Surface Ozone Observations Research Articles

Arctic Tropospheric Ozone Trends

AbstractObserved trends in tropospheric ozone, an important air pollutant and short‐lived climate forcer (SLCF), are estimated using available surface and ozonesonde profile data for 1993–2019, using a coherent methodology, and compared to modeled trends (1995–2015) from the Arctic Monitoring Assessment Program SLCF 2021 assessment. Increases in observed surface ozone at Arctic coastal sites, notably during winter, and concurrent decreasing trends in surface carbon monoxide, are generally captured by multi‐model median trends. Wintertime increases are also estimated in the free troposphere at most Arctic sites, with decreases during spring months. Winter trends tend to be overestimated by the multi‐model medians. Springtime surface ozone increases in northern coastal Alaska are not simulated while negative springtime trends in northern Scandinavia are not always reproduced. Possible reasons for observed changes and model performance are discussed including decreasing precursor emissions, changing ozone dry deposition, and variability in large‐scale meteorology.

Retrieval of tropospheric ozone profiles using ground-based MAX-DOAS

In recent years, surface ozone concentrations have increased in many cities in China. Ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) is a powerful technique for retrieving the profiles of tropospheric trace gases, such as NO2, SO2, and HCHO. However, since the difficulties in deducting the effects of stratospheric ozone, there are few studies on the retrieval of tropospheric ozone profiles using MAX-DOAS measurements. Here, we developed an accurate inversion method to retrieve tropospheric ozone concentrations during the PRIDE-GBA Campaign, wherein the ozone differential slant column densities (DSCDs) were retrieved in QDOAS software using the “time-interpolated zenith spectrum” as the reference spectrum. The tropospheric DSCDs (DSCDstrop) were then calculated by subtracting the simulated stratospheric DSCDs (DSCDsstr, simulated from the SCIATRAN model) from the DSCDs. Tropospheric ozone profiles were retrieved from the DSCDstrop using the optimal estimation method (OEM). The results showed that high values of tropospheric ozone were mainly distributed below 1 km, which is consistent with lidar measurements. In addition, the observed surface ozone concentrations were highly correlated with the in-situ measurements, with correlation coefficients (R) of 0.75 and 0.81, respectively. Combined with the retrieved NO2 and HCHO profiles using the MAX-DOAS measurements, we found that the planetary boundary layer ozone pollution of HeShan Observatory during the PRIDE-GBA Campaign are controlled by the NOx-limited regime. The results of this study indicate that the MAX-DOAS technique has the potential to retrieve tropospheric ozone profiles with high temporal and spatial resolution.

Мониторинг приземного озона в природном заповеднике «Карадагский» в период 2017–2021 гг.

The background environmental monitoring station in the Karadag Nature Reserve is the only point for routine measurements (since 2010) of surface ozone on the Black Sea coast of the Russian Federation. Surface ozone observations at the station over the past five years are discussed. An insignificant interannual variability of surface ozone is shown as compared to the previous years. Seasonal and diurnal variations in surface ozone averaged over the period of 2017–2021 generally coincide with the previous period and are comparable with the typical variations at foreign Mediterranean stations. The surface ozone regime at the analyzed station is characterized by an insignificant diurnal amplitude: it is about 5 μg/m3 in winter and increases by almost 3–4 times in the warm season. However, due to increased nighttime ozone, it remains significantly smaller than at lowland stations. In the dark time of the day, surface ozone is 50–55 µg/m3 in winter and autumn and about 70 µg/m3 in spring and summer. The surface ozone regime is significantly affected by the breeze circulation and, at night, by slope winds providing the influx of ozone from the free troposphere. At the background monitoring station, surface ozone values approaching the maximum single permissible concentration of ozone (MPCm.s.) are occasionally recorded. In spring, such events are associated with the movement of the upper-level frontal zone over the region. The registered cases of exceeding MPCm.s. were observed in the plumes of air polluted in remote areas and spreading to Crimea. Keywords: surface ozone, Karadag Nature Reserve, seasonal and diurnal variation, maximum permissible concentration of ozone

Variation of Ozone, Carbon Monoxide, and Oxides of Nitrogen at Bengaluru, India

The present study analyses the continuous in-situ observations of surface ozone (O3), carbon monoxide (CO), and nitrogen oxides (NOX) conducted in an urban location, Bengaluru, India, during the year 2019 (January to December). The seasonal concentration of O3 fluctuated with the highest concentrations in the summer (39.6 ppbv) and winter (40.4 ppbv) and the lowest concentrations during the monsoon (16.8 ppbv). The seasonal mixing ratio of CO showed the highest value in post-monsoon (1.71 ppmv) and lowest during monsoon (0.79 ppmv). The seasonal trend of NOX showed highest in winter (56.8 ppbv) and lowest in monsoon (22.5 ppbv). The monthly mixing ratios of O3, CO, and NOX showed distinct variability, which may be attributed to changing anthropogenic activities, planetary boundary layer processes, and local meteorology. O3 was significantly related to temperature but inversely associated with relative humidity and wind speed. The association between CO, NOX with relative humidity, temperature, wind speed showed discrete results. The (dO3/dt) in the morning and evening duration were about 5.0 ppbv/h and -4.1 ppbv/h respectively.

Surface UV-assisted retrieval of spatially continuous surface ozone with high spatial transferability

With the rapid development of artificial intelligence, machine-learning has been used broadly in surface ozone retrievals. However, the accuracies of current machine-learning-based models tend to rely largely on the spatial and temporal patterns of the site observations of surface ozone instead of the internal physical and chemical mechanisms of ozone, which also limit the transferability of the retrieval models. In this study, the surface ultraviolet (UV) at 380 nm, which is an important component of the rate-determining step of ozone photochemical production, is involved as an indicator in the newly developed surface ozone retrieval algorithm. And the imputation of missing satellite observations of ozone nitrogen dioxide (NO2) column and surface UV is conducted to obtain spatially continuous surface ozone. Different validation schemes and case studies are used to comprehensively evaluate the new algorithm. With the involvement of the surface UV, the new retrieval algorithm shows the high accuracy (R2 = 0.853 and RMSE = 17.09 μg/m3) in spatially continuous surface ozone estimation. More significantly, the new algorithm shows outstanding spatial transferability, which has been a critical challenge for machine-learning models on surface ozone estimation.

Deep Learning to Evaluate US NOx Emissions Using Surface Ozone Predictions

AbstractEmissions of nitrogen oxides (NOx = NO + NO2) in the United States have declined significantly during the past three decades. However, satellite observations since 2009 indicate total column NO2 is no longer declining even as bottom‐up inventories suggest continued decline in emissions. Multiple explanations have been proposed for this discrepancy including (a) the increasing relative importance of nonurban NOx to total column NO2, (b) differences between background and urban NOx lifetimes, and (c) that the actual NOx emissions are declining more slowly after 2009. Here, we use a deep learning model trained by NOx emissions and surface observations of ozone to assess consistency between the reported NOx trends between 2005 and 2014 and observations of surface ozone. We find that the satellite‐derived trends best reproduce ozone in low NOx emission (background) regions. The 2010–2014 trend from older satellite‐derived emission estimates produced at low spatial resolution results in the largest bias in surface ozone in regions with high NOx emissions, reflecting the blending of urban and background NOx in these low‐resolution top‐down analyses. In contrast, the trend from higher resolution satellite‐based estimates, which are more capable of capturing the urban emission signature, is in better agreement with ozone in high NOx emission regions, and is consistent with the trend based on surface observations of NO2. Our results confirm that the satellite‐derived trends reflect anthropogenic and background influences.

Biases in air quality models capturing ozone trends at the urban, regional and national scales: Impacts on Relative Response Factors (RRFs)

Chemical transport models (CTMs) are widely used in scientific studies and air quality management. A particular application is to assess how pollutant concentrations, particularly ozone and PM, respond to emission controls. As part of a dynamic evaluation, two widely used CTMs, CMAQ and CAMx, were evaluated for how well they captured trends in observed surface ozone, as well as the changes in associated concentrations, between 2001 to 2011 and 2011 to 2016. Those periods were chosen as additional effort was used to improve emissions estimates for all three years. Prior studies found that both emissions inventories and observations indicate reductions in both ozone and NO2 during these periods, but studies have found that NO2 observations have not agreed as well with estimated emissions. In general, given the efforts to harmonize model inputs, the two models performed very similarly and captured the ozone declines for both the 2001–2011 and 2011–2016 periods at the national level, though moderate biases were found in some locations. At the national scale, ozone trends for the 2011–2016 period were better captured by the models than those for the 2001–2011 period. Both models overestimated the observed decrease in ground level NO2 at the higher end of the concentration spectrum. This is linked to, and can cause, the detected tendency of the models to have larger than observed increases in ozone at the lower end over time. At the metropolitan statistical area (MSA) level, the performance of capturing the ozone trends varied. For some MSA's, the models estimated the wrong direction in the trends; e.g., in Denver, where maximum 8-hr average ozone levels increased from 2001 to 2011, the models predicted a decrease. The US EPA recommends location specific Relative Response Factors (RRFs) to adjust results when applying the models for air quality management purposes. At the regional level, the simulated ozone RRFs were up to 8% larger than the observed RRFs for the 2001–2011 period, suggesting that the models underestimated the ozone decrease. For the 2011–2016 period, with a couple of exceptions, the simulated RRFs were up to 9% smaller than observed RRFs, indicating a negative bias in the simulated trend. For individual MSAs, the ratio of the simulated to observed RRFs ranged between 0.86 (Knoxville for 2001–2011) and 1.40 (Los Angeles for 2001–2011). This is possibly linked to potential biases in the estimated emissions trends. Such high biases could lead to similar biases in the estimated emissions controls required for an area to attain the air quality standards. The results indicate that there are both spatial and temporal biases in how well the two models have captured the observed ozone and NO2 trends. Air quality planning agencies should aim to diagnose and reduce such biases before using model results for air quality management.

Concurrent heatwaves and extreme Ozone (O3) episodes: combined atmospheric patterns and impact on human health

BACKGROUND AND AIM: In the context of the global environmental change, more recurrent and/or intense heat waves and extreme ozone episodes are likely to occur during the next decades. The main purpose of this research is to assess how extreme temperature may trigger the appearance of high ozone levels and how the concurrence of these two extremes could be linked to the synoptic general circulation and impacts human health and wellbeing. METHODS: We have used surface maximum temperature and ozone observations during extended summers (April–September) in two cities from Morocco: Casablanca and Marrakech, between 2010 and 2019. A percentile-thresholding method was applied to identify heatwaves and ozone episodes and a statistical approach was used to assess the connection between the observed parameters and climate indexes (North Atlantic Oscillation (NAO), Mediterranean Oscillation (MO), and Saharan Oscillation (SaOI)). The weather types behind the concurrence of extremes were identified and Heat and Air Quality Indexes were assessed to determine their combined health risks. RESULTS:Our findings show that the concurrence of heatwaves and ozone episodes depends both on the specific city and the large-scale atmospheric circulation. The likely synoptic pattern behind the concurrence of these events occurs when the country is under the combined influence of an anticyclonic area in the north and the Saharan trough extending the depression centered in the south. This pattern generates a warm southern flow and may foster photochemical pollution and then impact human health. CONCLUSIONS:Our study is an attempt to assess extreme temperature and ozone episodes combined features inducing large-scale atmospheric circulation and health effects. It explores the hypothesis that particular weather patterns increase the vulnerability of individuals especially those sensitive to air pollution. This work is a first step to the establishment of an alert system and will help to provide recommendations for coping with concurrent heatwaves and air pollution episodes. KEYWORDS: Ozone, Temperature extremes, North Atlantic Oscillation, Mediterranean Oscillation, Saharan Oscillation

The impact of Los Angeles Basin pollution and stratospheric intrusions on the surrounding San Gabriel Mountains as seen by surface measurements, lidar, and numerical models

Abstract. In this work, the impact of Los Angeles Basin pollution transport and stratospheric intrusions on the surface ozone levels observed in the San Gabriel Mountains is investigated based on a combination of surface and lidar measurements as well as WRF-Chem (Weather Research and Forecasting with Chemistry) and WACCM (Whole Atmosphere Community Climate Model) runs. The number of days with observed surface ozone levels exceeding the National Ambient Air Quality Standards exhibit a clear seasonal pattern, with a maximum during summer, when models suggest a minimum influence of stratospheric intrusions and the largest impact from Los Angeles Basin pollution transport. Additionally, measured and modeled surface ozone and PM10 were analyzed as a function of season, time of the day, and wind direction. Measurements and models are in good qualitative agreement, with maximum surface ozone observed for southwest and west winds. For the prevailing summer wind direction, slightly south of the ozone maximum and corresponding to south-southwest winds, lower ozone levels were observed. Back trajectories suggest that this is associated with transport from the central Los Angeles Basin, where titration limits the amount of surface ozone. A quantitative comparison of the lidar profiles with WRF-Chem and WACCM models revealed good agreement near the surface, with models showing an increasing positive bias as function of altitude, reaching 75 % at 15 km above sea level. Finally, three selected case studies covering the different mechanisms affecting the near-surface ozone concentration over the San Gabriel Mountains, namely stratospheric intrusions and pollution transport, are analyzed based on surface and ozone lidar measurements, as well as co-located ceilometer measurements and models.

On the understanding of surface ozone variability, its precursors and their associations with atmospheric conditions over the Delhi region

The present study investigates the variability of surface ozone (O3) and its precursors (NOX, CO) at different time scales for 3-year period (2016–2018) using continuous ground-based observations from 25 stations located within the Indian Capital Delhi (28.83°N, 77.81°E). Observations indicate that there is a clear spatial heterogeneity and seasonality in surface O3 where larger magnitudes (40–60 ppb) are observed during pre-monsoon season, and 10–20 ppb in other seasons. The frequency distribution exhibits that 70% of O3 concentrations are in the range of 10–50 ppb, while 10% lies larger than 50 ppb and the remaining 20% is mostly confined within 50–80 ppb. Further, a detailed analysis on the diurnal cycle of surface ozone and its precursors performed over IMD Lodhi Road location. Analysis of diurnal cycles show maximum and minimum O3 concentrations during the morning and nocturnal hours, with a rate of change 4.8 ± 1.96 and −7.4 ± 0.95 ppb h−1, respectively. On the other hand, the rate of change of O3 during high ozone days (HOD) is found to be 5.61 ± 2.6 ppb h−1 in the morning and −8.04 ± 3.4 ppb h−1 in the evening hours, respectively. It is also noted that the increase in precursor concentrations and associated titration effects appear to play a considerable role in surface O3 depletion during the evening hours. It is also noted from the composites of HODs that there is an enhanced diurnal O3 (80–90 ppb) concentrations compared to 3-year average (40–60 ppb). Further evaluation of associations between surface O3 changes and meteorological parameters shows that observed surface O3 (Copernicus Atmosphere Monitoring Service-CAMS modelled O3) show positive correlations of R2 = 0.3 (0.42) with temperature, of R2 = 0.47 (0.60) with Solar Radiation (SR), of R2 = 0.39 (0.59) with boundary layer height. An overall assessment shows CAMS modelled three year daily averaged ozone values are generally in good agreement with surface ozone observations with a CC value of about R2 = 0.65.

Ozone Content over the Russian Federation in 2020

The review is based on the results of operation of the total ozone (TO) monitoring system in Russia and neighboring countries that functions in the operational mode at the Central Aerological Observatory. The monitoring system uses data from the national network equipped with M-124 filter ozonometers being under the methodological supervision of the Main Geophysical Observatory. The quality of the functioning of the entire system is operationally controlled using the OMI satellite equipment observations (NASA, USA). Basic TO observation data are generalized for each month of the fourth quarter of 2020, for the fourth quarter, and for the whole year. Data of routine observations of surface ozone in the Moscow region are also presented.

Ozone Content over the Russian Federation in 2020

The review is based on the results of operation of the total ozone (TO) monitoring system in Russia and neighboring countries that functions in the operational mode at the Central Aerological Observatory. The monitoring system uses data from the national network equipped with M-124 filter ozonometers being under the methodological supervision of the Main Geophysical Observatory. The quality of the functioning of the entire system is operationally controlled using the OMI satellite equipment observations (NASA, USA). Basic TO observation data are generalized for each month of the fourth quarter of 2020, for the fourth quarter, and for the whole year. Data of routine observations of surface ozone in the Moscow region are also presented.

Concurrence of heatwaves and extreme ozone episodes and impact on health and well-being

Climate change and air pollution are universal related problems and their combined monitoring, at national, regional and local scales, is a worldwide primary public concern. Industrial and traffic activities emit various pollutants that are harmful to health, among which we count ozone (O3) which concentrations may be accentuated under the influence of sunny days. Ozone in the air we breathe can cause respiratory diseases and can trigger a variety of health problems including chest pain, coughing, throat irritation, and airway inflammation. It can also reduce lung function and harm lung tissue. People most at risk include people with asthma, children, older adults, and people who are active outdoors. Temperature is the first meteorological factor to be directly involved in leading to ozone extreme events. In Morocco, the cities of Casablanca and Marrakech are metropolises whose rate of urbanization and population density are the highest in the country, they concentrate many large industrial units and a large vehicle fleet generating pollution. Urban and atmospheric conditions in both cities make them vulnerable to climate change extremes and air pollution episodes that may directly affect population health and wellbeing. In the context of global climate change, more frequent and/or persistent heatwaves and extreme ozone episodes are likely to occur during the next decades and a key question is about the concurrence of these extremes and their joint effect on human health. In this work, using surface temperature and ozone observations during extended summers (April-September), between 2009 and 2016, in Casablanca and Marrakech, we evaluate the thermal comfort and air quality health indexes related to concurrent heatwaves and ozone episodes in order to assess their impact on health and wellbeing. We show that the extremes in temperature and ozone cluster together in many cases and cause the two indexes to worsen.

Concentration and Isotopic Composition of Methane, Associated Gases, and Black Carbon over Russian Arctic Seas (Shipborne Measurements)

In Russian Arctic seas, observations of surface methane concentrations (CH4), ozone, nitrogen and carbon oxides, as well as the content of $${{\delta }^{{13}}}{{{\text{C}}}_{{{\text{C}}{{{\text{H}}}_{4}}}}}$$ isotope and black carbon (BC, soot aerosol), were carried out from onboard of the R/V Academician Mstislav Keldysh. The areas of local methane releasing from bottom sediments were investigated. It was shown that the studied methane releasing on the Arctic shelf are of a local nature and, on the whole, insignificantly affect the composition of the atmosphere in the region. The average concentration of methane in the surface air in the Arctic seas is mainly determined by large-scale processes of air mass transfer. An analysis of the distribution of black carbon along the route of the vessel was carried out. It was found that the excess in the concentration of black carbon over the background values is observed occasionally during advection of air masses from the mainland and from the areas of associated gases burning and forest fires. The effect of emissions from a ship’s chimney on the data obtained was analyzed.

Influences of oceanic ozone deposition on tropospheric photochemistry

Abstract. The deposition of ozone to seawater is an important ozone sink. Despite constituting as much as a third of the total ozone deposition, it receives significantly less attention than the deposition to terrestrial ecosystems. Models have typically calculated the deposition rate based on a resistance-in-series model with a uniform waterside resistance. This leads to models having an essentially uniform deposition velocity of approximately 0.05 cm s−1 to seawater, which is significantly higher than the limited observational dataset. Following from Luhar et al. (2018) we include a representation of the oceanic deposition of ozone in the GEOS-Chem model of atmospheric chemistry and transport based on its reaction with sea-surface iodide. The updated scheme halves the calculated annual area-weighted mean deposition velocity to water from 0.0464 cm s−1 (25th and 75th percentiles of 0.0461 cm s−1 and 0.0471 cm s−1 respectively) to 0.0231 cm s−1 (25th and 75th percentiles of 0.0121 cm s−1 and 0.0303 cm s−1 respectively). The calculated ozone deposition velocity varies from 0.009 cm s−1 in polar waters to 0.040 cm s−1 at the tropics. This improves comparisons to observations. The variability is driven mainly by the temperature-dependent rate constant for the reaction between iodide and ozone, the temperature dependence of the solubility, and variations in the ocean iodide concentration. The calculated annual deposition flux of ozone to the ocean is reduced from 222 to 122 Tg yr−1, and overall deposition of ozone to all surface types reduces from 862 to 758 Tg yr−1. Tropospheric ozone burdens and global mean OH increase from 324 to 328 Tg, and from 1.17×106 to 1.18×106 molec.cm-3, respectively. A total of 34 % of surface grid boxes experience a 10 % or greater increase in ozone concentration. Comparisons between observations of surface ozone and the model are improved with the new parameterization notably around the Southern Ocean. Process-level representation of oceanic deposition of ozone thus appears essential for representing the concentration of surface ozone over the planet.

Evaluation of El Niño-Southern oscillation influence on ozone exceedances along the United States Gulf Coast

An investigation into climate-scale influences on tropospheric ozone concentrations in cities along the United States Gulf Coast was performed using a thirty-year database of observed surface ozone data. Hourly surface ozone data were obtained from eight surface Environmental Protection Agency monitoring stations from Texas to Florida. Eight-hour peak concentrations were computed for each day in the record and a database created for further study. To account for regulatory changes, each time history was detrended over the thirty-year period. Days with precipitation, which tend to have low surface ozone concentrations due to low solar insolation and wet deposition of ozone precursors on surfaces, were also removed. The resulting database was stratified by Oceanic Niño Index, which is a measure of the state of the El Niño-Southern Oscillation (ENSO) based on water temperatures in the eastern Pacific Ocean. A common seasonal dependency was identified for all eight stations, with peaks in ozone concentrations in spring and autumn regardless of ENSO phase. The summer ozone minimum was more pronounced for the western Gulf of Mexico than the east. This extension of work done for the city of Houston, Texas confirmed that the influence of ENSO on surface air quality applies to the entire coast of the Gulf of Mexico. Statistically significant enhancements of 5–10 ppbv were found during the months of March and April for El Niño years relative to neutral years. Distributions fit to each city's dataset of 8-h peak ozone values for spring and fall confirmed the difference between the behavior of ozone concentrations in spring and fall for different ENSO phases. El Niño spring data possessed a broader distribution more centered at a higher mean value than the neutral or La Niña datasets for each city. Taken together, the higher mean and broader distribution of 8-h ozone concentrations lead to 30% more exceedances of the National Ambient Air Quality Standard of 70 ppbv for El Niño years relative to neutral years for the composite Gulf Coast dataset. These changes in the ozone climate of the Gulf Coast during El Niño years can be attributed in part to an area of anomalously low pressure in the eastern Gulf of Mexico which influences meridional and zonal flow along the coast. This feature leads to stagnation and a persistent continental airmass along the northern Gulf coast. Smaller but still statistically significant differences between El Niño and neutral autumn were also identified for some stations and were attributed to increased frontal activity during El Niño autumn which leads to an earlier, longer ozone season in previous research. This work extends and improves understanding of the influence of the El Niño-Southern Oscillation on air quality in a large and economically important region of the United States.

Observed ozone over the Chesapeake Bay land-water interface: The Hart-Miller Island Pilot Project

ABSTRACT Enhanced ozone concentrations at land-water interfaces create National Ambient Air Quality Standard (NAAQS) compliance issues across the United States. The northern Chesapeake Bay experiences higher ozone at sites adjacent to the Bay, creating ozone compliance concerns for the state of Maryland. Accordingly, the Maryland Department of the Environment sited an ozone monitor at Hart-Miller Island (HMI) within the northern Chesapeake Bay (NCB) and gathered a continuous ozone and meteorological record over 278 days within the 2016 and 2017 ozone seasons. The representative water site was the highest ozone monitor in the state 28% of all days and 75% when any ozone monitor in the state experienced ozone above the 2015 ozone NAAQS (70 ppbv), known as an exceedance day. In total, 24 exceedance days were observed at HMI. Numerical ozone predictions produced by an operational version of the Community Multi-scale Air Quality (CMAQ) model forecast 52 such days with a high bias of 15.5% in daily maximum ozone concentration during the same period. Trajectory modeling indicated over 70% of exceedance days possessed northwesterly transport over the Baltimore area, with HYSPLIT trajectories descending at least 500 m in greater than 80% of cases toward the NCB surface. These trajectories possessed a button-hook pattern during descent to create southerly surface winds at HMI that may impact coastal sites, creating ozone events at Maryland monitors such as Edgewood. Consequently, the NCB was influenced by the residual layer and from both regional long-range transport and locally sourced ozone precursors. Changes in local meteorology and emissions had a significant impact on over-water ozone concentrations and forecasts. Results of the multi-season ozone pilot study over the Chesapeake Bay provided a conceptual model of high ozone development over water downwind of a large urban center and guidance for future study of the NCB area. Implications: Multi-seasonal observations of surface ozone and meteorology over the water of the northern Chesapeake Bay showed specific conditions leading to degraded air quality. The novel data set collected offers a deeper understanding of over-water ozone magnitude, occurrence, and transport across the land-water interface and comparison to air quality models not before possible.

An evaluation of the ability of the Ozone Monitoring Instrument (OMI) to observe boundary layer ozone pollution across China: application to 2005–2017 ozone trends

Abstract. Nadir-viewing satellite observations of tropospheric ozone in the UV have been shown to have some sensitivity to boundary layer ozone pollution episodes, but so far they have not yet been compared to surface ozone observations collected by large-scale monitoring networks. Here we use 2013–2017 surface ozone data from China's new Ministry of Ecology and Environment (MEE) network of ∼ 1000 sites, together with vertical profiles from ozonesondes and aircraft, to quantify the ability of tropospheric ozone retrievals from the Ozone Monitoring Instrument (OMI) and to detect boundary layer ozone pollution in China. We focus on summer when ozone pollution in China is most severe and when OMI has the strongest sensitivity. After subtracting the Pacific background, we find that the 2013–2017 mean OMI ozone enhancements over eastern China have strong spatial correlation with the corresponding multiyear means in the surface afternoon observations (R=0.73), and that OMI can estimate these multiyear means in summer afternoon surface ozone with a precision of 8 ppb. The OMI data show significantly higher values on observed surface ozone episode days (>82 ppb) than on non-episode days. Day-to-day correlations with surface ozone are much weaker due to OMI noise and are stronger for sites in southern China (<34∘ N; R=0.3–0.6) than in northern China (R=0.1–0.3) because of weaker retrieval sensitivity and larger upper tropospheric variability in the north. Ozonesonde data show that much of the variability of OMI ozone over southern China in summer is driven by the boundary layer. Comparison of 2005–2009 and 2013–2017 OMI data indicates that mean summer afternoon surface ozone in southern China (including urban and rural regions) has increased by 3.5±3.0 ppb over the 8-year period and that the number of episode days per summer has increased by 2.2±0.4 (as diagnosed by an extreme value model), generally consistent with the few long-term surface records. Ozone increases have been particularly large in the Yangtze River Delta and in the Hubei, Guangxi and Hainan provinces.

So near yet so different: Surface ozone at three sites in Malaysia

Surface ozone is an air pollutant and greenhouse gas. This study attempts to identify some key challenges in ozone mitigation based on surface ozone observations at three locations within and near Klang Valley, Malaysia. Surface ozone data obtained from the Department of Environment Malaysia for Petaling Jaya (S1), Shah Alam (S2) and Cheras (S3) stations are analysed to show variation in ozone between stations that are in close proximity. Initial statistical analysis results indicate a large difference in the frequency of total daily maximum ozone concentration at S2 compared to S1 and S3 stations. The application of IoT could likely be a viable option for determining sensitivity to precursors and meteorological conditions once the technology has further matured.

A Case Study of Stratospheric Ozone Transport to the Northern San Francisco Bay Area and Sacramento Valley during CABOTS 2016

AbstractThe California Baseline Ozone Transport Study (CABOTS) was a major air quality study that collected ozone measurements aloft between mid-May and mid-August of 2016. Aircraft measurements, ground-based lidar measurements, and balloon-borne ozonesondes collected precise upper-air ozone measurements across the central and Southern California valley. Utilizing daily ozonesonde data from Bodega Bay, California, and Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2), reanalysis data for 25 July to 14 August 2016, three stratospheric intrusion events are identified over Northern California influencing air masses above Bodega Bay and Sacramento simultaneously. Calculated percent daily changes in afternoon ozonesonde observations indicate increasing ozone concentrations from the point of likely stratospheric air injection with the arrival of higher potential vorticity, confirmed by ensemble back trajectories. An analysis of the onsite surface monitoring ozone data indicates ozone increases in the observations for dates of plausible low-level stratospheric air influence. Further, a comparison of Bodega Bay surface ozone observations and 14 Sacramento Valley nonattainment zone surface sites show that the surface ozone observed at the higher-elevation surface sites in the lower Sierra Nevada foothills were positively correlated with elevated ozone captured by the ozonesondes within the lowest 0.5–1 km. The strongest correlations observed (~0.61) were between elevated Bodega Bay ozonesonde data and the Placerville (~612 m) afternoon surface ozone data, an indication that these regions separated by 200 km would be influence by the same ozone source. A comparison of daily changes in afternoon ozone show that the two locales often experience similar daily ozone increases or decreases. While this study leads to a basic quantification of stratospheric influence on surface ozone in the Sacramento nonattainment zone, a future campaign that examines ozone and winds aloft at both locales is suggested to improve the quantification of stratospheric ozone.