Implementing Gas-to-Particle Partitioning of Semi-Volatile Inorganic Compounds in UCLALES-SALSA

Abstract. We investigated the effect of inorganic semi-volatile compounds (SVC) HNO₃ and NH₃ on the cloud condensation nuclei (CCN) activity of aerosols and the subsequent changes in cloud properties. This was done by upgrading our state-of-the-art large eddy simulator – UCLALES-SALSA, which was modified to include the treatment of the condensation and dissolution of SVCs onto pre-existing aerosols and cloud droplets. The immediate effect of these SVCs on aerosols was to shift the aerosol dry size distribution towards larger sizes as a result of their co-condensation with water vapour. Since the dry size of a CCN is the dominant factor determining its CCN activity, a marked increase in cloud droplet number concentration(similar to the Twomey effect) was noted both in zero- and three-dimensional simulations when gas-phase concentrations of VCs were increased. As the overall amount of precipitation was small in the simulated stratocumulus case, the increase in droplet concentration led to a smaller mean size and reduced drizzle. Within clouds, the smaller droplets contain a relatively higher amount of nitrate than the larger ones, and as the drizzle is mainly formed through large droplets, the ammonium nitrate in-cloud scavenging is weaker than would be estimated based on average droplet composition. The model was also able to simulate the relatively more acidic interstitial particles than cloud droplets. However, below the cloud, condensation of gases on drizzling droplets quickly increases their overall wet scavenging efficiency compared to sulphate.