Study of formation of coarse particle nitrate aerosol

The results of analyses of six atmospheric aerosol samples collected in Helsinki, Finland, using a Berner low-pressure impactor, are reported. Formation of coarse particle nitrate was studied by evaluating various ion balances of the measured water-soluble ions (NH4+, Na+, K+, Mg2+, Ca2+, Cl−, NO3− and SO42−) and mass size distributions of Na+ Ca 2+, Cl− and NO−3. Atmospheric coarse particle NO−3 was found to be formed in the reaction of nitric acid with sea- and soil-derived coarse particles. The fraction of nitrate found on sea and soil particles followed the measured concentrations and size distributions of these two particle types: the concentration ratio of soil to sea particles increased with increasing particle size as did the fraction of nitrate on soil particles. The reaction with sea-salt particles resulted in the evaporation of Cl− as HCI. The presence of non-sea-salt SO42− indicates that also H2SO4 and/or SO2 reacted with sea salt and contributed to the Cl− loss. Overall, the percentage of evaporated Cl− decreased with increasing particle size, which suggests a surface reaction mechanism. Cl− evaporation was almost complete for the 1–2 um diameter size fraction indicating that all major sea-derived chlorides, NaCl, MgCl2, CaCl2 and KCI, may react with acidic species in the atmosphere. The results showed further some indications of possible reaction of HCI with soil particles.