughts alone offset 43% of the effects from air pollution control. Our results highlight that more stringent controls of O3 precursors are necessary to mitigate the higher risks of O3 pollution episodes by more droughts in a warming world.

Rapid increase in atmospheric glyoxal and methylglyoxal concentrations in Lhasa, Tibetan Plateau: Potential sources and implications

Glyoxal (Gly) and methylglyoxal (Mgly) are the intermediate products of several volatile organic compounds (VOCs) as well as the precursors of brown carbon and may play key roles in photochemical pollution and regional climate change in the Tibetan Plateau (TP). However, their sources and atmospheric behaviors in the TP remain unclear. During the second Tibetan Plateau Scientific Expedition and Research in the summer of 2020, the concentrations of Gly (0.40 ± 0.30 ppbv) and Mgly (0.57 ± 0.16 ppbv) observed in Lhasa, the most densely populated city in the TP, had increased by 20 and 15 times, respectively, compared to those measured a decade previously. Owing to the strong solar radiation, secondary formations are the dominant sources of both Gly (71%) and Mgly (62%) in Lhasa. In addition, primary anthropogenic sources also play important roles by emitting Gly and Mgly directly and providing abundant precursors (e.g., aromatics). During ozone pollution episodes, local anthropogenic sources (industries, vehicles, solvent usage, and combustion activities) contributed up to 41% and 45% in Gly and Mgly levels, respectively. During non-episode periods, anthropogenic emissions originating from the south of Himalayas also have non-negligible contributions. Our results suggest that in the previous decade, anthropogenic emissions have elevated the levels of Gly and Mgly in the TP dramatically. This study has important implications for understanding the impact of human activities on air quality and climate change in this ecologically fragile area.

The impact of atmospheric blocking on the compounding effect of ozone pollution and temperature: a copula-based approach

Abstract. Ozone pollution and high temperatures have adverse health impacts that can be amplified by the combined effects of ozone and temperature. Moreover, changes in weather patterns are expected to alter ozone pollution episodes and temperature extremes. In particular, atmospheric blocking is a high-impact, large-scale phenomenon at mid-high latitudes that has been associated with temperature extremes. This study examines the impact of atmospheric blocking on the ozone and temperature dependence among measurement stations over Europe during the period 1999–2015. We use a copula-based method to model the dependence between the two variables under blocking and non-blocking conditions. This approach allows us to examine the impact of blocks on the joint probability distribution. Our results showed that blocks lead to increasing strength in the upper tail dependence of ozone and temperature extremes (&gt; 95th percentile) in north-west and central Europe (e.g. the UK, Belgium, Netherlands, Luxembourg, Germany and the north-west of France). The analysis of the probability hazard scenarios revealed that blocks generally enhance the probability of compound ozone and temperature events by 20 % in a large number of stations over central Europe. The probability of ozone or temperature exceedances increases 30 % (on average) under the presence of atmospheric blocking. Furthermore, we found that, in a number of stations over north-western Europe, atmospheric blocking increases the probability of ozone exceedances by 30 % given high temperatures. Our results point out the strong influence of atmospheric blocking on the compounding effect of ozone and temperature events, suggesting that blocks might be considered a relevant predicting factor when assessing the risks of ozone-heat-related health effects.

Unexpected Fast Photochemistry Emerges in Cool Seasons China

Ozone is a crucial air pollutant that damages human health and vegetation. As it is formed by the sunlit oxidation of the nitrogen oxides and volatile organic compounds, the current ozone mitigation strategies are primarily focused on summer. By analysing ozone pollution episodes in eastern China, observed daily total oxidants accumulation in cool seasons (spring and autumn: 60 ppb and winter 40 ppb) is comparable to that measured in summer (60 ppb) indicating fast photochemical production of secondary pollutants including ozone over the year. Unrecognized fast radical primary productions is found to counteract the increased termination of hydroxyl radical and dis-benefit meteorological conditions to maintain the rapid total oxidant formations in cool seasons. Elucidating and regulate the primary radical sources may be critical for the secondary air pollution control in cool seasons.

The Characteristics of Heavy Ozone Pollution Episodes and Identification of the Primary Driving Factors Using a Generalized Additive Model (GAM) in an Industrial Megacity of Northern China

Tropospheric ozone is the only normal pollutant with a continuously increasing annual average concentration worldwide. In this study, data were monitored at the Nankai University Air Quality Research Supersite (NKAQRS) (38.99° N, 117.33° E) between 1 April, and 31 August from 2018 to 2020, 33 O3 episodes from 2018 to 2020 were analyzed to reveal the characteristics of O3, VOCs and OFP during O3 episodes and to evaluate the driving factors. The O3 episodes showed a decreasing trend in terms of pollution frequency, days, heavy pollution duration and peak concentration. Ethane, acetylene, cyclopentane, and methylcyclopentane were the major types in 2020, while 1-hexene was the main component in 2019. The main ozone-contributing species in 2020 were propene cyclopentane methylcyclopentane and ethylene. Alkenes were important contributors to ozone formation. Using generalized additive models (GAMs), the explanatory variables in the study are divided into environmental and meteorological factors, and 16 impact factors are selected as explanatory variables. We found that the influence of these meteorological factors on O3 pollution was nonlinear and impacted by the interaction between variables. O3 episodes were mainly driven by meteorological and precursor (NO) factors in 2018, while meteorological conditions (T), followed by precursor (NO2) were the driving factors in 2019 and 2020, suggesting that O3 episodes were mainly driven by meteorological conditions.

Characteristics of BTEX and Health Risk Assessment During Typical Pollution Episodes in Summer and Winter in Tianjin Urban Area

Real-time BTEX(including benzene, toluene, ethylbenzene, m-, p-, and o-xylenes) were measured continuously in Tianjin urban site in July 2019 and January 2020 using a Syntech Spectras GC955 analyzer. The BTEX concentration levels, composition, and evolutionary mechanisms during typical pollution episodes were investigated. The potential sources of BTEX were analyzed qualitatively using the diagnostic ratios method. Finally, the BTEX health risk was evaluated by using the human exposure analysis and evaluation method according to US EPA. The averaged total mixing ratio of BTEX were 1.32×10-9 and 4.83×10-9 during ozone pollution and haze episodes, respectively. Benzene was the most abundant species, followed by toluene. The mixing ratio of BTEX was largely affected by short southwestern distance transportation in January, while local emissions in July. In addition, the BTEX mixing ratio depended on the influence of temperature and relative humidity(RH) in July, while the concentration was more sensitive to changes in RH when the temperature was low in January. Diagnostic ratios and source implications suggested that the BTEX was affected mainly by biomass/biofuel/coal burning during haze episodes. The traffic related emissions also had an impact except for the influence of biomass/biofuel/coal burning in July. The averaged hazard quotient(HQ) values were 0.072 and 0.29 during ozone pollution and haze episodes, respectively, which were in the upper safety range limit recommended by the US EPA. The carcinogenic risk posed by benzene in both cleaning and pollution processes was higher than the safety threshold set by the US EPA, which should be monitored carefully.

Analysis of the photochemical characteristics and sensitivity of ozone pollution in Xi&amp;rsquo;an

<p indent="0mm">The Fenwei Plain is an important agricultural production base and transportation hub in China with a residential population of over 55 million. With rapid industrialization and urbanization, this area has experienced serious photochemical pollution in the summer season. In this study, a field campaign was conducted in summer 2019 with measurements of ozone and photochemical precursors in Xi’an, a megacity in the Fenwei Plain. An explicit case was performed to explore O₃-NO_{<italic>x</italic>}-VOCs sensitivity and the local ozone budget using an observation-based model. Results revealed that precursors significantly promoted ozone production. The positive matrix factorization (PMF) model combined with the relative incremental reactivity (RIR) method was applied to identify the key VOC sources that affect ozone formation. An ozone pollution episode lasting <sc>11 d</sc> was selected and divided into three periods: Initial (July 8–11), polluted (July 12–15), and sweep (July 16–18) periods. It was found that strong solar radiation intensity, high temperature, low relative humidity, and low pressure prevailed during the polluted period, which favored ozone production. The maximum O₃ concentrations at the upwind CB site and urban DHS site were found to be 125 ppbv during the polluted period, whereas at the QL site, the maximum O₃ concentrations were much lower (98 ppbv). The maximum local ozone production rate <sc>(<italic>P</italic>(O_{<italic>x</italic>}))</sc> of the three sites during the polluted period was significantly higher than that in the initial and sweep periods. The <italic>P</italic>(O_{<italic>x</italic>}) was 17.7, 33.9, and 14.9 ppbv/h for the CB, DHS, and QL sites, respectively. The <italic>in situ</italic> O_{<italic>x</italic>}<italic> </italic>budget demonstrated that ozone pollution at the CB and DHS sites was mainly attributed to local photochemical production, whereas the regional transportation effect could not be ignored at the QL site. The O₃-NO_{<italic>x</italic>}-VOCs sensitivity analysis revealed that ozone production regimes were transitional, VOCs-limited, and NO_{<italic>x</italic>}-limited at the CB, DHS, and QL sites, respectively, and the regimes stayed unchanged for all the three sites during the entire ozone pollution episode. However, during the initial period at the DHS site, NO_{<italic>x</italic>} reduction led to a negative effect on ozone control. The ozone formation regime obtained from the emission rate box model was consistent with that of the RIR method. From the proportion of grouped VOCs, the alkane and OVOCs were the dominant species in the mixing ratio (78%–86%), whereas the contribution of aromatics to OFP (16%–24%) was second only to that of OVOCs. Finally, by combining the PMF model and RIR method, ozone production was found to be more sensitive to precursors reduction in the initial period than in the polluted period at the three sites, which indicated that pollutants control measures for ozone pollution mitigation will be more effective in the initial than in polluted periods. Therefore, during an ozone pollution episode, the time to take action for ozone pollution control is not during the polluted period when the ozone concentrations peak, but should be brought forward, when controlling the concentration of precursors has the highest efficiency in reducing ozone production.

Development of a high-performance machine learning model to predict ground ozone pollution in typical cities of China

High ozone concentrations have adverse effects on human health and ecosystems. In recent years, the ambient ozone concentration in China has shown an upward trend, and high-quality prediction of ozone concentrations has become critical to support effective policymaking. In this study, a novel hybrid model combining wavelet decomposition (WD), a gated recurrent unit (GRU) neural network and a support vector regression (SVR) model was developed to predict the daily maximum 8 h ozone. We used the ground ozone observation data in six representative megacities across China from Jan. 1, 2015 to Jun. 15, 2020 for model training, and we used data from Jun. 15 to Dec. 31, 2020 for model testing. The results show that the developed model performs very well for megacities; against observations, the model obtains an average cross-validated R2 (coefficient of determination) ranging from 0.90 for Shanghai to 0.97 for Chengdu in the one-step predictions, thereby indicating that the model outperformed any single algorithm or other hybrid algorithms reported. The developed model can also capture high ozone pollution episodes with an average accuracy of 92% for the next five days in inland cities. This study will be useful for the environmental health community to prevent high ozone exposure more efficiently in megacities in China and shows great potential for accurate ozone prediction using machine learning approaches.

A numerical study of the urban green roof and cool roof strategies’ effects on boundary layer meteorology and ozone air quality in a megacity

Green roof and cool roof are two popular strategies to mitigate urban heat island especially in the summer time. The meteorological effect (cooling on the near surface temperature) of the two mitigation strategies has been proved by lots of studies while the environmental effect (effect on urban air quality) is still largely unknown. In this paper the WRF-Chem model coupled with a single layer urban canopy model is used to simulate three heat wave and heavy ozone pollution episodes in Shanghai, China. Sensitivity experiments of green roof and cool roof are carried out and the results show that both green roof and cool roof can effectively mitigate urban heat island, and their cooling efficiencies are similar even their cooling mechanisms are different. All of them can reduce the lasting time and area of high temperature and extreme hot, but the ozone pollution may become more serious. According to the different ambient wind field and ozone distribution, these three ozone pollution episodes can be divided into two conditions: urban dome and urban plume. The decrease of urban surface temperature can inhibit the turbulent activities and reduce the height of the boundary layer; the concentration of ozone in urban Zone A (below 750 m height) increases, but decreases in urban Zone B (750 m-2km height) for urban dome. By calculating the ozone fluxes at different heights, we find that the urban heat island circulation can transport surface ozone into city, horizontally; while in Zone B, the horizontal divergence transport ozone out of the city, and green roof and cool roof can decrease the temperature difference between urban and suburban, which can inhibit this ozone transportation of urban heat island circulation.

Impacts of Ozone‐Vegetation Interactions on Ozone Pollution Episodes in North China and the Yangtze River Delta

AbstractPersistent ozone (O3) pollution episodes (OPEs, with regionally averaged maximum daily 8‐h average (MDA8) O3 concentration exceeding 80 ppbv and lasting for 5 days or longer) occurred frequently in megalopolis over eastern China in recent years. Here, we apply a newly developed chemistry‐biosphere model (GEOS‐Chem‐YIBs) to comprehensively quantify the vegetation contributions to OPEs in North China and Yangtze River Delta (YRD) over May to October in 2014–2017. The simulated MDA8 O3 concentrations increases 16.7 and 16.6 ppbv) during OPEs in North China and YRD, of which about 1.4 ppbv (8.4%) and 3.8 ppbv (22.9%) are caused by the processes including increased BVOC emissions and reduced stomatal dry deposition. Furthermore, the O3 damages to vegetation stomata slightly increase seasonal‐mean O3 concentrations by &lt;1 ppbv, but such effects are not exacerbated during OPEs despite the high O3 exposure.

Atmospheric carbonyls in a heavy ozone pollution episode at a metropolis in Southwest China: Characteristics, health risk assessment, sources analysis

Ambient carbonyls were continuously observed in the field during a heavy ozone pollution episode in Chengdu, China from August 4 to August 19, 2019, and the pollution characteristics, atmospheric photochemical reactivity, human health risk, and sources of carbonyls were analyzed. Fifteen carbonyls were quantified with average total mixing ratios of 20.38 ppbv Formaldehyde (9.86 ppbv), acetone (4.41 ppbv), and acetaldehyde (3.57 ppbv) were the three most abundant carbonyls. During the heavy ozone pollution episode, the concentration of carbonyls was found to be higher on pollution days than on the clean days, and relatively higher in the daytime, especially at noon on the pollution days. This was influenced by the intensity of photochemical reactions and precipitation. The “weekend effect” with the concentration of carbonyls was higher on the weekends than on the weekdays was pointed out. Formaldehyde, acetaldehyde and hexaldehyde were the dominant oxidative species during the observation. The carcinogenic and non-carcinogenic risk values of formaldehyde and acetaldehyde were higher on pollution days than on clean days, and these values were higher compared with those of other cities in China and abroad. Long-term exposure to these compounds should therefore be avoided. Diagnostic ratios and correlation analysis together with backward trajectory analysis showed that primary emission and secondary formation accounted 66%–76% and 24%–34% of carbonyls in Chengdu, respectively, with primary emission being the main sources of carbonyls, and carbonyls from the surrounding cities and emission from natural sources also had a significant contribution to the carbonyls in Chengdu.

Heavy ozone pollution episodes in urban Beijing during the early summertime from 2014 to 2017: Implications for control strategy

Ground-level ozone (O3) has become the principal air pollutant in Beijing during recent summers. In this context, an investigation of ambient concentrations and variation characteristics of O3 and its precursors in May and June from 2014 to 2017 in a typical urban area of Beijing was carried out, and the formation sensitivity and different causes of heavy O3 pollution (HOP, daily maximum 8-h O3 (MDA8h O3)>124 ppbv) were analyzed. The results showed that the monthly assessment values of the O3 concentrations (the 90th percentile MDA8h O3 within one month) were highest in May or June from 2014 to 2017, and the values presented an overall increasing trend. During this period, the number of O3 pollution days (MDA8h O3 > 75 ppbv) also showed an increasing trend. During the HOP episodes, the concentrations of volatile organic compounds (VOCs), nitrogen oxides (NOX), and carbon monoxide (CO) were higher than their respective mean values in May and June, and the meteorological conditions were more conducive to atmospheric photochemical reactions. The HOP episodes were mainly caused by local photochemical formation. From 2014 to 2017, O3 formation during the HOP episodes shifted from VOC and NOX mixed-limited to VOC-limited conditions, and O3 formation was most sensitive to anthropogenic VOCs. Six categories of VOC sources were identified, among which vehicular exhaust contributed the most to anthropogenic VOCs. The VOC concentrations and OFPs of anthropogenic sources have decreased significantly in recent years, indicating that VOC control measures have been effective in Beijing. Nevertheless, NOX concentrations did not show an evident decreasing trend in the same period. Therefore, more attention should be devoted to O3 pollution control in May and June; control measure adjustments are needed according to the changes in O3 precursors, and coordinated control of VOCs and NOX should be strengthened in long-term planning.