Dynamics of water-soluble inorganic ions in Qinhuangdao: Particle size association and influences of environmental conditions

Abstract

The dynamics of water-soluble inorganic ions in particulate matter (PM) in Qinhuangdao (1 km from the sea) are studied, with a focus on particle size association and the influence of marine conditions. In ${\mathrm{PM}}_{2.5}$, sulfate (${\mathrm{SO}}_4^{2-}$) and nitrate (${\mathrm{NO}}_3^{-}$) were the predominant ions, while in ${\mathrm{PM}}_{2.5-10}$, ${\mathrm{NO}}_3^{-}$ concentrations were substantially higher than sulphate, and exceeded those in PM_2.5. Marine atmosphere conditions were found to significantly impact the concentration and distribution of these ions. Concentrations were generally higher in southerly air, reflective of air mass trajectories which start over land, but approach the coast from the sea. The contribution of nitrate is much increased on the days of higher pollution. Nitrogen Oxidation Ratios are higher for predominantly southerly wind conditions and for coarse than fine particles, and for coarse particles are higher for nitrate than sulphate. The highest Sulfur Oxidation Ratios are for predominantly northerly winds and fine particles, reflecting the very different precursor sources and/or mechanism of formation. Other key findings are that air from the marine sector is frequently more polluted than that approaching directly over land, with a significant contribution of regional transport to pollutant concentrations. Secondary pollutant formation in maritime air has a major influence.