Spatial pattern and seasonal variation of alkaline precipitation observed in the Gansu Province, NW China

In contrast to the term acid rain, alkaline precipitation or basic precipitation with pH value > 7 has been measured in many regions, and is considered to be an indicator of a regional atmospheric environment. Based on an observation network including 14 stations in northwestern China’s Gansu Province, the pH values in precipitation were measured during 2011–2015, and the spatial pattern and seasonal variation of precipitation acidity were investigated. In the Gansu Province, the precipitation is alkaline for most events, and precipitation with pH < 5.6 is very rare. The atmospheric environment of the study region is greatly influenced by the Asian dust sources in which the surface soil is alkaline, and the measured alkaline precipitation is related to the high dust loading. However, in some industrial cities like Jinchang and Lanzhou, the acidity in precipitation is usually stronger than the others. The seasonal variation of precipitation acidity is complex, which is jointly influenced by emission, transport and deposition. For extremely arid Northwest Gansu where deserts are located, the pH value is much high; for semi-arid and semi-humid East Gansu, the seasonal variation is not spatially coherent; for cold Southwest Gansu at high elevations, the precipitation acidity is relatively stable; for South Gansu in a humid climate, the large fluctuation is related to the complex moisture regimes of the monsoons.