1. Aerosol Lifetimes at High Latitudes Betty Croft 1, Jeff Pierce 1,2 and Randall Martin 1,3 1 Dalhousie University, Halifax, Canada 2 Colorado State University, Fort Collins, USA 3 Harvard-Smithsonian Center for Astrophysics, Cambridge, USA NETCARE Workshop 2013 November 18, 2013

2. Aerosol Lifetimes are Poorly Constrained in Global Models Black carbon (BC) global mean lifetime range: 3.3 and 10.6 days (Bond et al., 2013) Global mean BC lifetime [days] Direct radiative forcing (DRF) depends on lifetime (Schultz et al., 2006) E: emissions rate; L: lifetime governed by removal; MAC BC : mass absorption cross-section; AFE: absorption forcing efficiency Lifetime also controls geographic distribution and indirect aerosol forcing. Similarly, Arctic BC lifetimes are poorly constrained due to several uncertain processes (e.g. Browse et al., 2012; Wang et al., 2011; Koch et al., 2009).

3. Radionuclides as a Constraint on Aerosol Lifetime [Bq m -2 ] 137 Cs simulated with GEOS-Chem model  Fukushima Dai-Ichi accident site (white)  CTBTO measurement sites (black)  Dalhousie-CTBTO contract for access to radionuclide measurements Croft et al. (submitted), ACP Surface Layer 137 Cs/ 133 Xe 137 Cs Column Burden

4. [days] Simulated 137 Cs / 133 Xe Surface Layer E-folding Times Fit over days 20-80 after the March 11, 2011 earthquake Circles: Measurement values Site-mean measurement e-folding time: 13.9 days Site-mean simulated: e-folding time: 16.7 days Global mean simulated 137 Cs lifetime: 1.8 days April-May, 2011 Regionally Varying Lifetime Bias at High Latitude Sites Croft et al. (submitted), ACP Bias reflects uncertainty in aerosol removal simulation, particularly at high latitudes

5. Zonal Mean Lifetime with respect to Removal: May-August 2011 Black Carbon 137 Cs Latitude o N Altitude [km] 0 3 10 30 100 [days] Lifetime Gradients Reflect Differences in Removal Processes Rapid transition of removal efficiency with altitude north of 60 o N  Need measurements with altitude

6. Mixed phase and ice cloud uncertainties: ice nuclei number and behavior impaction scavenging aging seasonal changes influence of processes outside the Arctic Challenges in Simulating Aerosol Scavenging in Arctic Simulated Column Black Carbon Lifetime with respect to Removal Summer 0 3 10 30 100 [days] 60 o N-90 o N Simulation: Summer Winter Burden: 9.8 Gg 5.4 Gg Removal: 150 Gg 42 Gg (85% wet) (80% wet) Lifetime (removal): 5.9 d 11.6 d Winter

7. Summer Concentration vertical profiles are strongly sensitive to transitions between several uncertain processes related to scavenging. Arctic Scavenging Sensitivity Studies 75N-90N 180W-30W Aerosol fraction in cloud liquid and ice is important for scavenging. Typical assumptions: 100% over given T ranges or Verheggen et al. (2007) for T-dependent fraction Black carbon Measurements Needed to Constrain Uncertain Processes Need to understand variation of cloud-borne aerosol fraction with temp, size, and composition (how BC and OC differ).

8. Arctic Scavenging Sensitivity Studies Measurements of aerosol composition/mixing state are needed to better understand aging. Need measurements outside Arctic. What happens in the Arctic depends on what happens outside the Arctic! (e.g. winter aging sensitivity) - even tropical convective scavenging affects Arctic aerosol concentrations (Croft et al.,2012) Winter Summer Aging e-folding times: 1.15 day Reimer et al. 2007 (1.15 days if no sun and 2 hours if sunlit) 75N-90N 180W-30W Black carbon Ice Nuclei Number Strongly Sensitive to Aging Lee et al. (2013) assigned low uncertainty to aging for CCN prediction, but IN prediction is a different story.

9. Aerosol Scavenging is Strongly Size Dependent (Croft et al., 2010) Impaction by rain and snow Impaction by ice crystals (Croft et al., 2009) Size-resolved models (e.g. GEOS-Chem TOMAS and APM) allow closer coupling between cloud microphysics and scavenging. Need measurements of aerosol size and composition in interstitial and condensed phase. Ongoing need for measurements of collision and collection efficiency

10. Vertical Profiles Highly Sensitive to Scavenging – Need Constraints Black: Measurements Color: ECHAM5-HAM prescribed fraction (0.75) size-dependent scavenging with aerosol processing Black carbon concentration ng kg -1 Common to see two-order-of-magnitude inter-model differences in predicted black carbon vertical profiles in Arctic (e.g. Koch et al., 2009). Similar situation for organic carbon. Differences are strongly controlled by scavenging parameterizations. (Croft et al., 2010)

11. Outlook and Future Research Directions Scavenging plays a critical role in Arctic aerosol concentrations, but remains poorly constrained in global models. Measurements are needed to: 1) improve of aerosol scavenging parameterizations and understanding of processes (measurements of interstitial and condensed phase aerosol concentration, composition, size, CCN, IN, deposition and their variation with altitude, temperature and cloud properties) 2) evaluate global model simulations. Scientific focus: - Understanding and parameterizing aerosol size-dependent removal in the Arctic. - Understanding impacts of scavenging on predicted radiative forcing. - Coupling GEOS-Chem TOMAS (two moment aerosol sectional) microphysics model simulations with upcoming measurement campaigns.

13. Winter Summer 75N-90N 180W-30W Organic Carbon Arctic Scavenging Sensitivity Studies Should Cloud-Borne Fractions Differ Between OC and BC?