Abstract
Abstract. Highly time-resolved chemical characterization of nonrefractory submicrometer particulate matter (NR-PM1) was conducted in Seoul, the capital and largest metropolis of Korea, using an Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). The measurements were performed during winter, when elevated particulate matter (PM) pollution events are often observed. This is the first time that detailed real-time aerosol measurement results have been reported from Seoul, Korea, and they reveal valuable insights into the sources and atmospheric processes that contribute to PM pollution in this region. The average concentration of submicron aerosol (PM1 = NR-PM1+ black carbon (BC)) was 27.5 µg m−3, and the total mass was dominated by organics (44 %), followed by nitrate (24 %) and sulfate (10 %). The average atomic ratios of oxygen to carbon (O / C), hydrogen to carbon (H / C), and nitrogen to carbon (N / C) of organic aerosols (OA) were 0.37, 1.79, and 0.018, respectively, which result in an average organic mass-to-carbon (OM / OC) ratio of 1.67. The concentrations (2.6–90.7 µg m−3) and composition of PM1 varied dynamically during the measurement period due to the influences of different meteorological conditions, emission sources, and air mass origins. Five 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 = 0.06), cooking activities represented by a cooking OA factor (COA, O / C = 0.14), wood combustion represented by a biomass burning OA factor (BBOA, O / C = 0.34), and secondary organic aerosol (SOA) represented by a semivolatile oxygenated OA factor (SV-OOA, O / C = 0.56) and a low-volatility oxygenated OA factor (LV-OOA, O / C = 0.68). On average, primary OA (POA = HOA + COA + BBOA) accounted for 59 % the OA mass, whereas SV-OOA and LV-OOA contributed 15 and 26 %, respectively. Our results indicate that air quality in Seoul during winter is influenced strongly by secondary aerosol formation, with sulfate, nitrate, ammonium, SV-OOA, and LV-OOA together accounting for 64 % of the PM1 mass during this study. However, aerosol sources and composition were found to be significantly different between clean and polluted periods. During stagnant periods with low wind speed (WS) and high relative humidity (RH), PM concentration was generally high (average ±1σ = 43.6 ± 12.4 µg m−3) with enhanced fractions of nitrate (27 %) and SV-OOA (8 %), which suggested a strong influence from local production of secondary aerosol. Low-PM loading periods (12.6 ± 7.1 µg m−3) tended to occur under higher-WS and lower-RH conditions and appeared to be more strongly influenced by regional air masses, as indicated by higher mass fractions of sulfate (12 %) and LV-OOA (20 %) in PM1. Overall, our results indicate that PM pollutants in urban Korea originate from complex emission sources and atmospheric processes and that their concentrations and composition are controlled by various factors, including meteorological conditions, local anthropogenic emissions, and upwind sources.
Highlights
Ambient aerosols can reduce visibility, damage human health, and influence climate change directly by absorbing and reflecting solar radiation and indirectly by modifying cloud formation and properties (IPCC, 2013; Pope III and Dockery, 2006; Pöschl, 2005)
Assuming that PM1 represents approximately 80 % of PM2.5 mass (Lim et al, 2012), we found that 29 % of the measurement days (i.e., 14 days) violated the National Institute of Environmental Research (NIER)’s daily PM2.5 standard (50 μg m−3) and 58 % of the days (28 days) violated the WHO’s daily standard (25 μg m−3)
The average concentration of PM1 measured in Korea during this study was similar to or slightly lower than that measured in winter in several urban areas in China, including Shanghai (Huang et al, 2012), Shenzhen (He et al, 2011), Lanzhou (Xu et al, 2016), and Hong Kong (Li et al, 2015), but it was much lower than in Beijing where the winter mass concentrations of PM1 were found to be 7–10 times higher than in Seoul (Sun et al, 2014)
Summary
Ambient aerosols can reduce visibility, damage human health, and influence climate change directly by absorbing and reflecting solar radiation and indirectly by modifying cloud formation and properties (IPCC, 2013; Pope III and Dockery, 2006; Pöschl, 2005). The annual average concentration of PM2.5 in Seoul decreased from 28.5 μg m−3 in 2005 to 22.5 μg m−3 in 2012 since the enactment of the “Special Act on Seoul Metropolitan Air Quality Improvement” in 2005, which has led to emission reduction from diesel vehicles and fugitive dust emissions on roads and in open areas (Kang et al, 2016; KOSAE, 2009). The amount of reduction was not dramatic and the annual average PM2.5 concentration increased again in 2013 to 24.8 μg m−3. These values far exceeded the PM2.5 annual standards set by the US (15 μg m−3) and the WHO (World Health Organization; 10 μg m−3). Due to growing concerns over the adverse effects of atmospheric PM, the South Korean government established PM2.5 standards in 2015: 25 μg m−3 for annual average and 50 μg m−3 for 24 h average (NIER, 2014)
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