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NATURAL pH OF RAIN
Equilibrium with natural CO
2
(280 ppmv) results in a rain pH of 5.7:
2
2
2
2
2
7
2
2
3
1
2
10
3
3
2
M atm
M
M
H O
H
CO
g
CO
H O
K
CO
H O
HCO
H
K
HCO
CO
H
K
2
1/ 2
1
[
]
(
)
H
CO
H
K K
P
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-
“Acid rain” refers to rain with pH < 5
damage to ecosystems
PRECIPITATION PH OVER THE UNITED STATES
LONG-TERM TREND IN US SO
2
EMISSIONS
Sulfate wet deposition and aerosol concentrations, 1980-2010 Leibensperger et al. [2011]
Ammonium wet deposition and aerosol concentrations, 1980- Leibensperger et al. [2011]
TREND IN FREQUENCY OF ACID RAIN (pH < 5)
Lehmann et al. [2007]
BUT ECOSYSTEM ACIDIFICATION IS PARTLY A TITRATION PROBLEM
FROM ACID INPUT OVER MANY YEARS
Acid-neutralizing capacity (ANC)
from CaCO
3
and other bases
Acid fluxF
H+
0
acidification
t
H
F
dt
ANC
Electronic structure of mercury
Mass number = 80: 1s
2 2s
2
2p
6
3s
2
3p
6
3d
10
4s
2
4p
6
4d
10
4f
14
5s
2
5p
6
5d
10
6s
2
Complete filling of subshells gives elemental Hg(0) stability, high volatility
Two stable oxidation states: Hg(0) and Hg(II)
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Orbital energies vs.
atomic number
Energetic arrangement of orbitalsis such that mercury (Z=80) hasall its subshells filled
RISING MERCURY IN THE ENVIRONMENT
Global mercury deposition has roughly tripled since preindustrial times
Dietz et al. [2009[
HUMAN EXPOSURE TO MERCURY IS MAINLY FROM FISH CONSUMPTION
Tuna is the #1 contributor
Mercury biomagnification factor
State fish consumption advisories
EPA reference dose (RfD) is 0.
μ
g kg
d
(about 2 fish meals per week)
ELEMENTAL MERCURY IS GLOBALLY DISTRIBUTED IN ATMOSPHERE
Human emission (2006)
Streets et al. [2009];Soerensen et al. [2010]
Mean Hg(0) concentration in surface air: circles = observed, background = model
Transport around
northern mid-latitudes:
1 month
Transport to southern
hemisphere: 1 year
LOCAL POLLUTION INFLUENCE FROM EMISSION OF Hg(II)
High-temperature combustion emits both Hg(0) and Hg(II)
Hg(0)
Hg(II)
GLOBAL MERCURY POOL
NEAR-FIELDWET DEPOSITION
Photoreduction?
MERCURY DEPOSITION
“HOT SPOT”
Hg(II) concentrations in surface air: circles = observed, background=model
Large variability of Hg(II) implies atmospheric lifetime of only days
against deposition
Selin et al. [2007]
Thus mercury is BOTH a global
and a local pollutant!
