New particle formation characteristics and mechanisms of secondary PM_2.5 in Seoul and Seosan in the Republic of Korea during 2020–2022

Air pollution caused by particulate matter (PM_2.5) adversely affects environmental and human health. A lack of understanding of the characteristics and mechanisms of secondary aerosols has limited the mitigation of atmospheric PM_2.5. This study aims to investigate the spatiotemporal characteristics and mechanisms of new particle formation in the Republic of Korea (ROK). Four intensive monitoring campaigns were conducted at the Seoul (SE, megacity) and Seosan (SS, sub-urban) sites in different seasons during 2020–2022 based on real-time measurements. The mean growth rates at both sites were > 2 times higher during the summer, with a higher concentration of gaseous precursors than during the other seasons. Mean nucleation rates (NRs) were > 2 times higher during the winter at SE due to lower temperatures and condensation/coagulation sinks in this season than the other seasons. However, the mean NRs at SS were > 4 times higher during the summer due to higher sulfuric acid (H₂SO₄) concentrations than during the other seasons. Plotting NRs versus sulfuric acid concentrations revealed that the nucleation mechanisms of secondary PM_2.5 formation were in the middle of the H₂SO₄–ammonia (NH₃)–water and H₂SO₄–dimethylamine–water ternary systems during the summer and closer to the NH₃ ternary system during the other seasons. This seasonal difference may be caused by the decrease in the contribution of NH₃ ternary nucleation at elevated temperatures in the summer.