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. For the first time in the North China Plain (NCP) region, weinvestigated the seasonal variations in submicron particles (NR-PM1)and their chemical composition at a background mountainous site of Xinglongusing an Aerodyne high-resolution time-of-flight aerosol mass spectrometer. The average concentration of NR-PM1 was highest in autumn (15.1 µg m−3) and lowest in summer (12.4 µg 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. PM1 in Xinglong showed higher acidity in summer and moderate acidity in spring, autumn and winter, with average pH values of 2.7±0.6, 4.2±0.7, 3.5±0.5 and 3.7±0.6, respectively, which is higher than those estimated in the United States and Europe. The size distribution of all PM1 species showed a consistent accumulation mode peaking at approximately 600–800 nm (dva), 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 (&gt;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 (−0.45–0.10) and O/C (0.54–0.75) and OM/OC (1.86–2.13) ratios – compared with urbanstudies – were observed, with the highest oxidation state appearing insummer, likely because of the relatively stronger photochemical processingthat dominated the formation processes of both less oxidized OA (LO-OOA) andmore oxidized OA (MO-OOA). Aqueous-phase processing also contributed to theSOA formation and prevailed in winter, with the share to MO-OOA being moreimportant than that to LO-OOA. In addition, regional transport also playedan important role in the variations in SOA. Especially in summer,continuous increases in SOA concentration as a function of odd oxygen (Ox=O3+NO2) were found tobe associated with the increases in wind speed. Furthermore, backwardtrajectory analysis showed that higher concentrations of submicronparticles were associated with air masses transported short distancesfrom the southern regions in all four seasons, while long-range transportfrom Inner Mongolia (western and northern regions) also contributed tosummertime particulate pollution in the background areas of the NCP. Ourresults illustrate that the background particles in the NCP are influencedsignificantly by aging processes and regional transport, and the increasedcontribution of aerosol nitrate highlights how regional reductions innitrogen oxide emissions are critical for remedying occurrence ofnitrate-dominated haze events over the NCP.

atmospheric chemistry and physics

Seasonal variations in the highly time-resolved aerosol composition, sources and chemical processes of background submicron particles in the North China Plain