Impact of In‐Cloud Aqueous Processes on the Chemistry and Transport of Biogenic Volatile Organic Compounds

Abstract

AbstractWe investigate the impacts of cloud aqueous processes on the chemistry and transport of biogenic volatile organic compounds (BVOC) using the National Center for Atmospheric Research's large‐eddy simulation code with an updated chemical mechanism that includes both gas‐ and aqueous‐phase reactions. We simulate transport and chemistry for a meteorological case with a diurnal pattern of nonprecipitating cumulus clouds from the Baltimore‐Washington area DISCOVER‐AQ campaign. We evaluate two scenarios with and without aqueous‐phase chemical reactions. In the cloud layer (2–3 km), the addition of aqueous phase reactions decreases HCHO by 18% over the domain due to its solubility and the fast depletion from aqueous reactions, resulting in a corresponding decrease in radical oxidants (e.g., 18% decrease in OH). The decrease of OH increases the mixing ratios of isoprene and methacrolein (MACR) (100% and 15%, respectively) in the cloud layer because the reaction rate is lower. Aqueous‐phase reactions can modify the segregation between OH and BVOC by changing the sign of the segregation intensity, causing up to 55% reduction in the isoprene‐OH reaction rate and 40% reduction for the MACR‐OH reaction when clouds are present. Analysis of the isoprene‐OH covariance budget shows the chemistry term is the primary driver of the strong segregation in clouds, triggered by the decrease in OH. All organic acids except acetic acid are formed only through aqueous‐phase reactions. For acids formed in the aqueous phase, turbulence mixes these compounds on short time scales, with the near‐surface mixing ratios of these acids reaching 20% of the mixing ratios in the cloud layer within 1 h of cloud formation.