Impacts of meteorological parameters on the occurrence of air pollution episodes in the Sichuan basin.

Episodes of fine-particulate matter (PM2.5) pollution are a widespread and common occurrence in China, and have potentially serious implications for human health. Meteorological conditions play an important role in air quality and influence the formation of regional air pollution episodes. This study applied a new classification method and daily PM2.5 concentration data to (a) evaluate different levels of air pollution in the Sichuan-Chongqing region between 2015 and 2017, and (b) investigate their relationships with meteorological parameters including atmospheric boundary layer height, wind speed, temperature inversion, weather type, and atmospheric transport. We identified a total of 40 air pollution episodes, the most severe of which occurred in winter when the atmospheric layer height was low. These heavy pollution events also coincided with low surface (10 m) wind speeds and temperature inversions, weather conditions that generally result from low pressure over the northwestern Sichuan Basin (90-102° E, 28-36° N) and southerly atmospheric transport.

Seasonal to sub-seasonal variations of the Asian Tropopause Aerosols Layer affected by the deep convection, surface pollutants and precipitation.

The Asian Tropopause Aerosols Layer (ATAL) refers to an accumulation of aerosols in the upper troposphere and lower stratosphere during boreal summer over Asia, which has a fundamental impact on the monsoon system and climate change. In this study, we primarily analyze the seasonal to sub-seasonal variations of the ATAL and the factors potentially influencing those variations based on MERRA2 reanalysis. The ability of the reanalysis to reproduce the ATAL is well validated by CALIPSO observations from May to October 2016. The results reveal that the ATAL has a synchronous spatiotemporal pattern with the development and movement of the Asian Summer Monsoon. Significant enhancement of ATAL intensity is found during the prevailing monsoon period of July-August, with two maxima centered over South Asia and the Arabian Peninsula. Owing to the fluctuations of deep convection, the ATAL shows an episodic variation on a timescale of 7-12 days. Attribution analysis indicates that deep convection dominates the variability of the ATAL with a contribution of 62.7%, followed by a contribution of 36.6% from surface pollutants. The impact of precipitation is limited. The ATAL further shows a clear diurnal variation: the peak of ATAL intensity occurs from 17:30 to 23:30 local time (LT), when the deep convection becomes strongest; the minimum ATAL intensity occurs around 8:30 LT owing to the weakened deep convection and photochemical reactions in clouds. The aerosol components of the ATAL show different spatiotemporal patterns and imply that black carbon and organic carbon come mainly from India, whereas sulfate comes mainly from China during the prevailing monsoon period.

Snow albedo reductions induced by the internal/external mixing of black carbon and mineral dust, and different snow grain shapes across northern China.

Previous studies have indicated that black carbon (BC) potentially induces snow albedo reductions across northern China. However, the effects of other light-absorbing particles (LAPs, e.g., mineral dust, MD), snow grain shape, or BC-snow mixing state on snow albedo have been largely ignored. Here we evaluate the BC- and MD-induced snow albedo reductions and radiative forcings (RFs) using an updated Snow, Ice, and Aerosol Radiation radiative transfer model, considering all of the potential factors that can be derived from the field observations across northern China. The results highlight that the LAP-induced albedo reductions for nonspherical snow grains are 2%-30% less than those for spherical grains. Furthermore, BC-snow internal mixing can significantly enhance albedo reduction by a factor of 1.42-1.48 relative to external mixing, with snow grain radius ranging from 100 to 1000 µm. The mean regional BC + MD-induced snow albedo reductions are amplified by the increase of snow grain radius, ranging from 0.012 to 0.123 for fresh snow to 0.016-0.227 for old snow. Finally, we discuss the relative contributions of BC and MD to the albedo reductions and RFs, highlighting the dominant role of BC in reducing snow albedo across northern China.

Interannual variability of dust height and the dynamics of its formation over East Asia.

The vertical structure of dust layers is rarely investigated, despite its critical role in exploring the radiative and microphysical effects of dust aerosols. In this study, we primarily investigate the temporal variability of most probable dust height (MPDH) over dust source regions in East Asia and its interactions with climate parameters using CALIPSO lidar measurements under cloud-free conditions in spring from 2007 to 2018. The vertical profile of dust aerosols exhibits significant interannual variability over this time; dust is mainly concentrated below 7 km and associated with a dust occurrence frequency (DOF) of 0.6, and the DOF is much higher than that over the Sahara and West Asia. We also found that high Indian Ocean sea surface temperature (SST) significantly contributes to the transport of dust aerosols to downstream areas by changing the circulation field near the equator and in the mid-low latitudes of the Northern Hemisphere, which results in low MPDH over northern China. MPDH is significantly negatively correlated with 500-hPa westerly wind and precipitation, and is positively correlated with surface air temperature (SAT) and normalized difference vegetation index (NDVI). Furthermore, MPDH is positively correlated with the Arctic Oscillation (AO) and the Atlantic Multidecadal Oscillation (AMO), but negatively correlated with the El Niño-Southern Oscillation (ENSO). The correlation coefficient between AMO and MPDH is 0.71 after detrending, which indicates that the AMO also plays an important role in the interannual variability of MPDH over East Asia. Furthermore, the Indian Ocean SST is the main influencing factor of the interannual variability of MPDH over northern China, but zonal wind is probably only the intermediate mechanism.