Cell Model of In-cloud Scavenging of Highly Soluble Gases

TL;DR

This study models the in-cloud scavenging process of highly soluble gases like HNO3 and H2O2 by cloud droplets, accounting for diffusion interactions and droplet size distribution to understand gas removal efficiency.

Contribution

It introduces a cellular model that incorporates droplet interactions and size distribution to analyze gas scavenging dynamics in clouds.

Findings

Scavenging significantly reduces soluble gas concentrations in interstitial air.

Scavenging coefficient remains constant during most of the process, then sharply declines.

HNO3 is scavenged more effectively than H2O2 due to dissociation effects.

Abstract

We investigate mass transfer during absorption of highly soluble gases such as HNO_{3}, H_{2}O_{2} by stagnant cloud droplets in the presence of inert admixtures. Thermophysical properties of the gases and liquids are assumed to be constant. Diffusion interactions between droplets, caused by the overlap of depleted of soluble gas regions around the neighboring droplets, are taken into account in the approximation of a cellular model of a gas-droplet suspension whereby a suspension is viewed as a periodic structure consisting of the identical spherical cells with periodic boundary conditions at the cell boundary. Using this model we determined temporal and spatial dependencies of the concentration of the soluble trace gas in a gaseous phase and in a droplet and calculated the dependence of the scavenging coefficient on time. It is shown that scavenging of highly soluble gases by cloud droplets leads to essential decrease of soluble trace gas concentration in the interstitial air. We found that scavenging coefficient for gas absorption by cloud droplets remains constant and sharply decreases only at the final stage of absorption. In the calculations we employed gamma size distribution of cloud droplets. It was shown that despite of the comparable values of Henry's law constants for the hydrogen peroxide (H2O2) and the nitric acid (HNO3), the nitric acid is scavenged more effectively by cloud than the hydrogen peroxide due to a major affect of the dissociation reaction on HNO3 scavenging.