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Fundamentals of air Pollution Fundamentals of air Pollution – Acid Precipitation– Acid Precipitation
Yaacov MamaneYaacov Mamane
Visiting ScientistVisiting ScientistNCR, RomeNCR, Rome
Dec 2006 - May 2007Dec 2006 - May 2007CNR, Monterotondo, ItalyCNR, Monterotondo, Italy
Sandstone portal Figure on Herten Castle in Ruhr district of Germany, Sculpted 1702.
photographed in 1908
photographed in 1969
F U M I F U G I U M: or The Inconveniencie of theAER AND SMOAK of LONDON DISSIPATED.
By John Evelyn, 1661 It is this horrid Smoake which obscures our Churches, and makes our Palaces look old, which fouls our Clothes, and corrupts the waters, so as the very Rain, and refreshing Dews which fall in the several Seasons, precipitate this impure vapour, which, with its black and tenacious quality, spots and contaminates whatsoever is expos'd to it: It is this which scatters and strews about those black and smutty Atomes upon all things where it comes, insinuating it self into our very secret Cabinets, and most precious Repositories:I propose therefore, that by an Act of this present Parliament, this infernal Nuisance be reformed; enjoyning, that all those Works be removed five or six miles distant from London below the River of Thames; I say, five or six miles, or at the least so far as to stand behind that Promontory jetting out, and securing Greenwich from the pestilent Aer of Plumstead- Marshes: because, being placed at any lesser Interval beneath the City, it would not only prodigiously infect that his Majesties Royal Seat
pH Levels in the USA, 1999pH Levels in the USA, 1999
History
As early as 1852, R. A. Smith analyzed rain that near the industrial city of Manchester, England and found that urban aerosol particles tend to be composed primarily of sulfuric acid, but as the air is transported away from sources over more rural areas, the acid is neutralized by absorption of ammonia.
urban → suburban → rural
H₂SO₄ + NH₃ → (NH₄)HSO₄ (+NH₃) → (NH₄)₂SO₄
sulfuric acid → ammonium bisulfate → ammonium sulfate
Throughout the early part of the twentieth century, European scientists documented the sources and effects of atmospheric acids. It was not until 1958 that acidity of precipitation in the US was characterized (Junge and Werby, 1958)
Effects - SoilsEffects - Soils
Soils have colloidal molecules (clay particles) that have a layer of negative charge. They hold positively charged cations such as Al³ , K , ⁺ ⁺Mg² , and Ca⁺ ² . ⁺
K , Mg⁺ ² , Ca⁺ ² are essential plant nutrients while ⁺ Al³ is toxic. ⁺Hydrogen ions from acid deposition replace these cations on the outer
layer of colloidal molecules. The metal ions are then dissolved and leached into solution and can be washed away from the soil and into surface or ground water.
Soil fertilitiy is reduced and aluminum ions can replace calcium in the fish’s gills.
The impact of acids on soil fertility depends on the structure and composition of the clays in the soil. The surface of the US Midwest is predominantly limestone (CaCO₃), and lakes and streams have high neutralizing capacity. In the East granite dominates; soils and surface waters lacking buffering capacity, are highly sensitive to acidification.
4NH
Forests can be especially sensitive to nutrient loss. In Europe in 1993 about a quarter of the trees have died or are more than 25% defoliated. This “forest death” has been attributed at, least in part, to environmental degradation from a combination of acid deposition, ground-level ozone, and excess nutrification, primarily nitrogen. In the US, loss of forests has been so dramatic, although several species including ash and oak are sensitive to acidification of soils.
Lakes and Streams
The sensitivity surface waters depends critically on their neutralizing or buffering capacity. Alkaline materials such as CaCO₃, and MgCO₃ can neutralize acids.
22
23
2 CaCOCOCa2H
Materials
The Taj Mahal, the Parthenon, the Madonna in Herten, Germany, and the Lincoln Memorial are made of marble.
Marble, a particular crystalline form of calcite (CaCO₃), and sandstone, are subject to attack by sulfuric acid.
CaSO₄ is gypsum, which is 100 times more soluble than CaCO₃. Many priceless historic structures have been lost to acid deposition.
On a more pragmatic note, the rate of corrosion of galvanized (zinc coated) steel is 0.62 um/yr in the Adirondacks, 1.01 in Washington, DC, and 1.47 in Stubenville, OH.
24423 COCaSOSOHCaCO
Origins
Primarily power generation and ore smelting.
For example nickel is mined as nickel sulfide, NiS. In smelting, it is heated in air (Sudbury, Canada).
The molecular weight of nickel is 57 g/mole, so smelting produces more than a ton of SO₂ for each ton of nickel produced.
Formation and Composition
Gas Phase production of nitric acid:
OH + NO₂ + M → HNO₃ + M
Aqueous phase production of nitric acid:
NO₂ + O₃ → NO₃ + O₂
NO₃ + NO₂ + M = N₂O₅ + M
N₂O₅ + H₂O(l) → 2HNO₃(aq)
22 SONiONiS
This process is important only at night, and when air temperatures are low because the formation of N₂O₅ is reversible, and the equilibrium coefficient is highly temperature dependent. Also, NO₃ is rapidly photolyzed by visible radiation.
NO₃ + hv → NO₂ + O
Gas Phase production of sulfuric acid:
OH + SO₂ + M → HOSO₂ + M
Aqueous phase production of sulfuric acid:
SO₂ + H₂O₂ → H₂SO₄
Global Sulfur BudgetGlobal Sulfur Budget(Flux Terms In Tg S Yr(Flux Terms In Tg S Yr-1-1))
Phytoplankton
(CH3)2S
SO2
1.3d
(DMS)1.0d
OHNO3
Volcanoes CombustionSmelters
SO42-
3.9d
22
10 64
OH
cloud42
8184
dep27 dry20 wet
dep6 dry44 wet
H2SO4(g)
Global Sulfur Emission To The AtmosphereGlobal Sulfur Emission To The Atmosphere1990 annual mean1990 annual mean
Chin et al. [2000]
Trends In Sulfate And Nitrate Wet DepositionTrends In Sulfate And Nitrate Wet Deposition
Rain Chemistry in the East MediterraneanRain Chemistry in the East MediterraneanTrends of H+ and SO4=Trends of H+ and SO4= ,, eq/leq/l
L ine Plo t (Shee t1 in Imported acid ra in .stw 21v*182c)
H+(L ) SO 4=(exc.)(R )
17
-19
/11
/81
10
-11
/03
/82
04
-05
/12
/82
06
-07
/02
/83
14
-15
/01
/84
04
-05
/12
/84
23
-26
/04
/85
15
-16
/01
/86
05
-06
/11
/86
30
-31
/12
/86
16
-17
/03
/87
23
-24
/12
/87
17
-18
/02
/88
18
-19
/12
/88
-20
0
20
40
60
80
100
-200
0
200
400
600
800
1000
Rain Chemistry in the East MediterraneanRain Chemistry in the East MediterraneanTrends of H+ and SO4=Trends of H+ and SO4= ,, eq/leq/l
Sca tte rp lo t (Shee t1 in Importe d acid ra in .stw 21v*1 82c)H+ = 8 .6 346 -0 .020 3*x
-200 0 20 0 40 0 60 0 80 0 10 00
SO 4=(exc.)
-20
-10
0
10
20
30
40
50
60
70
H+
Scatte rp lo t (Air Po ll Course \acid rain Israe l.x ls 21v *182c)
Cations = 44.92+0.94*x, e q /l
0 20 0 40 0 60 0 80 0 10 00 12 00 14 00 16 00 18 00 20 00 22 00
An ions
0
20 0
40 0
60 0
80 0
10 00
12 00
14 00
16 00
18 00
20 00
22 00
Ca
tion
s
Rain Chemistry in the East MediterraneanRain Chemistry in the East MediterraneanAnions and CationsAnions and Cations ,, eq/leq/l
AQUEOUS-PHASE CHEMISTRYAQUEOUS-PHASE CHEMISTRY
HENRY’S LAWHENRY’S LAW
The mass of a gas that dissolves in a given amount of liquid as a given temperature is directly proportional to the partial pressure of the gas above the liquid. This law does not apply to gases that react with the liquid or ionized in the liquid.
GAS HENRY’S LAW CONSTANT
(M / atm at 298 K)
CO₂ 3.1 x 10 ²⁻SO₂ 1.3HNO₃ 2.1 x 10 ⁵⁺H₂O₂ 9.7 x 10 ⁴⁺
Use of Henry’s Law
CO H COg2 2 3( ) + H O 2
Assume that the atmosphere contains only N2, O2, and CO2 and
that rain is in equilibrium with CO2.
CO2 form a weak acid H2CO3, and it is in equilibrium with it.
We should remember that:
H2O = H + OH⁺ ⁻
[H ][OH ] = 1 x 10 14⁺ ⁻ ⁻
pH = -log [H ]⁺
In pure H2O, pH = 7.0
We can assume that [CO2] in the atmosphere is around 350
ppm. ca. 370 ppm
H + HCO3- K1C =
H+ HCO3-
H2CO3
HCO + CO3= K2C =
H+ CO3=
HCO3-
2 3
3
CO H
H
H O 2 H + OH+ -
K w = H OH = 10+ - -14
+
10 H log- pH
KHC =
H CO
P2 3
CO2
PCO210-6x350=
H = OH + HCO + 2 CO-3-
3=
22
+H
2PCO
2CK
1CK
HCK
+H
2PCO
1CK
HCK
H
WK
H
2
2
CO2C1C
+
CO1C
P K K
H P K
HC
HCW
K
KKHO
2
3
K1C = 4.310-7 mole/l K2C = 4.710-11 mole/l
KHC = 0.034 M/atm
H
Atm
2
6350 10
K + K K P
10 M + 4.3 10 M 0.034M
Atm
5.1 10 M
W HC 1C CO
-14 2 -7
-12 2
2
~
H = 2.3 10 M + -6
pH = 5.6
pH = 6 - 0.36 5.6
(CO2) Total = (H2CO3) + (HCO3-) + (CO3=)
2
2
+
COHC1C2C
+
COHC1C
CO
H
P K K K
H
P K K+P
2
2
2
HCTotal KCO
CO PTotal CO2 2 A +B 10 +C 100pH pH
SO H O H SO K
H SO HSO H
HSO SO H
HS2 2 2 3
2 3 3
3 3
P
SO2
K
K
S
S
1
2
= H HSO
H SO
= H SO
HSO
+3-
2 3
+3=
3-
K1S = 1.310-2 MK2S = 6.610-8 M
KHS = 1.23 M/atm
SO SO
d SO
dtSO
SO SOt
3 4
43
4 3
+ 1
2 O
= k
= k t + C
2
3
3
SO T2 = H SO + HSO + SO + SO
K P + K K P
H +
2 3 3-
3=
4=
HS SO1S HS SO
+22
+ K K K P
+
+ K K K K P
dt
2S 1S HS SO
32S 1S HS SO
t
t
2
2
o
H
H
2
2
What would be the pH of pure rain water in Rome
today?
Assume that the atmosphere contains only N2, O2, and
CO2 and that rain is in equilibrium with CO2.
Remember:
H2O = H + OH⁺ ⁻
[H ][OH ] = 1 x 10⁺ ⁻ ⁻14
pH = -log [H ]⁺
In pure H2O, pH = 7.0
We can assume that: [CO2] = ca. 370 ppm
Today’s barometric pressure is 993 hPa = 993/1013 atm = 0.98 atm. Thus the partial pressure of CO₂ is
In water CO₂ reacts slightly, but [H₂CO₃] remains constant as long as the partial pressure of CO₂ remains constant.
atm46CO 1063.3)98.0(10370P
2
M101.23
103.62103.4)P(COH][CO5
422aq2
7
32
3
332
3222
104.3Keq]CO[H
]][HCO[H
HCOHCOH
COHOHCO
We know that:
and
Thus
H+ = 2.3x10-6 → pH = -log(2.3x10-6) = 5.6
EXAMPLE 2
If fog water contains enough nitric acid (HNO₃) to have a pH of 4.7, can any appreciable amount nitric acid vapor return to the atmosphere? Another way to ask this question is to ask what partial pressure of HNO₃ is in equilibrium with typical “acid rain” i.e. water at pH 4.7? We will have to assume that HNO₃ is 50% ionized.
32
3
532
COH*Ka ][H
][HCO][H
M101.23]CO[H
This is equivalent to 90 ppt, a small amount for a polluted environment, but the actual [HNO₃] would be even lower because nitric acid ionized in solution. In other words, once nitric acid is in solution, it will not come back out again unless the droplet evaporates; conversely any vapor-phase nitric acid will be quickly absorbed into the aqueous-phase in the presence of cloud or fog water.
Which pollutants can be rained out?
atm109.0
10/2.1102
/H][HNOP
10210][H
]log[HpH
11
55
aq3HNO
54.7
3
What is the possible pH of water in a high cloud (alt. 5km) that ≃absorbed sulfur while in equilibrium with 100 ppb of SO₂?
The pressure decreases as a function of height. At 5km the ambient pressure is around half the atmospheric pressure: 0.54 atm.
This SO₂ will not stay as SO₂•H₂O, but participate in a aqueous phase reaction, that is it will dissociate.
5km2Total2SO
2
2222
]P[SO]P[SOP
100ppb][SO
OHSOOHSO
2
M107
HP][SO
atm105.40.5410100P
8
SOaq2
89SO
2
2
222 HOSOHOHSO
The concentration of SO₂•H₂O, however, remains constant because more SO₂ is entrained as SO₂•H₂O dissociates. The extent of dissociation depends on [H ] and thus pH, but the concentration of SO₂⁺ •H₂O will stay constant as long as the gaseous SO₂ concentration stays constant. What’s the pH for our mixture?
If most of the [H ] comes from SO₂⁺ •H₂O dissociation, then
Note that there about 400 times as much S in the form of HOSO₂ as in ⁻the form H₂O•SO₂. HOSO₂ is a very weak acid, ant the reaction ⁻stops here. The pH of cloudwater in contact with 100 ppb of SO₂ will be 4.5
]SOO[H
]][HOSO[HK
22
2a
522a
2
103]SOO[HK][H
][HOSO][H
Because SO₂ participates in aqueous-phase reactions, Eq. (I) above will give the correct [H₂O•SO₂], but will underestimate the total sulfur in solution. Taken together all the forms of S in this oxidation state are called sulfur four, or S(IV).
If all the S(IV) in the cloud water turns to S(VI) (sulfate) then the hydrogen ion concentration will approximately double because both protons come off H₂O•SO₄, in other words HSO₄ is a strong acid.⁻
This is fairly acidic, but we started with a very high concentration of SO₂, one that is characteristic of urban air. In more rural areas of the eastern US an SO₂ mixing ratio of a 1-5 ppb is more common. As SO₂•H₂O is oxidized to H₂O•SO₄, more SO₂ is drawn into the cloud water, and the acidity continue to rise. Hydrogen peroxide is the most common oxidant for forming sulfuric acid in solution.
