Seasonal variations in the highly time-resolved aerosol composition, sources and chemical processes of background submicron particles in the North China Plain
For the first time in the North China Plain (NCP) region, we investigated the seasonal variations in submicron particles (NR-PMinline-formula1) and their chemical composition at a background mountainous site of Xinglong using an Aerodyne high-resolution time-of-flight aerosol mass spectrometer. The average concentration of NR-PMinline-formula1 was highest in autumn (15.1 inline-formulaµg m−3) and lowest in summer (12.4 inline-formulaµg m−3), with a greater abundance of nitrate in spring (34 %), winter (31 %) and autumn (34 %) and elevated organics (40 %) and sulfate (38 %) in summer. PMinline-formula1 in Xinglong showed higher acidity in summer and moderate acidity in spring, autumn and winter, with average pH values of inline-formula2.7±0.6, inline-formula4.2±0.7, inline-formula3.5±0.5 and inline-formula3.7±0.6, respectively, which is higher than those estimated in the United States and Europe. The size distribution of all PMinline-formula1 species showed a consistent accumulation mode peaking at approximately 600–800 nm (inline-formuladva), indicating a highly aged and internally mixed nature of the background aerosols, which was further supported by the source appointment results using positive matrix factorization and multilinear engine analysis. Significant contributions of aged secondary organic aerosol (SOA) in organic aerosol (OA) were resolved in all seasons (inline-formula>77 %), especially in summer. The oxidation state and the process of evolution of OAs in the four seasons were further investigated, and an enhanced carbon oxidation state (inline-formula−0.45–0.10) and inline-formula
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