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:

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 O

HCO 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

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)

Phytoplankton

(CH3)2S

SO2

1.3d

DMS1.0d

OHNO3

Volcanoes CombustionSmelters

SO42-

3.9d

22

10 64

OH

cloud, H+, H2O2

42

8184

dep27 dry20 wet

dep6 dry44 wet

H2SO4(g)

SULFUR CHEMISTRY

Gas phase oxidation:SO2 + OH … H2SO4 slow, lifetime of SO2 ~weeks

2 2 2

24

16 3 2 2

16 13 14 15

[ ][ ][ ( )][ ]

SO H O

d SOk HSO H O aq H

dtk K K K P P

2

2 213

[ ]

SO

SO H OK

P

R16 very fast:Titrates either SO2 or H2O2 in a cloud

22 4 4H SO (aq) SO 2H

Aside: dissociation of sulfuric acid:

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:

etc….

Rate of aqueous phase sulfate formation therefore:

GLOBAL SULFUR EMISSION TO THE ATMOSPHERE

Chin et al. [2000]

2001 estimates (Tg S yr-1): Industrial 57 Volcanoes 5 Ocean 15 Biomass burning 1

22 4 4H SO (aq) SO 2H

Efficient scavengingof both HNO3(g) and nitrate aerosol

3 3HNO (aq) NO H

Efficient scavengingof both NH3(g) and ammonium aerosol

3 4NH (aq) H NH

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

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 cropsA 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%

[Boesch et al., 2001]

Watershed estimates of controllable N inputs to Chesapeake

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/Acid04.ppt

TRENDS IN U.S. EMISSIONS OF SO2

AMMONIUM AND SULFATE TRENDS, 1985-2004

NH4+

SO42-

Lehmann 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…