Lower pH of acid rain associated with lightning: evidence from sampling within 14 showers and storms in the Georgia Piedmont in summer 1996

Fourteen showers and storms in northeast Georgia were sampled at intervals as short as 90 s in the summer of 1996. The average pH of samples collected while thunder was heard was 3.63, significantly less than the average pH (4.05) of samples collected when no thunder was heard. That difference existed within rainfall events as well as across them. Of lesser but still considerable statistical significance was the difference between the average pH of samples collected during the day (3.62) and the average pH of samples taken at night (3.94). In addition, among samples collected when thunder was heard, those collected when wind blew from a relatively sparsely populated area to the east had a higher average pH than that of samples associated with winds from more densely populated and industrialized areas that include Atlanta. Previous work has focused on the role of solar-driven photochemical reactions in the generation of acid precipitation, but the greater acidity of rainwater associated with lightning suggests that oxidation of SO2 and NOx is also enhanced by oxidants produced by lightning.