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

3. Motivation Boundary layer (BL) nucleation mechanism: Primary-emitted particulate sulfate 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 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. It can increase aerosol number concentration significantly. Most of global models only include binary homogeneous nucleation (BHN, H 2 SO 4 -H 2 O) mechanism. But observed new particle formation events in the BL cannot be explained by BHN. Most of global models only include binary homogeneous nucleation (BHN, H 2 SO 4 -H 2 O) 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). 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). We examine how primary-emitted particulate sulfate and boundary nucleation mechanism affect 1st AIE In this study

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

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

6.  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 Cases: 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, R eff in PD, 1 st 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 R eff is large over polluted regions 1 st AIE is large over regions with large change in R eff and large cloud forcing

8. Change in R eff (PD–PI), and 1 st AIE 1 st AIE (w/m 2 ) Change in R eff between PD and PI (µm) Primary-emitted sulfate (PAR) increases 1 st AIE Boundary layer nucleation (BLN) decreases 1 st AIE So Why?

9. Primary-emitted sulfate (PAR): Increase 1 st AIE 1 st AIE: NO BLN: -1.55 to -2.02 w/m 2 BLN: -1.49 to -1.65 w/m 2 Change in 1 st AIE from PAR Change in anthropogenic fraction of CCN (0.2%): (PD-PI)/PD 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 1 st AIE Large decreases over ocean Small decreases over land (PAR) Increases over land (NOPAR) Global S_Ocean S_Land N_Ocean N_Land Change in 1st AIE (w/m 2 ) from BLN Change the spatial pattern of 1 st AIE

11. Boundary layer nucleation (BLN): Anthropogenic fraction of CCN (0.2%) Regime I: decreases. The relative increase of SO 2 from PI to PD is small and the relative increase of pre- existing particle number is large. Regime II: increases. The relative increase of SO 2 from PI to PD is large and the relative increase of the particle number is small. NOPAR ( BLN – REF ) PI PD Nucleation PI PD Nucleation

12. How sensitive is the 1 st AIE to cloud types, and assumed minimum droplet number, N min ? STD : case PAR, standard configurations (liquid cloud, total cloud fraction, Nmin=20/cm 3 ), four months (Jan., Apr., Jul., and Oct.) WARM: only T > 273K; STRAT: only stratiform clouds; N40: N min = 40/cm 3 ; N10: N min = 10/cm 3 1 st AIE (w/m 2 )

13. Conclusion The decreases in the effective radius from anthropogenic emission can range from -0.86 to -1.23 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 1 st AIE range from -1.49 to -2.03 w/m 2. Primary-emitted particulate sulfate increases the 1 st 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 1 st 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 1 st 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, R eff at cloud top in present day CDNC, R eff at cloud top in present day R eff (µm)CDNC (#/cm 3 ) Both boundary layer nucleation (BLN) and primary emitted sulfate (PAR) increase CDNC, decrease R eff

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

17. Anthropogenic fraction of CCN