A study on elevated concentrations of submicrometer particles in an urban atmosphere

Abstract

Mass concentrations of chemical constituents (organics, nitrate, sulfate, ammonium, chloride, and black carbon (BC)) and the number size distribution of submicrometer particles in the ambient atmosphere were continuously measured in urban Gwangju, Korea, during the Megacity Air Pollution Studies (MAPS)-Seoul campaign. Organics (9.1 μg/m3) were the most dominant species, followed by sulfate (4.7 μg/m3), nitrate (3.2 μg/m3), ammonium (2.6 μg/m3), and BC (1.3 μμg/m3) in submicrometer particles (particulate matter less than 1 μm (PM1)). The potential source regions of the sulfate were located in the South and East regions of China and South and East regions of Korea, while local sources were responsible for the elevated BC concentration. Diurnal variation showed that concentrations of organics, nitrate, ammonium, chloride, and BC decreased with increasing mixing layer and wind speed (dilution effect), while sulfate and oxidized organics increased possibly due to their strong photochemical production in the afternoon. During the campaign, an elevated mass concentration of PM1 (PM1 event) and number concentration (nanoparticle formation (NPF) event) were observed (one PM1 event and nine NPF events out of 28 days). The PM1 event occurred with Western and Southwestern air masses with increasing sulfate and organics. Long-range transported aerosols and stagnant meteorological conditions favored the elevated mass concentration of submicrometer particles. Most of the NPF events took place between 10:00 and 14:00, and the particle growth rates after the initial nanoparticle formation were 7.2-11.0 nm/h. The times for increased concentration of nanoparticles and their growth were consistent with those for elevated sulfate and oxidized organics in submicrometer particles under strong photochemical activity.