NATURAL pH OF RAIN

• Equilibrium with natural CO2 (280 ppmv) results in a rain pH of 5.7:

2 2 -12 2 2

7 2 2 3 1

2 103 3 2

( ) 3 10 M atm

9 10 M

7 10 M

H OHCO g CO H O K

CO H O HCO H K

HCO CO H K

2

1/ 21[ ] ( )H COH K K P

• This pH can be modified by natural acids (H2SO4, HNO3, RCOOH…) and bases (NH3, CaCO3) 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

CHEMICAL COMPOSITION OF PRECIPITATION

Neutralization by NH3 is illusory because NH4+ NH3 + H+ in ecosystem

CONTRAST TO THE AEROSOL DISTRIBUTION:

Highest concentrations in industrial Midwest(coal-fired power plants)

Sulfate Nitrate

Ammonium

Acidity

Park et al. [2006]

LONG-TERM TREND IN US SO2 EMISSIONS

AMMONIUM AND SULFATE TRENDS, 1985-2004

NH4+

SO42-

Lehmann et al. [2007]

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-neutralizing capacity (ANC)from CaCO3 and other bases

Acid fluxFH+

0

acidificationt

HF dt ANC

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