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

PRECIPITATION PH OVER THE UNITED STATES

CHEMICAL COMPOSITION OF PRECIPITATION

Neutralization by NH3 is illusory because NH4+ g NH3 + H+ in ecosystem

Sulfate wet deposition and aerosol concentrations, 1980-2010

Leibensperger et al. [2011]

Ammonium wet deposition and aerosol concentrations, 1980-2010

Leibensperger et al. [2011]

Nitrate wet deposition and aerosol concentrations, 1980-2010

Leibensperger et al. [2011]

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

Deposition processes

In-cloud scavenging(rainout)

Below-cloud scavenging(washout)

Dry deposition

SEA/LAND

Bi-directional exchange

Wet deposition (scavenging)

Aerosol scavenging processes

CCN activationcoalescence

raindrop

impaction

diffusion

interception

diffusion

interception

impaction

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)

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

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

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

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

Dry deposition velocity of ozoneMonthly mean July values, MOZART model

Louisa Emmons, NCAR

Dry deposition velocity of HNO3

Monthly mean July values, MOZART model

Louisa Emmons, NCAR

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

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

Nitrogen deposition processes

Annual deposition fluxes (2006, GEOS-Chem) Mean US daytimedry deposition velocities

Zhang et al. [2012]

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]