During recent years, aggressive air pollution mitigation measures in northern China have resulted in considerable changes in gas and aerosol chemical composition. But it is unclear whether aerosol water content and acidity respond to these changes. The two parameters have been shown to affect heterogenous production of winter haze aerosols. Here, we performed thermodynamic equilibrium modeling using chemical and meteorological data observed in urban Beijing for four recent winter seasons and quantified the changes in the mass growth factor and pH of inorganic aerosols. We focused on high relative humidity (> 60%) conditions when submicron particles have been shown to be in the liquid state. From 2014/2015 to 2018/2019, the modeled mass growth factor decreased by about 9%-17% due to changes in aerosol compositions (more nitrate and less sulfate and 18 Hence, it is useful to characterize these two parameters for understanding haze formation. 48 A general approach to measuring aerosol water content is by perturbing RH and detecting the 49 changes in aerosol physical properties, e.g., using nephelometers or hygroscopicity tandem 50 differential mobility analyzers, 3,19,20 but such measurements are not performed routinely in China. Another common approach involves thermodynamic equilibrium analyses between gas and aerosol phase for semivolatile species, requiring composition measurements and a thermodynamic model (e.g., E-AIM or ISORROPIA). 2,21,22 Intercomparisons indicate that water contents estimated

PM2.5 retrieval from satellite-observed aerosol optical depth (AOD) is still challenging due to the strong impact of meteorology. We investigate influences of meteorology changes on the inter-annual variations of AOD and surface PM2.5 in China between 2006 and 2017 using a nested 3D chemical transport model, GEOS-Chem, by fixing emissions at the 2006 level. We then identify major meteorological elements controlling the inter-annual variations of AOD and surface PM2.5 using multiple linear regression. We find larger influences of meteorology changes on trends of AOD than that of surface PM2.5. On the seasonal scale, meteorology changes are beneficial to AOD and surface PM2.5 reduction in spring (1–50%) but show an adverse effect on aerosol reduction in summer. In addition, major meteorological elements influencing variations of AOD and PM2.5 are similar between spring and fall. In winter, meteorology changes are favorable to AOD reduction (−0.007 yr−1, −1.2% yr−1; p < 0.05) but enhanced surface PM2.5 between 2006 and 2017. The difference in winter is mainly attributed to the stable boundary layer that isolates surface PM2.5 from aloft. The significant decrease in AOD over the years is related to the increase in meridional wind speed at 850 hPa in NCP (p < 0.05). The increase of surface PM2.5 in NCP in winter is possibly related to the increased temperature inversion and more stable stratification in the boundary layer. This suggests that previous estimates of wintertime surface PM2.5 using satellite measurements of AOD corrected by meteorological elements should be used with caution. Our findings provide potential meteorological elements that might improve the retrieval of surface PM2.5 from satellite-observed AOD on the seasonal scale.

Climatic features of baroclinic Rossby wave packets at 300 hPa in summers of 1979–2016 over Eurasia are investigated using the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis and daily precipitation observations in China. It is found that the waves with zonal wave‐numbers 5–7 are dominant in Northern Hemisphere. The meridional width (zonal length) of wave packets is up to 50°lat, 130°lon. The wave packets get stronger and incline a little northwestward from 850 up to 300 hPa. Most of strong wave packets are observed around the Alps, the Black Sea and the Caspian Sea, and they propagate eastward, or northeastward, or southeastward. Their zonal and meridional propagation velocities reach about 32 and 7 m/s, respectively. Interannual variations of June–July precipitation in different sub‐regions of southern China are significantly affected by the baroclinic wave packet activities in different regions of Eurasia. These results have important implications for in‐depth understanding wave behaviors and their climate impacts on southern China.