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
Authors: Kim Hwajin , Zhang Qi , Heo Jongbae 
Published: November 2017

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 April 14 to June 15, 2016, as a part of the Korea-U.S. 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 µg m−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. The concentration and composition of PM1 varied dynamically due to the influences of different meteorological conditions, emission sources, and air mass origins. 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), cooking activities represented by a cooking OA factor (COA; O / C = 0.19; 22 % of OA mass), and secondary organic aerosol (SOA) 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 (LV-OOA; O / C = 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, high temperature, elevated ozone concentrations, and photochemical reactions during daytime promoted the formation of SV-OOA, LV-OOA and sulfate whereas nocturnal processing of nitrogen oxides and daytime photochemical reactions promoted nitrate formation. In addition, gas-to-particle partitioning processes appeared to have enhanced nighttime SV-OOA and nitrate formation. During a period of 4 days (from May 20 to May 23), LV-OOA was significantly enhanced and accounted for up to 41 % of the PM1 mass. This intense LV-OOA formation event was associated with large enhancements of both anthropogenic and biogenic VOCs (e.g., isoprene, toluene), high concentration of Ox (= O3 + NO2), strong solar radiation, and stagnant conditions, suggesting that it was mainly driven by local photochemical formation. We have also investigated the formation and evolution mechanisms of severe haze episodes. Unlike the winter haze events which were mainly caused by intense local emissions coupled with stagnant meteorological conditions, the spring haze events appeared to be influenced by both regional and local factors. For example, there were episodes of long range transport of plumes followed by calm meteorology conditions, which promoted the formation and accumulation of local secondary species, leading to high concentrations of PM. Overall, our results indicate that PM pollutants in urban Korea originate from complex emission sources and atmospheric processes and that the concentrations and composition of PM are controlled by various factors including meteorological conditions, local anthropogenic emissions, and upwind sources. Therefore, understanding the high aerosol pollution followed by efficient strategies to remove precursors are important to control the air pollution.