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Due to a robust emission control policy, significant reductions in major air pollutants, such as PM 2.5 , SO 2 , NO 2 , and CO, were observed in China between 2015 and 2020. On the other hand, during the same period, there was a notable increase in ozone (O 3) concentrations, making it a prominent air pollutant in eastern China. The annual mean concentration of maximum daily 8 h average (MDA8) O 3 exhibited alarming linear increases of 2.4, 1.1, and 2.0 ppb yr -1 (ppb is for parts per billion) in three megacity clusters: Beijing–Tianjin–Hebei (BTH), the Yangtze River Delta (YRD), and the Pearl River Delta (PRD), respectively. Meanwhile, there was a significant 3-fold increase in the number of O 3 -exceeding days, defined as MDA8 O 3 > 75 ppb. Our analysis indicated that the upward increases in the annual mean concentration of MDA8 were primarily driven by the rise in consecutive O 3 -exceeding days. There were expansions of high O 3 in urban centers to rural areas accompanied by a saturation effect so that MDA8 O 3 concentrations at the high-O 3 stations in 2015 remained nearly constant at 100 ppb. Last, we found a close association between O 3 episodes with 4 or more consecutive O 3 -exceeding days and the position and strength of tropical cyclones (TCs) in the northwest Pacific and the West Pacific subtropical high (WPSH). The TC and WPSH contributed to meteorological conditions characterized by clear skies, subsiding air motion, high vertical stability in the lower troposphere, increased solar radiation, and a positive temperature anomaly at the surface. These favorable meteorological conditions greatly facilitated the formation of O 3. Thus, we propose that the worsening O 3 increases observed in the BTH, YRD, and PRD regions from 2015 to 2020 can be mostly attributed to enhanced photochemical O 3 production resulting from an increased occurrence of meteorological conditions with high solar radiation and positive temperature anomalies under the influence of the WPSH and TCs. [ABSTRACT FROM AUTHOR]

Due to a robust emission control policy, significant reductions in major air pollutants, such as PM2.5, SO2, NO2, and CO, were observed in China between 2015 to 2020. On the other hand, during the same period, there was a notable increase in ozone (O3) concentrations, making it a prominent air pollutant in eastern China. The annual mean concentration of maximum daily 8-hour average (MDA8) O3 exhibited alarming linear trends of 2.4, 1.1, and 2.0 ppb yr–1 in three megacity clusters: three-fold increase in the number of O3-exceeding days, defined as MDA8 O3 >75 ppb during the same period. Our analysis indicated that the upward trends in the annual mean concentration of MDA8 were primarily driven by the rise in consecutive O3-exceeding days. Furthermore, from 2015 to 2017, there was a widespread expansion of high O3 concentrations from urban centers to surrounding rural regions, resulting in a more uniform spatial distribution of O3 after 2017. Lastly, we discovered a close association between O3 episodes featuring four or more consecutive O3-exceeding days and the position and strength of Beijing-Tianjin-Hebei (BTH), Yangtze River Delta (YRD) and Pearl River Delta (PRD). Additionally, there was a significant the West Pacific subtropical high (WPSH). The WPSH contributed to meteorological conditions characterized by clear skies, subsiding air motion, high vertical stability in the lower troposphere, increased solar radiation, and positive temperature anomaly at the surface. These favorable meteorological conditions greatly facilitated the formation of O3. Thus, we propose that the worsening O3 trends observed in BTH, YRD and PRD from 2015 to 2020 can be attributed to enhanced photochemical O3 production resulting from an increased occurrence of meteorological conditions with high solar radiation and positive temperature anomalies under the influence of WPSH and tropical cyclones. [ABSTRACT FROM AUTHOR]

Multiple linear regression (MLR) models are used to assess the contributions of meteorology/climate and anthropogenic emission control to linear trends of PM 2.5 concentration during the period 2013–2018 in three regions in eastern China, namely Beijing–Tianjin–Hebei (BTH), the Yangtze River Delta (YRD), and the Pearl River Delta (PRD). We find that quantitative contributions to the linear trend of PM 2.5 derived based on MLR results alone are not credible because a good correlation in the MLR analysis does not imply any causal relationship. As an alternative, we propose that the correlation coefficient should be interpreted as the maximum possible contribution of the independent variable to the dependent variable and the residual should be interpreted as the minimum contribution of all other independent variables. Under the new interpretation, the previous MLR results become self-consistent. We also find that the results of a short-term (2013–2018) analysis are significantly different from those of a long-term (1985–2018) analysis for the period 2013–2018 in which they overlap, indicating that MLR results depend critically on the length of time analyzed. The long-term analysis renders a more precise assessment because of additional constraints provided by the long-term data. We therefore suggest that the best estimates of the contributions of emissions and non-emission processes (including meteorology/climate) to the linear trend in PM 2.5 during 2013–2018 are those from the long-term analyses: i.e., emission <51 % and non-emission >49 % for BTH, emission <44 % and non-emission >56 % for YRD, and emission <88 % and non-emission >12 % for PRD. [ABSTRACT FROM AUTHOR]

A record-breaking severe ozone (O3) pollution episode occurred in the Pearl River Delta (PRD) in early autumn 2019 when the PRD was under the influence of a Pacific subtropical high followed by Typhoon Mitag. In this study, we analyzed the effects of meteorological and photochemical processes on the O3 concentration in the PRD during this episode by carrying out the Weather Research Forecast–Community Multiscale Air Quality (WRF-CMAQ) model simulations. Results showed that low relative humidity, high boundary layer height, weak northerly surface wind, and strong downdrafts were the main meteorological factors contributing to O3 pollution. Moreover, delayed sea breezes that lasted into the night would transport O3 from the sea back to the land and resulted in secondary O3 maxima at night. In addition, O3 and its precursors stored in the residual layer above the surface layer at night can be mixed down to the surface in the next morning, further enhancing the daytime ground-level O3 concentration on the following day. Photochemical production of O3 , with a daytime average production rate of about 7.2 ppbh-1 (parts per billion), is found to be the predominate positive contributor to the O3 budget of the boundary layer (0–1260 m) during the entire O3 episode, while the horizontal and vertical transport fluxes are the dominant negative contributors. This O3 episode accounted for 10 out of the yearly total of 51 d when the maximum daily 8 h average (MDA8) O3 concentration exceeded the national standard of 75 ppb in the PRD in 2019. Based on these results, we propose that the enhanced photochemical production of O3 during the episode is a major cause of the most severe O3 pollution year since the official O3 observation started in the PRD in 2006. Moreover, since this O3 episode is a synoptic-scale phenomenon covering the entire eastern China, we also suggest that the enhanced photochemical production of O3 in this O3 episode is a major cause of the extraordinarily high O3 concentrations observed in eastern China in 2019. [ABSTRACT FROM AUTHOR]

An observation-based method (OBM) is developed to investigate the sensitivity of ozone formation to precursors during two persistent elevated ozone episodes observed at 77 stations in Guangdong. Average OH concentrations derived at the 77 stations between 08:00 and 13:00 local time stay within a narrow range of 2.5×106 to 5.5×106 cm -3 with a weak dependence on the NO x. These values are in good agreement with OH values observed at a rural station in the Pearl River Delta (PRD). They also agree well with a box model constrained by the ambient conditions observed during the two episodes. The OBM has been used to evaluate the ozone production efficiency, ε (NO x or volatile organic compound, VOC), defined as the number of O 3 molecules produced per molecule of NO x (or VOC) oxidized. Average values of ε (NO x) and ε (VOC) determined by the OBM are 3.0 and 2.1 ppb ppb -1 , respectively, and both compared well with values in previous studies. Approximately 67 % of the station days exhibit ozone formation sensitivity to NO x , and approximately 20 % of the station days are in the transitional regime sensitive to both NO x and VOC, and only approximately 13 % of the station days are sensitive to VOC. These results are in semi-quantitative agreement with the ozone formation sensitivity calculated by the box model constrained by ambient conditions observed during the two episodes. However, our OBM results differ from those of most previous investigations, which suggested that limiting the emission of VOC rather than NO x would be more effective in reducing ozone reduction in Guangdong. [ABSTRACT FROM AUTHOR]

Satellite observations (International Satellite Cloud Climatology Project (ISCCP), 1983–2009) of linear trends in cloud cover are compared to those in global precipitation (Global Precipitation Climatology Project (GPCP) pentad V2.2, 1983–2009), to investigate possible cause(s) of the linear trends in both cloud cover and precipitation. The spatial distributions of the linear trends in total cloud cover and precipitation are both characterized primarily by a broadening of the major ascending zone of Hadley circulation. Our correlation studies suggest that global warming, Atlantic Multidecadal Oscillation (AMO) and Pacific Decadal Oscillation (PDO) can explain 67 %, 49 % and 38 %, respectively, of the spatial variabilities in the linear trends in cloud cover, but causality is harder to establish. Further analysis of the broadening of the major ascending zone of Hadley circulation shows that the trend in global temperature, rather than those in AMO and PDO, is the primary contributor to the observed linear trends in total cloud cover and precipitation in 1983–2009. The underlying mechanism driving this broadening is proposed to be the moisture–convection–latent-heat feedback cycle under global warming conditions. The global analysis is extended by investigating connections between clouds and precipitation in China, based on a large number of long-running, high-quality surface weather stations in 1957–2005. This reveals a quantitative matching relationship between the reduction in light precipitation and the reduction in total cloud cover. Furthermore, our study suggests that the reduction in cloud cover in China is primarily driven by global temperature; PDO plays a secondary role, while the contribution from AMO and Niño3.4 is insignificant, consistent with the global analysis. [ABSTRACT FROM AUTHOR]

To investigate the atmospheric oxidative capacity (AOC) in forested high mountain areas adjacent to the photochemistry-active Pearl River Delta (PRD) region in southern China, one-month online observations of isoprene and its oxidation products methyl vinyl ketone (MVK) and methacrolein (MACR) were conducted at a national background station in Nanling Mountains in summer 2016. The results showed that the observed daytime isoprene levels (377±46 pptv) were significantly lower in comparison with other forest sites within and outside China, although the sampling site was surrounded with subtropical evergreen broad-leaved trees which are strong isoprene emitters. Also, high daytime (MVKCMACR) = isoprene ratios (1.9±0.5) were observed. Based on the observations, we hypothesized that the lower isoprene levels in the study forest might be attributable to a strong AOC in relation to the elevated regional complex air pollution. In further data analyses, high site-level concentrations of daytime OH (7.3±0.5 x 106 molecules cm-3) and nighttime NO3 radicals (6.0±0:5±108 molecules cm-3) were estimated by using a photochemical box model incorporating the master chemical mechanism (PBM-MCM), and high regional mixing ratios of OH (19.7±2.3 x 106 molecules cm-3/ during 09:00-15:00 LT were also obtained by applying a parameterization method with measured aromatic hydrocarbons. And besides, high initial mixing ratios (1213±108 pptv) and short atmospheric reaction time (0.27 h) of isoprene during the day were derived by a sequential reaction approach. All these indicate that isoprene was rapidly and highly oxidized in this forest, which supports our hypothesis. The study suggests that the complex air pollution in the PRD may have significantly elevated the background AOC of the adjacent forests, and probably affects the regional air quality and ecological environment in the long term. The feedback of forest ecosystems to the increasing AOC in southern China warrants further studies. [ABSTRACT FROM AUTHOR]