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

22Non-refractory submicrometer particulate matter (NR-PM1) was measured in the Seoul 23Metropolitan Area (SMA), Korea, using an Aerodyne high-resolution time-of-flight aerosol mass 24 spectrometer (HR-ToF-AMS) from April 14 to June 15, 2016, as a part of the Korea-U.S. Air 25 Atmos. Chem. Phys. Discuss., https://doi.org/10. 5194/acp-2017-947 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 8 November 2017 c Author(s) 2017. CC BY 4.0 License. 2Quality Study (KORUS-AQ) campaign. This was the first highly time-resolved, real-time 1 measurement study of springtime aerosol in SMA and the results reveal valuable insights into the 2 sources and atmospheric processes that contribute to PM pollution in this region. 3The average concentration of submicrometer aerosol (PM1 = NR-PM1 + black carbon (BC)) 4 was 22.1 µg m -3 , which was composed of 44% organics, 20% sulfate, 17% nitrate, 12 % 5 ammonium, and 7 % BC. Organics had an average atomic oxygen-to-carbon (O/C) ratio of 0.49 6 and an average organic mass-to-carbon (OM/OC) ratio of 1.82. The concentration and composition 7 of PM1 varied dynamically due to the influences of different meteorological conditions, emission 8 sources, and air mass origins. Four distinct sources of OA were identified via positive matrix 9 factorization (PMF) analysis of the HR-ToF-AMS data: vehicle emissions represented by a 10 hydrocarbon like OA factor (HOA; O/C = 0.15; 17% of OA mass), cooking activities represented 11 by a cooking OA factor (COA; O/C = 0.19; 22% of OA mass), and secondary organic aerosol 12 (SOA) represented by a semi-volatile oxygenated OA factor (SV-OOA; O/C = 0.44; 27% of OA 13 mass) and a low volatility oxygenated OA factor (LV-OOA; O/C = 0.91; 34% of OA mass). 14 Our results indicate that air quality in SMA during KORUS-AQ was influenced strongly by 15 secondary aerosol formation with sulfate, nitrate, ammonium, SV-OOA, and LV-OOA together 16 accounting for 76% of the PM1 mass. In particular, high temperature, elevated ozone 17 concentrations, and photochemical reactions during daytime promoted the formation of SV-OOA, 18 LV-OOA and sulfate whereas nocturnal processing of nitrogen oxides and daytime photochemical 19 reactions promoted nitrate formation. In addition, gas-to-particle partitioning processes appeared 20 to have enhanced nighttime SV-OOA and nitrate formation. During a period of 4 days (from May 21 20 to May 23), LV-OOA was significantly enhanced and accounted for up to 41% of the PM1 22 mass. This intense LV-OOA formation event was associated with large enhancements of both 23 anthropogenic and biogenic VOCs (e.g., isoprene, toluene), high concentration of Ox (= O3 + NO2), 24 strong solar radiation, and stagnant conditions, suggesting that it was mainly driven by local 25 photochemical formation. We have also investigated the formation and evolution mechanisms of 26 severe haze episodes. Unlike the winter haze events which were mainly caused by intense local 27 emissions coupled with stagnant meteorological conditions,...

Response to reviewers

“…Since there were no direct measurements of S/IVOC concentrations, an estimated (Robinson et al, 2007;Dzepina et al, 2009) relationship between the amount of gasphase S/IVOC co-emitted with POA at the typical temperatures (~20°C) and OA mass concentrations (~10 μg sm −3 ) observed over Seoul were used. The POA is taken from Figure 5b 325 and is within the range of values observed in other urban environments (Zhang et al, 2005;Hayes 326 et al, 2013;Ait-Helal et al, 2014;Kim et al, 2018) (13 μg sm −3 ppmv −1 in Seoul versus 4.5 -28.8 327 μg sm −3 ppmv −1 in other studies).…”

“…Finally, unlike Kim et al (2017), we did not observe clear indication for cooking organic aerosol (COA) in our PMF results. The COA was at a minimum (less than 1 μg m −3 ) at the surface 579 in Seoul during the times the DC-8 overpassed (Kim et al, 2018); thus, we speculate the amount 580 of COA sampled was a small fraction of OA and was mostly lumped into the HOA factor. This 581 does not affect our characterization of HOA as POA, since COA is also a primary aerosol emission.…”

Nitrate-driven urban haze pollution during summertime over the North China Plain

“…The scatter plot of the molar concentration of excess ammonium versus the molar concentration of nitrate showed that nitrate was usually completely neutralized by excess ammonium. When ammonium is in deficit, nitrate may associate with other alkaline species or be part of an acidic aerosol (Kouimtzis and Samara, 1995).…”

“…N 2 O 5 is produced by the reversible reaction between NO 2 and the NO 3 radical, whereby NO 2 reacts with O 3 to form the NO 3 radical. Assuming N 2 O 5 and the NO 3 radical are both in steady state considering their short lifetimes (Brown et al, 2006), the nighttime production of N 2 O 5 and HNO 3 is proportional to the concentration of NO 2 and O 3 ([NO 2 ][O 3 ]) (Young et al, 2016;Kim et al, 2017). For the different PM 1 concentration bins, we examined the NO 2 and O 3 data at 00:00 to assess the nighttime HNO 3 production rate.…”

Taehwa Research Forest: A receptor site for severe pollution events in Korea during 2016

“…Recent chemical simulations performed during the KORUS-AQ period by Miyazaki et al, 2018 also reveal that observed boundary 10 layer O3 can be as much as 30 ppbv different with dynamic and stagnant flow regimes. Supporting work at TRF has also shown that fast oxidation rates (Kim et al, 2018) and overall oxidation capacity (Jeong et al, 2018) at TRF can exacerbate severe pollution events.…”