Sulfate acidic episodes

Forms of precipitation other than rainfall may contain excess acidity. Acidic fog can be especially damaging because it is very penetrating. In early December 1982, Los Angeles experienced a severe, 2-day episode of acid fog. This fog consisted of a heavy concentration of acidic mist particles at ground level that reduced visibility and were very irritating to breathe. The pH of the water in these particles was 1.7, much lower than ever before recorded for acid precipitation. Another source of precipitation heavy in the ammonium, sulfate, and nitrate ions associated with atmospheric acid is acid rime. Rime is frozen cloudwater that may condense on snowflakes or exposed surfaces. Rime constitutes up to 60% of the snowpack in some mountainous areas, and the deposition of acidic constituents with rime may be a significant vector for the transfer of acidic atmospheric constituents to Earth s surface in some cases. 

Preliminary studies at Watertown, Massachusetts showed that high levels of acid sulfate pollution correlated with winds from the Ohio Valley/Midwest sector, and also contained high levels of selenium. Conversely, winds from the east, i.e., from the city of Boston, contained relatively low levels of acid sulfate and selenium. This suggests that episodes of high acid sulfate at the Watertown site are due to air masses from the Ohio Valley (where sources are predominantly coal-fired) rather than to local sources (which are predominantly oil-fired). The technique was also used at sites in New York State and again implicated midwestem sources of pollution. ... 

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