EFFECTS OF ACID RAIN

EFFECTS OF ACID RAIN
Freshwater biota Terrestrial vegetation Concrete, limestone, metal
Leaching of toxic metals,
body corrosion
Leaching of nutrients,
toxic metals
Major effects begin to be observed when pH < 5
Corrosion

NATURAL pH OF RAIN
• Equilibrium with natural CO 2 (280 ppmv) results in a rain pH of 5.7:
• This pH can be modified by natural acids (H 2 SO 4 , HNO 3 , RCOOH…)
and bases (NH 3 , CaCO 3 ) natural rain has a pH in range 5-7
“Acid rain” refers to rain with pH < 5 damage to ecosystems

PRECIPITATION PH OVER THE UNITED STATES

Graedel and Crutzen, Atmospheric
Change (1992)
ACID FOG
Like acid rain but more concentrated;
pH values as low as 2.
Effects on terrestrial ecosystems, structures
thatʼs me!

CHEMICAL COMPOSITION OF PRECIPITATION

Chemical reactions leading to acidity in rain
Gas phase oxidation of SO 2 ( 1 – 2 weeks)
SO 2 + OH + M HSO 3 + M
HSO 3 + O 2 SO 3 + HO 2
SO 3 + H 2O H 2SO 4
Liquid phase (in cloud) oxidation of SO 2 ( ~ days )
SO 2 SO 2 H 2 O HSO 3 - + H +
(add H 2 O 2 from atm)
HSO 3 - + H2O 2 + H + SO 4 = + 2H + + H2O
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Neutralization of acidity in rainwater by NH 3 is illusory because in the
ecosystem/soils
NH 4 + + 3/2 O2 NO 2 - + 2H + + H2 O
NO 2 - + ½ O2 NO 3 - (nitrification) in a system with lots of N
If taken up and turned into organic matter, effectively
NH 4 + "" NH3 + H +
Neutralization of acidity in rainwater by base cations from soil is effective.
CaCO 3 Ca +2 + CO 3 -2
CO 3 -2 + H + HCO3 -
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LONG-TERM TREND IN US SO 2 EMISSIONS

AMMONIUM AND SULFATE TRENDS, 1985-2004
Lehmann et al. [2007]
NH 4 +
SO 4 2-

TREND IN FREQUENCY OF ACID RAIN (pH < 5)
Lehmann et al. [2007]
1994-1996
2002-2004

BUT ECOSYSTEM ACIDIFICATION IS PARTLY A TITRATION PROBLEM
FROM ACID INPUT OVER MANY YEARS
Acid flux
F H+
Acid-neutralizing capacity (ANC)
from CaCO 3 and other bases

NO 3 -
pH
precipitation
SO 4 -2
stream water
SO 4 -2

DEPLETION OF BASE CATIONS FROM ACID RAIN
(Hubbard Brook Experimental Forest, New Hampshire)

stream
precip
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Acid precipitation is caused by emissions of SO 2 and
NO x, which are converted to mineral acids in the
atmosphere.
The influx of acidic precipitation can damage
ecosystems by disrupting nutrient cycles, availability of
base cations (Ca +2 , K + ) to plants, and by direct damage
to plant organs (mostly acid fog).
Cap-and-trade legislation has been very effective in
reducing acid deposition but emissions, and lasting
ecological effects from the past, remain.
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