Local influences of emission source characteristics and meteorological factors on nitrate and ammonium partitioning in Korea

Particulate air pollution in Korea is influenced by both local emissions and the transportation of particles and precursors in continental outflows. Alongside local high-NO_x and NH₃ conditions, the inorganic-rich and hygroscopic transported particles enhance the gas-particle partitioning of inorganic precursor gases (HNO₃, NH₃) into particulate nitrate and ammonium, synergistically resulting in heavy haze pollution. Local source characteristics can impact concentrations of precursor gases and particle acidity, which are crucial factors in inorganic gas-particle partitioning. However, the episodic or background effects of transported particles and precursors make it challenging to clearly identify the local influences of emission source characteristics on chemical composition and concentrations. In this study, we investigated local effects on the gas-particle partitioning into nitrate and ammonium using hourly data of PM_2.5 chemical compositions, NH₃ concentration, and meteorological variables from three different sites in Korea: a typical urban site in Seoul, an industrial site in Ansan, and a rural site affected by industrial sources in Seosan. To isolate the effect of local characteristics from the impact of long-range transport from China, backward trajectories and wind speed were additionally utilized. Partitioning ratios of total HNO₃ (HNO₃ gas + particulate nitrate) and total NH₃ (NH₃ gas + particulate ammonium) were analytically calculated by using particle acidity and liquid water contents from the ISORROPIA II thermodynamic model. Results show that the particle acidity from the three sites is not significantly different, despite a large difference in total HNO₃ and total NH₃ concentrations. However, high relative humidity and liquid water content at the coastal-industrial site, Ansan, can result in a larger nitrate fraction. Local characteristics of secondary inorganic aerosols depend not only on different local source characteristics but also on different local meteorological conditions, particularly relative humidity, which affects nitrate and ammonium partitioning.