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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.

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]