Ozone Enhancement Research Articles

Elucidating key factors in regulating budgets of ozone and its precursors in atmospheric boundary layer

The vertical variations and key drivers of ozone and its precursors, namely NOx and VOCs, in the atmospheric boundary layer, have vital impacts on surface ozone budgets but are poorly understood so far. Using online gradient measurements from a 356 m tower, we obtained continuous vertical profiles of ozone and its precursors, which exhibited strong gradients throughout the day. In the daytime, the vertical gradients of ozone precursors are significantly regulated by reactions with OH radicals. At night, our observations confirmed more intense VOC reactions with NO3 radicals in the residual layer than in the boundary layer. Additionally, we found that residual layer entrainment could contribute to over half of the boundary-layer ozone enhancements in the morning periods. Our results underscore the importance of considering vertical changes of ozone and its precursors in the atmospheric boundary layer when developing future ozone mitigation strategies.

Diurnal hourly near-surface ozone concentration derived from geostationary satellite in China

Near-surface O3 is a harmful atmospheric pollutant and a key component of urban photochemical pollution. The availability of satellite ozone concentration products is predominantly restricted to daytime, resulting in a lack of understanding of nighttime ozone pollution (e.g. nocturnal ozone enhancement events). This research leverages 5-km bright temperatures derived from Advanced Himawari Images (AHI) on the Himawari-8 satellite, in conjunction with auxiliary data, to estimate 24-h near-surface O3 concentrations in China at a resolution of 5 km for 2020. The model achieves an average 5-fold cross-validation R2 of 0.92. Comparative analysis with on-site observations reveals that the model has low relative errors between 8:00 and 21:00 LT. The estimated O3 maps depict a consistent 24-h variation pattern, characterized by high and most fluctuating concentrations during the daytime, reaching a peak around 16:00 LT, which is primarily due to the increased photochemical reactions and the O3 accumulation in the mid-afternoon. In the daytime of summer, high surface ozone concentrations are mainly contributed by June. The elevated levels of O3 are predominantly found in central China, particularly in the Beijing-Tianjin-Hebei region and Inner Mongolia. It can also be seen that although the highest average daytime surface O3 concentration occurs in summer, the highest nighttime concentration is observed in spring, which may be attributed to the frequent occurrence of horizontal transport and vertical mixing of O3. This study holds promise in providing comprehensive round-the-clock surface O3 data across China, thereby enhancing our understanding of diurnal ground-level O3 variations.

Cause of nocturnal surface ozone enhancement in the North China Plain

The North China Plain (NCP), known for its dense population, extensive urbanization, and developed industry and agriculture, faces one of the foremost ozone (O3) pollution issues nationwide and even globally. Currently, most studies focus on daytime peak O3 levels, with insufficient understanding of the increase in nighttime O3 concentrations. Based on data from 204 national atmospheric composition monitoring sites in the NCP from 2015 to 2023, we investigated the characteristics of nocturnal surface O3 enhancement (NSOE) events and explored potential formation mechanisms. The mean annual frequencies of single-site and regional NSOE event in the NCP between 2015 and 2023 are 42 % and 21 %, respectively. The daytime peak O3 concentrations before and after NSOE events exceeded those during the corresponding periods of non-NSOE events by 84 ± 19 and 32 ± 15 μg/m3, respectively. The overall effect of the NSOE events was to decelerate the rate of decline in nighttime O3 concentrations and resulted in a reduction of NO2 and CO concentrations from 22:00 onwards. Low level jet (LLJ) and vertical mixing were the main factors affecting NSOE events in the NCP. The proportion of NSOE events affected by LLJ in four representative cities ranged from 57.6 % to 79.5 %. Furthermore, the high concentration of O3 in the residual layer before the NSOE event and the reduction of atmospheric stability during the NSOE event favored downward mixing of upper layer O3. The primary weather systems influencing the four most severe regional NSOE events were LLJ, typhoon, and cold fronts. The first two events were dominated by vertical mixing of O3, while the latter two events were mainly affected by horizontal transport. Our findings provide the first overview of NSOE events in the NCP from characteristics to mechanisms, emphasizing the necessity for future detailed studies based on nocturnal vertical O3 observations.

Beyond self-healing: stabilizing and destabilizing photochemical adjustment of the ozone layer

Abstract. The ozone layer is often noted to exhibit self-healing, whereby depletion of ozone aloft induces ozone increases below, explained as resulting from enhanced ozone production due to the associated increase in ultraviolet (UV) radiation below. Similarly, ozone enhancement aloft can reduce ozone below (reverse self-healing). This paper considers self-healing and reverse self-healing to manifest a general mechanism we call photochemical adjustment, whereby ozone perturbations lead to a downward cascade of anomalies in UV and ozone. Conventional explanations for self-healing imply that photochemical adjustment is stabilizing, damping perturbations towards the surface. However, photochemical adjustment can be destabilizing if the enhanced UV disproportionately increases the ozone sink, as can occur if the enhanced UV photolyzes ozone to produce atomic oxygen, which speeds up catalytic destruction of ozone. We analyze photochemical adjustment in two linear ozone models (Cariolle v2.9 and LINOZ), finding that (1) photochemical adjustment is destabilizing above 40 km in the tropical stratosphere and (2) self-healing often represents only a small fraction of the total photochemical stabilization. The destabilizing regime above 40 km is reproduced in a much simpler model: the Chapman cycle augmented with destruction of O and O3 by generalized catalytic cycles and transport (the Chapman+2 model). The Chapman+2 model reveals that photochemical destabilization occurs where the ozone sink is more sensitive than the source to perturbations in overhead column ozone, which is found to occur when the window of overlapping absorption by O2 and O3 is optically unsaturated, i.e., when overhead slant column ozone is below approximately 1018 molec. cm−2.

Exploring the long-term variations and high concentration episodes of peroxyacetyl nitrate in Megacity Seoul

Over the past few years, peroxyacetyl nitrate (PAN) has drawn significant attention as a key indicator of photochemical pollution owing to its intimate relationship with ozone and associated health effects. This study presents measurements conducted at the Korea University campus in Seoul during the high-ozone seasons from 2018 to 2021. PAN concentration was measured using fast gas chromatography with luminol chemiluminescence detection (GC-LCD), alongside measurements of O3, volatile organic compounds (VOCs), NO, NO2, and meteorological variables, including boundary layer height (BLH).The mean concentrations of PAN and O3 over the years were 0.56 ppbv and 35 ppbv in 2018, 1.29 ppbv and 58 ppbv in 2019, 0.21 ppbv and 50 ppbv 2020, and 0.53 ppbv and 46 ppbv in 2021, respectively. The annual variation observed in Seoul is consistent with trends seen in major cities worldwide during the COVID19 pandemic, reflecting a substantial reduction in urban emissions. Notably, the mean concentration of NOx and VOCs decreased significantly by more than 50 % and 25%, respectively, from 2019 to 2021.At temperatures above 30 °C, PAN decomposition was accelerated, decoupling a consistent positive relationship between PAN and O3 in 2020 and 2021. The results of a 0-D photochemical model (F0AM) calculation demonstrated that PAN formation primarily stems from anthropogenic VOCs, particularly > C2 alkenes. Elevated PAN concentrations during nighttime were attributed to boundary layer expansion and upper-air entrainment. Instances where PAN concentrations surged to at least 3 ppbv or higher in 2019 were attributed to biomass burning impacted air, as evidenced by concurrent elevations in K+ and OC in PM2.5, and O3.This study underscores the complex interplay of factors influencing PAN and ozone enhancements under decreased precursor levels, with an emphasis on dynamic change in the boundary layer, and long-distance transport of non-fossil sources during agricultural burning seasons.

Investigation into the nocturnal ozone in a typical industrial city in North China Plain, China

Ozone (O3) concentrations usually peak at midday by photochemical reactions and gradually decline after sunset due to chemical destruction and dry deposition. However, an increase in the frequency of elevated nocturnal ozone enhancement (NOE) and high nocturnal ozone value (HNOV) has been frequently observed in urban areas of eastern China, but the reasons are not well understood. In this study, taking a typical industrial city, ZiBo as a case study, we analyzed the trends, characteristics, and causes of the NOE and HNOV events in historical years by combining observations and model simulations. During the warm season (April–September) of 2017–2023, HNOV events are accompanied by low humidity, high temperature, large friction velocity, and a high boundary layer (52 days in total), whereas NOE events coincide with increases in humidity, wind speed, friction velocity, and boundary layer height (141 days in total). During the HNOV and NOE events, the nighttime average concentrations of Ox were 77 ± 7 and 12 ± 6 μg m−3 higher than the non-nocturnal O3 period, indicating enhanced atmospheric oxidizing capacity during nighttime. The modeling results indicate that both the HNOV and NOE events were mainly driven by vertical mixing and regional transport. We selected a typical period with high O3 pollution and frequent NOE and HNOV events to conduct the modeling study. Three typical nocturnal O3 events are identified: Case I was mainly driven by horizontal transport; while in the two subsequent cases, the vertical transport contribution was 80 μg m−3 h−1 (20:00 LT on June 21, 2021) and 35 μg m−3 h−1 (02:00 LT on June 26, 2021), respectively. Our study reveals that the O3 pollution in industrial cities has been extending to nighttime, primarily attributed to vertical mixing and horizontal transport within the boundary layer. This highlights the critical role of implementing regional joint control action to reduce primary emissions and eliminate residual ozone.

Machine-learning-based corrections of CMIP6 historical surface ozone in China during 1950–2014

Due to a lack of long-term observations in China, reports on historical ozone concentration are severely limited. In this study, by combining observation, reanalysis and model simulation data, XGBoost machine learning algorithm is used to correct the surface ozone concentration from CMIP6 climate model, and the long-term and large-scale surface ozone concentration of China during 1950–2014 is obtained. The long-term evolutions and trends of ozone and meteorological effects on interannual ozone variations are further analyzed. The results reveal that CMIP6 historical simulations have a large underestimation in ozone concentrations and their trends. The XGB-derived ozone are closer to observations, with R2 value of 0.66 and 0.74 for daily and monthly retrievals, respectively. Both the concentrations and exceedances of ozone in most parts of China have shown increasing trends from 1950 to 2014. The daily mean ozone concentration without climate change effects is estimated to be 117 ppb in the year 1950 averaged over China. It indicates that the increase in anthropogenic emissions of China has a significant contribution to ozone enhancement between 1950 and 2014. The higher ozone growth rates of XGB retrievals than those from the model indicate a regional surface ozone penalty due to the warming climate. The relatively significant increment in ozone are estimated in the Central and Western China. Seasonally, the ozone enhancement is largest in spring, indicating a shift in seasonal variation of ozone. Given the uncertainty in simulating historical ozone by climate model, we show that machine learning approaches can provide improved assessment of evolution in surface ozone, along with valuable information to guide future model development and formulate future ozone pollution prevention and control policies.

The Spillover of Tropospheric Ozone Increases Has Hidden the Extent of Stratospheric Ozone Depletion by Halogens

AbstractStratospheric ozone depletion from halocarbons is partly countered by pollution‐driven increases in tropospheric ozone, with transport connecting the two. While recognizing this connection, the ozone assessment's evaluation of observations and processes have often split the chapters at the tropopause boundary. Using a chemistry‐transport model we find that air‐pollution ozone enhancements in the troposphere spill over into the stratosphere at significant rates, that is, 13%–34% of the excess tropospheric burden appears in the lowermost extra‐tropical stratosphere. As we track the anticipated recovery of the observed ozone depletion, we should recognize that two tenths of that recovery may come from the transport of increasing tropospheric ozone into the stratosphere.

Energetic particle precipitation influences global secondary ozone distribution

The secondary ozone layer is a global peak in ozone abundance in the upper mesosphere-lower thermosphere (UMLT) around 90-95 km. The effect of energetic particle precipitation (EPP) from geomagnetic processes on this UMLT ozone remains largely unexplored. In this research we investigated how the secondary ozone responds to EPP using satellite observations. In addition, the residual Mean Meridional Circulation (MMC) derived from model simulations and the atomic oxygen [O], atomic hydrogen [H], temperature measurements from satellite observations were used to characterise the residual circulation changes during EPP events. We report regions of secondary ozone enhancement or deficit across low, mid and high latitudes as a result of global circulation and transport changes induced by EPP. The results are supported by a sensitivity test using an empirical model.

Deciphering multi-temporal scale dynamics in the concentration, sources and processes of near surface ozone over different climatic regions of India.

This study aims to investigate the factors influencing seasonal and long-term (2003-2021) changes in the near surface ozone (850 hpa) concentrations over different climatic sub-regions of India. Detailed comparison of daily (2019-2021) near surface ozone values of ERA-5 and CAAQMS (Continuous Ambient Air Quality Monitoring Stations) ground-based measurements revealed that ERA-5 is temporally in phase with CAAQMS measurements falling indifferent climatic sub-regions of India. ERA-5 near surface ozone shows statistically significant long-term (2003-2021) positive trends [2-4 percent per decade (ppd)] over most of the climatic sub-regions, over Indo-Gangetic Planes (IGPs), Southern and Central India trends are particularly strong. Trends were also estimated for each season separately, which were largely positive (2-6 ppd) over Central and Southern India in the Autumn and Winter seasons. Extensive climatological analysis reveals that the reversal of winds in the Indian monsoonal system plays a vital role in such trend patterns across the Indian subcontinent. South-westerly winds from June through September presumably bring ozone deficit air of marine origin, thus causing a dilution effect while the North-easterly winds during late Autumn and early Winters plausibly bring ozone-rich air from the stratospheric-tropospheric efflux dominated Himalayan region. It allows near surface ozone enhancement over Central and Southern India. Seasonal Principal component analysis (PCA) revealed that precursor gases (CH4 and NO2) and climatic variables especially specific humidity (SH) are the primary drivers of near surface ozone variability in the Winter season, while in Spring, climatic variables like boundary layer height (BLH), temperature (T) and SH have a significant role. Principal component regression (PCR) reveals a long-term increase in near surface ozone levels mostly dominated by precursor concentration over IGPs and Southern sub-regions. Whereas, BLH, T and SH significantly explain near surface ozone trends over North-eastern and Coastal India.

Impact of stratospheric intrusions on surface ozone enhancement in Hong Kong in the lower troposphere: Implications for ozone control strategy

Understanding the impact of stratospheric intrusion (SI) is crucial for elucidating atmospheric complexities and implications for ozone (O3) control. However, current studies have not focused on the influence of SI on surface O3, thus limiting the effectiveness of control strategies. This study delves into an SI event that occurred from March 5 to 12, 2022, employing a comprehensive integration of the Weather Research and Forecasting model coupled with Chemistry and multi-reanalysis data. The lower tropospheric O3 enhancement in Hong Kong has been noticed, despite the absence of SIs in the upper troposphere. This study employed a comprehensive methodology, integrating the use of a box model to estimate the stratosphere–troposphere exchange (STE) O3 flux, integrated process rate (IPR) analysis to quantify the contributions from individual physical and chemical processes, and tracer methods to detect stratospheric O3. During the deep SI episode, the STE flux peaked at −81.33 × 10−8 kg m−2·s−1, surpassing the monthly average by 15.2-fold. The IPR results indicate that vertical transport within the 0–16 km range contributes between 18.4 and 39.0 ppbv h−1 during the SI event, whereas horizontal advection shows a negative contribution. Stratospheric O3 tagging revealed that SI contributes 15.5–31.3 ppb (29.6–50.2%, respectively, of surface O3) when stratospheric O3 mixes down. Five emission reduction paths were designed in response to the negative impacts of SI. The “anthropogenic VOC (AVOC) only” path was the most efficient; however, when SI contributed 31 ppb on March 9, the “NOx only” path required a 69% reduction, while the “AVOC only” path required an 85% reduction for efficiency. This research not only elucidates the complex interplay between SI and surface O3 concentrations but also emphasizes the significance of refining existing emission reduction paths.

A Comparative Investigation of the Characteristics of Nocturnal Ozone Enhancement Events and Their Effects on Ground-Level Ozone and PM2.5 in the Central City of the Yellow River Delta, China, in 2022 and 2023

In recent years, nocturnal ozone enhancement (NOE) events have emerged as a prominent research focus in the field of the atmospheric environment. By using statistical analysis methods, we conducted a comparative investigation of nocturnal ozone concentrations and NOE events in Dongying, the central city of the Yellow River Delta, China, in 2022 and 2023, and further explored the effects of NOE events on O3 and PM2.5 on the same night and the subsequent day. The results showed that from 2022 to 2023, in Dongying, the annual average nocturnal ozone concentrations increased from 51 μg/m3 to 59 μg/m3, and the frequency of NOE events was higher in the spring, summer, and autumn, and lower in the winter. The NOE events not only exhibited promoting effects on nocturnal O3 and Ox, and on the daily maximum 8 h average concentration of O3 (MDA8-O3) on the same day (comparatively noticeable in summer and autumn), but also demonstrated a clear impact on nocturnal PM2.5 and PM2.5-bounded NO3− and SO42− (especially in winter). Additionally, the NOE events also led to higher concentrations of O3 and Ox, as well as higher MDA8-O3 levels during the subsequent day, with more observable impacts in the summer. The results could strengthen our understanding about NOE events and provide a scientific basis for the collaborative control of PM2.5 and O3 in urban areas in the Yellow River Delta in China.

Temperature-Dependent Evaporative Anthropogenic VOC Emissions Significantly Exacerbate Regional Ozone Pollution.

The evaporative emissions of anthropogenic volatile organic compounds (AVOCs) are sensitive to ambient temperature. This sensitivity forms an air pollution-meteorology connection that has not been assessed on a regional scale. We parametrized the temperature dependence of evaporative AVOC fluxes in a regional air quality model and evaluated the impacts on surface ozone in the Beijing-Tianjin-Hebei (BTH) area of China during the summer of 2017. The temperature dependency of AVOC emissions drove an enhanced simulated ozone-temperature sensitivity of 1.0 to 1.8 μg m-3 K-1, comparable to the simulated ozone-temperature sensitivity driven by the temperature dependency of biogenic VOC emissions (1.7 to 2.4 μg m-3 K-1). Ozone enhancements driven by temperature-induced AVOC increases were localized to their point of emission and were relatively more important in urban areas than in rural regions. The inclusion of the temperature-dependent AVOC emissions in our model improved the simulated ozone-temperature sensitivities on days of ozone exceedance. Our results demonstrated the importance of temperature-dependent AVOC emissions on surface ozone pollution and its heretofore unrepresented role in air pollution-meteorology interactions.

Nocturnal ozone enhancement in Shandong Province, China, in 2020–2022: Spatiotemporal distribution and formation mechanisms

Nighttime ozone enhancement (NOE) can increase the oxidation capacity of the atmosphere by stimulating nitrate radical formation and subsequently facilitating the formation of secondary pollutants, thereby affecting air quality in the following days. Previous studies have demonstrated that when nocturnal ozone (O3) concentrations exceed 80 μg/m3, it leads to water loss and reduction of plant yields. In this study, the characteristics and mechanisms of NOE over Shandong Province as well as its 16 cities were analyzed based on observed hourly O3 concentrations from 2020 to 2022. The analysis results show that NOE predominantly occurred in the periods of 0:00–3:00 (41 %). The annual mean frequency of NOE events was ~64 days/year, approximately 4–7 days per month. The average concentration of nocturnal O3 peak (NOP) was ~72.6 μg/m3. Notably, high NOP was observed in the period from April to September with the maximum in June. Coastal cities experienced more NOE events. Typical NOE events characterized by high NOP concentrations in the coastal cities of QingDao, WeiHai and YanTai in June 2021 were selected for detailed analysis with a regional chemical transport model. The results showed that high levels of O3 in eastern coastal cities during NOE events primarily originate from horizontal transport over the sea, followed by vertical transport. During the daytime, O3 and its precursors are transported to the Yellow Sea by westerly winds, leading to the accumulation of O3 near the sea and coastline. Consequently, under the influence of prevailing winds, the movement of O3 pollution belts from the sea to land causes rapid increases in near-surface O3 levels. Meanwhile, vertical transport can also contribute to NOE in coastal areas. The high-level O3 in the upper atmosphere generally originates from long-distance transport and turbulent transport of O3 produced near the ground during the daytime. At night, the absence of chemicals that consume O3 in the upper air and descending air flow carries O3 to the near-surface. The impacts of other O3-depletion processes (such as dry deposition) on NOE are less pronounced than those of transport processes.

Secondary aerosol formation during a special dust transport event: impacts from unusually enhanced ozone and dust backflows over the ocean

Abstract. In the autumn of 2019, a 5 d long-lasting dust event was observed using a synergy of field measurement techniques in Shanghai. This particular dust event stood out from others due to its unique characteristics, including low wind speed, high relative humidity, elevated levels of gaseous precursors, and contrasting wind patterns at different altitudes. During this event, three distinct dust stages were identified. The first stage was a typical dust invasion characterized by high concentrations of particulate matters but relatively short duration. In contrast, the second stage exhibited an unusual enhancement of ozone, attributed to compound causes of a weak synoptic system, transport from the ocean, and subsidence of high-altitude ozone downdrafted by dust. Consequently, gas-phase oxidation served as the major formation pathway of sulfate and nitrate. In the third stage of dust, a noteworthy phenomenon known as dust backflow occurred. The dust plume originated from the Shandong Peninsula and slowly drifted over the Yellow Sea and the East China Sea before eventually returning to Shanghai. Evidence of this backflow was found through the enrichment of marine vessel emissions (V and Ni) and increased solubility of calcium. Under the influence of humid oceanic breezes, the formation of nitrate was dominated by aqueous processing. Additionally, parts of nitrate and sulfate were directly transported via sea salts, evidenced by their co-variation with Na+ and confirmed through thermodynamic modeling. The uptake of NH3 on particles, influenced by the contributions of alkali metal ions and aerosol pH, regulated the formation potential of secondary aerosol. By developing an upstream–receptor relationship method, the quantities of transported and secondarily formed aerosol species were separated. This study highlights that the transport pathway of dust, coupled with environmental conditions, can significantly modify the aerosol properties, especially at the complex land–sea interface.

Investigation of the summer 2018 European ozone air pollution episodes using novel satellite data and modelling

Abstract. In the summer of 2018, Europe experienced an intense heatwave which coincided with several persistent large-scale ozone (O3) pollution episodes. Novel satellite data of lower-tropospheric column O3 from the Global Ozone Monitoring Experiment-2 (GOME-2) and Infrared Atmospheric Sounding Interferometer (IASI) on the MetOp satellite showed substantial enhancements in 2018 relative to other years since 2012. Surface observations also showed ozone enhancements across large regions of continental Europe in summer 2018 compared to 2017. Enhancements to surface temperature and the O3 precursor gases carbon monoxide and methanol in 2018 were co-retrieved from MetOp observations by the same scheme. This analysis was supported by the TOMCAT chemistry transport model (CTM) to investigate processes driving the observed O3 enhancements. Through several targeted sensitivity experiments we show that meteorological processes, and emissions to a secondary order, were important for controlling the elevated O3 concentrations at the surface. However, mid-tropospheric (∼ 500 hPa) O3 enhancements were dominated by meteorological processes. We find that contributions from stratospheric O3 intrusions ranged between 15 %–40 %. Analysis of back trajectories indicates that the import of O3-enriched air masses into Europe originated over the North Atlantic, substantially increasing O3 in the 500 hPa layer during summer 2018.

Nighttime ozone in the lower boundary layer: insights from 3-year tower-based measurements in South China and regional air quality modeling

Abstract. Nighttime ozone in the lower boundary layer regulates atmospheric chemistry and surface ozone air quality, but our understanding of its vertical structure and impact is largely limited by the extreme sparsity of direct measurements. Here we present 3-year (2017–2019) measurements of ozone in the lower boundary layer (up to 500 m) from the Canton Tower in Guangzhou, the core megacity in South China, and interpret the measurements with a 1-month high-resolution chemical simulation from the Community Multiscale Air Quality (CMAQ) model. Measurements are available at 10, 118, 168, and 488 m, with the highest (488 m) measurement platform higher than the typical height of the nighttime stable boundary layer that allows direct measurements of ozone in the nighttime residual layer (RL). We find that ozone increases with altitude in the lower boundary layer throughout the day, with a vertical ozone gradient between the 10 and 488 m heights (ΔO3/ΔH10–488 m) of 3.6–6.4 ppbv hm−1 in nighttime and 4.4–5.8 ppbv hm−1 in daytime. We identify a high ozone residual ratio, defined as the ratio of ozone concentration averaged over nighttime to that in the afternoon (14:00–17:00 LT), of 69 %–90 % in January, April, and October, remarkably higher than that in the other three layers (29 %–51 %). Ozone in the afternoon convective mixing layer provides the source of ozone in the RL, and strong temperature inversion facilitates the ability of RL to store ozone from the daytime convective mixing layer. The tower-based measurement also indicates that the nighttime surface Ox (Ox= O3+NO2) level can be an effective indicator of RL ozone if direct measurement is not available. We further find significant influences of nocturnal RL ozone on both the nighttime and the following day's daytime surface ozone air quality. During the surface nighttime ozone enhancement (NOE) event, we observe a significant decrease in ozone and an increase in NO2 and CO at the 488 m height, in contrast to their changes at the surface, a typical feature of enhanced vertical mixing. The enhanced vertical mixing leads to an NOE event by introducing ozone-rich and NOx-poor air into the RL to enter the nighttime stable boundary layer. The CMAQ model simulations also demonstrate an enhanced positive contribution of vertical diffusion (ΔVDIF) to ozone at the 10 and 118 m heights and a negative contribution at the 168 and 488 m heights during the NOE event. We also observe a strong correlation between nighttime RL ozone and the following day's surface maximum daily 8 h average (MDA8) ozone. This is tied to enhanced vertical mixing with the collapse of nighttime RL and the development of a convective mixing layer, which is supported by the CMAQ diagnosis of the ozone budget, suggesting that the mixing of ozone-rich air from nighttime RL downward to the surface via the entrainment is an important mechanism for aggravating ozone pollution the following day. We find that the bias in CMAQ-simulated surface MDA8 ozone the following day shows a strong correlation coefficient (r= 0.74) with the bias in nighttime ozone in the RL, highlighting the necessity to correct air quality model bias in the nighttime RL ozone for accurate prediction of daytime ozone. Our study thus highlights the value of long-term tower-based measurements for understanding the coupling between nighttime ozone in the RL, surface ozone air quality, and boundary layer dynamics.

Soil Emissions of Reactive Nitrogen Accelerate Summertime Surface Ozone Increases in the North China Plain.

Summertime surface ozone in China has been increasing since 2013 despite the policy-driven reduction in fuel combustion emissions of nitrogen oxides (NOx). Here we examine the role of soil reactive nitrogen (Nr, including NOx and nitrous acid (HONO)) emissions in the 2013-2019 ozone increase over the North China Plain (NCP), using GEOS-Chem chemical transport model simulations. We update soil NOx emissions and add soil HONO emissions in GEOS-Chem based on observation-constrained parametrization schemes. The model estimates significant daily maximum 8 h average (MDA8) ozone enhancement from soil Nr emissions of 8.0 ppbv over the NCP and 5.5 ppbv over China in June-July 2019. We identify a strong competing effect between combustion and soil Nr sources on ozone production in the NCP region. We find that soil Nr emissions accelerate the 2013-2019 June-July ozone increase over the NCP by 3.0 ppbv. The increase in soil Nr ozone contribution, however, is not primarily driven by weather-induced increases in soil Nr emissions, but by the concurrent decreases in fuel combustion NOx emissions, which enhance ozone production efficiency from soil by pushing ozone production toward a more NOx-sensitive regime. Our results reveal an important indirect effect from fuel combustion NOx emission reduction on ozone trends by increasing ozone production from soil Nr emissions, highlighting the necessity to consider the interaction between anthropogenic and biogenic sources in ozone mitigation in the North China Plain.

The impact of COVID-19 lockdowns on urban photochemistry as inferred from TROPOMI

Movement restrictions were imposed in 2020 to mitigate the spread of Covid-19. These lock-down episodes provide a unique opportunity to study the sensitivity of urban photochemistry to temporary emission reductions and test air quality models. This study uses Tropospheric Monitoring Instrument (TROPOMI) nitrogen dioxide/carbon monoxide (NO2/CO) ratios in urban plumes in combination with an exponential fitting procedure to infer changes in the NOx lifetime (τNOx) during Covid-19 lock-downs in the cities of Denver, Chicago, New York, Riyadh, Wuhan and Sao Paulo compared with the year before.The strict lockdown policy in Wuhan led to a 65–80% reduction in NO2, compared to 30–50% in the other cities that were studied. In New York and Wuhan, CO concentration was reduced by 10–15%, whereas over Riyadh, Denver, Chicago, and Sao Paulo the CO background concentration increased by 2–5 ppb. τNOx has been derived for calm (0.0 < U (m/s) < 3.5) and windy (5.0 < U (m/s) < 8.5) days to study the influence of wind speed. We find reductions in τNOx during Covid-19 lockdowns in all six megacities during calm days. The largest change in τNOx during calm days is found for Sao-Paulo (31.8 ± 9.0%), whereas the smallest reduction is observed over Riyadh (22 ± 6.6%). During windy days, reductions in τNOx are observed during Covid-19 lockdowns in New York and Chicago. However, over Riyadh τNOx is almost similar for windy days during the Covid-19 lockdown and the year before.Ground-based measurements and the Chemistry Land-surface Atmosphere Soil Slab (CLASS) model have been used to validate the TROPOMI-derived results over Denver. CLASS simulates an enhancement of ozone (O3) by 4 ppb along with reductions in NO (38.7%), NO2 (25.7%) and CO (17.2%) during the Covid-19 lockdown in agreement with the ground-based measurements. In CLASS, decreased NOx emissions reduce the removal of OH in the NO2 + OH reaction, leading to higher OH concentrations and decreased τNOx. The reduction in τNOx inferred from TROPOMI (28 ± 9.0%) is in agreement with CLASS. These results indicate that TROPOMI derived NO2/CO ratios provide useful information about urban photochemistry and that changes in photochemical lifetimes can successfully be detected.

Targeted Balloon Observations of Stratosphere‐To‐Troposphere Transport From a Mesoscale Convective System

AbstractStratosphere‐troposphere exchange (STE) is important to the global radiation budget because it is capable of inducing significant changes in trace greenhouse gases such as ozone and water vapor. In the extratropics, stratosphere‐to‐troposphere—or downward—transport (STT) dominates and is routinely driven by upper‐troposphere jet circulations and cyclones. Tropopause‐overshooting convection provides a more rapid STE pathway, but STT in convection is less understood and targeted observations are limited to a handful of fortuitous aircraft encounters. In this study, we seek to improve understanding of STT near thunderstorm anvils using balloon observations of ozone, ground‐based radar observations, visible satellite imagery, and model output. Radar and satellite imagery are used to identify and ultimately target convection driving the “ozone‐wrapping” process. Herein, observations from a single mesoscale convective system in central Oklahoma are reported. A significant ozone enhancement 40%–60% greater than background concentrations in the free troposphere was observed during the balloon ascent underneath and within the anvil of the storm. Backwards particle trajectories were calculated to rule out other sources of this feature such as long‐range transport of stratospheric air and smoke pollution from nearby wildfires. To our knowledge, this work presents the first targeted ozonesonde launch and sampling of this STT process.