natural ph of rain
DESCRIPTION
NATURAL pH OF RAIN. Equilibrium with natural CO 2 (280 ppmv) results in a rain pH of 5.7:. This pH can be modified by natural acids (H 2 SO 4 , HNO 3 , RCOOH…) and bases (NH 3 , CaCO 3 ) e natural rain has a pH in range 5-7. “Acid rain” refers to rain with pH < 5 e damage to ecosystems. - PowerPoint PPT PresentationTRANSCRIPT
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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 OHCO 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) e natural rain has a pH in range 5-7
“Acid rain” refers to rain with pH < 5 e damage to ecosystems
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PRECIPITATION PH OVER THE UNITED STATES
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CHEMICAL COMPOSITION OF PRECIPITATION
Neutralization by NH3 is illusory because NH4+ g NH3 + H+ in ecosystem
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Sulfate wet deposition and aerosol concentrations, 1980-2010
Leibensperger et al. [2011]
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Ammonium wet deposition and aerosol concentrations, 1980-2010
Leibensperger et al. [2011]
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Nitrate wet deposition and aerosol concentrations, 1980-2010
Leibensperger et al. [2011]
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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
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Deposition processes
In-cloud scavenging(rainout)
Below-cloud scavenging(washout)
Dry deposition
SEA/LAND
Bi-directional exchange
Wet deposition (scavenging)
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Aerosol scavenging processes
CCN activationcoalescence
raindrop
impaction
diffusion
interception
diffusion
interception
impaction
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Scavenging of gases by liquid clouds and rain
( ) ( )X g X aqConsider equilibrium
where X(aq) includes all dissolved species in fast equilibrium. Define effective Henry’s law constant * [ ( )] /H XK X aq p
Then the fraction f of X incorporated into the liquid phase is
*
{ ( )} 11{ ( )} { ( )} 1
H
X aqfX 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)
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Effective Henry’s law constants and gas-cloud partitioning
Species KH*, M atm-1
(pH=4.5, T=280K)O3 1.8x10-2
PAN 1.1x101
CH3OOH 9.5x102
CH2O 1.4x104
H2O2 4.1x105
NH3 5.0x106
HNO3 4.3x1011
mostly in gas
mostly in cloud(L = 1x10-7 v/v)
In non-cloud aerosol, L < 10-9 v/v ; only HNO3 partitions into the aerosol and then only if the aerosol is not acidified
KH = 2.1x105 M atm-1
K1 = 12 M
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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]
H2O2
HNO3
Major focus of SEAC4RS aircraft campaign in Southeast Asia in summer 2013
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Aerosol extinction coefficient (km-1)
Alti
tude
(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
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Dry deposition processesStandard resistance-in-series model
Aerodynamic, Raero
Quasi-laminar,Rql
Stomata, Rstom Mesosphyll, Rmes
Cuticle, Rcut
Soil, Rsoil
Surface, Rsurf
Atmospheric Sourceatmosphere
aerodynamicresistance
RA
boundaryresistance
RB
surfaceresistance
RC
Deposition flux F = Vn(z)
where deposition velocity V = 1/(RA + RB + RC)
z
zo
0
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Dry deposition velocity of ozoneMonthly mean July values, MOZART model
Louisa Emmons, NCAR
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Dry deposition velocity of HNO3
Monthly mean July values, MOZART model
Louisa Emmons, NCAR
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Bi-directional exchange
ATMOSPHERE
SEA/LAND
nA
nA,O
nS,O
nS
Air resistanceRA
Sea resistanceRS = f(U)
Net deposition flux 1 ( )S
AS H
AH
nF nR KRK
sea-airexchangevelocity
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Nitrogen deposition in the USGEOS-Chem simulation for 2006-2008
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 NOx emissions
Critical loads for ecosystems
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Nitrogen deposition processes
Annual deposition fluxes (2006, GEOS-Chem) Mean US daytimedry deposition velocities
Zhang et al. [2012]
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Nitrogen critical load exceedances in US National ParksPresent-day, GEOS-Chem model
IPCC Representative Concentration Pathways (RCP) scenarios, 2050
NOx emissions are projected to decrease, NH3 emissions to increase
Ellis et al. [2012]