The relationship between aerosol optical depth (AOD) and PM_2.5 is often investigated in order to obtain surface PM_2.5 from satellite observation of AOD with a broad area coverage. However, various factors could affect the AOD–PM_2.5 regressions. Using both ground and satellite observations in Beijing from 2011 to 2015, this study analyzes the influential factors including the aerosol type, relative humidity (RH), planetary boundary layer height (PBLH), wind speed and direction, and the vertical structure of aerosol distribution. The ratio of PM_2.5 to AOD, which is defined as η, and the square of their correlation coefficient (R²) have been examined. It shows that η varies from 54.32 to 183.14, 87.32 to 104.79, 95.13 to 163.52, and 1.23 to 235.08 µg m⁻³ with aerosol type in spring, summer, fall, and winter, respectively. η is smaller for scattering-dominant aerosols than for absorbing-dominant aerosols, and smaller for coarse-mode aerosols than for fine-mode aerosols. Both RH and PBLH affect the η value significantly. The higher the RH, the smaller the η, and the higher the PBLH, the smaller the η. For AOD and PM_2.5 data with the correction of RH and PBLH compared to those without, R² of monthly averaged PM_2.5 and AOD at 14:00 LT increases from 0.63 to 0.76, and R² of multi-year averaged PM_2.5 and AOD by time of day increases from 0.01 to 0.93, 0.24 to 0.84, 0.85 to 0.91, and 0.84 to 0.93 in four seasons respectively. Wind direction is a key factor for the transport and spatial–temporal distribution of aerosols originated from different sources with distinctive physicochemical characteristics. Similar to the variation in AOD and PM_2.5, η also decreases with the increasing surface wind speed, indicating that the contribution of surface PM_2.5 concentrations to AOD decreases with surface wind speed. The vertical structure of aerosol exhibits a remarkable change with seasons, with most particles concentrated within about 500 m in summer and within 150 m in winter. Compared to the AOD of the whole atmosphere, AOD below 500 m has a better correlation with PM_2.5, for which R² is 0.77. This study suggests that all the above influential factors should be considered when we investigate the AOD–PM_2.5 relationships.

Observations of surface direct solar radiation (DSR) and visibility, particulate matter with aerodynamic diameters less than 2.5 µm (PM_2.5), together with the aerosol optical thickness (AOT) taken from Moderate‐Resolution Imaging Spectroradiometer and Multiangle Imaging Spectroradiometer, were investigated to gain insight into the impact of aerosol pollution on surface solar radiation in China. The surface DSR decreased during 2004–2014 compared with 1993~2003 over eastern China, but no clear reduction was observed in remote regions with cleaner air. Significant correlations of visibility, PM_2.5, and regionally averaged AOT with the surface DSR over eastern China indicate that aerosol pollution greatly affects the energy available at the surface. The net loss of surface solar radiation also reduces the surface ground temperature over eastern China. However, the slope of the linear variation of the radiation with respect to atmospheric visibility is distinctly different at different stations, implying that the main aerosol type varies regionally. The largest slope value occurs at Zhengzhou and indicates that the aerosol absorption in central China is the highest, and lower slope values suggest relatively weakly absorbing types of aerosols at other locations. The spatial distribution of the linear slopes agrees well with the geographical distribution of the absorbing aerosols derived from the Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observations and Ozone Monitoring Instrument over China. The regional correlation between a larger slope value and higher absorbance properties of aerosols indicates that the net effects of aerosols on the surface solar energy and corresponding climatic effects are dependent on both aerosol amount and optical properties.

Using six flights observations in September 2015 over Hebei, China, this study shows a robust negative aerosol‐cloud droplet effective radius (r_e) relationship for liquid clouds, which is different from previous studies that found positive aerosol‐cloud r_e relationship over East China using satellite observations. A total of 27 cloud samples was analyzed with the classification of clean and polluted conditions using lower and upper 1/3 aerosol concentration at 200 m below the cloud bases. By normalizing the profiles of cloud droplet r_e, we found significant smaller values under polluted than under clean condition at most heights. Moreover, the averaged profiles of cloud liquid water content (LWC) show larger values under polluted than clean conditions, indicating even stronger negative aerosol‐cloud r_e relationship if LWC is kept constant. The droplet size distributions further demonstrate that more droplets concentrate within smaller size ranges under polluted conditions. Quantitatively, the aerosol‐cloud interaction is found around 0.10–0.19 for the study region.

Beijing has been experiencing serious air pollution in recent years, resulting in serious impacts on the local environment and climate and on human health. In addition to individual pollution sources and weather systems, feedback between aerosols and downwelling solar radiation (DSR) and between aerosols and winds also contribute to heavy aerosol pollution. By using atmospheric visibility (VIS) to represent the relative amount of aerosol pollution during a 5 week observation around the Asia‐Pacific Economic Cooperation (APEC) period (22 October to 25 November 2014) over a site in south Beijing, China, we show clear positive relationships between DSR and VIS and between winds and VIS. The sensitivities of daily DSR and surface winds to VIS are approximately 15.42 W/m²/km and 0.068 m/s/km, respectively. The strengthening contributions to atmospheric visibility by surface DSR‐VIS interactions and between surface wind‐aerosol interactions are estimated at approximately 15% and 12%, respectively, in south Beijing around the APEC period.

The size of a tropical cyclone (TC), measured by the area of either rainfall or wind, is an important indicator for the potential damage by TC. Modeling studies suggested that aerosols tend to enhance rainfall in the outer rainbands, which enlarges the eyewall radius and expands the extent of rainfall area. However, no observational evidence has yet been reported. Using TC rainfall area and aerosol optical depth (AOD) data, we find that aerosols have a distinguishable footprint in the TC size. Other dynamical factors for TC size, such as relative SST and Coriolis parameter, are also quantified and discussed. We show that, on average, TC rainfall size increases 9–20 km for each 0.1 increase of AOD in the western North Pacific. This finding implies that anthropogenic aerosol pollution can increase not only TC rainfall rate, but also TC rainfall area, resulting in potentially more destructive flooding affecting larger areas.

A multi-axis differential optical absorption spectroscopy (MAX-DOAS) instrument was deployed in May and June 2016 at a monitoring station (37.18^∘ N, 114.36^∘ E) in the suburban area of Xingtai, which is one of the most polluted cities in the North China Plain (NCP), during the Atmosphere-Aerosol-Boundary Layer-Cloud (A²BC) experiment and Air chemistry Research In Asia (ARIAs) joint experiments to derive tropospheric vertical profiles of NO₂, SO₂, HONO, HCHO, CHOCHO and aerosols. Aerosol optical depths derived from MAX-DOAS were found to be consistent with collocated sun-photometer measurements. Also the derived near-surface aerosol extinction and HCHO mixing ratio agree well with the coincident visibility meter and in situ HCHO measurements, with mean HCHO near-surface mixing ratios of ∼3.5 ppb. Underestimations of MAX-DOAS results compared to in situ measurements of NO₂ (∼60 %) and SO₂ (∼20 %) are found expectedly due to vertical and horizontal inhomogeneity of trace gases. Vertical profiles of aerosols and NO₂ and SO₂ are reasonably consistent with those measured by a collocated Raman lidar and aircraft spirals over the station. The deviations can be attributed to differences in sensitivity as a function of altitude and substantial horizontal gradients of pollutants. Aerosols, HCHO and CHOCHO profiles typically extended to higher altitudes (with 75 % integrated column located below ∼1.4 km) than NO₂, SO₂ and HONO did (with 75 % integrated column below ∼0.5 km) under polluted conditions. Lifted layers were systematically observed for all species (except HONO), indicating accumulation, secondary formation or long-range transport of the pollutants at higher altitudes. Maximum values routinely occurred in the morning for NO₂, SO₂ and HONO but occurred at around noon for aerosols, HCHO and CHOCHO, mainly dominated by photochemistry, characteristic upslope–downslope circulation and planetary boundary layer (PBL) dynamics. Significant day-to-day variations are found for all species due to the effect of regional transport and changes in synoptic pattern analysed with the backward propagation approach based on HYSPLIT trajectories. Low pollution was often observed for air masses from the north-west (behind cold fronts), and high pollution was observed from the southern areas such as industrialized Wu'an. The contribution of regional transport for the pollutants measured at the site during the observation period was estimated to be about 20 % to 30 % for trace gases and about 50 % for aerosols. In addition, agricultural burning events impacted the day-to-day variations in HCHO, CHOCHO and aerosols. It needs to be noted that although several MAX-DOAS measurements of trace gases and aerosols in the NCP area have been reported in previous studies, this study is the first work to derive a comprehensive set of vertical profiles of NO₂, SO₂, HONO, HCHO, CHOCHO and aerosols from measurements of one MAX-DOAS instrument. Also, so far, the validation of MAX-DOAS profile results by comparison with various surface in situ measurements as well as profile measurements from lidar and aircraft is scarce. Moreover, the backward propagation approach for characterizing the contributions of regional transport of pollutants from different regions was applied to the MAX-DOAS results of trace gases and aerosols for the first time.