NATURAL pH OF RAIN

1. NATURAL pH OF RAIN • Equilibrium with natural CO2 (280 ppmv) results in a rain pH of 5.7: • 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

2. PRECIPITATION PH OVER THE UNITED STATES

3. CHEMICAL COMPOSITION OF PRECIPITATION Neutralization by NH3 is illusory because NH4+g NH3 + H+ in ecosystem

5. Sulfate wet deposition and aerosol concentrations, 1980-2010 Leibensperger et al. [2011]

6. Ammonium wet deposition and aerosol concentrations, 1980-2010 Leibensperger et al. [2011]

7. Nitrate wet deposition and aerosol concentrations, 1980-2010 Leibensperger et al. [2011]

8. BUT ECOSYSTEM ACIDIFICATION IS PARTLY A TITRATION PROBLEM FROM ACID INPUT OVER MANY YEARS Acid flux FH+ Acid-neutralizing capacity (ANC) from CaCO3 and other bases

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

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

11. 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)

12. Effective Henry’s law constants and gas-cloud partitioning 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

13. Variable gas/aerosol scavenging efficiencies in deep convection Major focus of SEAC4RS aircraft campaign in Southeast Asia in summer 2013 Model intercomparison deep convective outflow OUTFLOW H2O2 Cold cloud: co-condensation, surface uptake, aerosol scavenging? precipitation HNO3 Riming mixed cloud: retention efficiency upon drop freezing? ENTRAINMENT Warm cloud: scavenging relatively well understood Barth et al. [2007] INFLOW: soluble gases and aerosols

14. Scavenging is often less efficient than simulated in GEOS-Chem CALIOP satellite data show variable aerosol scavenging Mean aerosol vertical profiles, April 2008 Altitude (km) Aerosol extinction coefficient (km-1) Patrick Kim(Harvard)

15. Dry deposition processes Standard resistance-in-series model atmosphere z Deposition flux F = Vn(z) where deposition velocity V = 1/(RA + RB + RC) aerodynamic resistance RA zo boundary resistance RB 0 surface resistance RC

16. Dry deposition velocity of ozone Monthly mean July values, MOZART model Louisa Emmons, NCAR

17. Dry deposition velocity of HNO3 Monthly mean July values, MOZART model Louisa Emmons, NCAR

18. Bi-directional exchange nA Air resistance RA nA,O ATMOSPHERE SEA/LAND nS,O Sea resistance RS = f(U) nS sea-air exchange velocity Net deposition flux

19. GEOS-Chem simulation for 2006-2008 Critical loads for ecosystems Nitrogen deposition in the US • Nitrogen deposition exceeds critical loads in much of the country • Most of that deposition is as nitric acid originating from NOx emissions Zhang et al. [2012], Ellis et al. [2012]

20. Mean US daytime dry deposition velocities Annual deposition fluxes (2006, GEOS-Chem) Nitrogen deposition processes Zhang et al. [2012]

21. Nitrogen critical load exceedances in US National Parks Present-day, GEOS-Chem model NOx emissions are projected to decrease, NH3 emissions to increase IPCC Representative Concentration Pathways (RCP) scenarios, 2050 Ellis et al. [2012]