Gas and Aerosol Partitioning Over the Equatorial Pacific Wenxian Zhang April 21, 2009

2 In the Marine Boundary Layer… Sea salt aerosol - Main components: Na and Cl - Large water content - Diameter larger than 0.5um - Average lifetime: 6 days (Pham et al., 1995) - Emission rate around 1000 particles m -2 s -1 (Smith et al., 1993)

3 In the Marine Boundary Layer… DMS (CH3-S-CH3) -Produced by marine phytoplankton -The dominant sulfur compound emitted from the oceans -Volatile in the ocean -Oxidized by OH and NO3 radicals -Exclusive source for methane sulfonic acid (MSA) and the dominant source of SO2 in the marine atmosphere MSA (CH 3 (O)S(O)OH) -One of the product of DMS oxidation -Non-volatile and can exist in aqueous phase -MS/NSS ratio have been studies to understand the sulfur cycle.

4 Over the Marine Boundary Layer…  Buffer Layer -650~1500m -Intermittent turbulence -Reduced concentration of sea salt aerosols  Free Troposphere -Less vertical mixing -Aerosol size is less than 0.5um

5 Gas/Aerosol Partitioning Equilibrium Condition μ is the chemical potential whose difference is the cause for chemical reaction or for mass transfer from one phase to another. K is the equilibrium constant, which is highly depend on T α is the activity of a species, which depends on the concentration of this component

6 Deliquescent Relative Humidity  The threshold RH for the solid particle to absorb water spontaneously  Depends on the particle composition

7 ISORROPIA  A Thermodynamic equilibrium model for multiphase multicomponent inorganic aerosols  Components: Sodium, ammonium, chloride, sulfate, nitrate, and water  Possible species: Gas: NH 3, HNO 3, HCl, H 2 O Liquid: NH 4,Na,H,Cl,NO 3,SO 4,HSO 4,OH,H 2 O Solid:(NH 4 ) 2 SO 4,NH 4 HSO 4,(NH 4 ) 3 H(SO 4 ) 2,NH 4 NO 3, NH 4 Cl,NaCl,NaNO 3,NaHSO 4,Na 2 SO 4

8 Three box model  Assumptions - Lifetime of DMS is 0.5day - DMS is entirely converted to MS and NSS - tMS/tNSS= Aerosol sulfate comes from only non – sea salt sulfate T=285K RH=40% T=290K RH=90% T=298 RH=80% E DMS =2.3x10 9 molec cm -2 s -1 (Davis et al.,1999) k1= 10 m 2 s -1 k2= 1 m 2 s m 1500m

9 Total Concentrations DMSMSNSSClNaNOxNH4 FT BuL BL

10 Aerosol Composition in BL Sulfate poor, sodium rich Only solid is Na2SO4 HCl can exist as gas Most nitrate in the aerosol

11 Aerosol Composition in BuL Sulfate poor, sodium rich No solid Nitrate in aerosol

12 Aerosol Composition in FT tA/tS=1.25 tA/tS=2

13 Aerosol Composition in FT

14 pH and MSA gas MSA  MS - + H +

15 Summary  The marine atmosphere becomes more acidic with increase in altitude.  The results suggest that MSA can be released to gas phase in pronounced amount in the dry and acidic FT.  Ammonia does not significantly affect the aerosol/gas partitioning in BL and BuL. However, the aerosol component is sensitive to ammonia in FT.