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Over the past decade, surface ozone has emerged as a significant air pollutant in China, especially in the North China Plain (NCP). For effective ozone management in the NCP, it is crucial to accurately estimate the surface ozone levels and identify the primary influencing factors for ozone pollution in this region. This study utilized ozone precursors such as volatile organic compounds (VOCs) and nitrogen oxides (NOX), meteorological data, land cover, normalized difference vegetation index (NDVI), terrain, and population data to build an extreme gradient boosting (XGBoost)-based ozone estimation model in the NCP during 2019 to 2021. Four ozone estimation models were developed using different NO2 and formaldehyde (HCHO) datasets from the Sentinel-5 TROPOMI observations and Copernicus Atmosphere Monitoring Service (CAMS) reanalysis data. Site-based validation results of these four models showed high accuracy with R2 values above 0.86. Among these four models, two models with higher accuracy and higher spatial coverage ratio were selected, and their results were averaged to produce the final ozone estimation products. The results indicated that VOCs and NOX were the two main pollutants causing ozone pollution in the NCP, and their relative contributions accounted for more than 23.34% and 10.23%, respectively, while HCHO also played a significant role, contributing over 5.64%. Additionally, meteorological factors also had a notable impact, contributing 28.63% to ozone pollution, with each individual factor contributing more than 2.38%. The spatial distribution of ozone pollution identified the Hebei–Shandong–Henan junction as a pollution hotspot, with the peak occurring in summer, particularly in June. Therefore, for this hotspot region in the NCP, promoting the reduction in VOCs and NOx can play an important role in the mitigation of O3 pollution and the improvement in air quality in this region. [ABSTRACT FROM AUTHOR]

The present study investigated the temporal variability of surface O3 and its (NO, NO2 and CO) precursors at the proximity of a busy trafficked crossroad located in urban Tunis, Tunisia. It was performed during January to December, 2016. The results revealed that the diurnal profiles of selected precursors were characterized by maxima at traffic-peak hours linked to the effect of automobile traffic being a close source. At traffic-peak hours, the decrease of O3 concentration levels is due to the oxidation reaction of NO into NO2. Based on the diurnal profile per each month, the duration of O3 accumulation was shown close to five hours, with higher accumulation rates in the summer season (2.4 to 3 ppb/h) and also in March (3 ppb/h). The lower accumulation ones were, however, observed in the other months (1.1 to 1.9 ppb/h). The O3 excess pronounced from the 10th March to the 1st April, 2016 was due to the effect of the persistence of a Rex-type blocking anticyclone over Central Europe. Compared to extremely stable conditions occurring in hot seasons (summer and autumn), the recorded moderate stable conditions during such Rex was categorized by a marked deviation between day-night intercepts (41% against 23%). This could be attributed to the cumulative effect of the biogenic VOCs and the seasonal excess of CO interfering in the photochemical cycle. It, therefore, implies that the study site is CO and VOC-sensitive. [ABSTRACT FROM AUTHOR]

Over the past decade, surface ozone has emerged as a significant air pollutant in China, especially in the North China Plain (NCP). For effective ozone management in the NCP, it is crucial to accurately estimate the surface ozone levels and identify the primary influencing factors for ozone pollution in this region. This study utilized ozone precursors such as volatile organic compounds (VOCs) and nitrogen oxides (NOX), meteorological data, land cover, normalized difference vegetation index (NDVI), terrain, and population data to build an extreme gradient boosting (XGBoost)-based ozone estimation model in the NCP during 2019 to 2021. Four ozone estimation models were developed using different NO2 and formaldehyde (HCHO) datasets from the Sentinel-5 TROPOMI observations and Copernicus Atmosphere Monitoring Service (CAMS) reanalysis data. Site-based validation results of these four models showed high accuracy with R2 values above 0.86. Among these four models, two models with higher accuracy and higher spatial coverage ratio were selected, and their results were averaged to produce the final ozone estimation products. The results indicated that VOCs and NOX were the two main pollutants causing ozone pollution in the NCP, and their relative contributions accounted for more than 23.34% and 10.23%, respectively, while HCHO also played a significant role, contributing over 5.64%. Additionally, meteorological factors also had a notable impact, contributing 28.63% to ozone pollution, with each individual factor contributing more than 2.38%. The spatial distribution of ozone pollution identified the Hebei–Shandong–Henan junction as a pollution hotspot, with the peak occurring in summer, particularly in June. Therefore, for this hotspot region in the NCP, promoting the reduction in VOCs and NOx can play an important role in the mitigation of O3 pollution and the improvement in air quality in this region.

The East Asian summer monsoon (EASM) was extremely strong in 2018, which substantially affected surface ozone (O3) in China. Taking 2018 and the average synthesis of 2003 and 2010 to represent the strong and weak EASM cases, respectively, GEOS-Chem with constant anthropogenic emission was employed to investigate the impact of the EASM on surface O3 in the east of China. Simulations show that surface O3 decreased in the northeast and the eastern coast of China and increased in most of the remaining regions during strong EASM. The difference in surface O3 between strong and weak EASM was around −15~7 ppbv. After analyzing relevant meteorological fields, it is found that the decrease in northeast China was mainly attributed to the large increase in vertical upward transport. The considerable decrease in the Huang-Huai-Hai region depended on the dilution and diffusion of eastward anomalous horizontal circulation. The increase in Hunan-Hubei-Guangdong Province was largely due to input from the north. In addition, the vast areas between the Yangtze River and the Yellow River were supported by higher temperatures and stronger shortwave solar radiation that promoted photochemical reactions. The reasons for changes in Shanxi-Sichuan-Yunnan Province were relatively more complex and thus require more in-depth exploration. [ABSTRACT FROM AUTHOR]

In this study, the high surface O3 episodes in the hot summer in Japan in July 2018 are studied using two models, NHM-Chem and ASUCA-Chem. The model performances are similar to each other with both models tending to overestimate the observed surface O3 by ∼5–10 ppbv. The meteorological factors causing the interannual changes of surface O3 are investigated through sensitivity tests of NHM-Chem using meteorological fields of different years, i.e., July 2007 and 2012. July 2007 and 2012 represent cold and normal summers, respectively, with respect to the monthly mean surface temperature in Japan. The hot summer provides favorable conditions for the O3 formation, such as a higher temperature and a higher solar radiation. However, the monthly mean surface wind speed in July 2018 is greater than that of the other periods, causing a negative effect on the monthly surface O3 concentrations because of faster dilution and diffusion in certain locations. Over the Kanto plain, which is the most populated region of Japan, the monthly mean surface temperature is ∼2 K higher than the normal values; however, both observation and simulation demonstrate that the monthly mean surface O3 is lower than the normal temperature summer (July 2012) by 4–6 ppbv. The sensitivity tests indicate that the enhanced biogenic volatile organic compound emissions increase surface O3 by 4–5 ppbv, but the wind field changes decrease surface O3 by 9–10 ppbv. Heat wave is associated with high surface O3 episodes; however, the monthly mean value is a mixed result of many other meteorological events in addition to the heat wave. The interannual changes in the monthly surface O3 can vary depending on magnitude of the positive and negative effects at each location.

This paper highlights the variations of surface ozone (O3), total column ozone (TCO), oxides of nitrogen (NO and NO2), carbon monoxide (CO), sulphur dioxide (SO2), ammonia (NH3), volatile organic compounds (Benzene, Tolune, Ethyle Benzene, Xylenes (collectively called BTEX)), particulate matters (PM10 and PM2.5), and meteorological parameters at the time of an annular solar eclipse on 26 December 2019 at Kannur town in Kerala, South India. The maximum solar obscuration has resulted a decrease in solar radiation by 93%, air temperature by 16.3%, wind speed by 36.1% and an increase in relative humidity by 27.1% at this coastal location. Along with the reduction in solar radiation, the concentration of surface O3 (61.5%) and total column O3 (11.8%) have been observed to decrease at the maximum phase of solar eclipse. CO and NO2 concentration were found to be increased by 28.9% and 42.2%, respectively, while NO exhibited its typical diurnal variation. Further, a decrease in concentrations of SO2 by 17.6%, PM10 by 18.5%, and PM2.5 by 11.3% were observed. NH3 and BTEX were found to be higher than 11.3% and 22.6% of the concentrations in control days. All of these deviated parameters could be seen returning to their normal state after completing the eclipse episode. The variation of photodissociation coefficient j(NO2) values were theoretically calculated from the observed data, which shows a good agreement with the model simulated j(NO2) reduction. This is an extensive second observation on the variation of trace pollutants on solar eclipse, after the partial solar eclipse observed on 15 January 2010 at Kannur.

Tree-based machine learning and deep learning approaches are widely applied in ozone (O3) retrieval, but they cannot achieve high accuracy and interpretability simultaneously. To overcome this limitation, a tree-based ensemble deep learning model, named semi-SILDM, was proposed for O3 prediction at both national (5 km) and urban scales (250 m) in China. The Moderate Resolution Imaging Spectroradiometer (MODIS) Top of Atmosphere (TOA) measurements were first investigated through significant linear and nonlinear relationships with surface O3. To examine the actual predictive ability of the semi-SIDLM, time-based validation was employed to divide data chronologically by year into training (2018), validation (2017), and test data (2019). The semi-SIDLM predicted O3 in 2019 showed a coefficient of determination (R2) of 0.71 (0.69) and a Root Mean Square Error (RMSE) of 21.88 (26.59) µg/m3 at the national (urban) scale in China. In addition to its high accuracy, the semi-SIDLM has interpretability for retrieval results, which indicates the strong influence of the Fangshan and Tongzhou districts on the principle O3 Beijing urban area; the temporal characteristics reveal the higher contributions of May–July to O3 pollution compared to other months. The proposed model of this study will benefit further studies on O3 monitoring and deepen the understanding of its spatiotemporal characteristics.

Under the background of global warming, the summer temperature of the North and Northeast China (NNEC) has significantly increased since 2017, which was accompanied by the aggravated ozone (O 3) pollution. In 2018, the NNEC experienced a record-breaking summer of the past 40 years. Influenced by the abnormal high temperatures, a regional ozone event occurred on 2-3 August, over 63% of 79 selected cities in the NNEC were exposed to O 3 pollution, and the maximum value of MDA8 O 3 reached 268 μg/m3. Observations indicated that ozone concentrations agree well with the maximum temperature at 2 meters (MT2M) over NNEC with a correlation coefficient of 0.69. During the pollution episode, strong downdraft in the local high (35°N-42.5°N, 112.5°E-132.5°E; LH) over the NNEC created the favourable meteorological conditions for O 3 formation. By analyzing the horizontal wind and wave activity fluxes (WAFs) at 200 hPa, we found that the LH formation was resulted from the Rossby wave propagation from upstream along the mid-latitude Asian jet. The split polar vortex intrusion further strengthened the amplitude of the Rossby wave and reinforced the LH. Moreover, a secondary circulation between Typhoon Jongdari and the LH contributed to the enhanced LH with strong subsidence. On the other hand, the stratospheric intrusions under the deep subsidence also contributed to the enhanced surface O 3. In this study, the deep-seated meteorological dynamical mechanisms contributing to the abnormal high temperatures were investigated, which can lead to a better understanding of the regional O 3 pollution over NNEC under the global-warming background. During the record-breaking summer in 2018, the NNEC region experienced regional O3 pollution in 27 July-3 August under extremely high-temperature conditions with other favorable meteorological conditions, and the deep subsidence over NNEC suppressed the dispersion of pollutants, further aggravating the pollution. By investigating the meteorological dynamics of this process, we found that the sinking motion was primarily induced by a quasi-barotropic LH, which was closely related to the propagation of the Rossby wave. Moreover, the activities of polar vortex and the secondary circulation between the LC and Typhoon Jongdari both facilitated the development of the LC. It should be noted the deep sinking motion of the secondary circulation not only intensified the LH, but also could deliver O 3 -rich air from the UTLS to the near-surface ground and directly led to the enhancement of O 3 over the NNEC region.A schematic representation of the meteorological mechanism for the regional ozone pollution over the NNEC is shown in the graphic abstract. [Display omitted] [ABSTRACT FROM AUTHOR]

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.

The Antarctic continent is known to be an unpopulated region due to its extreme weather and climate conditions. However, the air quality over this continent can be affected by long-lived anthropogenic pollutants from the mainland. The Argentinian region of Ushuaia is often the main source area of accumulated hazardous gases over the Antarctic Peninsula. The main objective of this study is to report the first in situ observations yet known of surface ozone (O) over Ushuaia, the Drake Passage, and Coastal Antarctic Peninsula (CAP) on board the RV Australis during the Malaysian Antarctic Scientific Expedition Cruise 2016 (MASEC'16). Hourly O data was measured continuously for 23 days using an EcoTech O analyzer. To understand more about the distribution of surface O over the Antarctic, we present the spatial and temporal of surface O of long-term data (2009-2015) obtained online from the World Meteorology Organization of World Data Centre for greenhouse gases (WMO WDCGG). Furthermore, surface O satellite data from the free online NOAA-Atmospheric Infrared Sounder (AIRS) database and online data assimilation from the European Centre for Medium-Range Weather Forecasts (ECMWF)-Monitoring Atmospheric Composition and Climate (MACC) were used. The data from both online products are compared to document the data sets and to give an indication of its quality towards in situ data. Finally, we used past carbon monoxide (CO) data as a proxy of surface O formation over Ushuaia and the Antarctic region. Our key findings were that the surface O mixing ratio during MASEC'16 increased from a minimum of 5 ppb to ~ 10-13 ppb approaching the Drake Passage and the Coastal Antarctic Peninsula (CAP) region. The anthropogenic and biogenic O precursors from Ushuaia and the marine region influenced the mixing ratio of surface O over the Drake Passage and CAP region. The past data from WDCGG showed that the annual O cycle has a maximum during the winter of 30 to 35 ppb between June and August and a minimum during the summer (January to February) of 10 to 20 ppb. The surface O mixing ratio during the summer was controlled by photochemical processes in the presence of sunlight, leading to the depletion process. During the winter, the photochemical production of surface O was more dominant. The NOAA-AIRS and ECMWF-MACC analysis agreed well with the MASEC'16 data but twice were higher during the expedition period. Finally, the CO past data showed the surface O mixing ratio was influenced by the CO mixing ratio over both the Ushuaia and Antarctic regions. Peak surface O and CO hourly mixing ratios reached up to ~ 38 ppb (O) and ~ 500 ppb (CO) over Ushuaia. High CO over Ushuaia led to the depletion process of surface O over the region. Monthly CO mixing ratio over Antarctic (South Pole) were low, leading to the production of surface O over the Antarctic region. [ABSTRACT FROM AUTHOR]

The East Asian trough (EAT) is often regarded as an important controlling factor for the East Asian climate. However, there has been no detailed study of its effects on compound pollutants including PM 2.5 and O 3. This study investigates the possible effect of the intraseasonal EAT events on PM 2.5 and O 3 concentrations over eastern China in early winter (November–January). Studies show that 65% of the strong EAT evolved from a cyclonic anomaly over Kazakhstan have significant effects on PM 2.5 concentration in the North China Plain. From day −3 to day 0 prior the EAT peak, increased northerly wind and higher planetary boundary layer height caused by the strong EAT enhance atmospheric diffusion conditions, leading to lower PM 2.5 concentration. PM 2.5 concentration tends to decrease on day 4 prior to the peak of EAT events and reaches its minimum on the third day after that. In addition, surface O 3 concentrations in the Yangtze River basin experienced the first decrease and then increase with a turning point at day 1 after the peak of EAT events. These changes in the O 3 concentration are attributed to changes in the surface temperature anomaly and surface downward solar radiation associated with the EAT. Specifically, the EAT-related lower surface temperatures and less downward solar radiation before the peak of the EAT events (i.e., day −4 to day 1) favor lower O 3 concentrations and vice versa after the peak of the EAT (i.e., day 2 to day 4). Our findings could be of great value for the targeted emission reductions in eastern China in early winter. • The strong East Asian trough event has a significant dispersive effect on the wintertime PM 2.5 over the North China Plain. • Surface O 3 in the Yangtze River basin typically first decreases and then increases during the strong East Asian trough. • Changes in surface O3 are driven by anomalous surface air temperatures and solar radiation. [ABSTRACT FROM AUTHOR]

The North China Plain (NCP) is a major region of summer O 3 pollution in China. However, the contribution of stratosphere-to-troposphere transport (STT) to summertime surface O 3 in the NCP and the meteorological mechanisms are poorly understood. In this study, meteorological reanalysis data, O 3 observations, and Lagrange modeling were used to analyze the contribution of the STT to the change in surface O 3 in the NCP, the vertical transport pathways, and the associated meteorological mechanism. In summer 2019, the STT contributed an estimated 5.7%–18.8% to the surface O 3 concentrations in the NCP. The influence of the STT in the NCP is closely related to the changes in westerly circulation in the upper troposphere and lower stratosphere (UTLS) over the Eurasian region. The troughs and vortices in the westerlies led to the convergences of O 3 in the UTLS and tropopause folding, forming downward intrusions of O 3 -rich air from the UTLS to the lower troposphere. Controlled by the westerly circulation, the O 3 source regions of STT in the stratosphere are found between 40° and 70°N, oscillating zonally with changing transport periods, where a key source region of STT is situated steadily over 60°-100°E northwest of the NCP. In association with the cut-off low of westerly circulation in the UTLS over Siberia evolved from the splitting of the polar vortex, a slanted channel of O 3 downward intrusion was set up from the UTLS to the lower troposphere over the NCP in 7 days. As the cut-off low in westerly circulation moves southeastward close to the NCP, the channel of the O 3 downward intrusion is set upright with a shorter period of vertical transport to the NCP ground. Our results provide insights into the the O 3 source–receptor relationship in the STT with vertical O 3 transport structures in changes in the atmospheric environment. • STT contributes 5.7%–18.8% to surface O 3 in the North China Plain (NCP). • Stratospheric O 3 sources intruding NCP oscillate over 60–100°E and 40–70°N. • STT affects NCP with long-range slanted channels followed by upright transport. [ABSTRACT FROM AUTHOR]

Abstract: Surface ozone (O3) measurements were carried out in the marine environment of the Arabian Sea (AS) during the premonsoon months, April–May 2006, as part of the Integrated Campaign for Aerosols, gases and Radiation Budget. The O3 mixing ratio over the AS varied in the range ∼3–22 ppb with a mean of 13.5 ± 2 ppb. Comparatively high mixing ratios were observed over the southern AS and close to the coast. The spatial pattern did not show any evidence of transport from nearby landmass or in situ photochemistry. Longitudinally separated narrow regions of low and high O3 were seen over the southern AS. The role of aerosols in modifying the O3 concentration was examined based on the co-located measurement of aerosol mass loading, number density, size distribution and optical depth. The O3 mixing ratio showed positive correlation with aerosol loading. Over high O3 regions, large particle concentration showed significant enhancement. The role of chloride ion in depleting O3 was also investigated. The observed spatial features were compared with those measured during the earlier cruises conducted in different seasons and over various oceanic regions. A comparison has been made with the measurements over the Bay of Bengal during the same cruise. [Copyright &y& Elsevier]

10Be/7Be is a stratospheric sensitive tracer. In this paper, measurements of 10Be/7Be and surface O3 from October 2005 to May 2006 at Mt. Waliguan (hereafter WLG, 100.898°E, 39.287°N, 3810 m, a.s.l.), China global atmospheric watch (GAW) observatory, are introduced and used to investigate the stratosphere-troposphere transport (STT) and its impact on surface O3 on the Tibetan Plateau. The results show that the magnitude of STT is weak in winter, followed by strengthening from the end of winter to 10 7 10 7 the middle of spring (from mid February to mid April) with large increases in Be, Be, Be/ Be and surface O3. At the end of spring (from the end of April to mid May in this paper), the STT weakened, and the continuous increase of surface O3 at WLG is produced by tropospheric photochemistry reactions. [ABSTRACT FROM AUTHOR]

Abstract: Observations were made of surface O3, NO x , UV radiation, and meteorological parameters at DBR, Guangdong province, and the statistical relationship of O3 to its factors (NO x , water, scattering, UV radiation) was studied. An empirical method was applied to estimate O3. The calculated value agrees well with that observed under different sky conditions, and averaged relative biases of daily O3 concentrations in clear and all sky conditions were ⩽7%. A good correlation was found between O3 and the ratio of NO2/NO in clear and all sky conditions, and correlation coefficients between O3 and NO2/NO were more than 0.90. This empirical method can be used as a tool to analyze the relationship between O3 and the factors affecting it. The analysis shows that O3 is more sensitive to the change of NO and NO2 than that of other factors. [Copyright &y& Elsevier]

The World Health Organization, which proclaimed the COVID-19 a pandemic in early March 2020, imposed a partial lockdown by the Government of India on 21 March 2020. The aim of this investigation was to measure the change in air pollutants, including particulate matter (PM 2.5 and PM 10) and gaseous pollutants (NO 2 , CO and O 3) during COVID-19 lockdown (25th March to 14th April 2020) across four major polluted cities in North India. In all region, PM 2.5 , PM 10 , NO 2 and CO were significantly reduced while O 3 has been shown mixed variation with increased in Agra and decreased in all other stations during lockdown. PM 2.5 was reduced by ~20–50% and highly decreased in Noida. PM 10 was most significantly decreased by 49% in Delhi. NO 2 was reduced by ~10–70%, and high reduction was observed in Noida. Likewise, ~10–60% reduction was found in CO and most significantly decreased in Gurugram. However, an increased in O3 was observed in Agra by 98% while significantly reduced in other sites. Compared to the same timeframe in 2018–2019, PM 2.5 and PM 10 values for all sites were reduced by more than 40%. Unlabelled Image • Effect of Lockdown during COVID-19 on Air quality has been analysed. • The concentration of PM 2.5 and PM 10 were highly declined in Noida and Delhi. • The concentrations of NO 2 and CO were declined in at all observing sites. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Here, we present an assessment of long-term trends in the O3 weekend effect (WE) occurrences and spread within the Mexico City (MCMA), Guadalajara (GMA), and Monterrey (MMA) metropolitan areas, which are the three largest metropolitan areas (MAs) of Mexico and concentrate around 33% of the total population in the country. Daytime averages and peak differences in O3 concentrations from weekdays to weekends were used as a proxy of WE occurrence. All MAs exhibited the occurrence of WE in all years at least in one monitoring site. Substantial differences in O3 daytime averages and peaks from weekdays to weekends have decreased over time in all MAs, and since 1998 and 2013 for the MCMA and GMA, respectively, higher O3 levels during weekends are typical during most of the year. The largest variations in the O3 WE were observed at downwind and urban core sites of the MCMA and GMA. Significant increasing trends (p < 0.05) in the O3 WE magnitude were observed for Sundays at all sites within the MCMA, with trends in annual averages ranging between 0.33 and 1.29 ppb O3 yr−1. Within the GMA, for Sundays, fewer sites exhibited increasing trends in the WE occurrence and at lower growth rates (0.32 and 0.48 ppb yr−1, p < 0.1) than within the MCMA, while within the MMA no apparent trends were observed in marked contrast with the MCMA and GMA. Our findings suggest that policies implemented have been successful in controlling weekday ground-level O3 within the MCMA and GMA, but further actions must be introduced to control the increases in the O3 WE magnitude and spread. [ABSTRACT FROM AUTHOR]