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Aerosol Indirect Forcing: Effects of Particulate Sulfate and Boundary Layer Nucleation

This study examines the effects of primary-emitted particulate sulfate and boundary layer nucleation mechanism on aerosol indirect forcing. The coupled CAM-IMPACT model is used to simulate aerosol number concentration and compare with observations. Results show that primary-emitted sulfate increases aerosol number concentration and the boundary layer nucleation mechanism improves the comparison with observations.

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Aerosol Indirect Forcing: Effects of Particulate Sulfate and Boundary Layer Nucleation

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  1. Aerosol 1st indirect forcing in the coupled CAM-IMPACT model: effects from primary-emitted particulate sulfate and boundary layer nucleation Minghuai Wang and Joyce E. Penner Department of Atmospheric, Oceanic and Space Sciences University of Michigan AMWG 2008, Boulder, CO Thanks to Xiaohong Liu (PNNL)

  2. Free troposphere (FT) nucleation Growth Cloud Processing Activated Exchange Exchange Boundary layer (BL) Activated Cloud Processing Primary particles Growth Growth nucleation Introduction: Aerosol microphysics ~10 km ~1 km

  3. Motivation Boundary layer (BL) nucleation mechanism: • Most of global models only include binary homogeneous nucleation (BHN, H2SO4-H2O) mechanism. But observed new particle formation events in the BL cannot be explained by BHN. • A boundary layer nucleation mechanism is added into our model and increases the simulated aerosol number concentration, and improves the comparison with observations (Wang et al., in preparation). • Most of global models assumed a certain amount of anthropogenic sulfur emitted as particulate sulfate, to take account of sub-grid scale nucleation and growth near strong sources of sulfur emissions • It can increase aerosol number concentration significantly. Primary-emitted particulate sulfate In this study • We examine how primary-emitted particulate sulfate and boundary nucleation mechanism affect 1st AIE

  4. Calculation Method for Indirect Effect Size-resolved aerosol number concentration Coupled CAM-IMPACT NCAR CAM3 IMPACT (Liu et al., 2005) Aerosol microphysics Nucleation parameterization (Abdul-Razzak and Ghan, 2000; 2002) Cloud droplet number concentration, effective radius Updraft velocity (Morrison et al. 2005) re and rv relationship (Rotstayn and Liu 2003) Met fields (CAM-IMPACT) The aerosol first indirect forcing (1st AIE) Radiative transfer model (CAM3)

  5. Pure sulfate aerosol in 2 modes: • Nucleation (R<0.05 m), Accumulation(0.05m<R) • OM/BC fixed size distribution (R<1 m) • Sea salt and dust in 4 size bins between 0.05 and 10 µm • Binary homogeneous nucleation (BHN, Vehkamaki et al. 2002) • 2% of anthropogenic sulfur is emitted as particulate sulfate in two modes (PAR): • Mode 1: rg = 0.013µm, 1.6, 15%(m); • Mode 2: rg = 0.068µm, 2.0, 85%(m) IMPACT aerosol model (Liu et al., 2005) A boundary layer nucleation (BLN) mechanism is added: • BL particle formation rate (1nm) is taken from observations (Kulmala et al., 2006): • j1nm=A[H2SO4] , A=1.0e-6/s (Sihto et al. 2006)

  6. Cases: • REF: no primary-emitted sulfate particle and no BL nucleation • PAR: REF + primary-emitted sulfate particles • BLN: REF + BL nucleation in the BL. • BLN_PAR: BLN+ primary-emitted sulfate particles The effect of boundary layer nucleation: BLN vs. REF; BLN_PAR vs. PAR The effect of primary-emitted sulfate particles:: PAR vs. REF; BLN_PAR vs. BLN The coupled CAM-IMPACT model was run for two scenarios of aerosols: Present day (PD) and preindustrial period (PI).

  7. Cloud top CDNC, Reff in PD, 1st AIE in PAR (primary-emitted sulfate) High cloud droplet number over land, and low droplet number over ocean Small droplet effective radius over land, and large over ocean Change in Reff is large over polluted regions 1st AIE is large over regions with large change in Reff and large cloud forcing

  8. Change in Reff (PD–PI), and 1st AIE Change in Reff between PD and PI (µm) 1st AIE (w/m2) Primary-emitted sulfate (PAR) increases 1st AIE Boundary layer nucleation (BLN) decreases 1st AIE So Why?

  9. Primary-emitted sulfate (PAR): Increase 1st AIE Change in anthropogenic fraction of CCN (0.2%): (PD-PI)/PD Change in 1st AIE from PAR 1st AIE: NO BLN: -1.55 to -2.02 w/m2 BLN: -1.49 to -1.65 w/m2 Anthropogenic fraction of CCN: NO BLN: 49% to 59% BLN: 51% to 55% Reason: primary-emitted sulfate forms CCN-size particles more efficiently, and the percentage change between PD and PI in primary-emitted sulfate emissions is larger than the percentage change for other primary emissions Boundary layer nucleation offsets some effects of primary emitted sulfate

  10. Boundary layer nucleation (BLN): Decreases the 1st AIE Change in 1st AIE (w/m2) from BLN Global Large decreases over ocean Small decreases over land (PAR) Increases over land (NOPAR) S_Ocean S_Land Change the spatial pattern of 1st AIE N_Ocean N_Land

  11. Nucleation PD PI Nucleation PD PI Boundary layer nucleation (BLN): Anthropogenic fraction of CCN (0.2%) NOPAR (BLN – REF) Regime I:decreases. The relative increase of SO2 from PI to PD is small and the relative increase of pre-existing particle number is large. Regime II: increases. The relative increase of SO2 from PI to PD is large and the relative increase of the particle number is small.

  12. How sensitive is the 1st AIE to cloud types, and assumed minimum droplet number, Nmin? 1st AIE (w/m2) STD: case PAR, standard configurations (liquid cloud, total cloud fraction, Nmin=20/cm3), four months (Jan., Apr., Jul., and Oct.) WARM: onlyT > 273K; STRAT: only stratiform clouds; N40: Nmin = 40/cm3; N10: Nmin = 10/cm3

  13. Conclusion • The decreases in the effective radius from anthropogenic emission can range from -0.86 to -1.23 µm depending whether primary-emitted sulfate, and BLN is included. The results for the 1st AIE range from -1.49 to -2.03 w/m2. • Primary-emitted particulate sulfate increases the 1st AIE because it produces CCN-size particles more efficiently than does formation of particles by nucleation and because the percentage change between PD and PI in sulfur emissions is large. • Boundary layer nucleation decreases the 1st AIE over ocean. Over land it slightly decreases the 1st AIE when primary-emitted sulfate is included, but it increases the 1st AIE when primary-emitted sulfate is not included. It changes the pattern of the 1st AIE. • Different assumptions regarding cloud types and the minimum value for cloud droplet number concentration have large impact on the estimation of 1st AIE.

  14. Aerosol size distribution in the marine boundary layer: model vs. observations (Heintzenberg et al. 2000) Enhanced boundary layer nucleation rates are not able to explain the number concentrations of small particles between 30S and 60S, but number concentrations are reasonable in the NH. Primary emitted sulfate causes a large increase at northern latitudes.

  15. CDNC, Reff at cloud top in present day CDNC (#/cm3) Reff (µm) Both boundary layer nucleation (BLN) and primary emitted sulfate (PAR) increase CDNC, decrease Reff

  16. Boundary layer nucleation: Anthropogenic fraction of CCN at ~930 hPa PAR (BLN_PAR – PAR)

  17. Anthropogenic fraction of CCN

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