Surface Ozone Measurements Research Articles

Summertime response of ozone and fine particulate matter to mixing layer meteorology over the North China Plain

Abstract. Measurements of surface ozone (O3), PM2.5 and its major secondary components (SO42-, NO3-, NH4+, and organic carbon (OC)), mixing layer height (MLH), and other meteorological parameters were made in the North China Plain (NCP) during the warm season (June–July) in 2021. The observation results showed that the summertime regional maximum daily 8 h average ozone (MDA8 O3) initially increased and reached the maximum value (195.88 µg m−3) when the MLH ranged from approximately 900 to 1800 m, after which the concentration of O3 decreased with further increase in MLH. Interestingly, synchronous increases in PM2.5 concentration along with the development of the mixing layer (MLH <1200 m) were observed, and the positive response of PM2.5 to MLH was significantly associated with the increase in SO42- and OC. It was found that this increasing trend of PM2.5 with elevated MLH was driven not only by the wet deposition process but also by the enhanced secondary chemical formation, which was related to appropriate meteorological conditions (50 % < RH <70 %) and increased availability of atmospheric oxidants. Air temperature played a minor role in the change characteristics of PM2.5 concentration, but it greatly controlled the different change characteristics of SO42- and NO3-. The concentrations of PM2.5, its major secondary components, and the oxidation ratios of sulfate (SOR) and nitrate (NOR) increased synchronously with elevated MDA8 O3 concentrations, and the initial increase in PM2.5 along with increased MLH corresponded well with that of MDA8 O3. We highlight that the correlation between MLH and secondary air pollutants should be treated with care in hot weather, and the superposition-composite effects of PM2.5 and O3 along with the evolution of mixing layer should be considered when developing PM2.5–O3 coordinated control strategies.

Meteorological and chemical controls on surface ozone diurnal variability in Beijing: A clustering-based perspective

This study aimed to reveal the dominant, recurring patterns of surface ozone diurnal cycles in Beijing, their quantitative characteristics, namely, occurrence frequency and transition probability, and their potential linkages to meteorological and chemical processes based on multi-source observations (surface ozone, satellite-based ozone precursors, ground-based and sonde-based meteorological measurements) and reanalyses (ERA5 meteorological and TCR2 chemical reanalyses). By applying K-means clustering, four distinct ozone diurnal patterns (Cluster 1: all-day low anomaly; Cluster 2: nighttime high-daytime low anomaly; Cluster 3: nighttime low-daytime high anomaly; Cluster 4: extreme pollution pattern with all-day high anomaly) are objectively identified from 2013 to 2020 warm-season (May–September) surface ozone measurements. Despite a relatively low total occurrence, the extreme pollution pattern tended to persist over an extended period (average persistence of 2.3 days). Composite analyses of multi-source meteorology indicate that the elevated nocturnal ozone in Cluster 2 is associated with the compensating downdraft flow of nocturnal convective storms. By contrast, elevated nocturnal ozone in Cluster 4 was associated with the prevailing low-level jet (LLJ) over the whole North China Plain (NCP) region. The LLJ-related dynamical processes trigger regional transport in the residual layer and downward mixing from the residual layer, elevating the background ozone level in downstream Beijing. Moreover, Cluster 4 is under the control of 500-hPa anomalous high pressure, which leads to daytime hot-dry weather favoring the occurrence of extreme afternoon pollution via photochemical ozone production. From Cluster 1 to Cluster 4, satellite-based formaldehyde (CH2O) increased by 71% accompanied by less change in nitrogen dioxide (NO2), causing a shift in ozone production sensitivity. Because extreme pollution pattern typically occurs in the transition regime, synergetic emission control of volatile organic compounds (VOCs) and nitrogen oxides (NOx) would be effective in alleviating severe ozone pollution in Beijing.

16 Years (2006–2021) of Surface Ozone Measurements in Córdoba (Southern Spain): Trends and the Impact of the COVID-19 Lockdown

Surface ozone concentrations (O3) during the period 2006-2021 are analysed at Córdoba city (southern Iberian Peninsula) in suburban and urban sampling sites. The aims are to present the levels and temporal variations, to explore trends and to quantity the variation in O3 concentrations in the context of the COVID-19 lockdown. The O3 means are higher in the suburban station (62 µg m-3 and 51.3 µg m-3), being the information level threshold only exceeded twice during this period. The daily evolution shows a maximum at about 17:00 UTC, whereas the minimum is reached at about 9:00 UTC, with higher levels in the suburban station. The seasonal evolution of this daily cycle also presents monthly differences in shape and intensity between stations. The trends are analysed by means of daily averages and daily 5th and 95th percentiles, and they show a similar increase in all of these parameters, with special emphasis on the daily P95 concentrations, with 0.27 µg m-3 year-1 and 0.24 µg m-3 year-1. Finally, the impact of the COVID-19 lockdown shows a decline in O3 concentrations over 10%.

Relationship of Surface Ozone (O3) with its precursors and meteorological parameters over New Delhi, India

In the present study, continuous measurements of Surface Ozone (O3), Oxides of Nitrogen (NOx (NO+ NO2)), and carbon monoxide (CO), monitored at five different locations in Delhi National Capital Region have been studied for the period 2013 – 2019. The five monitoring locations used are namely IMD Lodi Road, IGI Airport Palam, CV Raman Dheerpur, CRRI Mathura Road, and NCMRWF Noida. The average hourly concentration of O3, NO, NO2, CO, NOx (NO + NO2), and OX(NO2 +O3) are found in the range of 32.44 ppb to 36.57 ppb, 19.46 to 28.09 ppb, 20.83 to 26.89 ppb,1.67 to 1.89 ppm,43.04 to 54.99 ppb, and 54.06 to 60.99 ppb respectively during the study period. Diurnal variation of NOx and CO Concentrations show higher values during the morning (0600-0900h) and late evening (1900-2400h) hours while the highest concentrations of ozone have been observed during afternoon hours. The relationship between NO, NO2, and surface O3 as a function of NOx has also been examined during daylight hours (0500hrs IST to 1900 hrs IST) and chemical coupling of the three species i.e. NO, NO2 and O3 have been studied. The ground-level concentration of Ozone has been found to decrease with increasing NOx concentration during the daytime. The variations in concentrations of oxidants (NO2 + O3) with the concentration of [NOx] have been studied to examine the contributing pollution sources of oxidants at all the study sites. The average rate of change of O3 concentrations (dO3/dt) has been examined at all five locations. The monthly and diurnal variation of oxidants [OX] at all the study locations has shown a strong positive correlation with temperature whereas a negative correlation with humidity.

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

The influence of the summer tropospheric circulation on the observed ozone mixing ratios at a coastal site in the Eastern Mediterranean

This study combines surface ozone (O3) measurements at the Finokalia site (Eastern Mediterranean) with backward trajectory modeling results, reanalysis, and satellite data to investigate how the summer tropospheric circulation affects the O3 mixing ratios in the Eastern Mediterranean. The whole dataset covers the summer months (June, July, August) of 2011–2018. Our findings show that north-northeastern flow, known as the Etesians, is a high frequent wind system over the Eastern Mediterranean during the summer period. Under this flow regime, polluted air masses from the continental central and Eastern Europe reach the monitoring site, increasing the O3 mixing ratios of the area. Contrary, western flows carrying maritime air masses from the Mediterranean Sea, though less frequent, have a central role in controlling the annual O3 mixing ratios in the Eastern Mediterranean. Furthermore, the vertical O3 profile analysis over the Finokalia site indicates enhanced O3 mixing ratios in the middle troposphere when northern flows prevail, confirming the significant relationship between the low-level summer atmospheric circulation and the downward transport of O3-rich air masses to the lower troposphere over Finokalia station, Eastern Mediterranean.

Measurements of surface ozone and its precursors in different microenvironments of coastal Indian metropolis of Mumbai

Measurements of surface ozone and its precursors in different microenvironments of coastal Indian metropolis of Mumbai

Photochemical reactions and surface ozone measurements in Tehran city center

Efforts have been made for surface ozone concentration measurements considering secondary reactions via actinometry. Pyro heliometry, pyranometry and spectrophotometry, idometry in Amir Abad station of Tehran city center in parallel. In actinometry method consideration were made to show solar radiation in all different filters of green, yellow, red and dark red by means of 525 nm, 630 nm, 695 nm and 721 nm in parallel by the same time during 1991–1992. Resulted as solar radiation reduction in all filters and concluded for secondary reactions at Amirabad station for the first time in Iran. Measurements were made daily and seasonally at midday in Amirabad station. Where in idometry and spectrophotometry method consideration were made in certain wavelengths of 276.5 nm and 301 nm, for surface ozone measurements during autumn winter considering, October, November, December 1991–1992 and 1999–2001, using rain samples. Which has shown a concentration range of 30–60 (ppb) and 80–115 (ppb), respectively. The concentration measurements of surface ozone were made as a function of photochemical reactions of NO2. NO and photon rays in agreement with the results of spectrometry method by the same time due to F.M. Shahrtash for the first time in Iran. This study was in agreements with the works in Montreal, Ca (1992). Other consideration was made for surface ozone data collection analysis of (MOI) from Mehrabad station of Tehran city center via Dobson method during summer–autumn 2015. Which has shown a range of concentration of 80–92 (ppb), in comparison with the measurements of Amirabad station. Besides consideration was made for recent research work in China, which has detected the surface ozone concentration of 70–100 ppb during 2013–2018, mainly in North China and Yangtze river plain” by means of “photochemical reactions and surface ozone” in agreement with this study as a whole.

Observation and modeling of high-<sup>7</sup>Be concentration events at the surface in northern Europe associated with the instability of the Arctic polar vortex in early 2003

Abstract. Events of very high concentrations of 7Be cosmogenic radionuclide have been recorded at low-elevation surface stations in the subpolar regions of Europe during the cold season. With an aim to investigate the mechanisms responsible for those peak 7Be events, and in particular to verify if they are associated with the fast descent of stratospheric air masses occurring during sudden stratospheric warming (SSW) events, we analyze 7Be observations at six sampling sites in Fennoscandia during January–March 2003 when very high 7Be concentrations were observed and the Arctic vortex was relatively unstable as a consequence of several SSW events. We use the GEOS-Chem chemistry and transport model driven by the MERRA-2 meteorological reanalysis to simulate tropospheric 7Be over northern Europe. We show that the model reasonably reproduces the temporal evolution of surface 7Be concentrations observed at the six sampling sites. Our analysis of model simulations, surface 7Be observations, atmospheric soundings of ozone and temperature and surface ozone measurements indicates that the 7Be peak observed in late February 2003 (between 20 and 28 February 2003) at the six sampling sites in Fennoscandia was associated with downward transport of stratospheric vortex air that originated during an SSW that occurred a few days earlier (between 18 and 21 February 2003).

Analysis of Ozone Concentrations between 2002–2020 in Urban Air in Northern Spain

This paper analyses surface ozone measurements at five stations in an urban area (Valladolid) in the upper Spanish plateau over the period 2002–2020. Temporal evolutions, the relationship between ozone and other pollutants such as nitrogen oxides, and the assessment of the ozone concentration trend during the study period were analysed. Daily evolution of ozone at all the stations showed mean maximum concentrations in the afternoon, 15:00 GMT, with values ranging between 73.8 and 80.9 µg m−3, evidencing the influence of photochemical processes favoured by solar radiation in ozone formation. The lowest levels were recorded at night and in the early morning, 7:00 GMT, and were between 23.4 and 32.3 µg m−3, related with the reduction by NO reactions and deposition processes. A broad spring–summer peak between May and July was seen, with the highest values in the latter, with a mean value of up to 73.8 µg m−3. The variation in the monthly mean ozone concentrations of the different percentiles was analysed using a harmonic model. The empirical equation described the experimental values satisfactorily, with a confidence level of 95% and coefficients of determination above 80%, confirming the major decreasing trend in the ozone peak values over the study period.

The Increasing Surface Ozone and Tropospheric Ozone in Antarctica and Their Possible Drivers.

A comprehensive analysis of the temporal evolution of tropospheric ozone in Antarctica using more than 25 years of surface ozone and ozonesonde measurements reveals significant changes in tropospheric ozone there. It shows a positive trend in ozone at the surface and lower and mid-troposphere, but a negative trend in the upper troposphere. We also find significant links between different climate modes and tropospheric ozone in Antarctica and observe that changes in residual overturning circulation, the strength of the polar vortex, and stratosphere-troposphere exchange make noticeable variability in tropospheric ozone. Therefore, this study alerts of increasing ozone concentration in Antarctica, which would have a profound impact on the future climate of the region as tropospheric ozone has warming feedback to the Earth's climate.

A study of the effect of aerosols on surface ozone through meteorology feedbacks over China

Abstract. Interactions between aerosols and gases in the atmosphere have been the focus of an increasing number of studies in recent years. Here, we focus on aerosol effects on tropospheric ozone that involve meteorological feedbacks induced by aerosol–radiation interactions. Specifically, we study the effects that involve aerosol influences on the transport of gaseous pollutants and on atmospheric moisture, both of which can impact ozone chemistry. For this purpose, we use the UK Earth System Model (UKESM1), with which we performed sensitivity simulations including and excluding the aerosol direct radiative effect (ADE) on atmospheric chemistry, and focused our analysis on an area with a high aerosol presence, namely China. By comparing the simulations, we found that ADE reduced shortwave radiation by 11 % in China and consequently led to lower turbulent kinetic energy, weaker horizontal winds and a shallower boundary layer (with a maximum of 102.28 m reduction in north China). On the one hand, the suppressed boundary layer limited the export and diffusion of pollutants and increased the concentration of CO, SO2, NO, NO2, PM2.5 and PM10 in the aerosol-rich regions. The NO/NO2 ratio generally increased and led to more ozone depletion. On the other hand, the boundary layer top acted as a barrier that trapped moisture at lower altitudes and reduced the moisture at higher altitudes (the specific humidity was reduced by 1.69 % at 1493 m on average in China). Due to reduced water vapour, fewer clouds were formed and more sunlight reached the surface, so the photolytical production of ozone increased. Under the combined effect of the two meteorology feedback methods, the annual average ozone concentration in China declined by 2.01 ppb (6.2 %), which was found to bring the model into closer agreement with surface ozone measurements from different parts of China.

Impact of aerosols on surface ozone during COVID-19 pandemic in southern India: A multi-instrumental approach from ground and satellite observations, and model simulations

The World Health Organization (WHO) declared the coronavirus disease of 2019 (COVID-19) as a pandemic due to its widespread global infection. This has resulted in lockdown under different phases in many nations, including India, around the globe. In the present study, we report the impact of aerosols on surface ozone in the context of pre-lockdown (01st - 24th March 2020 (PLD)), lockdown phase1 (25th March to 14th April 2020 (LDP1)), and lockdown phase 2 (15th April to 03rd May 2020 (LDP2)) on clear days at a semi-arid site, Anantapur in southern India using both in situ observations and model simulations. Collocated measurements of surface ozone (O3), aerosol optical depth (AOD), black carbon mass concentration (BC), total columnar ozone (TCO), solar radiation (SR), and ultraviolet radiation (UV-A) data were collected using an Ozone analyzer, MICROTOPS sunphotometer, Ozonometer, Aethalometer, and net radiometer during the study period. The diurnal variations of O3 and BC exhibited an opposite trend during three phases. The concentrations of ozone were ~10.7% higher during LDP1 (44.8 ± 5.2 ppbv) than the PLD (40.5 ± 6.0 ppbv), which mainly due to an unprecedented reduction in NOx emissions leading to a lower O3 titration by NO. The prominent increase in the surface zone during LDP1 is reasonably consistent with the observed photolysis frequencies (j (O1D)) through Tropospheric Ultraviolet and Visible (TUV) model. The results show that a pronounced spectral and temporal variability in the AOD during three lockdown phases is mainly due to distinct aerosol sources. The increase in AOD during LDP2 due to long-range transport can bring large amounts of mineral dust and smoke aerosols from the west Asian region and central India, and which is reasonably consistent with the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) air mass back trajectories and Moderate Resolution Imaging Spectroradiometer (MODIS) fire counts analysis over the measurement location. Overall, a drastic reduction in BC concentration (~8.4%) and AOD (10.8%) were observed in the semi-arid area during LDP1 with correspondence to PLD. The columnar aerosol size distributions retrieved from the spectral AODs followed power-law plus unimodal during three phases. The absorption angstrom exponent (AAE) analysis reveals a predominant contribution to the BC from biomass burning activities during the lockdown period over the measurement location.

Identification of Sea Breeze Recirculation and Its Effects on Ozone in Houston, TX, During DISCOVER‐AQ 2013

AbstractIn coastal environments, sea breeze recirculation has been found to be an important mesoscale meteorological phenomenon that causes high ozone episodes, yet the identification of this small‐scale circulation pattern remains difficult. In this study, a new method was developed to automatically identify sea breeze recirculation in Houston, TX, by applying K‐Means clustering algorithm to surface winds measurements at near‐coast sites during the DISCOVER‐AQ (Deriving Information on Surface Conditions from Column and Vertically Resolved Observations Relevant to Air Quality) field campaign period from August to October 2013. The key to the clustering algorithm is seven features derived from site‐based surface winds on each day, including zonal (U) and meridional (V) winds in the morning and afternoon, 24‐hr transport direction (θ) and the recirculation factor, which is the ratio of net transport distance (L) to wind run distance (S). For comparison, the same clustering was applied to San Antonio, TX, a noncoastal city yet within the synoptic‐scale distance from Houston. Four clusters were obtained for each region, including three synoptic patterns common to both regions and one mesoscale pattern that differs by region, classified as Stagnation and Sea Breeze Cluster for San Antonio and Houston, respectively. The clustering outputs were verified by wind profiler data in Houston. By linking the wind clusters with surface and aircraft ozone measurements, we revealed a clear connection between circulation patterns and daily ozone variability showing that maximum daily average 8‐hr (MDA8) ozone levels and spatial distributions differed by cluster type (e.g., the highest ozone found in the Stagnation/Sea Breeze Cluster and the lowest ozone in the Southerly Cluster). This automatable method of sea breeze identification we developed can be potentially applied to other coastal cities because it has low data requirement and no ad hoc location‐specific adjustments.

Impacts of global NO<sub><i>x</i></sub> inversions on NO<sub>2</sub> and ozone simulations

Abstract. Tropospheric NO2 and ozone simulations have large uncertainties, but their biases, seasonality, and trends can be improved with NO2 assimilations. We perform global top-down estimates of monthly NOx emissions using two Ozone Monitoring Instrument (OMI) NO2 retrievals (NASAv3 and DOMINOv2) from 2005 to 2016 through a hybrid 4D-Var/mass balance inversion. Discrepancy in NO2 retrieval products is a major source of uncertainties in the top-down NOx emission estimates. The different vertical sensitivities in the two NO2 retrievals affect both magnitude and seasonal variations of top-down NOx emissions. The 12-year averages of regional NOx budgets from the NASA posterior emissions are 37 % to 53 % smaller than the DOMINO posterior emissions. Consequently, the DOMINO posterior surface NO2 simulations greatly reduced the negative biases in China (by 15 %) and the US (by 22 %) compared to surface NO2 measurements. Posterior NOx emissions show consistent trends over China, the US, India, and Mexico constrained by the two retrievals. Emission trends are less robust over South America, Australia, western Europe, and Africa, where the two retrievals show less consistency. NO2 trends have more consistent decreases (by 26 %) with the measurements (by 32 %) in the US from 2006 to 2016 when using the NASA posterior emissions. The performance of posterior ozone simulations has spatial heterogeneities from region to region. On a global scale, ozone simulations using NASA-based emissions alleviate the double peak in the prior simulation of global ozone seasonality. The higher abundances of NO2 from the DOMINO posterior simulations increase the global background ozone concentrations and therefore reduce the negative biases more than the NASA posterior simulations using GEOS-Chem v12 at remote sites. Compared to surface ozone measurements, posterior simulations have more consistent magnitude and interannual variations than the prior estimates, but the performance from the NASA-based and DOMINO-based emissions varies across ozone metrics. The limited availability of remote-sensing data and the use of prior NOx diurnal variations hinder improvement of ozone diurnal variations from the assimilation, and therefore have mixed performance on improving different ozone metrics. Additional improvements in posterior NO2 and ozone simulations require more precise and consistent NO2 retrieval products, more accurate diurnal variations of NOx and VOC emissions, and improved simulations of ozone chemistry and depositions.

Increasing ambient surface ozone levels over the UK accompanied by fewer extreme events

Abstract We report a statistical analysis of surface ozone (O3) measurements over the UK, 1999–2019. Data are from the Automatic Urban and Rural Network (AURN) that forms the basis of checking nationwide compliance with ambient air quality directives. We determine the mean O3 trend across all AURN sites to be 0.41 ± 0.07 μg m−3yr−1, composed of larger trends in suburban and urban environments (0.36 ± 0.11 μg m−3yr−1–0.75 ± 0.16 μg m−3yr−1) than in the rural environment (0.16 ± 0.07 μg m−3yr−1); consequently the gap between urban and rural surface O3 across the UK has steadily declined from 20 μg m−3 to 12 μg m−3 until 2012 when it has since stalled. These trends in urban surface O3 are coincident with and likely caused by a pronounced decline in surface concentrations of NO2 (proxy for NOx), which we link to reduced emissions from the transport sector. Using a gamma probability distribution to describe changes in surface O3, we find an increase in the probability of high mean O3 and an associated decrease in the probability of extreme (elevated) O3 events. We find no discernible trend in the number of days per year that exceed the 8-h mean O3 value of 100 μg m−3 set by the UK Department for Environment, Food and Rural Affairs (Defra) or a similar metric of 120 μg m−3 set by the EU. These exceedances, typically occurring during summer months across the network, range from 499 exceedances in 2010 to in excess of 2300 exceedances in 2003 and 2006. We find that 50% of these exceedances occur at temperatures greater than 18∘C (21∘C) in rural (urban) environments. With 50% (95%) of the 66 million UK inhabitants living and working within 12 km (50 km) of an AURN site, our analysis is relevant to a large fraction of the UK population. Consequently, we may need to re-prioritize epidemiological studies to understand the impact of chronic exposure to elevated mean O3 concentrations on human health.

Long-Term Variations of Air Quality Influenced by Surface Ozone in a Coastal Site in India: Association with Synoptic Meteorological Conditions with Model Simulations

Atmospheric ozone (O3) in the surface level plays a central role in determining air quality and atmospheric oxidizing capacity. In this paper, we review our comprehensive results of simultaneous measurements of surface ozone (O3) and its precursor gas (NOx) and weather parameters that were carried out continuously for a span of six years (January 2013–December 2018) at a typical rural coastal site, Kannur (11.9° N, 75.4° E) in South India. Surface O3 concentration reached its maximum during daytime hours and minimum during the night time. The influence of solar radiation and water content on variations of O3 are discussed. A Multi-Layer Perceptron (MLP) artificial neural network technique has been used to understand the effect of atmospheric temperature on the increase in O3 over the past six years. This has been found that temperature has been a major contributor to the increase in O3 levels over the years. The National Centre for Atmospheric Research- Master Mechanism (NCAR-MM) Photochemical box model study was conducted to validate the variations of O3 in different seasons and years, and the results were shown to be in good agreement with observed trends.

Surface mesovortices in relation to the urban heat island effect over the Saint Louis metropolitan area

Using high spatial and temporal resolution mesonet data, a comprehensive surface mesoanalysis of Saint Louis's Urban Heat Island (UHI) effect was conducted. The synoptic and mesoscale conditions were evaluated in which the surface mesovortices were observed. A long-term analysis of the surface mesovorices and four cases are presented as well as a hypothesis developed to explain the formation of the surface mesovortices.Results suggest that surface mesovortices were often observed in conjunction with UHI events and low wind speeds within the Saint Louis metropolitan area. The surface mesovortices are caused in part by the direct thermal circulation produced by the UHI enhancing existing vorticity.The surface mesovortices are also hypothesized to have impact on local air quality; surface ozone measurements at Saint Louis Science Center were noticed to spike during one surface mesovortex event. The surface mesovortices may re-circulate ozone created outside the immediate Saint Louis metropolitan area back into the immediate Saint Louis metropolitan area.

Long-term changes of regional ozone in China: implications for human health and ecosystem impacts

The first Tropospheric Ozone Assessment Report (TOAR) provides information on present-day distributions and long-term trends of ozone metrics relevant for climate change, human health, and vegetation. However, only few results are available in TOAR for China due to limited long-term ozone observations. Here, we present an integrated analysis of long-term measurements of surface ozone from eight sites distributed in the North China Plain (NCP) and Yangtze River Delta (YRD), the relatively underdeveloped region Northeast China, and the remote regions in Northwest and Southwest China. Trends and present-day values for seven annual and five seasonal ozone metrics were calculated following the TOAR methodologies. We compare the seasonal and diurnal cycles of ozone concentrations as well as the present-day values of ozone among sites and discuss the long-term trends in the ozone metrics. Large and significant increases of ozone are detected at the background site in the NCP, moderate increases at the global baseline site in western China, significant decreases at the northwestern edge of China, and nearly no trend at other sites. Extremely high values of ozone occurred in the NCP and YRD, particularly in warmer seasons. The present-day levels of summer ozone metrics in the NCP are much higher than the thresholds set in TOAR for the highest value groups of ozone metrics. The summer ozone metrics at the Shangdianzi background site in the NCP indicate increases at rates of more than 2%/yr during 2004–2016. In contrast, ozone at the Lin’an background site in the YRD was constant over the period 2006–2016. Our results fill some knowledge gaps in spatiotemporal changes of ozone in China and may be of useful in the assessment of ozone impacts on human health and vegetation.

Intercomparison of lidar, aircraft, and surface ozone measurements in the San Joaquin Valley during the California Baseline Ozone Transport Study (CABOTS)

Abstract. The California Baseline Ozone Transport Study (CABOTS) was conducted in the late spring and summer of 2016 to investigate the influence of long-range transport and stratospheric intrusions on surface ozone (O3) concentrations in California with emphasis on the San Joaquin Valley (SJV), one of two extreme ozone non-attainment areas in the US. One of the major objectives of CABOTS was to characterize the vertical distribution of O3 and aerosols above the SJV to aid in the identification of elevated transport layers and assess their surface impacts. To this end, the NOAA Earth System Research Laboratory (ESRL) deployed the Tunable Optical Profiler for Aerosol and oZone (TOPAZ) mobile lidar to the Visalia Municipal Airport (36.315∘ N, 119.392∘ E) in the central SJV between 27 May and 7 August 2016. Here we compare the TOPAZ ozone retrievals with co-located in situ surface measurements and nearby regulatory monitors and also with airborne in situ measurements from the University of California at Davis–Scientific Aviation (SciAv) Mooney and NASA Alpha Jet Atmospheric eXperiment (AJAX) research aircraft. Our analysis shows that the lidar and aircraft measurements agree, on average to within 5 ppbv, the sum of their stated uncertainties of 3 and 2 ppbv, respectively.