High-time-resolution chemical composition and source apportionment of PM2.5 in northern Chinese cities: implications for policy

Three real-time measurement campaigns were conducted in Xi’an, Shijiazhuang, and Beijing to investigate the chemical characteristics and source contributions of PM_2.5 and explore the formation progress of heavy pollution for policy implications. Chemeical components of PM_2.5 including OA, NO₃^-, SO₄^2-, NH₄⁺, Cl-, BC, and elements were monitored by Q-ACSM, AE33, and Xact, respectively. The results show that chemical compositions of PM_2.5in the three cities are all dominated by organic aerosol (OA) and nitrate (NO₃^-). And results of source apportionment analyzed by the hybrid environmental receptor model (HERM) show that the secondary nitrate plus sulfate contributed higher to PM_2.5 compared to other primary sources. Morevoer, the potential formation mechanisms of secondary aerosol in three cities were further explored by establishing the correlations between secondary nitrate plus sulfate and aerosol liquid water content (ALWC), and O<em>_x</em> (O₃ + NO₂). The results show that photochemical oxidation and aqueous-phase reaction were two important pathways of secondary aerosol fromation. Additionally, this study compared the changes in chemical composition and source contributions of PM_2.5 in past decades, and suggested that not only the activities of clean energy replacements for the rural household is urgently enhanced to reduce the primary source emissions in northern China, but also collaborative control ozone and particulate matter need to be contiouslty promoted to weaken the atmosphere oxidation capacity for reducing secondary aerosol formation.