Chemical characteristics of PM2.5 ions measured by a semicontinuous measurement system during the fall season at a suburban site, Gwangju, Korea

Using a semicontinuous measurement system and filter/denuder system simultaneously, we investigated the chemical characteristics of transient fine particles at a suburban site in Gwangju, South Korea, from 5 to 13 October and 12 to 18 November 2003. The semicontinuous system measured soluble inorganic species, including Cl−, NO3−, SO42−, K+, and NH4+, in PM2.5 with about a 30-min resolution time. During two intensive sampling periods, PM2.5 and gaseous species were also collected two times a day (every 12 h) using a filter/denuder system and analyzed for particulate mass, inorganic, and gaseous species. Mean concentrations of inorganic species were measured as follows: SO42−, 3.86 ± 3.46 (1 σ: standard deviation); NH4+, 2.62 ± 1.88; NO3−, 2.89 ± 2.37; Cl−, 1.02 ± 1.15; and K+, 0.70 ± 0.50 µg m− 3. In the late afternoon on 15 November, maximum NH4+, NO3−, and SO42− concentrations of 16.3, 18.7, and 30.5 µg m− 3, respectively, were observed. This unusual PM2.5 episode was mainly attributed to long-range transport from China, based on air mass back trajectories, Cl−/Na+ concentration ratio, and NOx concentration on 15 November. K+, Cl−, NO3−, and SO42− concentrations increased rapidly when a biomass burning plume passed over the Gwangju Institute of Science and Technology campus on 12 October, with the increased K+ concentration (4 µg m− 3) especially remarkable. The distribution of NO3−, NH4+, and Cl− was due to the combined effects of precursor variations, photochemistry, and thermodynamics. Diurnal variations in these species were clear, as their phase conversions between gas and aerosol depended on the day/night temperature difference during the first sampling period. However, in the second sampling period, there were no clear diurnal variations of these semi-volatile species, as they existed mainly in the particle phase due to low temperatures and were highly associated with precursor levels.