Process analysis of PM_2.5 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_2.5 concentration in Qingdao during three representative PM_2.5 pollution events in the winter of 2015 and 2016. Compared with the observed surface PM_2.5 concentrations, CMAQ could reasonably reproduce the temporal and spatial variations of PM_2.5 during these three events. Process analysis results show that primary emissions accounted for 72.7%-93.2% of the accumulation of surface PM_2.5 before and after the events. When the events occurred, primary emissions were still the major contributor to the increase of PM_2.5 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_2.5 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_2.5 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_2.5, when precipitation occurred.