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 )  natural rain has a pH in range 5-7 “Acid rain” refers to rain with pH < 5  damage to ecosystems

PRECIPITATION PH OVER THE UNITED STATES

CHEMICAL COMPOSITION OF PRECIPITATION Neutralization by NH 3 is illusory because NH 4 +  NH 3 + H + in ecosystem

Sulfate wet deposition and aerosol concentrations, Leibensperger et al. [2011]

Ammonium wet deposition and aerosol concentrations, Leibensperger et al. [2011]

Nitrate wet deposition and aerosol concentrations, 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 flux F H+

Deposition processes In-cloud scavenging (rainout) Below-cloud scavenging (washout) Dry deposition SEA/LAND Bi-directional exchange Wet deposition (scavenging)

Aerosol scavenging processes CCN activation coalescence raindrop impaction diffusion interception diffusion interception impaction

Scavenging of gases by liquid clouds and rain Consider equilibrium where X(aq) includes all dissolved species in fast equilibrium. Define effective Henry’s law constant Then the fraction f of X incorporated into the liquid phase is 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 SpeciesK H *, M atm -1 (pH=4.5, T=280K) O3O3 1.8x10 -2 PAN1.1x10 1 CH 3 OOH9.5x10 2 CH 2 O1.4x10 4 H2O2H2O2 4.1x10 5 NH 3 5.0x10 6 HNO 3 4.3x10 11 mostly in gas mostly in cloud (L = 1x10 -7 v/v) In non-cloud aerosol, L < v/v ; only HNO 3 partitions into the aerosol and then only if the aerosol is not acidified K H = 2.1x10 5 M atm -1 K 1 = 12 M

Variable gas/aerosol scavenging efficiencies in deep convection INFLOW: soluble gases and aerosols precipitation ENTRAINMENT OUTFLOW Warm cloud: scavenging relatively well understood Riming mixed cloud: retention efficiency upon drop freezing? Cold cloud: co-condensation, surface uptake, aerosol scavenging? Model intercomparison deep convective outflow Barth et al. [2007] H2O2H2O2 HNO 3 Major focus of SEAC 4 RS aircraft campaign in Southeast Asia in summer 2013

Aerosol extinction coefficient (km -1 )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 processes Standard resistance-in-series model atmosphere aerodynamic resistance R A boundary resistance R B surface resistance R C Deposition flux F = Vn(z) where deposition velocity V = 1/(R A + R B + R C ) z zozo 0

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

Bi-directional exchange ATMOSPHERE SEA/LAND nAnA n A,O n S,O nSnS Air resistance R A Sea resistance R S = f(U) Net deposition flux sea-air exchange velocity

Nitrogen deposition in the US GEOS-Chem simulation for 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 daytime dry deposition velocities Zhang et al. [2012]

Nitrogen critical load exceedances in US National Parks Present-day, GEOS-Chem model IPCC Representative Concentration Pathways (RCP) scenarios, 2050 NO x emissions are projected to decrease, NH 3 emissions to increase Ellis et al. [2012]