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.
If you want to read up a bit on this and see data for rainwater from many
different locales globally, I suggest the book "Global Environment: water
air and geochemical cycles" by Berner and Berner (Prentice-Hall, 1996) or
a similar text
Dr. Kenneth Rubin, Associate Professor
Department of Geology and Geophysics
University of Hawaii, Honolulu HI 96822