Diurnal variation of surface PM_2.5 concentration (diurnal PM_2.5) could dramatically affect aerosol radiative and health impacts and can also well reflect the physical and chemical mechanisms of air pollution formation and evolution. So far, diurnal PM_2.5 and its modeling capability over East China have not been investigated and therefore are examined in this study. Based on the observations, the normalized diurnal amplitude of surface PM_2.5 concentrations averaged over East China is weakest (∼1.2) in winter and reaches ∼1.5 in other seasons. The diurnal PM_2.5 shows the peak concentration during the night in spring and fall and during the daytime in summer. The simulated diurnal PM_2.5 with WRF-Chem and its contributions from multiple physical and chemical processes are examined in the four seasons. The simulated diurnal PM_2.5 with WRF-Chem is primarily controlled by planetary boundary layer (PBL) mixing and emission variations and is significantly overestimated against the observation during the night. This modeling bias is likely primarily due to the inefficient PBL mixing of primary PM_2.5 during the night. The simulated diurnal PM_2.5 is sensitive to the PBL schemes and vertical-layer configurations with WRF-Chem. Besides the PBL height, the PBL mixing coefficient is also found to be the critical factor determining the PBL mixing of pollutants in WRF-Chem. With reasonable PBL height, the increase in the lower limit of the PBL mixing coefficient during the night can significantly reduce the modeling biases in diurnal PM_2.5 and also the mean concentrations, particularly in the major cities of East China. It can also reduce the modeling sensitivity to the PBL vertical-layer configurations. The diurnal variation and injection height of anthropogenic emissions also play roles in simulating diurnal PM_2.5, but the impact is relatively smaller than that from the PBL mixing. This study underscores that more efforts are needed to improve the boundary mixing process of pollutants in models with observations of PBL structure and mixing fluxes in addition to PBL height, in order to simulate reasonably the diurnal PM_2.5 over East China. The diurnal variation and injection height of anthropogenic emissions must also be included to simulate the diurnal PM_2.5 over East China.

Accompanied by unfavorable meteorological conditions with stable stratification in various haze regions of China, persistent heavy aerosol pollution episodes (HPEs) lasting more than 3 consecutive days frequently occur, particularly in winter. In the North China Plain (NCP), explosive growth of fine particulate matter smaller than 2.5 µm in diameter (PM_2.5), which occurs during some HPE_S, is dominated by a two-way feedback mechanism between more unfavorable meteorological conditions and cumulative aerosol pollution. However, the existence of a two-way feedback mechanism such as this in other key haze regions in China is uncertain; these regions include the Guanzhong Plain (GZP), the Yangtze River Delta (YRD) region, the Two Lakes Basin (TLB; a large outflow basin connected to Hubei Province and Hunan Province), the Pearl River Delta (PRD) region, the Sichuan Basin (SB), and the Northeast China Plain (NeCP). In this study, using surface PM_2.5 and radiation observations, radiosonde observations, and reanalysis data, we observed the existence of a two-way feedback mechanism in the six abovementioned regions. In the SB, this two-way feedback mechanism is weak due to the suppression of cloudy mid-upper layers. In the more polluted NCP, the GZP, and the NeCP, the feedback is more striking than that in the YRD, the TLB, and the PRD. In these regions, the feedback of worsened meteorological conditions on PM_2.5 explains 60 %–70 % of the increase in PM_2.5 during the cumulative stages (CSs). For each region, the low-level cooling bias becomes increasingly substantial with increasing aerosol pollution and a closer distance to the ground surface. With PM_2.5 mass concentrations greater than 400 µg m⁻³, the near-ground bias exceeded −4 ^∘C in Beijing and reached up to approximately −4 ^∘C in Xi'an; this result was caused by accumulated aerosol mass to some extent. In addition to the increase in PM_2.5 caused by the two-way feedback, these regions also suffer from the regional transport of pollutants, including inter-regional transport in the GZP, trans-regional transport from the NCP to the YRD and the TLB, and southwesterly transport in the NeCP.

Heavy PM_2.5 (particulate matter with aerodynamic diameter equal to or less than 2.5 μm) pollution and urban heat island (UHI) pose increasing threats to human health and living environment in populated cities. However, how PM_2.5 pollution affects the UHI intensity (UHII) has not been fully understood. The impacts of PM_2.5 on the wintertime UHII in the Beijing‐Tianjin‐Hebei megalopolis of China are explored during 2013–2017. The results show that the UHII at the time of daily maximum/minimum temperature (UHII_max/UHII_min) exhibits a decreasing/increasing tendency as PM_2.5 concentration increases, causing a continuous decrease in the diurnal temperature range. These effects are mediated via aerosol‐radiation interaction (aerosol‐cloud interaction) under clear‐sky (cloudy) condition. The changes in PM_2.5 concentration further cause different relative trends of UHII_max/UHII_min/diurnal temperature range across different cities in the Beijing‐Tianjin‐Hebei region, which are likely related to the differences in both the PM_2.5 composition and city size. This study provides insights on how air pollution affects urban climate and would help to design effective mitigation strategies.

Air quality in China has been gradually improving because of recent emission control policies, but synoptic circulations in the planetary boundary layer have become increasingly conducive to haze formation. The effect of persistent synoptic circulation (PSC) on long‐term variation in haze episodes remains to be fully elucidated. This study identified and analyzed PSC similarity and its influence on haze during the winter half year from 1961 to 2013. We analyzed sea level pressure and geopotential heights at the 850‐hPa level by using correlation coefficients and mean Hamming distance to quantify PSC similarity. A total of 1,754 PSC events were identified, of which 236 and 167 PSC events of 868 and 625 days, respectively, were associated with six polluted types and three clean types, respectively. The variations in occurrence frequencies of polluted PSC events exhibited different trends in the past decades over eastern China: an increase in 1961–1979, no obvious change in 1980–1999, and a rapid increase since 2000. The durations of polluted and clean PSC events in the planetary boundary layer over eastern China have tended to become longer and shorter, respectively, over the past decades. Our analyses suggest that the reduction in Arctic sea ice in autumn may be favorable for less cold activities and more polluted PSC events with longer durations in eastern China, positively contributing to the formation and maintenance of persistent haze pollution. This work provides convincing evidence that the occurrence frequencies and durations of haze episodes increased because of the increase in pollution‐related PSC events.

Heatwaves (HWs) paired with higher ozone (O₃) concentration at the surface level pose a serious threat to human health. Their combined modulation of synoptic patterns and urbanization remains unclear. Using 5 years of summertime temperature and O₃ concentration observation in Beijing, this study explored potential drivers of compound HWs and O₃ pollution events and their public health effects. Three favorable synoptic weather patterns were identified to dominate the compound HWs and O₃ pollution events. These weather patterns contributing to enhance those conditions are characterized by sinking air motion, low boundary layer height, and high temperatures. Under the synergy of HWs and O₃ pollution, the mortality risk from all non-accidental causes increased by approximately 12.31 % (95 % confidence interval: 4.66 %, 20.81 %). Urbanization caused a higher risk of HWs and O₃ in urban areas than at rural stations. Particularly, due to O₃ depletion caused by NO titration at traffic and urban stations, the health risks related to O₃ pollution in different regions are characterized as follows: suburban stations > urban stations > rural stations > traffic stations. In general, favorable synoptic patterns and urbanization enhanced the health risk of these compound events in Beijing by 33.09 % and 18.95 %, respectively. Our findings provide robust evidence and implications for forecasting compound HWs and O₃ pollution events and their health risks in Beijing or in other urban areas all over the world that have high concentrations of O₃ and high-density populations.