QUESTIONS
1. Is the rate of reaction of S(IV) more likely to be slower than
calculated for a cloud droplet or a rain droplet? Why?
2. If you wanted to determine whether a species would be a good
oxidant in the aqueous phase what are the 3 things you would need
to know?
CHAPTER 13: ACID RAIN
NATURAL pH OF RAIN
• Equilibrium with “natural” CO2 (280 ppmv) results in a rain pH of 5.7:
H 2O

 CO2  H 2O
CO2 ( g ) 

K H  3 102 M atm-1

 HCO3  H  K1  9 107 M
CO2  H 2O 


 CO32  H 
HCO3 

K 2  7 1010 M
[ H  ]  ( K1KH PCO2 )1/2
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
ENVIRONMENTAL IMPACTS OF ACIDITY
PRECIPITATION PH OVER THE UNITED STATES: 1994
CHEMICAL COMPOSITION OF PRECIPITATION
Electoneutrality condition for acid rain based on predominant ions:
[H+] + [NH4+] +2[Ca2+] = 2[SO42-] + [NO3-]
PH MEASURED IN CLOUD AND FOG WATER
Courtesy: Jeff Collett
GLOBAL SULFUR BUDGET [Chin et al., 1996]
(flux terms in Tg S yr-1)
cloud, H+, H2O2
42
SO2
4
NO3
18
t  3.9d
OH
t  1.3d 8
SO42-
H2SO4(g)
OH
(CH3)2S
DMS
t  1.0d
10
64
dep
27 dry
20 wet
22
Phytoplankton
Volcanoes
Combustion
Smelters
dep
6 dry
44 wet
SULFUR CHEMISTRY
Aside: dissociation of sulfuric acid:
H2SO4 (aq)  SO42  2H
Gas phase oxidation:
SO2 + OH  …  H2SO4 slow, lifetime of SO2 ~weeks
In cloud oxidation (focus here on H2O2 oxidation at low pH):
SO2(g)  SO2.H2O
(13)
SO2.H2O  HSO3- + H+
(14)
H2O2(g)  H2O2(aq)
(15)
HSO3- + H2O2(aq) + H+  SO42- + 2H+ + H2O
(16)
Remember equilibrium constants:
[SO2  H 2O]
K13 
PSO2
etc….
Rate of aqueous phase sulfate formation therefore:
d [ SO42 ]
 k16 [ HSO3 ][ H 2O2 (aq)][ H  ]
dt
 k16 K13 K14 K15 PSO2 PH 2O2
R16 very fast:
Titrates either SO2
or H2O2 in a cloud
GLOBAL SULFUR EMISSION TO THE ATMOSPHERE
2001 estimates (Tg S yr-1):
Industrial 57 Volcanoes 5
Ocean
15 Biomass burning 1
Chin et al. [2000]
H2SO4 (aq)  SO42  2H
Efficient scavenging
of both HNO3(g) and nitrate aerosol
HNO3 (aq)  NO3  H
Efficient scavenging
of both NH3(g) and ammonium aerosol
NH3 (aq)  H  NH4
BUT ECOSYSTEM ACIDIFICATION IS PARTLY A TITRATION PROBLEM
FROM ACID INPUT OVER MANY YEARS
Acid flux
FH+
Acid-neutralizing capacity (ANC)
from CaCO3 and other bases
t
F
H
0
dt  ANC  acidification
AREAS (IN BLACK) WITH LOW
ACID-NEUTRALIZING CAPACITY
ACID RAIN: US-CANADA ENVIRONMENTAL POLICY
ISSUE OF 1970’s - 1980’s
http://archives.cbc.ca/environment/pollution/topics/584/
Dying lakes, dying crops
A long awaited agreement
A policy debate that was ultimately addressed with domestic legislation (Eastern
Canada Acid Rain Program in 1985 and US amendment to Clean Air Act in 1991)
EXCESS NITROGEN DEPOSITION CAN ALSO LEAD TO
EUTROPHICATION OF LAKES AND RIVERS
Excessive deposition of assimilable N  eutrophication  accumulation of algae 
suppression of supply of O2 to deep water  hypoxia
N inputs to the Chesapeake Bay have increased 7-fold over natural!
 1987 agreement to reduce N inputs by 40%
Watershed estimates of controllable
N inputs to Chesapeake
[Boesch et al., 2001]
SOLUTIONS TO ACID DEPOSITION?
CHEMICAL:
Liming – addition of calcium
carbonate.
Works, but is expensive and
only a short term solution
BIOLOGICAL:
Long-term solution –
reduce emissions and let
lakes recover naturally
www.life.uiuc.edu/ib/349/lectures/Aci
d04.ppt
TRENDS IN U.S. EMISSIONS OF SO2
AMMONIUM AND SULFATE TRENDS, 1985-2004
NH4+
SO42Lehmann et al. [2007]
CHANGE IN
PRECIPITATION PH
FROM 1994 TO 2008
DEPLETION OF BASE CATIONS FROM ACID RAIN
(Hubbard Brook Experimental Forest, New Hampshire)
STILL A MAJOR CONCERN IN INDUSTRIALIZING NATIONS…