Chemical Differences Between PM₁ and PM_2.5 in Highly Polluted Environment and Implications in Air Pollution Studies

Abstract

Abstract Submicron aerosol (PM 1 ) species measured by aerosol mass spectrometers have been widely used to validate chemical transport models; however, the uncertainties due to chemical differences between PM 1 and PM 2.5 are poorly constrained. Here we characterized such differences in a highly polluted environment in north China in winter. Our results showed that the changes in PM 1 /PM 2.5 ratios as a function of relative humidity (RH) were largely different for primary and secondary species. Secondary organic and inorganic aerosol (SOA and SIA) presented clear decreases in PM 1 /PM 2.5 ratios at RH > 60% during periods with high SIA contributions (>50%), likely driven by the changes in aerosol hygroscopicity and phase states, while the traffic and coal combustion OA had limited dependence on RH. Thermodynamic modeling showed negligible impacts of PM differences on predictions of particle acidity, yet these impacts can cause a difference in aerosol water content by up to 50–70%.