Investigation of sulfate and nitrate formation on mineral dust particles by receptor modeling

The formation of sulfate and nitrate by heterogeneous reactions of gaseous precursors on mineral dust particles was investigated using positive matrix factorization (PMF) of coarse PM10 (particulate diameters from 2.2 to 10 μm) collected at urban (Hanoi) and rural (Lucnam) sites in northern Vietnam. Air samples were analyzed for ionic and elemental components using ion chromatography and proton induced X-ray emission methods. PMF revealed six similar sources/types of coarse PM10 at the two sites, namely soil dust containing nitrate and sulfate, coal fly ash from distant and local sources, soil dust containing organic matter and ammonium sulfate and marine aerosol. Traffic (road) dust was found only at the urban site.From the PMF factor models, the yields of NO3-, SO42- and NH4+ can be estimated and their possible chemical forms in different particulate types can be suggested. The yields of nitrate and sulfate formation on mineral dust particles increase with the [Ca]/[Si] ratio, which is greater in soil dust than in coal fly ash. Nitrate is bound to Ca-richest soil dust particles. Ammonium was found in dust particles containing soil organic matter, which also hold the largest amount of sulfate. The comparison of urban and rural receptor models provided synergy for the source identification and insights into the properties of mineral dust particles relevant to their interactions with acidic gases in ambient air.