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

Abstract

Abstract. For the first time in the North China Plain (NCP), we investigated the seasonal variations of submicron particles (NR-PM1) and its chemical composition at a background mountain station using Aerodyne high-resolution time-of-flight aerosol mass spectrometry (HR-ToF-AMS). The averaged NR-PM1 were highest in autumn (15.1 μg m−3) and lowest in summer (12.4 μg m−3), with the abundance of more nitrate in spring (34 %), winter (31 %), and autumn (34 %), and elevated organics (40 %) and sulfate (38 %) proportion in summer. The submicron particles were almost neutralized by excess ammonium in all four seasons except summer, when the aerosol particles appeared to be slightly acidic. The size distribution of all PM1 species showed a consistent accumulation mode peaked at approximately 600–800 nm (dva), indicating the highly aged and internally mixed nature of the background aerosols, which further supported by the source appointment using multilinear engine (ME-2) and significant contributions of aged secondary organic aerosol (SOA) in organic aerosol (OA) were resolved in all seasons (> 77 %), especially in summer (95 %). The oxidation degree and evolution process of OAs in the four seasons were further investigated, and enhanced carbon oxidation state (−0.45–0.10), O / C (0.54–0.75) and OM / OC (1.86–2.13) ratios compared with urban studies were observed, with the highest oxidation degree of which appeared in summer, likely due to the relatively stronger photochemical processing which dominated the processes of both less oxidized OA (LO-OOA) and more oxidized OA (MO-OOA) formations. Aqueous-phase processing also contributed to the SOA formation but prevailed in autumn and winter and the role of which to MO-OOA and LO-OOA also varied in different seasons. In addition, compared with the urban atmosphere, LO-OOA formation in the background atmosphere exhibited more regional characteristics, as photochemical and aqueous-phase processing enhanced during the transport in summer and autumn, respectively. Furthermore, the backward trajectories analysis showed that higher submicron particles were associated with air mass for short distance transported from the southern regions in four seasons, while the long-range transport from Inner Mongolia (west and north regions) also contributed to the summer particle pollutions in the background areas of NCP. Our results illustrate the background particles in NCP are influenced significantly by aging processing and transport, and the more neutralized state of submicron particles with the abundance of nitrate compared with those in the background atmosphere in southern and western China, highlighting the regional reductions in emissions of nitrogen oxide and ammonia are critical for remedying the increased occurrence of nitrate-dominated haze event in the NCP.

Measurement report: Effects of photochemical aging on the formation and evolution of summertime secondary aerosol in Beijing

“…The average increase rate of sulfate was the largest (0.4 µg m −3 h −1 ) among NR-PM 1 species, while that of NR-PM 1 was 0.8 µg m −3 h −1 . These results also suggested that the photochemical aging process plays an important role in sulfate formation in summertime (Li et al, 2020). Previous studies have revealed that an aqueous-phase process plays an important role in sulfate formation (Sun et al, , 2013a.…”

“…Typically, the fraction of OA (48.3 %) in suburban Beijing is higher than those in urban Beijing (33 %-38 %), while the contribution of SIA (51.7 %) in suburban Beijing is smaller than those in urban Beijing (58 %-64 %) (Hu et al, 2017;Huang et al, 2010;Sun et al, 2010Sun et al, , 2012. The reason may be the difference in the emissions of gaseous precursors (e.g., SO 2 and NO x ) and their conversion ratio to SIA (Hu et al, 2017;Li et al, 2020). Meanwhile, the inorganic sulfate, nitrate, and chloride could be well neutralized by ammonium during this field observation since there is an extremely good linear relationship between NO 3 + 2 • SO 4 + Cl and NH 4 (R 2 = 0.998, slope = 0.992 ± 0.002, Fig.…”

Measurement report: Effects of photochemical aging on the formation and evolution of summertime secondary aerosol in Beijing

“…The average increase rate of SO4 2was the largest (0.4 μg m -3 per hour) among NR-PM1 species, while that of NR-PM1 was 0.8 μg m -3 per hour. These results also suggested that the photochemical aging process plays an important role in SO4 2formation in summertime (Li et al, 2020). Previous studies have revealed that an aqueous-phase process plays an important role in SO4 2formation Sun et al, 2013a).…”

“…their conversion ratio to SIA (Li et al, 2020;Hu et al, 2017). Meanwhile, the inorganic SO4 2-, NO3 -, and Clcould be well neutralized by NH4 + during this field observation since there is an extremely good linear relationship between NO3 -+2×SO4 2-+Cland NH4 + (R 2 = 0.998, slope = 0.992 ± 0.002, Fig.…”