Measurement report: On the contribution of long-distance transport to the secondary aerosol formation and aging

To investigate the physio-chemical properties of aerosol transported from major pollution regions in China, observations were conducted ∼ 200 m above the ground at the junction location of the North China Plain and Fenwei Basin, which are two regions of top priority for China's Blue Sky Campaign. We identified three pollution transport sectors including those from Beijing–Tianjin–Hebei (BTH), urban Guanzhong Basin (GZB) and northern China and one clean transport sector from the rural Guanzhong Basin region. Secondary inorganic aerosol (SIA) constituted a major fraction (39 %–46 %) in all pollution transport sectors, with a high sulfur oxidation ratio (0.44–0.58) and a high nitrogen oxidation ratio (0.24–0.29), suggesting efficient formation of secondary inorganic aerosol during regional transport. More oxidized oxygenated organic aerosol (MO-OOA) played a dominant role in the source of organic aerosol in all sectors including the clean one, accounting for 42 %–58 % of total organic aerosol. Elemental analysis (O and C) shows that aerosol particles at this receptor site were much more oxidized than in urban regions, pointing that long-range transport contributed markedly to the organic aerosol oxidation and aging. Case studies of pollution events with high sulfate, nitrate and more-oxidized oxygenated organic aerosol production rate indicate the strong formation efficiency of secondary aerosol during regional transport in the Beijing–Tianjin–Hebei transport sector.