High Ozone Episodes Research Articles

National and transboundary contributions to surface ozone concentration across European countries

Tropospheric ozone impacts health, climate, and ecosystems. Effective ozone mitigation policies are challenged by limited quantitative understanding of national versus transboundary contributions to surface ozone. This study uses a chemical transport model with a source apportionment algorithm to analyze ozone contributions across Europe from 2015 to 2017 during peak ozone season. We quantify country-level ozone production and imported ozone, distinguishing contributions from 35 European countries, neighboring countries, seas, and hemispheric influences. Results show substantial contributions from outside the 35 European countries, with hemispheric contributions playing a significant role. European contributions are crucial during high ozone episodes, especially from Germany, France, Italy, the UK, Poland, and Spain. Spain, northern Italy, and northwest France are identified as areas where national precursor reductions would be more effective in improving national air quality. Furthermore, 25 of the 35 European countries studied are net importers of cumulative ozone mass, with the Netherlands, Belgium, and the UK acting as major exporters. These findings highlight the need for comprehensive air quality policies and cross-border cooperation.

Effects of VOC emission reduction on atmospheric photochemistry and ozone concentrations during high-ozone episodes

We studied atmospheric volatile organic compounds (VOCs) characteristics and sources during fifteen high ozone (O3) episodes (daily 1-h maximum >100 ppbv and maximum 8-h average > 80 ppbv) in Nanjing, China, and used a photochemical box model to study the O3 formation sensitivity of VOCs, O3 photochemistry, radical budget, and reactivity for different emission reduction scenarios together with the base case ones. Overall, emission reduction from biogenic sources, followed by combustion sources, multiple sources, and solvent usage, had the major impacts on the O3 concentrations. According to photochemical box model simulations, isoprene was the most sensitive or influential VOC in the formation of O3, followed by ethylene, o-xylene, and cis-2-pentene. A 20.6 ± 11.7% concentration reduction of the sixteen most sensitive VOCs (SV16) for the production of O3 could bring the O3 concentration below the national standard level (daily 1-h maximum <100 ppbv). Besides, a 30% emission reduction from all the anthropogenic sources was more than enough to bring the O3 concentration below the national standard level. The total emission reduction of alkenes was always enough to bring the O3 concentration below the national standard level, which was not the case for aromatics under severe pollution conditions. The simulated OH reactivity analysis illustrates the importance of alkenes and aromatics, which account for about half of the total OH reactivity. The highest OH reactivity decreased due to the emission reduction for biogenic sources, followed by combustion and multiple sources. Alkenes played a major role in the OH, HO2, and RO2 radical's formation in the study area. The biggest drop in OH, HO2, and RO2 radical concentrations was noticed for biogenic emission reduction, followed by combustion and multiple sources. Due to the emission reduction from VOC sources, the highest O3 formation rates decreased for biogenic sources, followed by combustion and multiple sources. The acquired information is valuable to the scientific community and policymakers for formulating and implementing O3 pollution control strategies.

Benefits of net-zero policies for future ozone pollution in China

Abstract. Net-zero emission policies principally target climate change but may have a profound influence on surface ozone pollution. To investigate this, we use a chemistry–climate model to simulate surface ozone changes in China under a net-zero pathway and examine the different drivers that govern these changes. We find large monthly mean surface ozone decreases of up to 16 ppb in summer and small ozone decreases of 1 ppb in winter. Local emissions are shown to have the largest influence on future ozone changes, outweighing the effects of changes in emissions outside China, changes in global methane concentrations, and a warmer climate. Impacts of local and external emissions show strong seasonality, with the largest contributions to surface ozone in summer, while changes in global methane concentrations have a more uniform effect throughout the year. We find that while a warmer climate has a minor impact on ozone change compared to the net-zero scenario, it will alter the spatial patterns of ozone in China, leading to ozone increases in the south and ozone decreases in the north. We also apply a deep learning model to correct biases in our ozone simulations and to provide a more robust assessment of ozone changes. We find that emission controls may lead to a surface ozone decrease of 5 ppb in summer. The number of days with high-ozone episodes with daily mean ozone greater than 50 ppb will be reduced by 65 % on average. This is smaller than that simulated with the chemistry–climate model, reflecting overestimated ozone formation under present-day conditions. Nevertheless, this assessment clearly shows that the strict emission policies needed to reach net zero will have a major benefit in reducing surface ozone pollution and the occurrence of high-ozone episodes, particularly in high-emission regions in China.

Ozone containment through selective mitigation measures on precursors of volatile organic compounds

Abatement of volatile organic compounds (VOCs) ozone reduction is usually carried out by reducing the total amount of VOCs without considering reactivity between different species. This study incorporates the concept of maximum incremental reactivity (MIR) and speciation profiles into the industrial emission inventory of Taiwan to target organic species from industrial sources with the greatest ozone formation potentials (OFPs). These high OFP sources/species are then mitigated to assess the O3 reduction amount (ΔO3) with Community Multiscale Air Quality (CMAQ) modeling under VOC-limited conditions. The objective is to minimize the number of target sources/species and their tonnage while achieving maximum O3 reduction. This approach is referred to as the Selective Precursor Mitigation (SPM).A case study of a high ozone episode (September 4–10, 2020) was chosen for illustration, during which a relatively stagnant atmospheric condition with minimal transboundary ozone occurred. A series of scenarios to target the highest OFP chemicals/industries for mitigation are compared for the achievable max. ΔO3, areas affected (area coverage), and reduction efficiency. For instance, by reducing the ten leading industry classes with the island's highest OFPs (OFPind), up to 19 % of max. 1-h ΔO3 can be expected. If, however, the same tonnage of VOCs as that of OFPind is distributed to all industries without considering the reactivity, called the overall mitigation (OM), comparable results to those of OFPind were found, but the number of sources needed to be managed with OM would increase by nearly three times (29,662 for OM vs. 11,981 for OFPind). Further reducing the management scale by only zooming in the ten highest OFP chemicals within the ten leading OFP industries (OFPsp) would result in relatively limited area coverage. Still, major ozone hot spots could be alleviated.Although the domain is set on the island of Taiwan, the SPM approach is universally applicable to other regions worldwide to gain the maximum ozone reduction effect at a minimized societal cost.

Evaluating WRF-GC v2.0 predictions of boundary layer height and vertical ozone profile during the 2021 TRACER-AQ campaign in Houston, Texas

Abstract. The TRacking Aerosol Convection ExpeRiment – Air Quality (TRACER-AQ) campaign probed Houston air quality with a comprehensive suite of ground-based and airborne remote sensing measurements during the intensive operating period in September 2021. Two post-frontal high-ozone episodes (6–11 and 23–26 September) were recorded during the aforementioned period. In this study, we evaluated the simulation of the planetary boundary layer (PBL) height and the vertical ozone profile by a high-resolution (1.33 km) 3-D photochemical model, the Weather Research and Forecasting (WRF)-driven GEOS-Chem (WRF-GC). We evaluated the PBL heights with a ceilometer at the coastal site La Porte and the airborne High Spectral Resolution Lidar 2 (HSRL-2) flying over urban Houston and adjacent waters. Compared with the ceilometer at La Porte, the model captures the diurnal variations in the PBL heights with a very strong temporal correlation (R&gt;0.7) and ±20 % biases. Compared with the airborne HSRL-2, the model exhibits a moderate to strong spatial correlation (R=0.26–0.68), with ±20 % biases during the noon and afternoon hours during ozone episodes. For land–water differences in PBL heights, the water has shallower PBL heights compared to land. The model predicts larger land–water differences than the observations because the model consistently underestimates the PBL heights over land compared to water. We evaluated vertical ozone distributions by comparing the model against vertical measurements from the TROPospheric OZone lidar (TROPOZ), the HSRL-2, and ozonesondes, as well as surface measurements at La Porte from a model 49i ozone analyzer and one Continuous Ambient Monitoring Station (CAMS). The model underestimates free-tropospheric ozone (2–3 km aloft) by 9 %–22 % but overestimates near-ground ozone (&lt;50 m aloft) by 6 %-39 % during the two ozone episodes. Boundary layer ozone (0.5–1 km aloft) is underestimated by 1 %–11 % during 8–11 September but overestimated by 0 %–7 % during 23–26 September. Based on these evaluations, we identified two model limitations, namely the single-layer PBL representation and the free-tropospheric ozone underestimation. These limitations have implications for the predictivity of ozone's vertical mixing and distribution in other models.

Attribution of surface ozone to NOx and volatile organic compound sources during two different high ozone events

Abstract. Increased tropospheric ozone (O3) and high temperatures affect human health during heat waves. Here, we perform a source attribution that considers separately the formation of German surface ozone from emitted NOx and volatile organic compound (VOC) precursors during two peak ozone events that took place in 2015 and 2018 which were associated with elevated temperatures. Results showed that peak ozone concentrations can be primarily attributed to nearby emissions of anthropogenic NOx (from Germany and immediately neighboring countries) and biogenic VOC. Outside of these high ozone episodes, baseline ozone concentrations are attributed primarily to long-range transport, with ozone due to remote anthropogenic NOx emissions and methane oxidation adding to the tropospheric ozone background. We show that a significant contribution to modeled O3 coming from German NOx or VOC emissions occurs mostly in southern Germany, emphasizing that the production of ozone depends on the local interplay between NOx and VOC precursors. Shipping activities in the Baltic and North seas have a large impact on ozone predicted in coastal areas, yet a small amount of ozone from these sources can also be seen far inland, showing the importance of transported ozone on pollution levels. We have also shown that changes in circulation patterns during the peak O3 episodes observed in Germany during the 2015 and 2018 heat waves can affect the contribution of different NOx emission sources to total O3; thus, the possible influence of multiple upwind source regions should be accounted for when mitigation strategies are designed. Our study also highlights the good correlation between ozone coming from German biogenic VOC emissions and total ozone, although the diurnal variation in the ozone coming from biogenic sources is not dominated by the diurnal variation in biogenic emissions, and the peaks of ozone from biogenic sources are disconnected from local emission peaks. This suggests that the formation of O3 from local German biogenic VOC emissions is not the sole factor that influences the ozone formation, and other meteorological and chemical processes affect the diel variation of ozone with a biogenic origin. Overall, this study helps to demonstrate the importance of a source attribution method to understand the sources of O3 in Germany and can be a useful tool that will help to design effective mitigation strategies.

Influences of stratospheric intrusions to high summer surface ozone over a heavily industrialized region in northern China

The stratospheric contribution to tropospheric ozone has long been a topic of much debate over the past few decades. In this study, we leveraged multiple datasets from surface, sounding and satellite observations to reanalysis datasets, along with a global chemical transport model (Global Nested Air Quality Prediction Modelling System, GNAQPMS) to investigate the impact of a stratospheric-to-tropospheric transport (STT) event characterized by long duration and wide range in the summer on surface high ozone episodes over heavily industrialized regions in northern China. In 14–18 August 2019, the ERA5 reanalysis datasets showed a potential vorticity (PV) tongue and a deep, upper-level trough penetrate towards 35° N over the North China Plain (NCP), indicating the occurrence of a stratospheric intrusion. From Atmospheric Infrared Sounder (AIRS) measurements, we found that the ozone-rich, stratospheric air mass had been injected into the lower altitudes. The GNAQPMS generally captured the featured layers, although there was a slight underestimation in the low troposphere. The averaged magnitudes of stratospheric contribution (O3S) and percentage (O3F) simulated by GNAQPMS were 3–20 μg m−3 and 6%–20%, respectively, while the Whole Atmosphere Community Climate Model (WACCM) indicated a higher stratospheric contribution by 3–5 μg m−3. Through this study, we give our opinions on the controversial topic of a more thorough understanding of the influence of natural processes apart from anthropogenic emissions, even in a heavily polluted region during summer.

Composition and reactivity of volatile organic compounds in the South Coast Air Basin and San Joaquin Valley of California

Abstract. Comprehensive aircraft measurements of volatile organic compounds (VOCs) covering the South Coast Air Basin (SoCAB) and San Joaquin Valley (SJV) of California were obtained in the summer of 2019. Combined with the CO, CH4, and NOx data, the total calculated gas-phase hydroxyl radical reactivity (cOHRTOTAL) was quantified to be 6.1 and 4.6 s−1 for the SoCAB and SJV, respectively. VOCs accounted for ∼ 60 %–70 % of the cOHRTOTAL in both basins. In particular, oxygenated VOCs (OVOCs) contributed &gt;60 % of the cOHR of total VOCs (cOHRVOC) and the total observed VOC mixing ratio. Primary biogenic VOCs (BVOCs) represented a minor fraction (&lt;2 %) of the total VOC mixing ratio but accounted for 21 % and 6 % of the cOHRVOC in the SoCAB and SJV, respectively. Furthermore, the contribution of BVOCs to the cOHRVOC increased with increasing cOHRVOC in the SoCAB, suggesting that BVOCs were important ozone precursors during high ozone episodes. Spatially, the trace gases were heterogeneously distributed in the SoCAB, with their mixing ratios and cOHR being significantly greater over the inland regions than the coast, while their levels were more evenly distributed in SJV. The results highlight that a better grasp of the emission rates and sources of OVOCs and BVOCs is essential for a predictive understanding of the ozone abundance and distribution in California.

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.

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.

The impact of the aerosol reduction on the worsening ozone pollution over the Beijing-Tianjin-Hebei region via influencing photolysis rates

Due to the implementation of the toughest-ever emission control actions across China from 2013 to present, the aerosols are decreasing annually but ozone is simultaneously increasing, especially over the Beijing-Tianjin-Hebei (BTH) region, where ozone pollution can even spread into winter. Quantifying each impact of aerosols on ozone in all seasons is urgent for the worsening ozone pollution in the improved aerosol air quality. In this study, we focused on the impact of aerosols on ozone via influencing photolysis rates. The air pollutants were simulated over the Central East China (CEC) in 2018 by using the Weather Research and Forecasting with Chemistry (WRF-Chem) model. By implementing emissions with base years of 2014 and 2018, we quantified the increase in ozone (ΔOzone_photolysis) caused by the decreasing aerosol concentrations (ΔPM2.5) by influencing photolysis rates over the BTH region in all seasons. Furthermore, combined with the ozone observations, the contribution of ΔOzone_photolysis to the total changes in ozone (ΔOzone_total) in all seasons was quantitatively discussed. Our results showed that ΔPM2.5 showed obvious seasonal variations, which PM2.5 decreased more significantly in winter and autumn than in spring and summer, although significant reductions in anthropogenic emissions were observed in all seasons. Consistent seasonal variations were also observed in ΔOzone_photolysis, and the mean increases reached 5.5 μg m−3, 2.6 μg m−3, 1.2 μg m−3, and 1.4 μg m−3 in winter, autumn, spring, spring, and summer, respectively. Compared with ΔOzone_total, ΔOzone_photolysis accounted for 36.3%, 17.2%, 3.5% and 10.6% of ΔOzone_total in winter, autumn, spring, and summer, respectively, suggesting that ΔOzone_photolysis was not the primary contributor to the current changes in ozone over the BTH region. However, the 36.3% contribution to ΔOzone_total in winter suggested that ΔOzone_photolysis is still an important contributor and should not be ignored when discussing the formation of high ozone episodes occurring in winter.

Multi-Scale Atmospheric Emissions, Circulation and Meteorological Drivers of Ozone Episodes in El Paso-Juárez Airshed

Ozone pollution has been prevalent in the El Paso-Juárez Airshed (EPJA), especially in the past few decades, and it has been on the rise recently. The spatial and temporal distribution of the tropospheric ozone and several key meteorological factors that influence its concentration has not been adequately understood. Therefore, this investigation comprehensively examined 57 high and 48 low ozone episodes occurring in this region during 2013–2019. We found that the interannual ozone concentration in EPJA was strongly affected by anthropogenic emissions. On the other hand, seasonal ozone variations are due to meteorological variables (among them, solar radiation, planetary boundary layer, and winds) in addition to biogenic emission factors. High ozone events are characterized by calm winds, shallow planetary boundary layer (PBL), whereas low ozone events were marked with strong winds, precipitation, and deep PBL. Synoptic and mesoscale wind patterns for these ozone episodes were identified and characterized. Most of the high ozone episodes occurred when an anticyclonic circulation aloft was associated with a 500-mile middle and upper tropospheric high-pressure region over the EPJA. During these events, stable air masses with convective available potential energies (CAPE) values of less than 450 J/kg were found. The importance of surface topography is illustrated by the fact that stations close to the Rio Grande River show a bimodal distribution of wind direction according to the valley axis. High ozone episodes occur with a surface easterly wind that is decoupled from winds above the Franklin mountains.

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.

Impacts of lake breeze meteorology on ozone gradient observations along Lake Michigan shorelines in Wisconsin

Daytime onshore lake breezes are a critical factor controlling ozone abundance at coastal sites around Lake Michigan. Coastal counties along the western shore of Lake Michigan have historically observed high ozone episodes dating to the 1970s. We classified ozone episode days based on the extent or absence of the lake breeze (i.e., “inland”, “near-shore” or “no” lake breeze) to establish a climatology of these events. This work demonstrated variable gradients in ozone abundances based on these different types of meteorology, with the sharpest ozone concentration gradients on days with a near-shore lake breeze. On 76–82% of days in which ozone reached 70 ppb for at least 1 h, a lake breeze was present. Evidence of ozone gradients from multiple observation platforms during the 2017 Lake Michigan Ozone Study (LMOS 2017) are shown for two days with different depths of lake breezes.

Atmospheric oxidation capacity and its contribution tosecondary pollutants formation

<p indent="0mm">Atmospheric oxidation capacity (AOC) is defined as the oxidation reaction rates of primary pollutants in the atmospheric chemical process, which is characterized by the oxidants concentration or the reaction rates. Atmospheric oxidation plays an important role in the formation of secondary pollutants, and works as an important indicator for the study of secondary pollution. Since 2013, the Chinese Government implemented unprecedented strident regulations to improve air quality. Although the overall levels of fine particulate matter (PM_2.5) have dropped considerably throughout China, it remains unclear how secondary pollutants have responded to the emission reductions in view of complex gas-precursor relationship and gas-particle interactions. Previous studies also reported the positive correlation between the AOC level and secondary pollutant concentrations. The updated Community Multi-scale Air Quality (CMAQ) model with the SAPRC 11 photo-chemical mechanism as well as the observation data was applied in this study to investigate the relationship of the secondary pollutants (ozone and secondary aerosols) and their precursors with the major oxidants (HO₂, OH and NO₃ radicals) in years 2013 and 2020 in China. And the process analysis tool was implemented in the CMAQ model to determine the key oxidants budget, which could better understand the primary sources and sinks of these oxidants. This analysis was able to calculate the chemical reaction rates at each integration time step. Radical source reactions were generally photolysis reactions, and the sink reactions removed radicals through the formation of stable products. The anthropogenic emissions were from Multiresolution Emission Inventory for China (MEIC) database, and the biogenic emissions were calculated from the Model of Emissions of Gas and Aerosols from Nature (MEGAN). The meteorology inputs were generated from the Weather Research and Forecasting (WRF) model. The CMAQ model predictions agree well with the observations, with most of the model performance within the criteria. The results show that the AOC level keeps identical from 2013 to 2020 throughout China. In the North China Plain (NCP) and the Pearl River Delta (PRD), the slightly increasing trends of AOC are even reported, which may aggravate the secondary pollution. The enhanced AOC level in the NCP is mainly from the reduced PM_2.5 concentration, which is regarded as the important sink of AOC. In addition, the concentration of major oxidants also represents different regional characteristics. The highest concentrations of HO_{<italic>x</italic>} and NO₃ radical are reported in the Sichuan Basin (SCB) and NCP, respectively. The ozone photolysis and the formation of HNO₃ (OH+NO₂) are reported as the most important AOC primary sources and sinks, respectively. In the NCP, the HONO photolysis is found as an important AOC source, which is consistent with previous studies. The AOC level increases significantly during the high ozone episode (increasing rate up to 475%). Also, the high level of AOC is corresponding to the significant increase of the secondary organic aerosols, indicating its important role in the formation of secondary pollutants. In summer, the reduction of NO_{<italic>x</italic>} emissions can reduce the AOC level. Meanwhile, a more complex response between the NO_{<italic>x</italic>} emissions and AOC level is found in winter. More rational emission reduction ratio of NO_{<italic>x</italic>} and volatile organic compounds (VOCs) should be applied to better control the AOC in China. In the future, more effective AOC control strategies should be established to reduce the PM_2.5 and O₃ coordinately in the 14th Five-Year Plan of China.

Effects of synoptic patterns on the vertical structure of ozone in Hong Kong using lidar measurement

Long-term ozone lidar routine measurements are sparse in China. In this study, we used one year of continuous ozone lidar measurements to systematically investigate the effects of synoptic patterns on the vertical structure of ozone during high ozone episodes in Hong Kong. The height of the near-surface ozone layer was identified and compared with that of the mixing layer. The results showed that Hong Kong was greatly affected by ozone transport from the Pearl River Delta region when a tropical cyclone (“C” pattern) or a trough (“T” pattern) existed. When a high-pressure center was located to the north (“H” pattern) or when both a high-pressure system and a tropical cyclone existed (“CH” pattern), Hong Kong was affected by ozone transport from southeastern or southern China. Substantial amounts of ozone were transported within a layer near the ground with an average height of 0.83, 0.75, 0.94, and 0.85 km under the C, H, CH, and T synoptic patterns, respectively. Variation in the near-surface ozone layer height was consistent with that of the mixing layer height. Weak vertical mixing resulted in a shallow near-surface ozone layer under the C and H synoptic patterns. Due to the combined effect of near-surface transport and suppressed vertical mixing, the ozone concentrations were extremely high near the ground but rapidly declined with height under the C synoptic pattern. Our investigations of the vertical structure of ozone contribute to filling the knowledge gap between synoptic patterns and the evolution of high surface ozone episodes.

Using wavelet transform and dynamic time warping to identify the limitations of the CNN model as an air quality forecasting system

Abstract. As the deep learning algorithm has become a popular data analysis technique, atmospheric scientists should have a balanced perception of its strengths and limitations so that they can provide a powerful analysis of complex data with well-established procedures. Despite the enormous success of the algorithm in numerous applications, certain issues related to its applications in air quality forecasting (AQF) require further analysis and discussion. This study addresses significant limitations of an advanced deep learning algorithm, the convolutional neural network (CNN), in two common applications: (i) a real-time AQF model and (ii) a post-processing tool in a dynamical AQF model, the Community Multi-scale Air Quality Model (CMAQ). In both cases, the CNN model shows promising accuracy for ozone prediction 24 h in advance in both the United States of America and South Korea (with an overall index of agreement exceeding 0.8). For the first case, we use the wavelet transform to determine the reasons behind the poor performance of CNN during the nighttime, cold months, and high-ozone episodes. We find that when fine wavelet modes (hourly and daily) are relatively weak or when coarse wavelet modes (weekly) are strong, the CNN model produces less accurate forecasts. For the second case, we use the dynamic time warping (DTW) distance analysis to compare post-processed results with their CMAQ counterparts (as a base model). For CMAQ results that show a consistent DTW distance from the observation, the post-processing approach properly addresses the modeling bias with predicted indexes of agreement exceeding 0.85. When the DTW distance of CMAQ versus observation is irregular, the post-processing approach is unlikely to perform satisfactorily. Awareness of the limitations in CNN models will enable scientists to develop more accurate regional or local air quality forecasting systems by identifying the affecting factors in high-concentration episodes.

Long-term trends of surface ozone in Korea

To gain a better understanding of the state of the art of surface ozone in Korea, the trends of several indicators were presented based on the extensive surface ozone monitoring data in 16 administrative districts and Baekryeong island, a background area. Important indicators such as the annual mean of hourly mean surface ozone (AVG1), the daily maximum 8-h values (MDA8), and the AVG1 during daytime (D-AVG1) were high in background areas while low in urban areas including the Seoul Metropolitan Area (SMA). It implies that regional background level largely determines the ozone characteristics in Korea while local photochemical reactions affect the ozone levels in individual district. This hypothesis was validated by the frequency of the high ozone episodes by district. Spatial and temporal heterogeneity of surface ozone in Korea were high, thus, there should be control policy district by district based on the target indicator(s).

Episodes of high tropospheric ozone reduce nodulation, seed production and quality in soybean (Glycine max (L.) merr.) on low fertility soils.

Driven by human activities, air pollution and soil degradation are threatening food production systems. Rising ozone in the troposphere can affect several physiological processes in plants and their interaction with symbiotic microorganisms. Plant responses to ozone may depend on both soil fertility and the ontogenetic stage in which they are exposed. In this work, we studied the effects of ozone episodes and soil fertility on soybean plants. We analysed soybean plant responses in the production of aboveground and belowground biomass, structural and functional attributes of rhizobia, and seed production and quality. The experiment was performed with plants grown in two substrates with different fertility (commercial soil, and soil diluted (50%, v/v) with sand). Plants were exposed to acute episodes of ozone during vegetative and reproductive stages. We observed that ozone significantly reduced belowground biomass (≈25%), nodule biomass (≈30%), and biological nitrogen fixation (≈21%). Plants exposed to ozone during reproductive stage growing in soil with reduced fertility had lower seed production (≈10% lower) and seed protein (≈12% lower). These responses on yield and quality can be explained by the observed changes in belowground biomass and nitrogen fixation. The negative impact of ozone on the symbiotic interaction with rhizobia, seed production and quality in soybean plants were greater in soils with reduced fertility. Our results indicate that food security could be at risk in the future if trends in ozone concentration and soil degradation processes continue to increase.

Characteristics and sources of volatile organic compounds during high ozone episodes: A case study at a site in the eastern Guanzhong Plain, China

This study performed continuous measurements of 105 volatile organic compounds (VOCs) in Weinan in the eastern Guanzhong Plain from 1 July to September 19, 2019. Ozone (O3) episode and non-episode days were identified according to China Ambient Air Quality Standard, and the concentrations of total quantified VOCs (TVOCs) were 33.43 ± 13.64 ppbv and 29.13 ± 14.31 ppbv, respectively. During different O3 pollution episodes, alkanes comprised the highest proportion to TVOC concentrations, while alkenes contributed the most to ozone formation potential (OFP). In addition, O3 episode days were mainly caused by enhanced emissions of precursors and meteorological conditions favorable to O3 production. Based on Empirical Kinetic Modelling Approach (EKMA), the O3 formation in Weinan was found in the transitional regime, in which the synergistic reduction of VOCs and nitrogen oxide (NOx) would be more effective for O3 reduction. Eight sources were identified by positive matrix factorization (PMF) model, with natural gas (NG)/liquefied petroleum gas (LPG) usage as the most significant contributor to VOC concentration, followed by vehicle exhaust, biomass burning, solvent usage, fuel evaporation, rubber/plastic industrial emissions, biogenic source, and mixed industrial emissions. Furthermore, rubber/plastic industrial emissions, solvent usage, fuel evaporation, and vehicle exhaust were the most significant sources to O3 formation. Based on conditional bivariate probability function (CBPF), vehicle exhaust, fuel evaporation, and solvent usage were mainly local emissions, while other sources were mainly affected by regional transport. This study provides useful reference for research on the atmospheric photochemical formation of O3 and evidence for regional O3 reduction strategies.