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-PM1) 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-PM1 was highest in autumn (15.1 mu g m(-3)) and lowest in summer (12.4 mu g 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 (d(va)), 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 (> 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 urban studies - were observed, with the highest oxidation state appearing in summer, likely because of the relatively stronger photochemical processing that dominated the formation processes of both less oxidized OA (LO-OOA) and more oxidized OA (MO-OOA). Aqueous-phase processing also contributed to the SOA formation and prevailed in winter, with the share to MO-OOA being more important than that to LO-OOA. In addition, regional transport also played an important role in the variations in SOA. Especially in summer, continuous increases in SOA concentration as a function of odd oxygen (O-x = O-3 + NO2) were found to be associated with the increases in wind speed. Furthermore, backward trajectory analysis showed that higher concentrations of submicron particles were associated with air masses transported short distances from the southern regions in all four seasons, while long-range transport from Inner Mongolia (western and northern regions) also contributed to summertime particulate pollution in the background areas of the NCP. Our results illustrate that the background particles in the NCP are influenced significantly by aging processes and regional transport, and the increased contribution of aerosol nitrate highlights how regional reductions in nitrogen oxide emissions are critical for remedying occurrence of nitrate-dominated haze events over the NCP.

Automated summary

The study revealed that the seasonal variations in submicron particles at the Xinglong background mountainous site in the NCP are significantly influenced by aging processes and regional transport, with an increased contribution of aerosol nitrate in summer highlighting the critical importance of regional reductions in nitrogen oxide emissions for alleviating nitrate-dominated haze events over the NCP.