Urban O3 Research Articles

Urban Surface Ozone Concentration in Mainland China during 2015-2020: Spatial Clustering and Temporal Dynamics.

Urban ozone (O3) pollution in the atmosphere has become increasingly prominent on a national scale in mainland China, although the atmospheric particulate matter pollution has been significantly reduced in recent years. The clustering and dynamic variation characteristics of the O3 concentrations in cities across the country, however, have not been accurately explored at relevant spatiotemporal scales. In this study, a standard deviational ellipse analysis and multiscale geographically weighted regression models were applied to explore the migration process and influencing factors of O3 pollution based on measured data from urban monitoring sites in mainland China. The results suggested that the urban O3 concentration in mainland China reached its peak in 2018, and the annual O3 concentration reached 157 ± 27 μg/m3 from 2015 to 2020. On the scale of the whole Chinese mainland, the distribution of O3 exhibited spatial dependence and aggregation. On the regional scale, the areas of high O3 concentrations were mainly concentrated in Beijing-Tianjin-Hebei, Shandong, Jiangsu, Henan, and other regions. In addition, the standard deviation ellipse of the urban O3 concentration covered the entire eastern part of mainland China. Overall, the geographic center of ozone pollution has a tendency to move to the south with the time variation. The interaction between sunshine hours and other factors (precipitation, NO2, DEM, SO2, PM2.5) significantly affected the variation of urban O3 concentration. In Southwest China, Northwest China, and Central China, the suppression effect of vegetation on local O3 was more obvious than that in other regions. Therefore, this study clarified for the first time the migration path of the gravity center of the urban O3 pollution and identified the key areas for the prevention and control of O3 pollution in mainland China.

Role of meteorology-driven regional transport on O3 pollution over the Chengdu Plain, southwestern China

Owing to the complex mesoscale meteorological dynamics, deep basin topography and pollutants emissions, the Chengdu Plain (CDP) situated at west border of the Sichuan Basin (SCB) have suffered excessive ozone (O3) pollution in the warm season. However, the meteorology-driven mechanism and associated dominant processes underlying the elevated O3 levels are not well elucidated due to limited ambient measurements. Here, we adopted trace gases data measured by basin-wide network in combination with a high-resolution chemical transport model for characterizing a regional continuous O3 episode occurred from 10 to 18 August 2019. The extensive monitoring network shapes a much-detailed evolution map of O3 concentrations which depicts that O3 hotspots extend from metropolitan areas to suburban areas across the CDP. It is found that both advection of O3-rich air masses in the residual layer (RL) and regional transport of O3 and its precursors under prevailing winds exert profound influences on the elevation of ambient O3 across the CDP. As the convective boundary layer is destroyed after sunrise, vertical mixing effects cause the entrainment of O3-rich air masses in RL to the surface, affecting the redistribution of surface O3 concentration. In addition, process analysis (PA) and integrated source apportionment method (ISAM) results of the CMAQ model indicated that vertical transport and gas-phase chemistry were the main sources of O3 during the day, while dry deposition and horizontal advection were the main sinks of surface O3. O3 transport by boundary condition (BCO) was the largest contributor to all urban O3 concentrations in the CDP. Our findings highlight the regional transport mechanism of O3 in SCB under complex topographic conditions and point out the physical and chemical causes and sources of typical summer O3 pollution episodes in the SCB.

Influence of Wildfire on Urban Ozone: An Observationally Constrained Box Modeling Study at a Site in the Colorado Front Range.

Increasing trends in biomass burning emissions significantly impact air quality in North America. Enhanced mixing ratios of ozone (O3) in urban areas during smoke-impacted periods occur through transport of O3 produced within the smoke or through mixing of pyrogenic volatile organic compounds (PVOCs) with urban nitrogen oxides (NOx = NO + NO2) to enhance local O3 production. Here, we analyze a set of detailed chemical measurements, including carbon monoxide (CO), NOx, and speciated volatile organic compounds (VOCs), to evaluate the effects of smoke transported from relatively local and long-range fires on O3 measured at a site in Boulder, Colorado, during summer 2020. Relative to the smoke-free period, CO, background O3, OH reactivity, and total VOCs increased during both the local and long-range smoke periods, but NOx mixing ratios remained approximately constant. These observations are consistent with transport of PVOCs (comprised primarily of oxygenates) but not NOx with the smoke and with the influence of O3 produced within the smoke upwind of the urban area. Box-model calculations show that local O3 production during all three periods was in the NOx-sensitive regime. Consequently, this locally produced O3 was similar in all three periods and was relatively insensitive to the increase in PVOCs. However, calculated NOx sensitivities show that PVOCs substantially increase O3 production in the transition and NOx-saturated (VOC-sensitive) regimes. These results suggest that (1) O3 produced during smoke transport is the main driver for O3 increases in NOx-sensitive urban areas and (2) smoke may cause an additional increase in local O3 production in NOx-saturated (VOC-sensitive) urban areas. Additional detailed VOC and NOx measurements in smoke impacted urban areas are necessary to broadly quantify the effects of wildfire smoke on urban O3 and develop effective mitigation strategies.

Ozone Pollution of Megacity Shanghai during City-Wide Lockdown Assessed Using TROPOMI Observations of NO2 and HCHO

An unprecedented city-wide lockdown took place in Shanghai from April to May 2022 to curb the spread of COVID-19, which caused socio-economic disruption but a significant reduction of anthropogenic emissions in this metropolis. However, the ground-based monitoring data showed that the concentration of ozone (O3) remained at a high level. This study applied Tropospheric Monitoring Instrument (TROPOMI) observations to examine changes in tropospheric vertical column density (VCD) of nitrogen dioxide (NO2) and formaldehyde (HCHO), which are precursors of O3. Compared with the same period in 2019–2021, VCDs of NO2 and HCHO decreased respectively by ~50% and ~20%. Multiple regression analysis showed that the lockdown effect played a dominant role in this dramatic decline rather than meteorological impacts. Using the exponentially-modified Gaussian method, this study quantified nitrogen oxides (NOX) emission in Shanghai as 32.60 mol/s with a decrease of 50–80%, which was mainly contributed by the transportation and industrial sectors. The significant reduction of NOX emission in Shanghai is much higher than that of volatile organic compounds (VOCs), which led to dramatic changes in formaldehyde-to-nitrogen dioxide ratio (HCHO/NO2, FNR). Thus, when enforcing regulation on NOx emission control in the future, coordinately reducing VOCs emission should be implemented to mitigate urban O3 pollution.

Important revelations of different degrees of COVID-19 lockdown on improving regional air quality: a case study of Shijiazhuang, China.

To control the spread of COVID-19, Shijiazhuang implemented two lockdowns of different magnitudes in 2020 (lockdown I) and 2021 (lockdown II). We analyzed the changes in air quality index (AQI), PM2.5, O3, and VOCs during the two lockdowns and the same period in 2019 and quantified the effects of anthropogenic sources during the lockdowns. The results show that AQI decreased by 13.2% and 32.4%, and PM2.5 concentrations decreased by 12.9% and 42.4% during lockdown I and lockdown II, respectively, due to the decrease in urban traffic mobility and industrial activity levels. However, the sudden and unreasonable emission reductions led to an increase in O3 concentrations by 160.6% and 108.4%, respectively, during the lockdown period. To explore the causes of the O3 surge, the major precursors NOx and VOCs were studied separately, and the main VOCs species affecting ozone formation during the lockdown period and the source variation of VOCs were identified, and it is important to note that the relationship between diurnal variation characteristics of VOCs and cooking became apparent during the lockdown period. These findings suggest that regional air quality can be improved by limiting production, but attention should be paid to the surge of O3 caused by unreasonable emission reductions, clarifying the control priorities for urban O3 management.

The Spatial Variation of the Influence of Lockdown on Air Quality across China and Its Major Influencing Factors during COVID-19

China has experienced a series of COVID-19 recurrences in different cities across the country since 2020, and relatively strict (full lockdown) or lenient closure (semi-lockdown) strategies have been employed accordingly in each city. The differences in detailed transmission control measures during lockdown periods led to distinct effects on air quality, which has rarely been studied. To fill this gap, we examined the effects of semi-lockdown and full lockdown on six major airborne pollutants, based on 55 lockdown cases. For all lockdown cases, the concentrations of PM2.5, PM10, SO2, NO2 and CO were much lower than in previous years. Specifically, due to the stricter transmission control, the concentration of the five airborne pollutants experienced a much sharper decline during full lockdown. However, O3 presented a different variation pattern during lockdown periods. Generally, O3 concentrations presented a slight increase in semi-lockdown cases and a notable increase in full lockdown cases. Meanwhile, O3 increased notably in northern China, particularly in the Beijing–Tianjin–Hebei region, while O3 had a slight variation in southern China. The unique variation of O3 across regions and lockdown types was mainly attributed to the spatial heterogeneity of O3 formation regimes, especially the VOCs-controlled O3 formation in northern China. Based on Geographical Detector, we examined the spatial continuity of natural and socio-economic factors on the variation of airborne pollutants during lockdown. In terms of meteorological factors, humidity and precipitation were the dominant factors for PM2.5 and PM10, respectively, while humidity and temperature were the dominant factors for O3. In terms of socio-economic factors, the numbers of taxis and private cars were the dominant factors for PM2.5 and O3 variations during lockdown. GD also revealed that the combination of natural and socio-economic factors had a significantly enhanced effect on airborne pollutants during lockdown. The combination of relative humidity and total area of urban built-up areas exerted the strongest interactive effects on both PM2.5 and O3. This research highlighted the challenge for urban O3 management, and suggested the control of VOCs emissions should be preferably considered, especially in northern China.

Changes in physical and chemical properties of urban atmospheric aerosols and ozone during the COVID-19 lockdown in a semi-arid region

The synergistic response of urban atmospheric aerosols and ozone (O3) to reduction of anthropogenic emissions is complicated and still needs further study. Thus, the changes in physical and chemical properties of urban atmospheric aerosols and O3 during the Coronavirus Disease 2019 (COVID-19) lockdown were investigated at three urban sites and one rural site in Lanzhou with semi-arid climate. Fine particulate matter (PM2.5) decreased at four sites by ∼ 20% while O3 increased by >100% at two urban sites during the COVID-19 lockdown. Both primary emissions and secondary formation of PM2.5 decreased during the lockdown. Significant increase in both sulfur and nitrogen oxidation ratios was found in the afternoon, which accounted for 48.7% of the total sulfate and 40.4% of the total nitrate, respectively. The positive matrix factorization source apportionment revealed increased contribution of secondary formation and decreased contribution of vehicle emissions. Aerosol scattering and absorption decreased by 33.6% and 45.3%, resulting in an increase in visibility by 30% and single scattering albedo (SSA) at 520 nm slightly increased by 0.02. The enhanced O3 production was explained by increased volatile organic compounds to nitrogen oxides ratio, decreased aerosol, as well as increased SSA. The primary emissions of secondary aerosol precursors significantly decreased while Ox (i.e., NO2 and O3) exhibited little change. Consequently, Ox to CO ratio, PM2.5 to elemental carbon (EC) ratio, secondary inorganic aerosols to EC ratio, and secondary organic carbon to EC ratio increased, confirming enhanced secondary aerosol production efficiency during the lockdown. Positive feedback among O3 concentration, secondary aerosol formation, and SSA was revealed to further promote O3 production and secondary aerosol formation. These results provide scientific guidance for collaborative management of O3 and particulate matter pollution for cities with semi-arid climate.

Long-Term Variations of Meteorological and Precursor Influences on Ground Ozone Concentrations in Jinan, North China Plain, from 2010 to 2020

Ground-level ozone (O3) pollution in the North China Plain has become a serious environmental problem over the last few decades. The influence of anthropogenic emissions and meteorological conditions on ozone trends have become the focus of widespread research. We studied the long-term ozone trends at urban and suburban sites in a typical city in North China and quantified the contributions of anthropogenic and meteorological factors. The results show that urban O3 increased and suburban O3 decreased from 2010 to 2020. The annual 90th percentile of the maximum daily 8-h average of ozone in urban areas increased by 3.01 μgm−3year−1 and, in suburban areas, it decreased by 3.74 μgm−3year−1. In contrast to the meteorological contributions, anthropogenic impacts are the decisive reason for the different ozone trends in urban and suburban areas. The rapid decline in nitrogen oxides (NOX) in urban and suburban areas has had various effects. In urban areas, this leads to a weaker titration of NOX and enhanced O3 formation, while in suburban areas, this weakens the photochemical production of O3. Sensitivity analysis shows that the O3 formation regime is in a transition state in both the urban and suburban areas. However, this tends to be limited to volatile organic compounds (VOCs) in urban areas and to NOX in suburban areas. One reasonable approach to controlling ozone pollution should be to reduce nitrogen oxide emissions while strengthening the control of VOCs.

Exploring the link between ozone pollution and stratospheric intrusion under the influence of tropical cyclone Ampil

Tropical cyclones (TCs) are synoptic-scale systems with a diameter of up to 2000 km, and may persist for several days to weeks. They can significantly affect the atmospheric conditions and ozone (O3) concentrations in coastal areas. They also induce stratospheric intrusion (SI, the injection of stratospheric O3 into the troposphere) by changing the height of the tropopause. Focusing on the Yangtze River Delta (YRD) region, a highly urbanized coastal area with severe O3 pollution, we systematically analyze the characteristics of O3 pollution and SIs as well as their connections under the influence of tropical cyclone Ampil. Based on surface observations, 120-h backward trajectories and ERA5 reanalysis meteorological data, the urban O3 pollution affected by tropical cyclone Ampil mainly resulted from active photochemical reactions inside the boundary layer and poor diffusion conditions characterized by weak winds. SIs induced by tropical cyclone Ampil were important for the upper and middle troposphere, where O3 concentration could increase up to 180 ppbv. But they hardly reached the ground over the YRD. Therefore, there was no direct connection between O3 pollution and SIs. However, the location of SIs can predict urban O3 pollution. SIs moved with tropical cyclone Ampil but appeared on the northwest side of Ampil, usually 500–1000 km away from tropical cyclone Ampil center. At 200 hPa, they corresponded to a high-PV (>2 PVU) air mass rich in O3 (>200 ppbv). Below this stratospheric PV signature, urban O3 concentration was usually high.

Model analysis of vertical exchange of boundary layer ozone and its impact on surface air quality over the North China Plain

In addition to photochemical production and horizontal regional transport, surface O3 concentration can also likely be affected by vertical transport, which is not well known so far. The process analysis was conducted by using the Regional Atmospheric Modeling System Community Multiscale Air Quality (RAMS-CMAQ) model to investigate photochemical production and the vertical transport mechanism of boundary-layer O3 during a typical O3 pollution episode in the North China Plain (NCP), and further quantify the contribution of vertical transport to surface O3. The diurnal variations of vertical budgets of O3 and NO2 in the boundary layer at multiple sites showed that there were substantial differences in the vertical distribution of O3 production and transport between urban and suburban/rural areas. In urban areas, surface O3 is consumed by titration reaction to generate NO2, which is then transported to the upper boundary layer and produces O3 by photochemical reaction. With the development of the boundary layer, the upper-layer O3 stored in the residual layer at nighttime can be transported vertically to the surface as the turbulent diffusion intensifies the next morning. While in suburban and rural areas, the vertical transport is relatively weaker because the photochemical formation of O3 occurs in the whole boundary layer, although it decreases slightly with the altitude. Model simulation showed that 20.6–27.9% of urban surface O3 changes in the morning (09:00–10:00 LST) was attributable to the downward transport from the residual layer, while it is 15.0–22.1% at suburban site. The vertical transport from above the boundary layer contributed 24.0–63.6% to daytime urban surface O3 changes, which was weak in suburban areas. Differences and similarities in O3 formation and transport mechanism in urban and suburban regions revealed here highlight the importance of earlier control and regional collaboration.

Land use and anthropogenic heat modulate ozone by meteorology: a perspective from the Yangtze River Delta region

Abstract. With the rapid advance in urbanization, land use and anthropogenic heat (AH) dictated by human activities significantly modify the urban climate and in turn the air quality. Focusing on the Yangtze River Delta (YRD) region, a highly urbanized coastal area with severe ozone (O3) pollution, we estimate the impacts of land use and AH on meteorology and O3 using the Weather Research and Forecasting model coupled to Chemistry (WRF-Chem). These results enhance our understanding of the formation of O3 pollution in rapidly developing city clusters with place-specific topography, as most of our results can be supported by previous studies conducted in other regions around the world. Regional O3 pollution episodes occurred frequently (∼ 26 times per year) in the YRD from 2015 to 2019. These O3 pollution episodes are usually in calm conditions characterized by high temperature (over 20 ∘C), low relative humidity (less than 80 %), light wind (less than 3 m s−1) and shallow cloud cover (less than 5 okta). In this case, O3 pollution belts tend to appear in the converging airflows associated with the sea and the lake breezes. On the other hand, rapid urbanization has significantly changed land use and AH in this region, which subsequently affects meteorology and O3 concentration. The largest change in land use comes from urban expansion, which causes an increase in 2 m temperature (T2) by a maximum of 3 ∘C, an increase in planetary boundary layer height (PBLH) by a maximum of 500 m, a decrease in 10 m wind speed (WS10) by a maximum of 1.5 m s−1 and an increase in surface O3 by a maximum of 20 µg m−3. With regard to the sea and lake breezes, the expansion of coastal cities, like Shanghai, can enhance the sea breeze circulation by ∼ 1 m s−1. During the advance of the sea breeze front inland, the updraft induced by the front causes strong vertical mixing of O3. However, once the sea breeze is fully developed in the afternoon (∼ 17:00 LT), further progression inland will stall. Then O3 removal by the low sea breeze will be weakened, and surface O3 can be 10 µg m−3 higher in the case with cities than in the case with no cities. The expansion of lakeside cities, such as Wuxi and Suzhou, can extend the lifetime of lake breezes from noon to afternoon. Since the offshore flow of the lake breeze transports high O3 from the land to the lake, the onshore flow brings high O3 back to the land. Surface O3 in lakeside cities can increase by as much as 30 µg m−3. Compared to land use, the effects of AH are relatively small. The changes mainly appear in and around cities where AH fluxes are large. There are increases in T2, PBLH, WS10 and surface O3 when AH fluxes are taken into account, with increments of approximately 0.2 ∘C, 75 m, 0.3 m s−1 and 4 µg m−3, respectively. AH contributes largely to the urban environment, altering meteorological factors, O3 concentration and urban breeze circulation, but its effect on the sea and the lake breezes seems to be limited.

Enhanced drought resistance of vegetation growth in cities due to urban heat, CO2 domes and O3 troughs

Sustained increase in atmospheric CO2 is strongly coupled with rising temperature and persistent droughts. While elevated CO2 promotes photosynthesis and growth of vegetation, drier and warmer climate can potentially negate this benefit, complicating the prediction of future terrestrial carbon dynamics. Manipulative studies such as free air CO2 enrichment (FACE) experiments have been useful for studying the joint effect of global change factors on vegetation growth; however, their results do not easily transfer to natural ecosystems partly due to their short-duration nature and limited consideration of climatic gradients and potential confounding factors, such as O3. Urban environments serve as a useful small-scale analogy of future climate at least in reference to CO2 and temperature enhancements. Here, we develop a data-driven approach using urban environments as test beds for revealing the joint effect of changing temperature and CO2 on vegetation response to drought. Using 75 urban-rural paired plots from three climate zones over the conterminous United States (CONUS), we find vegetation in urban areas exhibits a much stronger resistance to drought than in rural areas. Statistical analysis suggests the drought resistance enhancement of urban vegetation across CONUS is attributed to rising temperature (with a partial correlation coefficient of 0.36) and CO2 (with a partial correlation coefficient of 0.31) and reduced O3 concentration (with a partial correlation coefficient of −0.12) in cities. The controlling factor(s) responsible for urban-rural differences in drought resistance of vegetation vary across climate regions, such as surface O3 gradients in the arid climate, and surface CO2 and O3 gradients in the temperate and continental climates. Thus, our study provides new observational insights on the impacts of competing factors on vegetation growth at a large scale, and ultimately, helps reduce uncertainties in understanding terrestrial carbon dynamics.

Spatial Resolved Surface Ozone with Urban and Rural Differentiation during 1990-2019: A Space-Time Bayesian Neural Network Downscaler.

Long-term exposure to ambient ozone (O3) can lead to a series of chronic diseases and associated premature deaths, and thus population-level environmental health studies hanker after the high-resolution surface O3 concentration database. In response to this demand, we innovatively construct a space-time Bayesian neural network parametric regressor to fuse TOAR historical observations, CMIP6 multimodel simulation ensemble, population distributions, land cover properties, and emission inventories altogether and downscale to 10 km × 10 km spatial resolution with high methodological reliability (R2 = 0.89-0.97, RMSE = 1.97-3.42 ppbV), fair prediction accuracy (R2 = 0.69-0.77, RMSE = 5.63-7.97 ppbV), and commendable spatiotemporal extrapolation capabilities (R2 = 0.62-0.76, RMSE = 5.38-11.7 ppbV). Based on our predictions in 8-h maximum daily average metric, the rural-site surface O3 are 15.1±7.4 ppbV higher than urban globally averaged across 30 historical years during 1990-2019, with developing countries being of the most evident differences. The globe-wide urban surface O3 are climbing by 1.9±2.3 ppbV per decade, except for the decreasing trends in eastern United States. On the other hand, the global rural surface O3 tend to be relatively stable, except for the rising tendencies in China and India. Using CMIP6 model simulations directly without urban-rural differentiation will lead to underestimations of population O3 exposure by 2.0±0.8 ppbV averaged over each historical year. Our original Bayesian neural network framework contributes to the deep-learning-driven environmental studies methodologically by providing a brand-new feasible way to realize data fusion and downscaling, which maintains high interpretability by conforming to the principles of spatial statistics without compromising the prediction accuracy. Moreover, the 30-year highly spatial resolved monthly surface O3 database with multiple metrics fills in the literature gap for long-term surface O3 exposure tracing.

Contrasting chemical environments in summertime for atmospheric ozone across major Chinese industrial regions: the effectiveness of emission control strategies

Abstract. The United Kingdom Chemistry and Aerosols (UKCA) chemistry–climate model is used to quantify the differences in chemical environment for surface O3 for six major industrial regions across China in summer 2016. We first enhance the UKCA gas-phase chemistry scheme by incorporating reactive volatile organic compound (VOC) tracers that are necessary to represent urban and regional-scale O3 photochemistry. We demonstrate that the model with the improved chemistry scheme captures the observed magnitudes and diurnal patterns of surface O3 concentrations across these regions well. Simulated O3 concentrations are highest in Beijing and Shijiazhuang on the North China Plain and in Chongqing, lower in Shanghai and Nanjing in the Yangtze River Delta, and lowest in Guangzhou in the Pearl River Delta despite the highest daytime O3 production rates in Guangzhou. NOx / VOC and H2O2 / HNO3 ratios indicate that O3 production across all regions except Chongqing is VOC limited. We confirm this by constructing O3 response surfaces for each region changing NOx and VOC emissions and further contrast the effectiveness of measures to reduce surface O3 concentrations. In VOC-limited regions, reducing NOx emissions by 20 % leads to a substantial O3 increase (11 %) in Shanghai. We find that reductions in NOx emissions alone of more than 70 % are required to decrease O3 concentrations across all regions. Reductions in VOC emissions alone of 20 % produce the largest decrease (−11 %) in O3 levels in Shanghai and Guangzhou and the smallest decrease (−1 %) in Chongqing. These responses are substantially different from those currently found in highly populated regions in other parts of the world, likely due to higher NOx emission levels in these Chinese regions. Our work provides an assessment of the effectiveness of emission control strategies to mitigate surface O3 pollution in these major industrial regions and emphasises that combined NOx and VOC emission controls play a pivotal role in effectively offsetting high O3 levels. It also demonstrates new capabilities in capturing regional air pollution that will permit this model to be used for future studies of regional air-quality–climate interactions.

Increased ozone pollution alongside reduced nitrogen dioxide concentrations during Vienna’s first COVID-19 lockdown: Significance for air quality management

BackgroundLockdowns amid the COVID-19 pandemic have offered a real-world opportunity to better understand air quality responses to previously unseen anthropogenic emission reductions. Methods and main objectiveThis work examines the impact of Vienna’s first lockdown on ground-level concentrations of nitrogen dioxide (NO2), ozone (O3) and total oxidant (Ox). The analysis runs over January to September 2020 and considers business as usual scenarios created with machine learning models to provide a baseline for robustly diagnosing lockdown-related air quality changes. Models were also developed to normalise the air pollutant time series, enabling facilitated intervention assessment. Core findingsNO2 concentrations were on average −20.1% [13.7–30.4%] lower during the lockdown. However, this benefit was offset by amplified O3 pollution of +8.5% [3.7–11.0%] in the same period. The consistency in the direction of change indicates that the NO2 reductions and O3 increases were ubiquitous over Vienna. Ox concentrations increased slightly by +4.3% [1.8–6.4%], suggesting that a significant part of the drops in NO2 was compensated by gains in O3. Accordingly, 82% of lockdown days with lowered NO2 were accompanied by 81% of days with amplified O3. The recovery shapes of the pollutant concentrations were depicted and discussed. The business as usual-related outcomes were broadly consistent with the patterns outlined by the normalised time series. These findings allowed to argue further that the detected changes in air quality were of anthropogenic and not of meteorological reason. Pollutant changes on the machine learning baseline revealed that the impact of the lockdown on urban air quality were lower than the raw measurements show. Besides, measured traffic drops in major Austrian roads were more significant for light-duty than for heavy-duty vehicles. It was also noted that the use of mobility reports based on cell phone movement as activity data can overestimate the reduction of emissions for the road transport sector, particularly for heavy-duty vehicles. As heavy-duty vehicles can make up a large fraction of the fleet emissions of nitrogen oxides, the change in the volume of these vehicles on the roads may be the main driver to explain the change in NO2 concentrations. Interpretation and implicationsA probable future with emissions of volatile organic compounds (VOCs) dropping slower than emissions of nitrogen oxides could risk worsened urban O3 pollution under a VOC-limited photochemical regime. More holistic policies will be needed to achieve improved air quality levels across different regions and criteria pollutants.

COVID-19 lockdowns highlight a risk of increasing ozone pollution in European urban areas

Abstract. In March 2020, non-pharmaceutical intervention measures in the form of lockdowns were applied across Europe to urgently reduce the transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the virus which causes the COVID-19 disease. The aggressive curtailing of the European economy had widespread impacts on the atmospheric composition, particularly for nitrogen dioxide (NO2) and ozone (O3). To investigate these changes, we analyse data from 246 ambient air pollution monitoring sites in 102 urban areas and 34 countries in Europe between February and July 2020. Counterfactual, business-as-usual air quality time series are created using machine-learning models to account for natural weather variability. Across Europe, we estimate that NO2 concentrations were 34 % and 32 % lower than expected for respective traffic and urban background locations, whereas O3 was 30 % and 21 % higher (in the same respective environments) at the point of maximum restriction on mobility. To put the 2020 changes into context, average NO2 trends since 2010 were calculated, and the changes experienced across European urban areas in 2020 was equivalent to 7.6 years of average NO2 reduction (or concentrations which might be anticipated in 2028). Despite NO2 concentrations decreasing by approximately a third, total oxidant (Ox) changed little, suggesting that the reductions in NO2 were substituted by increases in O3. The lockdown period demonstrated that the expected future reductions in NO2 in European urban areas are likely to lead to widespread increases in urban O3 pollution unless additional mitigation measures are introduced.

Lessons from the COVID-19 air pollution decrease in Spain: Now what?

We offer an overview of the COVID-19 -driven air quality changes across 11 metropolises in Spain with the focus on lessons learned on how continuing abating pollution. Traffic flow decreased by up to 80% during the lockdown and remained relatively low during the full relaxation (June and July). After the lockdown a significant shift from public transport to private vehicles (+21% in Barcelona) persisted due to the pervasive fear that using public transport might increase the risk of SARS-CoV-2 infection, which need to be reverted as soon as possible. NO2 levels fell below 50% of the WHO annual air quality guidelines (WHOAQGs), but those of PM2.5 were reduced less than expected due to the lower contributions from traffic, increased contributions from agricultural and domestic biomass burning, or meteorological conditions favoring high secondary aerosol formation yields. Even during the lockdown, the annual PM2.5 WHOAQG was exceeded in cities within the NE and E regions with high NH3 emissions from farming and agriculture. Decreases in PM10 levels were greater than in PM2.5 due to reduced emissions from road dust, vehicle wear, and construction/demolition. Averaged O3 daily maximum 8-h (8hDM) experienced a generalized decrease in the rural receptor sites in the relaxation (June–July) with −20% reduced mobility. For urban areas O3 8hDM responses were heterogeneous, with increases or decreases depending on the period and location. Thus, after canceling out the effect of meteorology, 5 out of 11 cities experienced O3 decreases during the lockdown, while the remaining 6 either did not experience relevant reductions or increased. During the relaxation period and coinciding with the growing O3 season (June–July), most cities experienced decreases. However, the O3 WHOAQG was still exceeded during the lockdown and full relaxation periods in several cities. For secondary pollutants, such as O3 and PM2.5, further chemical and dispersion modeling along with source apportionment techniques to identify major precursor reduction targets are required to evaluate their abatement potential.

Urban Ground-Level O3 Trends: Lessons from Portuguese Cities, 2010–2018

Big datasets of air-quality pollutants and weather data allow us to review trends of NO2, NO, O3, and global radiation (GR), for Lisbon, Porto and Coimbra, with regard to the historical period of 2010–2018. GR is expected to have a considerable impact on photochemical reactions of the O3 formation mechanism. We aim to characterize daily, monthly, and yearly trends. We explore Weekday (WD) versus weekend (WE), and seasonality of O3 and NO2. We are interested to know these pollutant peak concentration variations over the years and investigate if parallels can be drawn between urban mobility indicators and these pollutants. For this purpose, economic data, European emission standards, and car stock data (fuel, age, and number of vehicles) are cross-analyzed. How are they correlated? Has it impacted NO2 and O3 variations? How do different air-quality monitoring stations (AQMS), traffic and non-traffic, compare? How is Lisbon NOx-O3 correlated? What are its implications for future scenarios? Results show that urban mobility trends and economic events are correlated with NO2 and O3 variability. Weekend effect has a partial relationship with urban mobility trends and economy as it is relatively well correlated for Lisbon but not for Porto and Coimbra. Nonetheless, weekend effect for the period of 2010–2018 is overall trending upwards for all cities. In Lisbon and Coimbra, O3 concentrations also trend upwards during the same 2010–2018 period but for Porto they do not. Regardless, for the period of 2015–2018, after the economic recession, the upwards trends of both weekend effect and overall O3 concentrations are clear for all AQMS. For AQMS peak values comparison, Lisbon traffic AQMS registered an annual averaged 8-hour daily max O3 concentration of 34.4 ppb while Lisbon non-traffic AQMS presented 39.1 ppb. Altogether, annual 8-hour daily maximum values for 2010–2018 traffic AQMS in Lisbon show an inverse relationship with fuel sales, and have concentrations fluctuating between 28–35 ppb, which is slightly higher than the 2001–2010 historical European range of 27–31 ppb. Lastly, for the 8 years data in Lisbon, it has been shown that a negative NOx-O3 correlation exists, and the study location might be VOC–sensitive. This means that as NOx concentrations decrease, O3 concentrations become exponentially higher. Further research into VOCs with better data availability is required to make more concise claims. Regardless, it can be inferred that in a future scenario where mitigation continues to escalate, through O3 emission standards and an aggressive shift of car stock to electric vehicles, achieving unprecedented rises in O3 concentrations could be observed.

Interpretation of decadal-scale ozone production efficiency in the Seoul Metropolitan Area: Implication for ozone abatement

Understanding the drivers of urban ozone (O3) formation is challenging and requires uncovering the non-linear relationship between O3 and its precursors. We used a novel method to differentiate between background O3 levels and local photochemical O3 generation using the observation-based O3 production efficiency (OPE) from decade-long observations including nitrogen dioxide (NO2), nitrogen monoxide (NO), reactive nitrogen compounds (NOy), O3, and 56 volatile organic compounds (VOCs) covering eight years (2009–2016). In this study, we employed Ox (=O3+NO2), and defined OPEx as the number of Ox molecules produced from the oxidation of a single NOx molecule, and estimates the local contribution (in %) to total O3 from the slope of the Ox−NOz(=NOy–NOx) linear regression. We thus identified the local urban O3 formation regime using the relationship of several factors, including nitrogen oxide (NOx), VOC/NOx, NO2, NOz, and long-range transported Ox, and suggested how O3 can be reduced in the Seoul Metropolitan Area (SMA).The results show that the regional background dominates the annual level of Ox with a contribution of about 66 ± 11%, while photochemical production acts as a stronger factor influencing the annual variability of Ox than regional background. Regional background varies by transport evidenced by its good correlation with the west wind fraction, whereas photochemical production depends on the VOC/NOx ratio, which is high in summer and low in winter. The relationship between photochemical production and VOC/NOx indicates that the transition of the Ox formation regime occurs at a VOC/NOx ratio of approximately 4 in the SMA. Urban environments with NOx > 20 ppb typically lie in low VOC/NOx zone, where reduced NO2 emissions lead to decreased NOz but increased OPEx, with no discernible change in photochemical Ox production. Thus, NOx emission reductions in urban environments would have a negligible effect on Ox reduction. In contrast, suburban environments such as usually NOx < 10 ppb are located in the higher VOC/NOx zone, where decreased NO2 leads to decreased OPEx with no discernible change in NOz, consequently suppressing photochemical Ox production. Together, these results suggest that any urban O3 policy in the SMA should consider significant cuts in NOx emissions to allow the O3 regime to shift. The OPEx method can also be applied to other urban areas for the development of effective O3 reduction policies.

Unprecedented Temporary Reduction in Global Air Pollution Associated with COVID-19 Forced Confinement: A Continental and City Scale Analysis

Shortly after the outbreak of the novel infectious disease (COVID-19) started at the end of 2019, it turned into a global pandemic, which caused the lockdown of many countries across the world. Various strict measures were adopted to reduce anthropogenic activities in order to prevent further spread and infection of the disease. In this study, we utilized continental scale remotely sensed data along with city scale in situ air quality observations for 2020 as well as data from the baseline period (2015–2019) to provide an early insight on air pollution changes in response to the COVID-19 pandemic lockdown, by combining both continental and city scales. For the continental scale analysis, data of NO2, SO2, and O3 were acquired from the ozone monitoring instrument (OMI) and data of aerosol optical depth (AOD) were collected from the moderate resolution imaging spectroradiometer (MODIS). For city scale analysis, data of NO2, CO, PM2.5, O3, and SO2 were derived from ground-based air quality observations. Results from satellite observations at the continental scale showed that concentrations of NO2, SO2, and AOD substantially dropped in 2020 during the lockdown period compared to their averages for the baseline period over all continents, with a maximum reduction of ~33% for NO2 in East Asia, ~41% for SO2 in East Asia, and ~37% for AOD in South Asia. In the case of O3, the maximum overall reduction was observed as ~11% in Europe, followed by ~10% in North America, while a slight increase was found in other study regions. These findings align with ground-based air quality observations, which showed that pollutants such as NO2, CO, PM2.5, and SO2 during the 2020 lockdown period decreased significantly except that O3 had varying patterns in different cities. Specifically, a maximum reduction of ~49% in NO2 was found in London, ~43% in CO in Wuhan, ~38% in PM2.5 in Chennai, and ~48% in SO2 in Beijing. In the case of urban O3, a maximum reduction of ~43% was found in Wuhan, but a significant increase of ~47% was observed in Chennai. It is obvious that restricted human activities during the lockdown have reduced the anthropogenic emissions and subsequently improved air quality, especially across the metropolitan cities.