Viscosity controls humidity dependence of N₂O₅ uptake to citric acid aerosol

• Main Authors: G. Gržinić, T. Bartels-Rausch, T. Berkemeier, A. Türler, M. Ammann
• Format: Article
• Language: English
• Published: Copernicus Publications 2015-12-01
• Series: Atmospheric Chemistry and Physics
• Online Access: http://www.atmos-chem-phys.net/15/13615/2015/acp-15-13615-2015.pdf
• ISSN: 1680-7316; 1680-7324

The heterogeneous loss of dinitrogen pentoxide (N₂O₅) to aerosol particles has a significant impact on the night-time nitrogen oxide cycle and therefore the oxidative capacity in the troposphere. Using a ¹³N short-lived radioactive tracer method, we studied the uptake kinetics of N₂O₅ on citric acid aerosol particles as a function of relative humidity (RH). The results show that citric acid exhibits lower reactivity than similar dicarboxylic and polycarboxylic acids, with uptake coefficients between ∼ 3 × 10^-4–∼ 3 × 10^-3 depending on humidity (17–70 % RH). At RH above 50 %, the magnitude and the humidity dependence can be best explained by the viscosity of citric acid as compared to aqueous solutions of simpler organic and inorganic solutes and the variation of viscosity with RH and, hence, diffusivity in the organic matrix. Since the diffusion rates of N₂O₅ in highly concentrated citric acid solutions are not well established, we present four different parameterizations of N₂O₅ diffusivity based on the available literature data or estimates for viscosity and diffusivity of H₂O. Above 50 % RH, uptake is consistent with the reacto-diffusive kinetic regime whereas below 50 % RH, the uptake coefficient is higher than expected from hydrolysis of N₂O₅ within the bulk of the particles, and the uptake kinetics is most likely limited by loss on the surface only. This study demonstrates the impact of viscosity in highly oxidized and highly functionalized secondary organic aerosol material on the heterogeneous chemistry of N₂O₅ and may explain some of the unexpectedly low loss rates to aerosol derived from field studies.