Reactive uptake of N2O5 to internally mixed inorganic and organic particles: The role of organic carbon oxidation state and inferred organic phase separations

Abstract

We measured N2O5 reactive uptake onto mixed organic/inorganic submicron particles using organic compounds with a variety of oxidation states (using mainly atomic O: C ratios as a proxy) and molecular weights. The organic mass fraction, organic molecular composition, and relative humidity (RH) were varied to assess their effects separately on the N 2O5 uptake coefficient, γ (N2O 5). At a constant RH, mixtures of organic components having an O:C < 0.5 with ammonium bisulfate significantly suppressed the uptake of N 2O5(g) compared to pure ammonium bisulfate, even at small organic mass fractions (e.g., ≤15 %). The effect of the organic component became less pronounced at higher RH. In general, highly oxygenated organic components (O : C > 0.8) had a smaller or even negligible impact on N 2O5(g) uptake at all RHs probed; however, a few exceptions were observed. Notably, γ (N2O5) for mixtures of ammonium bisulfate with polyethylene glycol (PEG), PEG-300 (O :C=0.56), decreased nearly linearly as the PEG mass fraction increased at constant RH until leveling off at the value measured for pure PEG. The response of γ (N2O5) to increasing PEG mass fraction was similar to that measured on ambient atmospheric particles as a function of organic mass fraction. The effects of the organic mass fraction on γ (N 2O5), for mixtures having an O: C