Typical Rainwater Composition

What is a chemical salt recipe for 'typical' rainwater?

Rainwater gets its compositions largely by dissolving particulate materials in the atmosphere (upper troposhere) when droplets of water nucleate on atmospheric particulates, and secondarily by dissolving gasses from the atmosphere. Rainwater compositions vary geographically. In open ocean and coastal areas they have a salt content essentially like that of sea water (same ionic proportions but much more dilute) plus CO2 as bicarbonate anion (acidic pH). Terrestrial rain compositions vary siginificantly from place to place because the regional geology can greatly affect the types of particulates that get added to the atmosphere. Likewise, sources of gaesous acids (SO3, NO2) and bases (NH3) vary as a function of biome factors and anthopogenic land use practices. Each of these gasses can be added in varying proportions from natural and non natural input sources (non-natural sources of SO3 and NO2 far outweigh natural ones). Particulate load to the atmosphere can also be greatly affected by human activities. Finally, local climate (especially the amount of precipitation in one area compared to another) will affect the solute concentrations in terrestrial rainwaters. The result is highly variable compositions, so there isn't one simple formula.

Dr. Kenneth Rubin, Associate Professor
Department of Geology and Geophysics
University of Hawaii, Honolulu HI 96822