Surface Ozone Trends Research Articles

Changes in South American surface ozone trends: exploring the influences of precursors and extreme events

Abstract. In this study, trends of 21st-century ground-level ozone and ozone precursors were examined across South America, a less-studied region where trend estimates have rarely been comprehensively addressed. Therefore, we provided an updated regional analysis based on validated surface observations. We tested the hypothesis that the recent increasing ozone trends, mostly in urban environments, resulted from intense wildfires driven by extreme meteorological events impacting cities where preexisting volatile organic compound (VOC)-limited regimes dominate. We applied the quantile regression method based on monthly anomalies to estimate trends, quantify their uncertainties and detect trend change points. Additionally, the maximum daily 8 h average (MDA8) and peak-season metrics were used to assess short- and long-term exposure levels, respectively, for the present day (2017–2021). Our results showed lower levels in tropical cities (Bogotá and Quito), varying between 39 and 43 nmol mol−1 for short-term exposure and between 26 and 27 nmol mol−1 for long-term exposure. In contrast, ozone mixing ratios were higher in extratropical cities (Santiago and São Paulo), with a short-term exposure level of 61 nmol mol−1 and long-term exposure levels varying between 40 and 41 nmol mol−1. Santiago (since 2017) and São Paulo (since 2008) exhibited positive trends of 0.6 and 0.3 nmol mol−1 yr−1, respectively, with very high certainty. We attributed these upward trends, or no evidence of variation, such as in Bogotá and Quito, to a well-established VOC-limited regime. However, we attributed the greater increase in the extreme percentile trends (≥ 90th) to heat waves and, in the case of southwestern South America, to wildfires associated with extreme meteorological events.

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.

Trends of surface ozone based on hourly concentrations in the Beijing–Tianjin–Hebei region during 2017–2021

As a typical secondary air pollutant, surface ozone has been monitored routinely since 2013 in China. Most studies on the spatiotemporal variation of ozone have been centered around the daily maximum 8-h average, with little attention paid to the trends of hourly ozone, especially hourly ozone exceedances. Focusing on hourly ozone exceedances and peak values, the spatiotemporal trends of hourly ozone at 77 sites in 13 cities of the Beijing–Tianjin–Hebei (BTH) region during 2017–2021 were analyzed in this study. The number of hours with exceedances (NH200) in 2019 was nearly three times that of 2021. On a five-year average, the percentage of cumulative NH200 in June accounted for up to 40.5 % of all hourly exceedances. Cities in central Hebei Province had the highest cumulative annual NH200. June had the highest average hourly ozone exceeded multiples of 0.158. The top two cities with the highest average exceeded multiple were Tangshan (0.166) and Beijing (0.158). Tangshan and Xingtai ranked as the top two in terms of the mean of the 10 highest daily maximum ozone concentrations (MTDM), with 286.74 and 285.37 µg m−3, respectively. The gap between the MTDM and the daily maximum of hourly ozone averaged over all sites had narrowed to 97.88 µg m−3 in 2021, much lower than that in other years, which indicated that the stability and convergence of ozone pollution in BTH region had been enhanced in 2021 to some extent.摘要本文以京津冀地区为研究对象, 采用国际常用的评价指标, 在分析2017–2021年臭氧小时浓度变化趋势的基础上, 重点讨论了小时浓度超标情况及其峰值水平. 结果显示, 2019年臭氧小时浓度超标数约为2021年的3倍, 6月份小时浓度超标数占比高达40.5%, 河北中部城市小时浓度超标情况突出; 2017年和2019年臭氧小时浓度超标倍数在高位区间的占比相对最高, 唐山和邢台的小时浓度峰值水平相对较高. 本文可为后续进一步研究局地臭氧污染特征和异常污染事件提供基础参考.

Comprehensive analysis of long-term trends, meteorological influences, and ozone formation sensitivity in the Jakarta Greater Area

Jakarta Greater Area (JGA) has encountered recurrent challenges of air pollution, notably, high ozone levels. We investigate the trends of surface ozone (O3) changes from the air quality monitoring stations and resolve the contribution of meteorological drivers in urban Jakarta (2010–2019) and rural Bogor sites (2017–2019) using stepwise Multi Linear Regression. During 10 years of measurement, 41% of 1-h O3 concentrations exceeded Indonesia’ s national threshold in Jakarta. In Bogor, 0.1% surpassed the threshold during 3 years of available data records. The monthly average of maximum daily 8-h average (MDA8) O3 anomalies exhibited a downward trend at Jakarta sites while increasing at the rural site of Bogor. Meteorological and anthropogenic drivers contribute 30% and 70%, respectively, to the interannual O3 anomalies in Jakarta. Ozone formation sensitivity with satellite demonstrates that a slight decrease in NO2 and an increase in HCHO contributed to declining O3 in Jakarta with 10 years average of HCHO to NO2 ratio (FNR) of 3.7. Conversely, O3 increases in rural areas with a higher FNR of 4.4, likely due to the contribution from the natural emission of O3 precursors and the influence of meteorological factors that magnify the concentration.

Surface ozone trends reversal for June and December in an Atlantic natural coastal environment.

Surface ozone and temperature trends were investigated using records from 2000 to 2021 in Southwestern Europe, at El Arenosillo observatory, focusing on June and December. The ozone trends for daily percentiles were increasing in June for lower percentiles (2.5 ± 1.2ppb decade-1 for the 5th percentile) and decreasing for higher (- 2.2 ± 1.4ppb decade-1 for the 95th percentile); in December, the trends were growing in the entire range of percentiles, with a peak of 2.2 ± 0.8ppb decade-1. A declining trend was obtained for the geopotential height at the pressure level of 850hPa (Z850) in June while highlighting the upward trend in December (26.3 ± 6.5m decade-1). The hourly trends for ozone and temperature were also explored in these months. In June, the nocturnal ozone trends were growing (4.0 ± 1.2ppb decade-1 or 10% decade-1 at 8:00 UTC) associated with temperature rises while in the daytime, a decrease in temperature was observed along with an ozone decreasing trend (- 2.6 ± 1.6ppb decade-1 or - 5% decade-1 at 18:00 UTC). Hourly ozone and temperature trends in December were increasing with peaks of 3.0 ± 0.9ppb decade-1 (~ 8% decade-1) at 12:00 UTC and 1.6 ± 0.3°C decade-1 at 19:00 UTC. Two representative scenarios of these months were studied. The ozone decreases in June could be associated with several factors, decreasing in temperatures and a possible weakening of the anticyclonic conditions leading to changes in the mesoscale processes' development. The strengthening of the Azores anticyclone in December could be enhancing the upward ozone trend observed. It is unknown whether the reversal ozone pattern trends found in this region are a local phenomenon; although we suggest that it could be happening on a larger scale as well, future studies should be carried out.

Influence of stratosphere-troposphere exchange on long-term trends of surface ozone in CMIP6

Surface ozone, which is a major air pollutant, has gained widespread public attention due to its significant harm to human health and ecosystems. Here we find that long-term trends of surface ozone are largely influenced by stratosphere-troposphere exchange (STE) using the historical simulations in the CMIP6 multi-model ensemble. It is found that STE contributes up to 39.5% of climatological surface ozone in the extratropics of the winter hemisphere. In the Southern Hemisphere (SH), prominent stratospheric ozone depletion tends to reduce tropospheric and surface ozone due to STE over 1960–1997, which has been reversed during 1997–2014. In the Northern Hemisphere (NH), STE enhances surface ozone during both ozone depletion and recovery periods. About 29.5% of the positive trends of surface ozone over 1960–2014 can be attributed to STE processes in boreal winter in the NH extratropics. The enhancement of STE is related to the changes in atmospheric circulation associated with global warming. Our results suggest that STE will further worsen surface ozone pollution in the future associated with the stratospheric ozone recovery and global warming.

Changes in surface ozone in South Korea on diurnal to decadal timescales for the period of 2001–2021

Abstract. Several studies have reported an increasing trend of surface ozone in South Korea over the past few decades, using different measurement metrics. In this study, we examined the surface ozone trends in South Korea by analyzing the hourly or daily maximum 8 h average ozone concentrations (MDA8) measured at the surface from 2001 to 2021. We studied the diurnal, seasonal, and multi-decadal variations of these parameters at city, province, and background sites. We found that the fourth-highest MDA8 values exhibited positive trends in seven cities, nine provinces, and two background sites from 2001 to 2021. For the majority of sites, there was an annual increase of approximately 1–2 ppb. After early 2010, all sites consistently recorded MDA8 values exceeding 70 ppb, despite reductions in precursor pollutants such as NO2 and CO. The diurnal and seasonal characteristics of ozone exceedances, defined as the percentage of data points with hourly ozone concentrations exceeding 70 ppb, differed between the Seoul Metropolitan Area (SMA) and the background sites. In the SMA, the exceedances were more prevalent during summer compared to spring, whereas the background sites experienced higher exceedances in spring than in summer. This indicates the efficient local production of ozone in the SMA during summer and the strong influence of long-range transport during spring. The rest of the sites showed similar exceedance patterns during both spring and summer. The peak exceedances occurred around 16:00–17:00 in the SMA and most locations, while the background sites primarily recorded exceedances throughout the night. During the spring of the COVID-19 pandemic (2020–2021), ozone exceedances decreased at most locations, potentially due to significant reductions in NOx emissions in South Korea and China compared to the period of 2010–2019. The largest decreases in exceedances were observed at the background sites during spring. For instance, in Gosung, Gangwondo (approximately 600 m above sea level), the exceedances dropped from 30 % to around 5 % during the COVID-19 pandemic. Regional model simulations confirmed the concept of decreased ozone levels in the boundary layer in Seoul and Gangwon-do in response to emission reductions. However, these reductions in ozone exceedances were not observed in major cities and provinces during the summer of the COVID-19 pandemic, as the decreases in NOx emissions in South Korea and China were much smaller compared to spring. This study highlights the distinctions between spring and summer in the formation and transport of surface ozone in South Korea, emphasizing the importance of monitoring and modeling specific processes for each season or finer timescales.

Investigation of the multi-year trend of surface ozone and ozone-precursor relationship in Hong Kong

As a highly urbanized city in East Asia, Hong Kong has faced severe photochemical ozone (O3) pollution for years. A comprehensive understanding of pollution characteristics is crucial for effective mitigation of the O3 problem. This study systematically investigated the multi-year trends of surface O3 over the last decade using measurements from 11 selected Air Quality Monitoring Stations (AQMS). The analysis reveals upward trends in the annual average and daily maximum 8-h average (DMA8) O3 from 2011 to 2022, with rates of 0.34 ppb year−1 (p < 0.05) and 0.22 ppb year−1 (p = 0.19), respectively. Though upward trends of DMA8 O3 were observed at urban stations (0.10–0.88 ppb year−1), the significant decreases in NO2 levels at all stations (−0.18 to −1.75 ppb year−1) led to a decline of total oxidants (Ox, O3+NO2). In addition to the daytime O3 peak increase, a clear nighttime O3 peak was present with a significant increasing trend, which was associated with the regional transport of O3-rich plumes and weakened NO titration effect. The relative contributions of local production and regional transport were investigated, and results indicated that local control efforts have predictably alleviated the locally produced Ox, while the regional background Ox still contributes significantly but appears to decline. Furthermore, an observation-based model was employed to examine the evolution of O3-precursors relationships. The results indicate that O3 formation was still VOC-limited in Hong Kong, and the suppressive effect of NOx on O3 formation has decreased in recent years. In terms of abatement strategies, apart from aromatics, we emphasize the significance of carbonyl compounds due to their high reactivity to O3 production.

Surface ozone trends and related mortality across the climate regions of the contiguous United States during the most recent climate period, 1991–2020

In this study, we leverage multiple linear regression and quantile regression combined with a novel deep learning tool (SHapley Additive exPlanations) to isolate the impact of meteorology on surface ozone pollution and to assess the effectiveness of emission reduction measures across the Contiguous United States (US) during the latest climate period (1991–2020). The findings demonstrate that all regions except the Northern Rockies and the Southwest experienced decreasing trends in median values during the warm season, with rural stations in the Southeast and urban stations in the Northeast experiencing the greatest declines of −1.29 ± 0.07 and −0.85 ± 0.08 ppb.a−1, respectively. Similar to the original data, the median values of adjusted MDA8 (Maximum Daily 8-h Average) ozone show negative trends in all regions except for Southwest urban stations, with the highest recorded in rural stations of the Southeast (−1.13 ± 0.05 ppb.a−1) and urban stations of the Northeast (−0.79 ± 0.06 ppb.a−1). In addition, the 95th percentile values of original and adjusted MDA8 ozone decreased in all regions in which Northeast urban stations had the greatest reduction (original: 3.53 ± 0.29 ppb.a−1, adjusted: 2.96 ± 0.27 ppb.a−1). Our results suggest that meteorological inter-annual variability reduces the ozone burden during the warm season in the eastern US and southern California; at the same time, it contributes to increased ozone pollution in the central US, Southwest, and northern California, indicating that efforts to reduce air pollution may be hindered by climate change. Our analysis of the impact of short-term exposure to ozone on health shows that the South was the most positively impacted by emission control policies implemented after 2000, and the Northeast had the highest number of prevented deaths (30.45 deaths prevented/million people) resulting from respiratory diseases. The results of this study should benefit air quality managers and policymakers, particularly in their efforts to update ozone mitigation strategies.

First long-term surface ozone variations at an agricultural site in the North China Plain: Evolution under changing meteorology and emissions

Significant upward trends in surface ozone (O3) have been widely reported in China during recent years, especially during warm seasons in the North China Plain (NCP), exerting adverse environmental effects on human health and agriculture. Quantifying long-term O3 variations and their attributions helps to understand the causes of regional O3 pollution and to formulate according control strategy. In this study, we present long-term trends of O3 in the warm seasons (April–September) during 2006–2019 at an agricultural site in the NCP and investigate the relative contributions of meteorological and anthropogenic factors. Overall, the maximum daily 8-h average (MDA8) O3 exhibited a weak decreasing trend with large interannual variability. < 6 % of the observed trend could be explained by changes in meteorological conditions, while the remaining 94 % was attributed to anthropogenic impacts. However, the interannual variability of warm season MDA8 O3 was driven by both meteorology (36 ± 28 %) and anthropogenic factors (64 ± 27 %). Daily maximum temperature was the most essential factor affecting O3 variations, followed by ultraviolet radiation b (UVB) and boundary layer height (BLH), with rising temperature trends inducing O3 inclines throughout April to August, while UVB mainly influenced O3 during summer months. Under changes in emissions and air quality, warm season O3 production regime gradually shifted from dominantly VOCs-limited during 2006–2015 to NOx-limited afterwards. Relatively steady HCHO and remarkably rising NOx levels resulted in the fast decreasing MDA8 O3 (−2.87 ppb yr−1) during 2006–2012. Rapidly decreasing NOx, flat or slightly increasing HCHO promoted O3 increases during 2012–2015 (9.76 ppb yr−1). While afterwards, slow increases in HCHO and downwards fluctuating NOx led to decreases in MDA8 O3 (−4.97 ppb yr−1). Additionally, continuous warming trends might promote natural emissions of O3 precursors and magnify their impacts on agricultural O3 by inducing high variability, which would require even more anthropogenic reduction to compensate for.

Quantifying the drivers of surface ozone anomalies in the urban areas over the Qinghai-Tibet Plateau

Abstract. Improved knowledge of the chemistry and drivers of surface ozone over the Qinghai-Tibet Plateau (QTP) is significant for regulatory and control purposes in this high-altitude region in the Himalayas. In this study, we investigate the processes and drivers of surface ozone anomalies (defined as deviations of ozone levels relative to their seasonal means) between 2015 and 2020 in urban areas over the QTP. We separate quantitatively the contributions of anthropogenic emissions and meteorology to surface ozone anomalies by using the random forest (RF) machine-learning model-based meteorological normalization method. Diurnal and seasonal surface ozone anomalies over the QTP were mainly driven by meteorological conditions, such as temperature, planetary boundary layer height, surface incoming shortwave flux, downward transport velocity and inter-annual anomalies were mainly driven by anthropogenic emission. Depending on region and measurement hour, diurnal surface ozone anomalies varied over −27.82 to 37.11 µg m−3, whereas meteorological and anthropogenic contributions varied over −33.88 to 35.86 µg m−3 and −4.32 to 4.05 µg m−3 respectively. Exceptional meteorology drove 97 % of surface ozone non-attainment events from 2015 to 2020 in the urban areas over the QTP. Monthly averaged surface ozone anomalies from 2015 to 2020 varied with much smaller amplitudes than their diurnal anomalies, whereas meteorological and anthropogenic contributions varied over 7.63 to 55.61 µg m−3 and 3.67 to 35.28 µg m−3 respectively. The inter-annual trends of surface ozone in Ngari, Lhasa, Naqu, Qamdo, Diqing, Haixi and Guoluo can be attributed to anthropogenic emissions in 95.77 %, 96.30 %, 97.83 %, 82.30 %, 99.26 % and 87.85 %, and meteorology in 4.23 %, 3.70 %, 2.17 %, 3.19 %, 0.74 % and 12.15 % respectively. The inter-annual trends of surface ozone in other cities were fully driven by anthropogenic emission, whereas the increasing inter-annual trends would have larger values if not for the favorable meteorological conditions. This study can not only improve our knowledge with respect to spatiotemporal variability of surface ozone but also provide valuable implications for ozone mitigation over the QTP.

Surface ozone trends at El Arenosillo observatory from a new perspective

Surface ozone trends observed at El Arenosillo observatory for the last 22 years (2000–2021) were investigated. The trends for daily averages and daily 5th and 95th percentiles were 1.2 ± 0.3 ppb decade−1, 2.2 ± 0.3 ppb decade−1 and -0.03 ± 0.43 ppb decade−1, respectively, thus showing a significant increase of background ozone. The surface temperature trends were also explored, obtaining trends of 0.5 ± 0.2 ⁰C decade−1, 1.1 ± 0.2 ⁰C decade−1 and -0.3 ± 0.2 ⁰C decade−1 for daily averages, 5th and 95th percentiles, respectively. To identify potential changes in the ozone drivers, the weather pattern shifts were analyzed through the horizontal distribution trends of temperature at 2 m and geopotential height at 850 hPa. A strengthening of the Azores anticyclone and a regional warming were detected, which could contribute to the ozone trends obtained. The surface ozone trend in every month was explored, identifying a monthly pattern, with remarkable opposite trends in December–January (2.4 ± 0.9 ppb decade−1) vs July–August (−0.5 ± 1.1 ppb decade−1). The surface ozone trends for every hour of the day were also explored, identifying two clear patterns. The first pattern occurred from spring to autumn and was characterized by a behavior opposite to the typical daily ozone cycle. The second pattern was observed in winter, and it shows two relative peaks in the ozone trends (around 13:00 and 19:00 UTC). In a context of ozone precursor's depletion, changes in the weather conditions and warmer climate, to improve our knowledge of the ozone trends, we suggest exploring them based on daily and hourly averages.

Meteorological influences on daily variation and trend of summertime surface ozone over years of 2015–2020: Quantification for cities in the Yangtze River Delta

We quantify the meteorological influences on daily variations and trends of maximum daily 8-h average ozone (MDA8 O3) concentrations by using multiple linear regression (MLR) and Lindeman, Merenda, and Gold (LMG) approaches. Different from previous region-based studies, we pay special attention to meteorological influences at city scale. Over 2015–2019, daily changes in key meteorological parameters could explain 47%–74% of the observed daily variations in summertime MDA8 O3 concentrations in Yangtze River Delta (YRD) and four cities (Shanghai, Nanjing, Hangzhou, and Hefei), with RH being the top driver. Over years of 2015–2020, daily concentrations of MDA8 O3 obtained from MLR equations (MDA8O3_MLR) of the local cities always had better performance than those of YRD. Compared with the observed daily MDA8 O3 in June–July–August (JJA) over the studied period, daily MDA8O3_MLR of the local cities (of YRD) had correlation coefficients of 0.73 (0.63), 0.75 (0.74), 0.79 (0.78), and 0.76 (0.73) in Shanghai, Nanjing, Hangzhou, and Hefei, respectively, and the MDA8O3_MLR of the local cities (of YRD) captured 54% (17%), 63% (51%), 52% (27%) of the observed O3-polluted days (days with MDA8 O3 concentration exceeding 160 μg m−3) in Shanghai, Nanjing, and Hangzhou, respectively. The meteorologically driven trends (Trend_Met) in MDA8 O3 were calculated using the established MLR equations. Over 2015–2019, the observed trends (Trend_Obs) and Trend_Met in MDA8 O3 were mostly positive in YRD, Nanjing, Hangzhou, and Hefei. In Shanghai, Trend_Obs, Trend_Met, and anthropogenically driven trend (estimated as Trend_Obs minus Trend_Met) of MDA8 O3 in JJA over 2015–2019 were −1.3, +1.0, and −2.3 μg m−3 y−1, respectively, indicating that the emission control measures alleviated O3 pollution in this city. Our results suggest that it is necessary to establish MLR equations at city scale to account for the role of meteorology in the actions of O3 pollution control.

Increasing but Variable Trend of Surface Ozone in the Yangtze River Delta Region of China

Surface ozone (O3) increased by ∼20% in the Yangtze River Delta (YRD) region of China during 2014–2020, but the aggravating trend is highly variable on interannual time and city-level space scales. Here, we employed multiple air quality observations and numerical simulation to describe the increasing but variable trend of O3 and to reveal the main driving factors behind it. In 2014–2017, the governmental air pollution control action plan was mostly against PM2.5 (mainly to control the emissions of SO2, NOx, and primary PM2.5) and effectively reduced the PM2.5 concentration by 18%–45%. However, O3 pollution worsened in the same period with an increasing rate of 4.9 μg m−3 yr−1, especially in the Anhui province, where the growth rate even reached 14.7 μg m−3 yr−1. After 2018, owing to the coordinated prevention and control of both PM2.5 and O3, volatile organic compound (VOC) emissions in the YRD region has also been controlled with a great concern, and the O3 aggravating trend in the same period has been obviously alleviated (1.1 μg m−3 yr−1). We further combined the precursor concentration and the corresponding O3 formation regime to explain the observed trend of O3 in 2014–2020. The leading O3 formation regime in 2014–2017 is diagnosed as VOC-limited (21%) or mix-limited (58%), with the help of a simulated indicator HCHO/NOy. Under such condition, the decreasing NO2 (2.8% yr−1) and increasing VOCs (3.6% yr−1) in 2014–2017 led to a rapid increment of O3. With the continuous reduction in NOx emission and further in ambient NOx/VOCs, the O3 production regime along the Yangtze River has been shifting from VOC-limited to mix-limited, and after 2018, the mix-limited regime has become the dominant O3 formation regime for 55% of the YRD cities. Consequently, the decreases of both NOx (3.3% yr−1) and VOCs (7.7% yr−1) in 2018–2020 obviously slowed down the aggravating trend of O3. Our study argues that with the implementation of coordinated regional reduction of NOx and VOCs, an effective O3 control is emerging in the YRD region.

Surface ozone trends over a 21-year period at El Arenosillo observatory (Southwestern Europe)

The near-surface ozone measured at El Arenosillo observatory (in Southwestern Europe) over 21 years (2000−2020) was analyzed to explore its trends. Using monthly averages and their anomalies, we obtained trends of 0.9 ± 0.7 ppb decade−1 (p = 0.211) and 1.2 ± 0.4 ppb decade−1 (p = 0.001), respectively. Daytime and nighttime trends were 0.6 ± 0.5 ppb decade−1 (p = 0.198) and 1.4 ± 0.4 ppb decade−1 (p < 0.001), respectively. The seasonal trends showed upward values for winter, spring and autumn between 1.6 and 1.8 ppb decade−1, with downward values for summer. The surface temperature and geopotential height at the 500 hPa level (Z500) showed upward trends, which were higher in winter and autumn, while NOx and VOC emissions presented downward trends. The surface ozone trend could be attributed mostly to the cold months and occurred mainly in the nocturnal period. The higher temperatures and anticyclonic conditions, i.e., a warmer climate, along with the reduction of the precursor emissions, could be leading to less destruction by NO titration in cold seasons and nighttime, whereas the reduction of VOC would not favor the ozone formation and accumulation processes in warm seasons and diurnal time. In the near future, new studies will be necessary to explore other processes involved in this upward ozone trend.

Upward trend and formation of surface ozone in the Guanzhong Basin, Northwest China

Increase trend of surface ozone (O3) was observed in the Guanzhong Basin (GZB) from 2014 to 2020 with growth rates of 3.9–6.4 μg m−3 yr−1 for the maximum daily average 8 h (MDA8) O3 concentrations. To further understand the formation of O3, investigation of volatile organic compounds (VOCs) was carried out in the summer of 2018. High levels of VOCs were observed in both residential area and industrialized cities. Elevated concentrations of none-methane Hydrocarbon (NMHCs) were observed in rush hours, which indicated dominated roles of traffic activities on the loading of ambient VOCs. In the nighttime, both of NMHCs and oxygenated VOCs (OVOCs) were raised, and the peaks of VOCs kept pace with accumulation of O3. Wind field indicated that northward and westward air mass, which passed through the remote forest and industrial area in east of the GZB, was responsible to elevated ambient VOCs in the GZB. Traffic emission, fuel evaporation, and solvent using were key contributors to ambient NMHCs, while solvent using and secondary formation dominated the loading of OVOCs. The present study indicated that both local management and regional collaborative control on active VOCs species from typical sources is urgently needed in GZB.

Contrasting Trends of Surface PM2.5, O3, and NO2 and Their Relationships with Meteorological Parameters in Typical Coastal and Inland Cities in the Yangtze River Delta.

The contrasting trends of surface particulate matter (PM2.5), ozone (O3), and nitrogen dioxide (NO2) and their relationships with meteorological parameters from 2015 to 2019 were investigated in the coastal city of Shanghai (SH) and the inland city of Hefei (HF), located in the Yangtze River Delta (YRD). In both cities, PM2.5 declined substantially, while O3 and NO2 showed peak values during 2017 when the most frequent extreme high-temperature events occurred. Wind speed was correlated most negatively with PM2.5 and NO2 concentrations, while surface temperature and relative humidity were most closely related to O3. All of the studied pollutants were reduced by rainfall scavenging, with the greatest reduction seen in PM2.5, followed by NO2 and O3. By contrast, air pollutants in the two cities were moderately strongly correlated, although PM2.5 concentrations were much lower and Ox (O3 + NO2) concentrations were higher in SH. Additionally, complex air pollution hours occurred more frequently in SH. Air pollutant concentrations changed more with wind direction in SH. A more effective washout effect was observed in HF, likely due to the more frequent strong convection and thunderstorms in inland areas. This research suggests pertinent air quality control measures should be designed accordingly for specific geographical locations.

Long-term trend of ozone in southern China reveals future mitigation strategy for air pollution

The 14-year trends of surface ozone were examined using observations from 12 urban sites and 4 regional sites over the Pearl River Delta (PRD) region in China over the 2006–2019 period. Surface ozone exhibits upward trends in the range of 0.28–1.02 ppb yr−1 at urban sites and mixed trends in the range of −0.29 – 0.21 ppb yr−1 at regional sites. Meanwhile, surface Ox (Ox = O3+NO2) mixing ratios exhibit downward trends at 9 urban sites and 3 regional sites in the range of −0.01 to −0.54 ppb yr−1, revealing that the ozone rise at urban sites is predominantly attributed to the reduced titration effect by nitric oxide (NO). The further decline of surface NO2 mixing ratios could drive the decreases in Ox mixing ratios and the increases in ozone mixing ratios at urban sites. In addition, surface Ox concentrations show a good correlation with aerosol concentrations, implying the dominant contributions of secondary production to aerosol and the consistent control strategies of ozone and aerosol pollution in PRD. For the abatement of ozone pollution, our results highlight the synergistic control of key precursors including nitrogen oxides and volatile organic compounds to counteract the reduced NO titration effect by inhibiting the photochemical production of ozone. At last, we also highlight the importance of establishing more regional sites in comprehensively assessing spatiotemporal changes in surface air pollutants in China.

Surface ozone interannual variability, trends, and extremes in CCMI models

Tropospheric ozone exhibits variations at interannual to decadal timescales tied closely to climate variability. The fidelity of current-generation global chemistry-climate models (CCMs) in representing observed variations and extremes of troposphere ozone is an important aspect of model abilities to predict ozone but has not been comprehensively assessed. Here we evaluate multiple state-of-the-art CCMs participating in the Chemistry-Climate Model Initiative phase 1 (CCMI-1) in simulating the interannual variability (IAV), decadal trends, and extremes of surface ozone during 1990–2010 at northern mid-latitudes. We find that current CCMs tend to underestimate observed ozone IAV over the U.S., Europe, and East Asia in both spring and summer and fail to capture the springtime ozone trends in the eastern U.S. (EUS) and Europe. Specified-dynamics simulations constrained by reanalysis data outperform the free-running simulations with CCMs in terms of IAV and decadal trends of surface ozone. Observations show large surface ozone IAV in summer over the EUS, Europe, and East Asia (standard deviation = 3−6 ppbv or relative standard deviation = 8−15%) while all models substantially underestimate observed IAV by a factor of 2–3. Analysis in these populous/polluted regions unveils that the models significantly underestimate ozone extreme anomalies during heatwaves/droughts by 2–9 ppbv (17–55%) over the EUS and by 7–10 ppbv (42–59%) over Europe during the past two decades. These underestimates in summertime extreme anomalies contribute to the model biases in surface ozone IAV. Future model developments are needed to improve the ability of current CCMs to predict ozone extremes during heatwaves and droughts.

Chinese Regulations Are Working—Why Is Surface Ozone Over Industrialized Areas Still High? Applying Lessons From Northeast US Air Quality Evolution

AbstractObservational data indicate increasing trends of surface ozone (O3) in China, despite emission controls that have resulted in reductions of precursor emissions. Here, we explore the cause of this contradiction, through analysis of surface observations (2014–2019) in China and historical observation record in the United States (US, 1990–2019). Our observation‐based analysis indicates that the reductions of nitrogen oxides (NOx) emissions led to increase of surface O3 in North China Plain (NCP) and Yangtze River Delta (YRD) of around 8 ppb. However, NOx controls resulted in shift of O3 chemical regimes over NCP and YRD, with turning points between NOx‐ and volatile organic compound (VOC)‐limited regimes around 2019, while model simulations suggest transitional or NOx‐limited regimes over the rest of China. The impacts of high fine particles (PM2.5) on O3 formation has declined because of the reduction of PM2.5 concentrations. Stricter NOx controls can mitigate O3 pollutions over industrialized areas in China.