Sulfate formation in atmospheric ultrafine particles at Canadian inland and coastal rural environments

Abstract

[1] Sulfate formation and growth in the various modes of ultrafine particles were explored using samples collected at seven inland rural sites and a remote coastal site in Canada. Two concentration regimes demarcated at a value of 0.03 μg m−3 were identified for nucleation-mode (aerodynamic diameter <0.05 μm) sulfate. In the ≥0.03 μg m−3 regime at the inland sites, strong nucleation of sulfate particles from the enhanced chemical conversion of SO2 to gaseous sulfuric acid was evident, as supported by the moderate to good correlations between the nucleation-mode sulfate and the ambient SO2 concentration as well as the elevated concentrations of O3 and NOy. However, the nucleation- and Aitken-mode sulfate did not correlate with SO2 in the ≥0.03 μg m−3 regime at the coastal site. At the sites where the ≥0.03 μg m−3 regime occurred frequently, the Aitken-mode (aerodynamic diameter 0.05–0.1 μm) sulfate was mainly from the growth of nucleation mode; the condensational growth is found to be more important than the coagulation growth at inland sites, but the coagulation growth became more important at the coastal site. The equivalent ratios of NH4+ to (SO42− + NO3−) during high-concentration episodes varied from 0.7 to 1.5 in the nucleation mode and were almost unity in the Aitken mode. The equivalent ratio in the nucleation mode ranged from 1.2 to 1.5 at a few inland sites, suggesting that ammonium organic acid salts could be present and could play important roles in the growth of the nucleation-mode particles, but they were unlikely important in the growth of the Aitken mode particles.