Environmental Engineering Reference
In-Depth Information
chemistry (
Likens 1989
). Od
´
n had begun investigations of how emissions of SO
2
, from
more urbanized and industrialized regions of Europe, were acidifying precipitation in
Sweden. Sulfur dioxide, along with nitrogen oxides (NO
X
), are major precursors for the
strong acids, H
2
SO
4
and HNO
3
, found in anthropogenically-acidified precipitation. Both
SO
2
and NO
X
emissions result from the combustion of fossil fuels, primarily coal and oil.
We published the first paper on acid rain in North America in 1972, entitling it “Acid
Rain” after much thought and deliberation about the title and its potential impact (
Likens
et al. 1972
). We didn't know at that time that R. A. Smith had referred to the acidity of
rain around London some 100 years previously. We then published two papers in 1974
about the regional nature of acid rain, including one in
Science
, which was picked up by
The New York Times
for a front-page story. That visibility helped bring this environmental
issue to be known around the world and helped to focus much of my subsequent research
onto acid rain. This research has been sustained at the HBEF for more than four decades.
Initially, we considered acid rain to be the acidification of rain and snow from pollutant
sources of SO
2
, which acidified lakes and streams and killed fish and other aquatic organ-
isms. It is now known that this serious environmental problem is much more complicated
(
Figure 15.1
) and widespread, affecting ecosystems around the world. Acid rain has
become a common and popular term for the mix of anthropogenically-acidified rain,
snow, sleet, hail, fog, and cloud water, and the dry deposition of acidifying gases and
particles. This definition makes clear that acid rain is more than just rain! Also, the increas-
ing role of NO
X
emissions in contributing to nitric acid in precipitation has been growing
in ecological importance. Indeed, recent data show that nitric acid could become the
dominant acid in precipitation in eastern North America within the next 10 years or so.
As the problem of acid rain unfolded from those early days in the 1960s and 1970s,
there were several simple, but fundamental, questions that needed to be answered, such
as “What is the pH of unpolluted rain?” and “How long has rain been anthropogenically
acidified?” Rain and snow are not distilled water; they contain a variety of dissolved and
particulate substances. So, how do these impurities affect the pH?
To answer these questions, colleagues and I went to some of the most remote places on
the planet to collect and analyze samples of rainfall to estimate what its chemistry might
have been prior to the Industrial Revolution. Sites included Poker Flat, Alaska; Torres del
Paine, Chile; Katherine, Australia; Amsterdam Island in the South Pacific Ocean; Cape
Point at the southern tip of Africa; and interior China (
Galloway et al. 1982; Likens et al.
1987
). This research showed that unpolluted rain collected in these areas, remote from
human activity, had a pH derived from mineral acids of about 5.1 (
Likens et al. 1987
).
In North America, using the few historical data that existed we deduced that acid rain,
as a phenomenon, actually began in the mid-1950s (
Cogbill and Likens 1974; Butler et al.
1984; Cogbill et al. 1984; Likens and Butler 1981
). Certainly, anthropogenic emissions of
SO
2
and NO
X
began to increase markedly with the Industrial Revolution, but apparently
the inherent acid-neutralizing capacity of the atmosphere (dust particles, etc.) prevented
any major change in the acidity of precipitation until the mid-1950s or so. Because of
decreasing emissions of acid precursors, particularly SO
2
, as a result of federal and state
legislation designed to reduce emissions, the acidity of precipitation is now decreasing in
eastern North America. Our long-term studies at the HBEF show this decrease, yet precipi-
tation is still about two times more acid than it would be if the atmosphere were not
