The investigations on organic sources and inorganic formation processes and their implications on haze during late winter in Seoul, Korea.

This study investigated the sources and formation processes of particulate matter (PM) with an aerodynamic diameter ≤1 μm (PM) and black carbon (BC) in Seoul during late winter via high-resolution time-of-flight aerosol mass spectrometry (HR-ToF-AMS) and positive matrix factorization (PMF) analysis. In this study, secondary aerosols (75.1%) exhibited higher contributions than did primary aerosols (24.9%), suggesting the importance of secondary aerosol formation over primary aerosol emissions for NR-PM+BC during late winter. Frequent haze episodes were observed and these were found to proceed in two distinct stages each with different pattern of sulfur oxidation ratio (SOR), nitrogen oxidation ratio (NOR) and meteorological conditions, such as the wind speed, direction and relative humidity (RH). Haze formation during stage 1 was caused mainly by local accumulation of primary aerosols and formation of local secondary aerosols under stagnant conditions. However, there were some impacts of down mixing of regional transport. Stage 2 took place during the night following stage 1 and was characterized by enhanced secondary aerosol formation. Enhancement of SOR might be due to accelerated aqueous phase reactions under higher RH and enhanced NOR is probably because of the heterogeneous uptake of NO by ammonium sulfate aerosols ensued after sulfate formation. These findings suggest that the winter air quality in Seoul depends on complex processes, from not only emissions and transport from upwind areas but also from significant impacts of meteorological condition.