The aerosol–cloud interaction (ACI) and aerosol–radiation interaction (ARI) have notable influences on clouds, but their effects on fog are rarely analyzed before. Previous studies indicate that fog frequency in East China has been decreasing, and we further reveal that fog duration increases during 1960–2010. We hypothesize that this trend is related to the increase of aerosol pollution and perform WRF‐Chem simulations to study the ARI and ACI effects. Results show that ACI significantly advances fog formation, delays fog dissipation, and increases fog duration by about 1 h in a case study, while ARI has negligible effect. The more but smaller fog droplets produced in polluted conditions significantly inhibit droplet sedimentation and reduce solar radiation, therefore provide favorable conditions for the duration of fog. Under extremely polluted conditions, ACI effect also far overweighs ARI effect. To shorten the duration of dense fog, our findings suggest the necessity for mitigating emissions.

Long‐range transport of PM_2.5 from the North China Plain (NCP) by cold frontal passage is considered to be an important haze formation mechanism in the Yangtze River Delta (YRD). However, detailed 3D distributions of PM_2.5 and meteorological fields during the front moving through are not fully understood. Herein, in situ observations showed that most haze events in autumn and winter in the YRD were associated with cold frontal passages. A typical cold front episode on November 2–3, 2017, was investigated with unmanned aerial vehicle sounding and model simulations, which both revealed that PM_2.5 in the upper air (∼1.0 km) reached the YRD earlier than that moving at the ground level. When the cold front reached the NCP, air pollutants ahead of the front was lifted from the ground level to the upper air. Subsequently, increased PM_2.5 concentrations and higher wind speed in the upper air resulted in stronger PM_2.5 outflow from the NCP to YRD compared to that at ground level. The maximum PM_2.5 mass contribution from the NCP to YRD was 15.6 μg m⁻³ (accounted for 31.5% of the total PM_2.5 in the YRD), which appeared at ∼1.0 km altitude. Compared to the noncold front period, the PM_2.5 mass transported from the NCP was 10–15 times higher during the cold front episode. Results indicate that the combination of multisite sounding and elaborate simulation can effectively explain the impact of synoptic processes on the transport, distribution, and evolution of air pollution and provide precise pollution control directives.

Two episodes with heavy air pollution in Nanjing, China, one in the summer and another one in the winter of 2017, were selected to study aerosol properties using sun photometer and ground-based measurements, together with source region analysis. The aerosol properties, the meteorological conditions, and the source regions during these two episodes were very different. The episodes were selected based on the air quality index (AQI), which reached a maximum value of 193 during the summer episode (26 May–3 June) and 304 during the winter episode (21–31 December). The particulate matter (PM) concentrations during the winter episode reached maximum values for PM_2.5/10 of 254 μg m⁻³ and 345 μg m⁻³, much higher than those during the summer (73 and 185 μg m⁻³). In contrast, the value of aerosol optical depth (AOD) at 500 nm was higher during the summer episode (2.52 ± 0.19) than during that in the winter (1.38 ± 0.18). A high AOD value does not necessarily correspond to a high PM concentration but is also affected by factors, such as wind, Planetary Boundary Layer Height (PBLH), and relative humidity. The mean value of the Ångström Exponent (AE) varied from 0.91–1.42, suggesting that the aerosol is a mixture of invaded dust and black carbon. The absorption was stronger during the summer than during the winter, with a minimum value of the single scattering albedo (SSA) at 440 nm of 0.86 on 28 May. Low values of asymmetry factor (ASY) (0.65 at 440 nm and 0.58 at 1020 nm) suggest a large number of anthropogenic aerosols, which are absorbing fine-mode particles. The Imaginary part of the Refractive Index (IRI) was higher during the summer than during the winter, indicating there was absorbing aerosol during the summer. These differences in aerosol properties during the summer and winter episodes are discussed in terms of meteorological conditions and transport. The extreme values of PM and AOD were reached during both episodes in conditions with stable atmospheric stratification and low surface wind speed, which are conducive for the accumulation of pollutants. Potential source contribution function (PSCF) and concentration weighted trajectory (CWT) analysis show that fine mode absorbing aerosols dominate during the summer season, mainly due to emissions of local and near-by sources. In the winter, part of the air masses was arriving from arid/semi-arid regions (Shaanxi, Ningxia, Gansu, and Inner Mongolia provinces) covering long distances and transporting coarse particles to the study area, which increased the scattering characteristics of aerosols.

Valley winds and urban breezes induced by thermal circulation have been the subject of numerous studies; however, their interactions deserve further observational study under different geographical conditions globally. We propose quantitative criteria for analysis of observations, including identification of valley and mountain–plain wind episodes, determination of the direction and duration of valley winds, and exclusion of suspect cases induced by strong synoptic systems. We selected a subset of 345 valley wind days that occurred during 2013–2015 in Hangzhou, China. Valley winds in the study area occur mainly in autumn and early winter (annual frequency: 32%). To the west (east) of the urban center of Hangzhou are small mountains (is a plain). At night, the directions of mountain–plain circulation and urban thermal circulation are consistent in the region from the mountains to western parts of the city, resulting in a strengthened mountain wind at the surface. Their interaction also delays the reversal of the mountain wind to the plain wind in the morning, and slightly advances the reversal of the plain wind to the mountain wind in the evening. Conversely, in the plain area to the east of Hangzhou, the interaction delays the time of reversal of the plain wind to the mountain wind in the evening. Our results indicate that interactions between the urban breeze and local thermal circulations could have distinct impact on the environment of the city and surrounding areas, and thus they should be taken into consideration in relation to urban planning and management.

In recent years, ozone (O₃) pollution has become a prominent air quality concern in the Sichuan Basin (SCB). Based on surface O₃ measurements from 22 cities between 2015 and 2024, this study investigates the evolution of extreme O₃ pollution events and their underlying causes. While the average O₃ concentration, the number of affected cities, and the total O₃ pollution hours have all increased during the past decade, extreme O₃ concentrations have shown a significant decline since 2020. These trends suggest that O₃ pollution in the SCB has become more spatially extensive and less intense. Decomposition analysis attributed ~75% of the post-2020 decline in extreme O₃ concentrations to precursor emission reductions, with meteorological variability explaining the remaining ~25%. Satellite observations of formaldehyde (HCHO) and nitrogen dioxide (NO₂) column densities indicate a regional shift in O₃ formation regimes across the SCB, with many areas transitioning from VOC (volatile organic compound)-limited to transitional or NO_x (nitrogen oxide)-limited conditions. This shift likely contributed to the broader spatial extent and longer duration of O₃ pollution in recent years. Model sensitivity simulations and Integrated Reaction Rate (IRR) analysis demonstrate that reductions in precursor emissions, particularly NO_x, directly weakened daytime photochemical O₃ production and disrupted NO_x-driven radical propagation under transition and NO_x-limited conditions, collectively driving the observed decline in extreme O₃ concentrations.

Regional-scale field observations of fine particles (PM_2.5) were carried out at urban, suburban and regional background sites across the Yangtze River Delta (YRD) from 15–30 January 2015. The coefficients of divergence (CD) values reveal the similarity of dataset at the three sites. The PM_2.5 concentrations and meteorological data exhibit temporal synchronization. From January 15 to 26, the YRD experienced severe PM_2.5 pollution resulting from a cold front moving through and high-pressure control. Then, a 4-day intermittent rain event from 27–30 January significantly scavenged PM_2.5. For the chemical components in PM_2.5, secondary inorganic ions were dominant, and they accounted for larger proportions at the urban and suburban sites than at the regional background site. The OC/EC ratios were higher in daytime than at night, and were lower on polluted days than on clean (rainy) days. The principal sources of PM_2.5 were secondary nitrate (38%) and sulfate (23%) formation, biomass burning (14%), and marine source (8%). Marine (16%) and sulfate (30%) sources were enhanced on clean (rainy) days, indicating the notable effect of marine air masses on PM_2.5 chemical components. The open burning source contribution at the regional site was the largest during the polluted period because more air masses arrived from combustion zones.