N2O5 Hydrolysis on a Liquid Surface Coated by Long Chain Fatty Acid and Alcohol monolayers

Heterogeneous reactions between aerosol particles and gas phase species play a crucial role in the atmosphere and can influence the composition of the atmosphere. Heterogeneous reactions of N2O5 with aqueous particles have a significant effect on the global NOx budget, which influences tropospheric concentrations of O3 and OH. The hydrolysis of N2O5 on sulfuric acid particles has been studied extensively and is found to be very efficient. However, surface active organic molecules are common constituents of aqueous atmospheric particles and their effect on heterogeneous reactions is still poorly understood. Previous studies have shown that organic coatings on atmospheric droplets can change the uptake of gaseous species substantially. It is important to know heterogeneous reaction rates for the hydrolysis of N2O5 on organic coated aqueous particles in order to accurately predict the oxidative capacity of the troposphere. Here we present the investigation of heterogeneous reactions of N2O5 on aqueous sulfuric acid coated by an organic monolayer. A new parallel plate flow reactor (PPFR) has been developed to allow heterogeneous reactions to be studied on a planar aqueous surface coated with an organic monolayer. In addition, a theoretical framework has been developed to derive heterogeneous kinetics taking into consideration the effects of diffusion. The PPFR coupled to a chemical ionization mass spectrometer was employed to measure reactive uptake coefficients for N2O5 on aqueous H2SO4 solutions coated with long chain fatty acids and alcohols. One key feature of this new technique is the ability to characterize the monolayer prior to kinetic measurements to determine the surface pressure and packing density of the organic molecules on the surface. Monolayers of octadecanol, hexadecanol, stearic acid, and phytanic acid were studied on 60 wt % H2SO4 solutions at 274 K. It was found that a monolayer of octadecanol reduced the uptake coefficient of N2O5 with H2SO4 by approximately a factor of 60. Conversely, phytanic acid (a branched molecule) caused no significant decrease in reactivity. Results from these studies will be reported and the atmospheric implications will be discussed.