Fine particle pH and sensitivity to NH₃ and HNO₃ over summertime South Korea during KORUS-AQ

Abstract. Using a new approach that constrains thermodynamic modeling of aerosol composition with measured gas-to-particle partitioning of inorganic nitrate, we estimate the acidity levels for aerosol sampled in the South Korean planetary boundary layer during the NASA/NIER KORUS-AQ field campaign. The pH (mean ± 1σ = 2.43 ± 0.68) and aerosol liquid water content determined were then used to determine the chemical regime of the inorganic fraction of particulate matter (PM) sensitivity to ammonia and nitrate availability. We found that the aerosol formation is always sensitive to HNO₃ levels, especially in highly polluted regions, while it is only exclusively sensitive to NH₃ in some rural/remote regions. Nitrate levels are further promoted because dry deposition velocity is low and allows its accumulation in the boundary layer. Because of this, HNO₃ reductions achieved by NO_x controls prove to be the most effective approach for all conditions examined, and that NH₃ emissions can only partially affect PM reduction for the specific season and region. Despite the benefits of controlling PM formation to reduce ammonium-nitrate aerosol and PM mass, changes in the acidity domain can significantly affect other processes and sources of aerosol toxicity (such as e.g., solubilization of Fe, Cu and other metals) as well as the deposition patterns of these trace species and reactive nitrate.