Is rain or snow a more efficient scavenger of organic chemicals?

Calculations of equilibrium partitioning in the atmosphere are used to quantify and compare the capacity of rain and snow to scavenge gaseous and particle-bound organic contaminants from the atmosphere at different temperatures. Whether snow or rain is more efficient in scavenging organic contaminants depends on the characteristics of the chemical, the characteristics of the snow and atmospheric temperature. At 0°C rain is typically more effective in scavenging the vapors of small organic molecules than snow, because the capacity of the snow surface to sorb such chemicals is smaller than that of liquid water droplets. Snow will, however, be a more effective scavenger for vapors of larger, non-polar organic compounds, which are only sparingly water soluble. The mode of scavenging (vapor vs. particles) and the total scavenging efficiency for such substances will be highly variable and dependent on temperature and the snow characteristics, namely the particle scavenging ratio and the specific snow surface area. To be effectively scavenged by snow, chemicals either need to have an adsorption coefficient at the snow surface KSA of >0.1 m or a particle–air partition coefficient KParticle/Air of >1011 at temperatures below 0°C. Many organic contaminants of concern have such properties. As temperature decreases, more of a chemical partitions from the vapor phase to liquid water droplets, atmospheric particles and the snow surface. This temperature effect is the main reason that snow scavenging ratios are often higher than rain scavenging ratios, especially below −10°C. It is also the reason why wet deposition processes become increasingly important with decreasing temperature, and even constitute the dominant deposition mechanism for some organic contaminants in cold environments.