N 2O 5 reaction on submicron sea salt aerosol: Kinetics, products, and the effect of surface active organics

Abstract

The reaction of N 2O 5 on sea salt aerosol is a sink for atmospheric nitrogen oxides and a source of the Cl radical. We present room-temperature measurements of the N 2O 5 loss rate on submicron artificial seawater (ASW) aerosol, performed with an entrained aerosol flow tube coupled to a chemical ionization mass spectrometer, as a function of aerosol phase (aqueous or partially crystalline), liquid water content, and size. We also present an analysis of the product growth kinetics showing that ClNO 2 is produced at a rate equal to N 2O 5 loss, with an estimated lower limit yield of 50% at 50% relative humidity (RH). The reaction probability for N 2O 5γ, N2O5, depends strongly on the particle phase, being 0.005 ± 0.004 on partially crystalline ASW aerosol at 30% RH and 0.03 ± 0.008 on aqueous ASW aerosol at 65% RH. At 50% RH, N 2O 5 loss is relatively insensitive to particle size for radii greater than 100 nm, and γ N2O5 displays a statistically insignificant increase from 0.022 to ∼0.03 for aqueous ASW aerosol over the RH range of 43-70%. We find that the presence of millimolar levels of hexanoic acid in the aerosol bulk decreases the γ N2O5 at 70% RH by a factor of 3-4 from ∼0.025 to 0.008 ± 0.004. This reduction is likely due to the partitioning of hexanoic acid to the gas-aerosol interface at a surface coverage that we estimate to be equivalent to a monolayer. This result is the first evidence that a monolayer coating of aqueous organic surfactant can slow the reactive uptake of atmospheric trace gases to aerosol. © 2005 American Chemical Society.