Aerosol Size-Dependent Below-Cloud Scavenging in the ECHAM5-HAM GCM

1. Aerosol Size-Dependent Below-Cloud Scavenging in the ECHAM5-HAM GCM 1B. Croft, 2U. Lohmann, 1R. Martin, 3P. Stier, 4J. Feichter, 5S. Wurzler, and 2S. Ferrachat 1Dalhousie University, Dept. Physics & Atmospheric Science, Halifax, Canada, croft@mathstat.dal.ca;2ETH Zurich, Zurich, Switzerland; 3California Institute of Technology, Pasadena, U.S.A.; 4Max Planck Institute for Meteorology, Hamburg, Germany; 5Landesumweltamt Nordrhein-Westfalen, Essen, Germany AGU Fall Meeting 2006, Presentation Number: A33B-0983 4. Annual-Global Aerosol Deposition Processes A) Constant Modal Scavenging B) Aerosol Size-Dependent Scavenging I) Sulphate Burdens: A) 0.86 Tg S B) 0.81 Tg S II) Black Carbon Burdens: A) 0.113 Tg C B) 0.106 Tg C III) Dust Burdens: A) 18.7 Tg B) 16.5 Tg IV) Sea Salt A) 11.6 Tg B) 11.0 Tg • Abstract • Wet scavenging processes largely control the global aerosol abundance and, ultimately, aerosol effects on the radiation budget of the climate system. Present-day general circulation models (GCMs) treat the below-cloud scavenging processes in a manner that neglects much of the significant dependency of aerosol scavenging on size. A below-cloud aerosol scavenging parameterization that represents the details of the aerosol size-dependency of the rain scavenging coefficients is presented and applied in the ECHAM5-HAM GCM. The relative contribution of below-cloud scavenging to the total deposition is quantified for sulphate, black carbon, sea salt, and dust. A comparison is made with the common assumption that constant scavenging coefficients may be applied in GCMs for the nucleation, Aitken, accumulation, and coarse aerosol size modes. Comparison with observed deposition is also shown. 6. Comparison with Observed Deposition A) Constant Modal Scavenging B) Aerosol Size-Dependent Scavenging Figure 4: Relative contributions of wet deposition (in-cloud and below-cloud components), dry deposition, and sedimentation to the total mass deposition of the various aerosol species, including sulphate, black carbon, dust and sea salt. Same legend applies to all figures. Figure 6: Year 2000observed U.S. sulphate deposition from the U.S. National Atmospheric Deposition Program (NADP) is compared to the sulphate deposition predicted by the ECHAM5-HAM GCM. One to one correspondence is shown by the dashed red line, and the solid red lines show a factor of two. The regression line is shown in blue. Red and green data points show that the model over-estimated, or under-estimated the observed precipitation by a factor of two or more, respectively. The green and red points are excluded from the regression and correlation coefficients in order to remove deposition that is biased due to errors in the modeled precipitation. 2. Introduction and Model Description The ECHAM5 GCM (Roeckner et al., 2003), coupled to the HAM aerosol model (Stier et al., 2005), predicts aerosol modal radii for seven lognormal modes. All results shown are from a one-year simulation following a three month spin-up period. In the standard model version, the below-cloud constant modal scavenging coefficients, normalized by the precipitation rate, are 5x10-4, 1x10-4, 1x10-3, and 1x10-1 mm-1 for the nucleation, Aitken, accumulation and coarse modes, respectively. 7. Thermophoretic Effects on Scavenging Black Carbon Deposition: Legend: Below-cloud In-cloud Dry Deposition Sedimentation Figure 1: The 7 aerosol modes of the ECHAM5-HAM GCM. Mass and number transfer follow the green arrows. 5. Geographic Distribution of Aerosol Deposition A) Constant Modal Scavenging B) Aerosol Size-Dependent Scavenging 3. Below-Cloud Scavenging Coefficients . The mean mass below-cloud scavenging coefficients are where and the Marshall-Palmer distribution is assumed for the raindrops and a lognormal distribution is assumed for the aerosols. Figure 7: Scavenging coefficients using the collision efficiencies of Wang et al. (1978) that account for particle motion due to thermal gradients associated with evaporative cooling. The far right panel shows deposition following same legend as Figure 4. Figure 5: Geographic distribution of sulphate ion and black carbon wet deposition is shown by the upper and lower panels, respectively. The left and right panels show deposition for the two below-cloud scavenging parameterizations. 8. Conclusion and Summary The ECHAM5-HAM GCM is the first atmospheric general circulation model to predict aerosol modal radius for seven lognormal modes. As a result, the size dependency of below-cloud scavenging can now be explicitly represented in the model. As opposed to using constant modal below-cloud scavenging coefficients that are scaled by rainfall rate, the revised scavenging approach suggests that previous studies may have under-estimated the importance of below-cloud scavenging as a process for the removal of aerosols from the atmosphere. Figure 3: Mean mass (solid) and number (dashed) scavenging coefficients as a function of aerosol modal size and rainfall rate. Figure 2: Collection efficiency (E) as a function of aerosol size (d) and collector drop size (D). 9. Acknowledgements: References Roeckner et al., The atmospheric general circulation model ECHAM5, Part I:Model description, Report 349, MaxPlanck Institute for Meteorology, Hamburg, Germany, available at http://www.mpimet.mpg.de, 2003. Stier et al., The aerosol-climate model ECHAM5-HAM, Atmos. Chem. Phys., 5, 1125-1156, 2005. Wang et al., On the effect of electric charges on the scavenging of aerosol particles by clouds and small raindrops, J. Atmos. Sci., 35, 1735-1743, 1978