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Debra Weisenstein 1, Sebastian Eastham 2, Jianxiong Sheng 3, Steven Barrett 2, Thomas Peter 3, David Keith 1 1 Harvard University, Cambridge, MA, U.S.A.,

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Presentation on theme: "Debra Weisenstein 1, Sebastian Eastham 2, Jianxiong Sheng 3, Steven Barrett 2, Thomas Peter 3, David Keith 1 1 Harvard University, Cambridge, MA, U.S.A.,"— Presentation transcript:

1 Debra Weisenstein 1, Sebastian Eastham 2, Jianxiong Sheng 3, Steven Barrett 2, Thomas Peter 3, David Keith 1 1 Harvard University, Cambridge, MA, U.S.A., 2 Massachusetts Institute of Technology, Cambridge, MA, 3 ETH-Zurich, Zurich, Switzerland SSiRC Meeting 28-30 October 2013 Modeling Stratospheric Aerosols at Background Levels: New Results from SOCOL and GEOS-CHEM

2 Why study background aerosols? Background and perturbed conditions are two different regimes Perturbed conditions decay to background conditions Transport of sulfur gases and aerosol across the tropopause uncertain Smaller background particles harder to measure Calculated size distributions under background condition don’t match well to available observations

3 Motivation for Model Development Aerosol-Climate Studies: Geoengineering, Volcanoes Sulfur chemistry, aerosol microphysics Ozone interactions Strat-trop exchange: impact on tropospheric chem + clouds Climate response

4 Models Used in This Study SOCOL CCM: ETH – AER Collaboration Chemistry-Climate model at ETH Aerosol microphysics from AER 2-D Add aerosol microphysics to SOCOL SOCOL/AER  Chemistry-Climate-Aerosol-Radiation interactions GEOS-CHEM CTM: Harvard – MIT Collaboration Comprehensive, validated tropospheric chemistry Multi-component aerosol microphysics package APM Extend chemistry into stratosphere UCX  Extend microphysics into stratosphere  Chemistry-Aerosol-Radiation interactions for trop + strat  No interactive climate response

5 SOCOL/AER Chemistry-climate model from ETH-Zurich MA-ECHAM GCM + MEZON chemistry Aerosol microphysics: Sulfate only scheme following AER 2-D model Improved H 2 SO 4 photolysis rate (Vaida et al. 2003) 40 sectional bins (wet radius 0.4 nm – 3.2  m) Size-dependent composition (H 2 SO 4 /H 2 O): Kelvin Effect Binary homogeneous nucleation (Vehkemaki et al. 2002) Coagulation (standard efficiency) Condensation and Evaporation Sedimentation  Aerosol – Radiative feedback (chemical and dynamical)

6 GEOS-CHEM Harvard’s 3-D tropospheric chemistry model Assimilated winds from GEOS-5, GISS, etc. Not a climate model, but off-line climate model interactions possible Two versions of aerosol microphysics implemented: Sulfate, sea salt, dust, OC, BC for troposphere APM – Fangqun Yu, SUNY-Albany –Sectional microphysics, 88 aerosol tracers TOMAS – Peter Adams, Carnegie Melon –Sectional 2-moment microphysics, 360 aerosol tracers

7 GEOS-CHEM with APM Part of standard GEOS-CHEM distribution – optional compilation Size-resolved aerosols: 40 sulfate bins (dry radius 0.6nm -5.8  m) 20 sea salt bins, 15 dust bins, 8 modes for OC/BC Aerosol type interactions: sulfate scavenging onto dust, sea salt, OC/BC Equilibrium uptake of ammonium and nitrates via ISORROPIA II Ion-mediated nucleation scheme Coagulation and Condensation Tested and validated for troposphere APM microphysics to be extended into stratsphere model: add strat nucleation, radiative interactions

8 Stratospheric GEOS-CHEM (UCX) Stratospheric chemistry extension developed by Steven Barrett’s group at MIT, Seb Eastham primary developer 72 vertical levels to 0.01 mb (chem to 60 km) Sources gases added: OCS, N 2 O, CFCs, HCFCs, etc. Stratospheric photolysis via FastJX Full ozone chemistry included from NO x, ClO x, BrO x, HO x Bulk sulfate and PSCs in stratosphere Submitted paper to Atmos. Env. To become part of future GEOS-CHEM public release APM microphysics to be integrated soon by D. Weisenstein (Harvard)

9 UCX Stratospheric Chemistry N TROPOPAUSE PSC/LBS S SOURCE Br org OCSN2ON2O hνhν 1D1D Cl Br BrONO 2 ClONO 2 ClO x HCl NO x Cl 2 O 2 BrClBrO x HBr BrNO 2 SO 2 H 2 SO 4 CH 4 HNO 3 H2OH2O Catalytic 0 3 loss Gravitational settling Release of active species Cl org

10 UCX Aerosol domains In troposphere: ISORROPIA II does equilibrium condensation of ammonium and nitrates into sulfate particles In stratosphere: Ammonium ignored (advected normally) Gas/liquid partitioning of H 2 SO 4 applied: Liquid H 2 SO4 particles below ~35 km Gas phase H 2 SO 4 above ~35 km Photolysis of gas-phase H 2 SO 4 yields SO 2 Equilibrium condensation of H 2 O/HNO 3 /HCl/HBr into particles to form PSCs PSC types: STS, NAT, Ice Supersaturation of 3K for NAT formation

11 Aerosol/Gas Interactions Photolysis rates impacted by aerosol scattering Heterogeneous reactions on solid and liquid aerosols –Shifts in mid-latitude NOx/ClOx partitioning –chlorine activation during polar winter/spring

12 2006 Antarctic Ozone Hole GEOS-CHEM UCX Simulation

13 Comparison of 3 Models GEOS-CHEM/UCX 2007 GEOS-CHEM/APM 2005 SOCOL/AER 2005 # Gas Species13210449 # Reactions342240283 Stratospheric Aerosol Types Sulfate (LBS, STS), PSCs (NAT, Ice) Sulfate (40 bins) Tropospheric Aerosol Types Sulfate, Dust (4), Sea Salt (2), BC/OC (4), SOA (optional) Sulfate (40), Dust (15), Sea Salt (20), BC/OC (8), sulfate on dust/seasalt/OC/BC, SOA (optional) Sulfate (40 bins) Model Top0.01 hPa Chemistry Top60 km + linearized chemistry above 20 km + linearized chemistry above 80 km (same as dynamics) Model Grid4°x5°,72 Levels4°x5°, 47 Levels3.75°x3.7°, 39 Levels

14 Sulfur Gas Emissions and Boundary Conditions GEOS-CHEM UCXGEOS-CHEM APMSOCOL/AER SO 2 EmissionsAnthropogenic Shipping Aircraft Biofuel Volcanic Anthropogenic Shipping Aircraft Biofuel Volcanic Anthropogenic=46 Tg Shipping = 4.9 Tg Biomass burning = 1.9 Volcanic = 12.6 Total = 65 Tg/yr DMSOceanic emission Oceanic = 18 Tg/yr CS2CS2 None 1 Tg/yr H2SH2SNone 8 Tg/yr OCS500 pptv fixed mixing ratio None500 pptv fixed mixing ratio

15 Modeled OCS + ATMOS Observation

16 Modeled SO 2 + ATMOS Observation

17 SOCOL/GEOS-CHEM Comparison OCS removal in tropical mid-strat as source of SO 2 CS 2, DMS, H 2 S convective transport to tropical mid-trop as source of SO 2. Scavenging removal efficiency? H 2 SO 4 + hv  SO 2

18 SOCOL/GEOS-CHEM Sulfate Comparison APM Aerosol Sulfate Ion-mediated nucleation in boundary layer

19 SOCOL/AER Sulfur Budget

20 Aerosol Size Distributions Equator, 20 km, October SOCOLGOES-CHEM APM Effective nucleation near tropical tropopause. Mixing of aged particles Less nucleation near tropical tropopause. No aged stratospheric particles above.

21 SOCOL Size Distributions in March Equator 45°N 45°S

22 Comparisons of SOCOL and OPC 2000-2010 Laramie SOCOL calculates too many particles above 20 km.

23 Extinctions from SOCOL and SAGE II Equator, April and October SOCOL overpredicts 1.02  m extinction above 20 km.

24 Extinctions from SOCOL and SAGE II 45N, January and July

25 0.525  m Extinction from SOCOL at 20 km in September

26 Summary SOCOL/AER CCM with microphysics –Robust results –OCS, SO 2 compare well with observations –Good representation of background stratospheric aerosol conditions –Too many particles above 20 km, 1.02  m extinction overestimated GEOS-CHEM extension into stratosphere –Promising results with bulk sulfate model –APM microphysics to be implemented Future Testing and Validation –SO 2 comparisons with MIPAS and other observations –Aerosol extinction comparisons with satellite observations –Evaluation of tropospheric convection and scavenging as controls of stratospheric sulfur –Volcanic simulations (Nabro, etc)

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