High O3 Episodes Research Articles

Source apportionment of Volatile Organic Compounds (VOCs) in the South Coast Air Basin (SoCAB) During RECAP-CA

Ozone (O3) concentrations in the South Coast Air Basin (SoCAB) surrounding Los Angeles remain at unhealthy levels despite multiple decades of control programs designed to reduce emissions of precursor Volatile Organic Compounds (VOCs). Here we report on comprehensive VOC measurements made at Redlands, which has the highest measured O3 concentrations in SoCAB, as part of the Re-Evaluating the Chemistry of Air Pollutants in California (RECAP-CA) field campaign (July–October 2021). Positive matrix factorization (PMF) analysis was applied to identify nine VOC factors. A photochemical chamber model initialized by field measurements was configured with a tagging technique to quantify the VOC factor contributions to O3 formation in Redlands. Biogenic VOCs (BVOCs) made the largest contribution (26.6%) to O3 formation, followed by traffic VOCs (21.2%), volatile chemical products (VCPs) (19%), and plant decomposition (14.9%). High O3 episodes were not driven by increased VOC emissions from any single source, but rather were associated with stagnation events that concentrated VOCs from all sources and high temperature days that enhanced O3 formation efficiency. This implies that VOC controls optimized to reduce O3 concentrations would look similar in both the NOx-limited and VOC-limited regimes that can occur at Redlands. These results suggest that control strategies that reduce VOC and NOx emissions from the on-road vehicle fleet, such as increasing electrification, may yield O3 reductions on days in both the NOx-limited and VOC-limited chemical regimes at Redlands.

Unravelling the impacts of stratospheric intrusions on near-surface ozone during the springtime ozone pollution episodes in Lhasa, China

While air pollution due to fine particulate matter (PM2.5) has been effectively controlled in China; the photochemical pollution characterized by elevated ozone (O3) has emerged as a major concern for air quality improvement. Except for ozone, Lhasa is one of the cleanest cities in China with the lowest annual PM2.5 concentration in 2017. The levels of major air pollutants in Lhasa are much lower than those of other cities in the eastern region of China, especially in May, when the O3 concentration peaks. This study was based on multi-source observations in combination with the Goddard Earth Observing System coupled with chemistry (GEOS-Chem) and the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) models to explore the causes of O3 pollution in Lhasa in May 2023. The results indicated that during the high O3 episodes, the concentrations of O3 precursors were low in Lhasa. Surrounding cities in other parts of the Tibetan Plateau also experienced high ozone concentrations despite being in different airsheds, suggesting that the O3 pollution in Lhasa was caused by regional transport rather than purely local emissions. Stratospheric intrusion events modulated by the westerly jet led to elevated ozone in the troposphere above Lhasa City. The results of the GEOS-Chem model indicated that horizontal advection, turbulence, diffusion, and other effects led to high concentrations of ozone in the near-surface above Lhasa. Vertical transport was the dominant factor leading to ozone concentration increases during high ozone days, with a contribution of 6.33 Gg/day. In addition, high-altitude air masses with a maximum altitude of over 8000 m, were observed arriving in Lhasa during the high ozone days. This study revealed that stratospheric intrusions have a greater contribution to the high O3 concentration in Lhasa in spring and provided a scientific basis for mitigating O3 pollution in the plateau cities.

Wintertime ozone surges: The critical role of alkene ozonolysis

Ozone (O3) pollution is usually linked to warm weather and strong solar radiation, making it uncommon in cold winters. However, an unusual occurrence of four high O3 episode days (with maximum hourly concentrations exceeding 100 ppbv and peaking at 121 ppbv) was recorded in January 2018 in Lanzhou city, China. During these episodes, the average daytime concentration of total non-methane volatile organic compounds (TVOCs) reached 153.4 ± 19.0 ppbv, with alkenes—largely emitted from the local petrochemical industry—comprising 82.3 ± 13.1 ppbv. Here we show a photochemical box model coupled with a Master Chemical Mechanism to elucidate the mechanisms behind this unusual wintertime O3 pollution. We find that the typically low temperatures (−1.7 ± 1.3 °C) and weak solar radiation (263.6 ± 60.7 W m-2) of those winter episode days had a minimal effect on the reactivity of VOCs with OH radicals. Instead, the ozonolysis of alkenes generated Criegee intermediates, which rapidly decomposed into substantial ROx radicals (OH, HO2, and RO2) without sunlight. This radical production led to the oxidation of VOCs, with alkene ozonolysis ultimately contributing to 89.6 ± 8.7% of the O3 formation during these episodes. This mechanism did not activate at night due to the depletion of O3 by the NO titration effect. Furthermore, the findings indicate that a reduction of alkenes by 28.6% or NOx by 27.7% in the early afternoon could significantly mitigate wintertime O3 pollution. Overall, this study unravels the unique mechanism of alkene-induced winter O3 pollution and offers a reference for winter O3 reduction strategies in the petrochemical industrial regions.

High O3 pollution initiated by cold front passage over Pearl River Estuary

The influence of cold fronts on the formation of ozone (O3) pollution, particularly the contributions of physical and chemical processes to O3 abundance during the passage of cold fronts in winter over the Pearl River Estuary (PRE) region was still unclear. This study provided an in-depth analysis on the occurrence of high O3 pollution based on data from field measurements and simulation of a chemical transport model (i.e., WRF-Chem) over the PRE region during winter 2021 when cold fronts were frequently observed. Totally 7 high O3 episode days were observed during pre- and post-frontal stages at DWS (i.e., Da Wan Shan island), a downwind island site of the inland Pearl River Delta (PRD) region, which were accompanied with the prevailing northerly wind patterns with insignificant variations of wind speeds in a stable boundary layer over the whole PRE region. The quantitative analysis based on simulation results indicated that horizontal transport and chemical processes accounted for the buildup of O3 abundance at DWS, with the highest mean rates of 7.3 and 5.6 ppbv/h at pre- and post-frontal stages, respectively. The high contributions from horizontal transport to O3 levels at DWS were associated with the persistent northerly winds with small variations, while the contributions of chemical processes were varied with the abundance of precursors and meteorological conditions. However, vertical transport made negative contributions to O3 abundance, with the most significant mean diffusion rate of −2.6 ppbv/h at the frontal stage because of the control of high-pressure system, while the less influence of vertical transport at pre- and post-frontal stages was associated with the weak pressure gradient forces and wind patterns. The sensitivity analysis further demonstrated that the emissions of precursors from the inland PRD region contributed 50–76% to O3 abundance over PRE at different stages. Overall, the above results quantitatively confirmed the dominant role of transport from inland PRD to the O3 pollution over the downwind PRE region and were helpful for better understanding of O3 pollution in winter over PRE.

Summertime Ozone Production at Carlsbad Caverns National Park, New Mexico: Influence of Oil and Natural Gas Development

AbstractSoutheastern New Mexico's Carlsbad Caverns National Park (CAVE) has increasingly experienced summertime ozone (O3) exceeding an 8‐hr average of 70 parts per billion by volume (ppbv). The park is located in the western part of the Permian oil and natural gas (O&G) basin, where production rates have increased fivefold in the last decade. We investigate O3–precursor relationships by constraining the F0AM box model to observations of nitrogen oxides (NOx = NO + NO2) and a suite of volatile organic compounds (VOCs) collected at CAVE during summer 2019. O&G‐related VOCs dominated the calculated VOC reactivity with hydroxyl radicals (OH) on days when O3 concentrations were primarily controlled by local photochemistry. Radical budget analysis showed that NOx levels were high enough to impose VOC sensitivity on O3 production in the morning hours, while subsequent NOx loss through photochemical consumption led to NOx‐sensitive conditions in the afternoon. Maximum daily O3 was responsive to both NOx and O&G‐related VOC reductions, with NOx reductions proving most effective. The model underestimated observed O3 during a 5‐day high O3 episode that was influenced by photochemically aged O&G emissions, as indicated by back‐trajectory analysis, low i‐/n‐pentane ratios, enhanced secondary VOCs, and low ratios of NOx to total reactive oxidized nitrogen (NOy). Model‐observation agreement was improved by constraining model NOx with observed NOy, which approximates NOx at the time of emission, indicating that a large fraction of O3 during this episode was formed nonlocally.

Summertime tropospheric ozone source apportionment study in the Madrid region (Spain).

The design of emission abatement measures to effectively reduce high ground-level ozone (O3) concentrations in urban areas is very complex. In addition to the strongly non-linear chemistry of this secondary pollutant, precursors can be released by a variety of sources in different regions, and locally produced O3 is mixed with that transported from the regional or continental scales. All of these processes depend also on the specific meteorological conditions and topography of the study area. Consequently, high-resolution comprehensive modeling tools are needed to understand the drivers of photochemical pollution and to assess the potential of local strategies to reduce adverse impacts from high tropospheric O3 levels. In this study, we apply the Integrated Source Apportionment Method (ISAM) implemented in the Community Multiscale Air Quality (CMAQ v5.3.2) model to investigate the origin of summertime O3 in the Madrid region (Spain). Consistent with previous studies, our results confirm that O3 levels are dominated by non-local contributions, representing around 70 % of mean values across the region. Nonetheless, precursors emitted by local sources, mainly road traffic, play a more important role during O3 peaks, with contributions as high as 25 ppb. The potential impact of local measures is higher under unfavorable meteorological conditions associated with regional accumulation patterns. These findings suggest that this modeling system may be used in the future to simulate the potential outcomes of specific emission abatement measures to prevent high-O3 episodes in the Madrid metropolitan area.

Influence of Wind Flows on Surface O3 Variation over a Coastal Province in Southeast China

Surface ozone (O3) is influenced not only by anthropogenic emissions but also by meteorological factors, with wind direction being one of the most overlooked factors. Here, we combine the observational data of both O3 and wind flow to compare the variation in surface O3 with wind direction between coastal and inland regions of Fujian, a province in the southeast coast of China with complicated topography. We further conduct a numerical simulation using a global chemical transport model, GEOS-Chem, to interpret the observational results, explore the linkages between these O3 variations and wind flows, and identify the dominant processes for the occurrence of high O3 that varies with wind flows. The results from the observations over 2015–2021 suggest that, over coastal regions, surface O3 concentrations show a strong dependence on wind flow changes. On average, during the daytime, when southeasterly winds prevail, the mean of O3 concentrations reaches 83.5 μg/m3, which is 5.0 μg/m3 higher than its baseline values (the mean O3 concentrations), while the northwesterly winds tend to reduce surface O3 by 6.4 μg/m3. The positive O3 anomalies with southeasterly wind are higher in the autumn and summer than in the spring and winter. During the nighttime, the onshore northeasterly winds are associated with enhanced O3 levels, likely due to the airmass containing less NO2, alleviating the titration effects. Over inland regions, however, surface O3 variations are less sensitive to wind flow changes. The GEOS-Chem simulations show that the prevailing southeasterly and southwesterly winds lead to the positive anomaly of chemical reactions of O3 over coastal regions, suggesting enhanced photochemical production rates. Furthermore, southeasterly winds also aid in transporting more O3 from the outer regions into the coastal regions of Fujian, which jointly results in elevated surface O3 when southeasterly winds dominates. When affected by wind flows in different directions, the chemical reaction and transport in the inland regions do not exhibit significant differences regarding their impact on O3. This could be one of the reasons for the difference in O3 distribution between coastal and inland regions. This study could help to deepen our understanding of O3 pollution and aid in providing an effective warning of high-O3 episodes.

Ozone alters the chemical signal required for plant – insect pollination: The case of the Mediterranean fig tree and its specific pollinator

Tropospheric ozone (O3) is likely to affect the chemical signal emitted by flowers to attract their pollinators through its effects on the emission of volatile organic compounds (VOCs) and its high reactivity with these compounds in the atmosphere. We investigated these possible effects using a plant-pollinator interaction where the VOCs responsible for pollinator attraction are known and which is commonly exposed to high O3 concentration episodes: the Mediterranean fig tree (Ficus carica) and its unique pollinator, the fig wasp (Blastophaga psenes). In controlled conditions, we exposed fig trees bearing receptive figs to a high-O3 episode (5 h) of 200 ppb and analyzed VOC emission. In addition, we investigated the chemical reactions occurring in the atmosphere between O3 and pollinator-attractive VOCs using real-time monitoring. Finally, we tested the response of fig wasps to the chemical signal when exposed to increasing O3 mixing ratios (0, 40, 80, 120 and 200 ppb). The exposure of the fig tree to high O3 levels induced a significant decrease in leaf stomatal conductance, a limited change in the emission by receptive figs of VOCs not involved in pollinator attraction, but a major change in the relative abundances of the compounds among pollinator-attractive VOCs in O3-enriched atmosphere. Fig VOCs reacted with O3 in the atmosphere even at the lowest level tested (40 ppb) and the resulting changes in VOC composition significantly disrupted the attraction of the specific pollinator. These results strongly suggest that current O3 episodes are probably already affecting the interaction between the fig tree and its specific pollinator.

An observation-based analysis of atmospheric oxidation capacity in Guangdong, China

In this study, an observation-based method (OBM) for evaluating the atmospheric oxidation capacity (AOC) in Guangdong province, China has been developed. We employ the OBM to calculate hydroxyl radical (OH), and in turn to evaluate AOC in seven representative districts/cities in Guangdong province. Three types of AOC are investigated in this study: the commonly defined AOC_ODT which is the rate atmospheric reductants being oxidized; our newly defined AOC_TOX which is the rate atmospheric oxidants being reduced; and the originally suggested AOC_CON which defines the oxidant concentration directly as the AOC index. Key characteristics of these three AOCs are analyzed and compared with each other as well as OH, O3 and NO2 under various atmospheric environments. We found that AOC contributed by OH radicals accounted for the largest portion of total AOC in all three definitions of AOC. In addition, all AOCs were higher during high O3 episodes relative to those of non-episode conditions. More importantly, we found that anthropogenic emissions had enhanced the AOC_CON_OH, or the OH concentration, in the atmosphere surface layer in Guangdong province by a factor of 5.7 over the OH concentration of pre-industrial environment. The corresponding enhancement factor for AOC_TOX_OH was even greater at about 570 because NO2 has increased by about 100 times. The enhancement factor for AOC_ODT_OH was about 30 because constant natural VOC emissions were assumed. This is a highly significant point in terms of the impact of the AOC on human health as well as the ecosystem.

Evaluation of different machine learning approaches for predicting high concentration episodes of ground-level ozone: A case study in Catalonia, Spain

Ground-level ozone (O3) is a pollutant with a great impact on human health and the environment. As a secondary air contaminant of photochemical origin, those areas with greater exposure to solar radiation, such as Spain and other Mediterranean countries, are considerably affected. With the aggravation of O3 pollution, it is important to provide reliable forecasting tools to help stakeholders implement more effective policies to mitigate the negative impact associated with this problem. In this regard, Machine Learning-based models have emerged in recent years, since they are able to identify complex relationships between ozone levels and relevant variables. However, their application to capture the most extreme events remains difficult. In this work, different ML approaches for predicting daily maximum 8-h average ozone (O3,MDA8) were compared, investigating their ability to forecast the highest concentration levels recorded. Two variants of the Random Forest algorithm (regression and classification) were applied to a specific area of Catalonia, Spain, with a special interest due to the high number of episodes of exceedance of O3 concentration levels. The predictive models were built with a 1 day time horizon, using datasets from 2002 to 2020. The variables used as inputs were other air pollutants concentrations and meteorological processes, monitored the day before to the target day to be predicted, and time information. Although results showed reasonable overall performances, low accuracy was achieved when forecasting the highest episodes of O3,MDA8. To improve the capacity of the models in predicting high-O3,MDA8 concentration levels, a methodology was proposed to fine-tuning the original predictions of the ML models according to a classification metric, G-Mean, which allows adjusting the balance between the correct predictions of different classes. Using the Sensitivity and Specificity metrics, the classical approaches were compared with the original ones proposed in the present study. The results obtained, for all the cases analysed, showed a mean increase in Sensitivity of 0.28, associated with a greater number of True Positives (correct predictions of high O3-episodes). On the other hand, the average Specificity value decreased, due to the appearance of a greater number of False Positives, although this reduction was only 0.05. The proposed criteria showed promising results, better balancing classification metrics and increasing the ratio of correct predictions linked to the higher ranges of O3.

Characterization of photochemical losses of volatile organic compounds and their implications for ozone formation potential and source apportionment during summer in suburban Jinan, China

Volatile organic compounds (VOCs) undergo substantial photochemical losses during their transport from emission sources to receptor sites, resulting in serious implications for their source apportionment and ozone (O3) formation. Based on the continuous measurements of VOCs in suburban Jinan in August 2022, the effects of photochemical losses on VOC source contributions and O3 formation were evaluated in this study. The observed and initial concentrations of total VOCs (TVOC) were 12.0 ± 5.1 and 16.0 ± 7.4 ppbv, respectively. Throughout the observation period, alkenes had the most prominent photochemical losses (58.2%), followed by aromatic hydrocarbons (23.1%), accounting for 80.6% and 6.9% of the total losses, respectively. During high O3 episodes, the photochemical loss of VOCs was 6.9 times higher than that during the cleaning period. Alkene losses (exceeding 67.3%), specifically losses of isoprene, propylene, ethylene, and n-butene, dominated the total losses of VOCs during the O3 increase period. Eight sources of VOCs were identified by positive matrix factorization (PMF) based on the observed and initial concentration data (OC-PMF and IC-PMF, respectively). Concentrations of all emission sources in the OC-PMF were underestimated by 2.4%–57.1%. Moreover, the contribution of each emission source was over- or underestimated compared with that in case of the IC-PMF. The contributions of biogenic and motor vehicle exhaust emissions were underestimated by 5.3 and 2.8 percentage points, respectively, which was associated with substantial oxidation of the emitted high-reactive species. The contributions of coal/biomass burning and natural gas were overestimated by 2.4 and 3.9 percentage points, respectively, which were related to the emission of low-reactive species (acetylene, ethane, and propane). Based on our results, the photochemical losses of VOCs grossly affect their source apportionment and O3 formation. Thus, photochemical losses of VOCs must be thoroughly accounted to establish a precise scientific foundation for air-pollution control strategies.

Seasonal variations of O3 formation mechanism and atmospheric photochemical reactivity during severe high O3 pollution episodes in the Pearl River Delta region

In this study, the seasonal differences of O3 formation and photochemical reactivity were investigated during the high O3 episodes observed in autumn of 2014 and winter of 2017 at a downwind rural site of Pearl River Delta region (Heshan site). The mixing ratios of O3 and NO2 were highest in the Period I of autumn, followed by the Period II of autumn and the winter period. Though the total concentrations of VOCs were comparable in these periods, alkenes made greater fraction of VOCs during Period I of autumn, while those of OVOCs and aromatics were higher in winter period, resulting in the largest contributions of alkenes (ROH(alkenes)) and OVOCs (ROVOCs) to the photochemical reactivity in autumn and winter, respectively. The ozonolysis of alkenes (especially for 1-pentene and isoprene) dominated HOx production during Period I of autumn, while in winter period, photolysis of OVOCs increased before noon, and made the greatest contribution to HOx production in late afternoon. As for the HOx destruction, reaction of OH + NO2 dominated in both seasons, with higher rates in Period I of autumn because of higher levels of radical and NO2. Furthermore, the variations in O3 precursors and radical budget resulted in different O3 formation and destruction rates, with the highest contribution from RO2 and NO to O3 formation, as well as photolysis and ozonolysis of alkenes for O3 destruction in Period I of autumn. The net O3 production from photochemistry and contributions of physical processes indicated that local photochemistry dominated the O3 accumulation at noon and in the early afternoon, while dilution in O3 levels caused by physical process increased before noon and in the late afternoon. O3 formation sensitivity could be changed during the photochemical consumption of precursors, which was more NOx-limited in the late afternoon. Overall, our findings highlighted that control measures of photochemical pollution should consider the seasonal variations in the abundance of O3 and its precursors, and O3 formation sensitivity regime.

Consumption Based Source Apportionment Indicates Different Regional Contributions to O3 Concentrations and Health Effects

China is confronting aggravated ozone (O3) pollution, leading to adverse health impacts. This study quantifies the regional contributions to O3 in China using two approaches; estimating ① where goods are produced (the production method), and ② where goods are consumed (the consumption method). The production method predicts higher local source contribution than the consumption method; this difference can be attributed to exports. Occurrence of high-O3 episodes suggests a major contribution to O3 concentration as a result of trade activities. Based on the consumption method, 9219 out of 18 532 daily premature mortalities were caused by local sources in north China, while it increased to 14 471 of the production method when neglecting contributions due to export and consumption in other regions. This study suggests that O3 control should consider both where goods are consumed and emissions are emitted, especially taking account of international trade activities.

Photochemical Mechanism and Control Strategy Optimization for Summertime Ozone Pollution in an Industrial City in the North China Plain

To investigate the characteristics and formation mechanism of ozone (O3) pollution in an industrial city, an extensive one-month field campaign focusing on O3 and its precursors (e.g., volatile organic compounds[VOC] and nitrogen oxides[NOx]) was conducted in Zibo City, a highly industrializd city in the North China Plain, in June 2021. The 0-D box model incorporating the latest explicit chemical mechanism (MCMv3.3.1) was applied using an observation dataset (e.g., VOC, NOx, HONO, and PAN) as model contraints to explore the optimal reduction strategy for O3 and its precursors. The results showed that ① during high-O3 episodes, stagnant weather conditions with high temperature and solar radiation as well as low relative humidity were observed, and oxygenated VOCs and alkenes from anthropogenic VOCs contributed the most to the total ozone formation potential and OH reactivity (k·OH). ② The in-situ O3 variation was primarily affected by local photochemical production and export process horizontal to downwind areas or vertical to the upper layer. The reduction in local emissions was essential to alleviate O3 pollution in this region. ③ During high-O3 episodes, high concentrations of ·OH (10×106 cm-3) and HO2· (14×108 cm-3) radical drove and generated a high O3 production rate (daytime peak value reached 36×10-9 h-1). The reaction pathways of HO2·+NO and ·OH+NO2 contributed the most to the in-situ gross Ox photochemical production (63%) and photochemical destruction (50%), respectively. ④ Compared to those during low-O3 episodes, the photochemical regimes during high-O3 episodes were more inclined to be considered as the NOx-limited regime. Detailed mechanism modeling based on multiple scenarios further suggested that the synergic emission reduction strategy of NOx and VOC, while focusing on NOx emission alleviation, would be practical options for controlling local O3 pollutions. This method could also provide policy-related guidance for the precise O3 pollution prevention and control in other industrialized Chinese cities.

Photochemical model representation of ozone and precursors during the 2017 Lake Michigan ozone study (LMOS)

Several areas in the Lake Michigan region are violating the human health-based ozone (O3) National Ambient Air Quality Standard. Land-water meteorology driven build-up of precursor pollutants (NOX and VOC) from mobile and stationary sources undergo photochemical O3 production and result in seasonal high O3 episodes during the spring and summertime. Routine and specialized surface measurements coupled with airborne and remotely sensed measurements from the 2017 Lake Michigan Ozone Study (LMOS) provide an opportunity to evaluate photochemical grid model representation of these processes. The Community Multiscale Air Quality Model (CMAQ) was applied at a 4-km grid resolution for the Lake Michigan region and compared against routine and field study measurements to determine how well the modeling system captures emissions, meteorology, and chemical production during O3 episodes.The model captured day to day and diurnal variability in observed O3 along Lake Michigan but often missed peak O3. Surface level NO2 was generally well characterized, but the model seemed to be missing a significant source of VOC, likely both regional and local (not necessarily the same source sector) based on model sensitivities. On June 2, 2017, the model captured the timing of the lake to land breeze but underestimated near-surface wind speed which coincided with peak O3. The underestimation of surface O3 north of Chicago at Zion, Illinois, on this afternoon may be related to over-water vertical mixing, the modeled lake breeze being too weak to transport ozone and precursors back onshore, regional and aloft underprediction of O3, or some combination of each of these factors. Despite underpredictions of peak O3 at some monitors on certain days, the modeling system is useful for developing control scenarios for this region. Model-predicted O3 sensitivity to precursors matched other assessments and suggests both NOX and VOC anthropogenic emissions are important to reduce O3 in the Chicago area.

Does Ozone Pollution Share the Same Formation Mechanisms in the Bay Areas of China?

As important regions of transition between land and sea, the three bay areas of Bohai Bay (BHB), Hangzhou Bay (HZB), and Pearl River Estuary (PRE) in China often suffer from severe photochemical pollution despite scarce anthropogenic emissions. To understand the causes of high ozone (O3) concentrations, the high O3 episode days associated with special synoptic systems in the three bays were identified via observations and simulated by the weather research and forecasting coupled with community multiscale air quality (WRF-CMAQ) model. It was revealed that the interaction between synoptic winds and mesoscale breezes resulted in slow wind speeds over the HZB and PRE, where air pollutants transported from upwind cities gained a long residence time and subsequently participated in intensive photochemical reactions. The net O3 production rates within the bay areas were even comparable to those in surrounding cities. This finding was also applicable to BHB but with lower net O3 production rates, while high levels of background O3 and the regional transport from farther upwind BHB partially elevated the O3 concentrations. Hence, these three bay areas served as O3 "pools" which caused the accumulation of air pollutants via atmospheric dynamics and subsequent intense photochemical reactions under certain meteorological conditions. The results may be applicable to other similar ecotones around the world.

Impact of regional transport on high ozone episodes in southeast coastal regions of China

Severe ozone (O3) pollution can be harmful to human health and ecosystems. The O3 level in Fujian, a coast province in southeast China, is generally 30–60 μg m−3 lower than in highly polluted regions in China, including North China Plain (NCP), Yangtze River Delta (YRD), and Pearl River Delta (PRD). Yet, Fujian still experiences high O3 episodes that exceed the national standard. Here we investigate whether and how these high O3 episodes are impacted by regional O3 transport from polluted regions. A backward trajectory model, HYSPLIT, is combined with observational data to identify transport pathways and O3 source regions. The results show that the exceedance days (8 h O3 concentrations above 160 μg m−3), averaged over 2015–2020, range 12–34 days for coastal cities and 3–9 days for inland cities, with two peaks in late-spring and early-autumn. On average, O3 transport from non-Fujian regions can impact 29–38% of exceedance days in coastal cities, and 44–50% of exceedance days in inland cities. The dominant transport pathways are from YRD and NCP to the East China Sea near Fujian in the lower troposphere, and then to Fujian near the ground level. Regional transport from PRD, mainly in spring, also happens. Using synoptic weather pattern (SWP) classification, we identify three SWPs in spring and two SWPs in autumn that are favorable to regional O3 transport. In summer, such transport is mainly associated with typhoon activities. This study underscores the importance of regional O3 transport to a clean coastal region and helps predict regional air pollution.

Improving predictability of high-ozone episodes through dynamic boundary conditions, emission refresh and chemical data assimilation during the Long Island Sound Tropospheric Ozone Study (LISTOS) field campaign

Abstract. Although air quality in the United States has improved remarkably in the past decades, ground-level ozone (O3) often rises in exceedance of the national ambient air quality standard in nonattainment areas, including the Long Island Sound (LIS) and its surrounding areas. Accurate prediction of high-ozone episodes is needed to assist government agencies and the public in mitigating harmful effects of air pollution. In this study, we have developed a suite of potential forecast improvements, including dynamic boundary conditions, rapid emission refresh and chemical data assimilation, in a 3 km resolution Community Multiscale Air Quality (CMAQ) modeling system. The purpose is to evaluate and assess the effectiveness of these forecasting techniques, individually or in combination, in improving forecast guidance for two major air pollutants: surface O3 and nitrogen dioxide (NO2). Experiments were conducted for a high-O3 episode (28–29 August 2018) during the Long Island Sound Tropospheric Ozone Study (LISTOS) field campaign, which provides abundant observations for evaluating model performance. The results show that these forecast system updates are useful in enhancing the capability of this 3 km forecasting model with varying effectiveness for different pollutants. For O3 prediction, the most significant improvement comes from the dynamic boundary conditions derived from the NOAA operational forecast system, National Air Quality Forecast Capability (NAQFC), which increases the correlation coefficient (R) from 0.81 to 0.93 and reduces the root mean square error (RMSE) from 14.97 to 8.22 ppbv, compared to that with the static boundary conditions (BCs). The NO2 from all high-resolution simulations outperforms that from the operational 12 km NAQFC simulation, regardless of the BCs used, highlighting the importance of spatially resolved emission and meteorology inputs for the prediction of short-lived pollutants. The effectiveness of improved initial concentrations through optimal interpolation (OI) is shown to be high in urban areas with high emission density. The influence of OI adjustment, however, is maintained for a longer period in rural areas, where emissions and chemical transformation make a smaller contribution to the O3 budget than that in high-emission areas. Following the assessment of individual updates, the forecasting system is configured with dynamic boundary conditions, optimal interpolation of initial concentrations and emission adjustment, to simulate a high-ozone episode during the 2018 LISTOS field campaign. The newly developed forecasting system significantly reduces the bias of surface NO2 prediction. When compared with the NASA Langley GeoCAPE Airborne Simulator (GCAS) vertical column density (VCD), this system is able to reproduce the NO2 VCD with a higher correlation (0.74), lower normalized mean bias (40 %) and normalized mean error (61 %) than NAQFC (0.57, 45 % and 76 %, respectively). The 3 km system captures magnitude and timing of surface O3 peaks and valleys better. In comparison with lidar, O3 profile variability of the vertical O3 is captured better by the new system (correlation coefficient of 0.71) than by NAQFC (correlation coefficient of 0.54). Although the experiments are limited to one pollution episode over the Long Island Sound, this study demonstrates feasible approaches to improve the predictability of high-O3 episodes in contemporary urban environments.

Enhancement of ozone formation by increased vehicles emission and reduced coal combustion emission in Taiyuan, a traditional industrial city in northern China

Coal combustion and industrial processes were often the main sources of atmospheric pollutants in traditional industrial cities in northern China. Studies on sensitivity analysis and source apportionment of ozone formation, which are the basis of ozone pollution controlling, are limited in those cities. The systematic observations of ambient Volatile Organic Compounds (VOCs) were performed to better investigate the major anthropogenic sources of summertime O3 during different O3 episodes in Taiyuan, a traditional industrial city in northern China. Average mixing ratio of VOCs was 21.80 ppbv during the summer sampling period, and the highest value was measured at the traffic-intensive area. Source apportionment carried by positive matrix factorization model (PMF) showed that VOCs from vehicular emissions exceeded those from coal combustion, and their contributions to the total ambient VOCs were 28.12% and 25.95%, respectively. VOCs limited regime of summertime O3 formation was revealed by using a photochemistry model. Alkenes group was the most crucial contributor to O3 formation. Vehicular emissions made the largest anthropogenic contribution (25.35%) to total O3 formation, while industrial processes with the contribution of 29.81% to O3 was also unacceptable in high O3 episodes. Multi-sources joint prevention and control, especially the joint of vehicular emissions and industrial processes emissions, may effectively reduce the summertime O3 pollution in Taiyuan. Given the vigorous implementation of the coal reform policies and future economic and energy restructuring, effects of vehicular emissions on VOCs and summertime O3 should become more remarkable, indicating that the implementation of more stringent control strategies of vehicular emissions may be the key to alleviate O3 pollution in traditional industrial cities in the future.

Large variability of O3-precursor relationship during severe ozone polluted period in an industry-driven cluster city (Zibo) of North China Plain

Zibo, a highly industrialized cluster city in the central area of the North China Plain (NCP), has suffered from serious ozone pollution in recent years. In this study, a comprehensive field campaign was conducted during July 2019 at three representative and diversified areas in Zibo, including two suburban sites (Tianzhen (TZ) and Xindian (XD)) and one urban site (Beijiao (BJ)). The O3 variation patterns were overall similar among the three sites, while its precursors (i.e. volatile organic compounds (VOCs) and nitrogen oxides (NOx)) showed distinct site-to-site differences. A 0-D box model incorporating the latest Master Chemical Mechanism (MCMv3.3.1) was applied using observations as constraints, and NOx-limited, VOC-limited and transitional regimes were generally identified for TZ, BJ and XD respectively, particularly during high-O3 episodes. We believe that such large variability of O3-precursor relationship found in Zibo may exist widely in many other cities, which challenges the current O3 pollution control. It was further concluded that both high in-situ O3 production rate (daytime peak 21–27 ppbv h−1) and O3 transported from residual layer are responsible for the high ground-level O3. Detailed mechanism modelling with many scenarios suggested that a synergic reduction plan for NOx and VOC would be the most feasible and effective option to achieve O3-attainment levels. It is suggested that the minimum reduction in NOx should be by more than 40% (TZ), 60% (BJ) and 20% (XD), while anthropogenic VOC should simultaneously be cut more than 60% (TZ), 40% (BJ) and 60% (XD). Our fundamental chemical mechanism modelling provided an improved understanding of O3-precursor relationship, and the derived policy-relevant guidance can assist the local government like Zibo to deal with O3 challenge. These results could also provide insights for regional O3 control across the North China Plain.