Process analysis of PM pollution events in a coastal city of China using CMAQ.

US EPA's Community Multiscale Air Quality modeling system (CMAQ) with Process Analysis tool was used to simulate and quantify the contribution of individual atmospheric processes to PM concentration in Qingdao during three representative PM pollution events in the winter of 2015 and 2016. Compared with the observed surface PM concentrations, CMAQ could reasonably reproduce the temporal and spatial variations of PM during these three events. Process analysis results show that primary emissions accounted for 72.7%-93.2% of the accumulation of surface PM before and after the events. When the events occurred, primary emissions were still the major contributor to the increase of PM in Qingdao, however the contribution percentage reduced significantly, which only account for 51.4%-71.8%. Net contribution from horizontal and vertical transport to the accumulation of PM was also positive and its percentage increased when events occurred. Only 1.1%-4.6% of aerosol accumulation was due to PM processes and aqueous chemical processes before and after events. When the events occurred, contribution from PM processes and aqueous chemistry increased to 6.0%-11.8%. Loss of PM was mainly through horizontal transport, vertical transport and dry deposition, no matter during or outside the events. Wet deposition would become the main removal pathway of PM, when precipitation occurred.