What effect does precipitation have on air pollutant concentrations?

• A. It can wash out soluble pollutants and reduce concentrations.
• B. It increases concentrations by concentrating pollutants.
• C. It converts pollutants to heavier gases that persist.
• D. It has no effect on air pollutants.

Answer: (A)

Precipitation acts as a washing process for the atmosphere. Rain and snow scavenges pollutants from the air, with soluble gases and fine particles dissolving in the rainwater and being deposited on the surface. This wet deposition removes pollutants from the atmosphere, so ambient concentrations drop after a rain event. It doesn’t create heavier gases or leave pollutants unchanged, and while not all pollutants are removed equally (some are less soluble), the overall effect is a decrease in air pollutant concentrations due to washing out.