Physical and chemical mechanisms of the daily-to-seasonal variation of PM in Korea.

In this study, the CSEOF technique is used to investigate the physical and chemical mechanisms associated with the weekly PM variation in South Korea. For this end, 9 years of hourly measurements of PM in South Korea is used together with other gaseous contaminants (NO, SO, CO and O) and traffic counts at the toll gates. The diurnal variation of PM concentrations indicates a significant correlation with human activities; higher concentrations in densely populated areas with a large volume of traffics and vice versa. In particular, the diurnal cycles of PM, NO and CO show bimodal structures with maxima corresponding to the morning and evening rush hours. While NO and CO maxima are observed at ~9 AM, with some delay from the traffic maximum, PM peak is observed ~11 AM, indicating roughly 2 h of conversion process from gaseous pollutants to particulate matter. After the sunset, gas-to-particle conversion efficiency is significantly reduced and PM concentration begins to increase slowly. SO exhibits a slightly different feature, showing diurnal variation with a single peak at 11 AM and relatively minor contribution to the diurnal variation of PM. O participates in the photochemical reactions of the gaseous pollutants, providing OH radical as an oxidant. Physical factors (e.g. boundary layer height, 10 m wind gust, wind convergence) affecting horizontal and vertical mixing of polluted air are responsible for dispersion of accumulated PM in the afternoon. As a result, strong morning-evening asymmetry is seen in the diurnal variation of PM concentrations. Also, notable seasonal dependency is observed in the concentrations of PM and the gaseous contaminants due to the seasonal variation of emission and the physical factors.