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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
Sulfate wet deposition and aerosol concentrations, 1980-
Leibensperger et al. [2011]
Nitrate wet deposition and aerosol concentrations, 1980-
Leibensperger et al. [2011]
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
Aerosol scavenging processes
CCN activationcoalescence
raindrop
impaction
diffusion
interception
diffusion
interception
impaction
Scavenging of gases by liquid clouds and rain
( )
(
)
X g
X aq
Consider equilibriumwhere X(aq) includes all dissolved species in fast equilibrium. Defineeffective Henry’s law constant
[
)] /
H
X
K
X aq
p
Then the fraction
f
of X incorporated into the liquid phase is
{
(
)}
1
1
{
(
)}
{
(
)}
1
H
X
aq
f
X
g
X
aq
K
LRT
where { } is concentration in moles per liter of air and
L
is the liquid water
content (volume water per volume of air)
Variable gas/aerosol scavenging efficiencies in deep convection
INFLOW:soluble gasesand aerosols
precipitation
ENTRAINMENT
OUTFLOW
Warm cloud:scavenging relativelywell understood Riming mixed cloud:retention efficiencyupon drop freezing?
Cold cloud:co-condensation, surface uptake,aerosol scavenging?
Model intercomparisondeep convective outflow
Barth et al. [2007]
H
2
O
2
HNO
3
Major focus of SEAC
4
RS aircraft campaign in Southeast Asia in summer 2013
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Aerosol extinction coefficient (km
Altitude (km)
Mean aerosol vertical profiles, April 2008
CALIOP satellite data show variable aerosol scavenging
Patrick Kim(Harvard)
Scavenging is often less efficient than simulated in GEOS-Chem
Dry deposition velocity of ozone
Monthly mean July values, MOZART model
Louisa Emmons, NCAR
Dry deposition velocity of HNO
3
Monthly mean July values, MOZART model
Louisa Emmons, NCAR
Nitrogen deposition in the US
GEOS-Chem simulation for 2006-
Zhang et al. [2012], Ellis et al. [2012]
Nitrogen deposition exceeds critical loads in much of the country
Most of that deposition is as nitric acid originating from NO
x
emissions
Critical loads for ecosystems
Nitrogen deposition processes
Annual deposition fluxes (2006, GEOS-Chem)
Mean US daytimedry deposition velocities
Zhang et al. [2012]
