Classification of diurnal patterns of particulate inorganic ions downwind of metropolitan Seoul

The inorganic ions in PM2.5 were measured downwind of metropolitan Seoul using a particle-into-liquid sampler over three periods. Five diurnal patterns, including a low-concentration pattern (L) and a high-concentration pattern with a decreasing trend during the day (H-), were distinguished for the first period from February to June 2012. The sum of ion concentrations increased primarily due to NO3- with decreasing temperature and increasing relative humidity, which caused preferential partitioning of NO3- into the particulate phase. The peak concentration occurred during the morning rush hour for L but was delayed until the next morning for H- due to the time required to form secondary inorganic ions from accumulated pollutants under lower wind speeds. The characteristic features of the patterns observed during the first period were generally similar with those obtained for the other two periods. However, for the second period, comprised of colder months, changes in the diurnal patterns from L to H- were accompanied by increasing temperature, as the effect of photochemical formation of NO3- was larger than that of volatilization. Although the role of long-range transport was not distinct on the whole, the inflow of air masses from cleaner sectors was observed to lower the concentrations.