Influence of intense secondary aerosol formation and long-range transport on aerosol chemistry and properties in the Seoul Metropolitan Area during spring time: results from KORUS-AQ

Abstract

Non-refractory submicrometer particulate matter (NR-PM1) was measured in the Seoul Metropolitan Area (SMA), Korea, using an Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) from 14 April to 15 June 2016, as a part of the Korea-US Air Quality Study (KORUS-AQ) campaign. This was the first highly time-resolved, real-time measurement study of springtime aerosol in SMA and the results reveal valuable insights into the sources and atmospheric processes that contribute to PM pollution in this region.

The average concentration of submicrometer aerosol (PM1 = NR-PM1 + black carbon (BC)) was 22.1 mu gm(-3), which was composed of 44% organics, 20% sulfate, 17% nitrate, 12% ammonium, and 7% BC. Organics had an average atomic oxygen-to-carbon (O/C) ratio of 0.49 and an average organic mass-to-carbon (OM/OC) ratio of 1.82. Four distinct sources of OA were identified via positive matrix factorization (PMF) analysis of the HR-ToF-AMS data: vehicle emissions represented by a hydrocarbon-like OA factor (HOA

O/C = 0.15

17% of OA mass), food cooking activities represented by a cooking-influenced OA factor (COA

O/C = 0.19

22% of OA mass), and secondary organic aerool (SO Lambda) represented by a semi-volatile oxygenated OA factor (SV-OOA

O/C = 0.44

27% of OA mass) and a low-volatility oxygenated OA factor (SV-OOA

O = C D 0.44

27% of OA mass) and a low-volatility oxygenated OA factor (LV-OOA

O = C D 0.91

34% of OA mass).

Our results indicate that air quality in SMA during KORUS-AQ was influenced strongly by secondary aerosol formation, with sulfate, nitrate, ammonium, SV-OOA, and LV-OOA together accounting for 76% of the PM1 mass. In particular, the formation of LV-OOA and sulfate was mainly promoted by elevated ozone concentrations and photochemical reactions during daytime, whereas SV-OOA and nitrate formation was contributed by both nocturnal processing of VOC and nitrogen oxides, respect