acid rain

in atmospheric chemistry
https://doi.org/10.1351/goldbook.A00083
Rain with \(\text{pH}\) values < about 5; commonly results from acids formed from pollutants. 'Pure' rain water equilibrated with atmospheric CO2 and naturally occurring acids in relatively clean air usually has a \(\text{pH} > 5\).
Source:
PAC, 1990, 62, 2167. (Glossary of atmospheric chemistry terms (Recommendations 1990)) on page 2170 [Terms] [Paper]