1. Processes Controlling the Seasonal Cycle of Arctic Aerosol Number and Size B. Croft 1, J. R. Pierce 2, R. V. Martin 1,3, R. Leaitch 4, T. Breider 5, A. Morrow 1 and S. D’Andrea 2 1 Dalhousie University, Halifax, Canada 2 Colorado State University, Fort Collins, USA 3 Harvard-Smithsonian Center for Astrophysics, Cambridge, USA 4 Science and Technology Branch, Environment Canada, Toronto, Canada 5 Harvard University, Cambridge, USA NETCARE Workshop November 4, 2014

2. Aerosol Number and Size are Important to Aerosol Effects on Climate Aerosol size and number also contribute to regional air quality. CCN and IN Scattering and absorption of radiation, and cloud property modification.

3. Strong Seasonal Cycle in Alert SMPS Measurements Similar cycle at Zeppelin 2008 and Alert 2012 D p [nm] Alert 2012 SMPS Measurements dN/dlogD p [cm -3 ] Accumulation Mode Aitken Mode Effective Diameter Cycle

4. Processes Controlling Aerosol Size Distributions Wet Scavenging Below clouds In clouds New Particle Formation and Emissions Aerosol Growth Coagulation Condensation Cloud Evaporation Precipitation Formation Activation, Collisions, Freezing Transport Deposition 1 2 3

5. Initial GEOS-Chem Simulation Over-Predicts Aerosol Number at Alert (similar findings for Zeppelin and ARCTAS 2008) Model over- predicts accumulation mode number in all seasons Poorest accumulation mode simulation in summer DJFMAM JJASON dN/dlogD p D p [nm]

6. Model Revision 1: High Efficiency Wet Scavenging Vigorous Wet Scavenging Reduces Accumulation Mode in Summer Model improvement only in summer. Non-summer number over-prediction continues. Alert Integrated SMPS [cm -3 ] Model still has too many aerosols, except summer is improved. Number [cm -3 ] Month DJF MAM JJA SON 30 0

7. Arctic has Strong Seasonal Cycle of Aerosol Lifetime GEOS-Chem Zonal Mean Arctic has lowest lifetimes in summer Revised scavenging reduces lifetimes Arctic has greatest inter-seasonal range of lifetimes Solid: New scavenging Dashed: Original

8. Model Revision 2: Sensitivity Test: Shut off new particle formation (NPF) globally in model >75% of summer number due to new particle formation Also, not physically reasonable to shut off all new particle formation Problem: Over-erosion of accumulation mode Nucleation change does not effectively fix what is wrong in model. DJF MAM JJASON No NPF Original NPF

9. Alert Integrated SMPS Contribution of New Particle Formation to Aerosol Number Alert Integrated SMPS Number [cm -3 ] 250 Month No new particle formation Standard new particle formation Measurements New particle formation

10. Hypothesis: Aerosol Coagulation by Cloud Processing is Needed Scenario 1: Outside of clouds: Scenario 2: Inside clouds: Activated 80 nm aerosol behaves like it is 10 μm, not 80nm! areas of persistent super-cooled cloud of low efficiency wet scavenging. This cloud processing is expected to be relevant in 40 nm aerosol 80 nm aerosol Inefficient coagulation Efficient coagulation

11. Model Revision 3: Cloud-Fraction-Weighted Coagulation Kernels Ω i,j(new) = (1 – α) Ω i,j(original) + α Ω i,j(activated_size) where Ω = coagulation kernel between aerosol i and j α = cloud fraction from met fields Assumption: In a cloud, aerosols >80 nm act as if 10 μm for coagulation kernel calculation.

12. Geographic impact Global % Change in Total Number of Aerosols > 3 nm in diameter (N3) GEOS-Chem November Mean with Revised Coagulation Reductions in N3 of > 40% in Arctic, a region of persistent, extensive super-cooled liquid clouds and low efficiency scavenging.

13. Revised Coagulation: Closer Agreement with Alert SMPS Winter, spring and fall simulation agrees better with Alert SMPS Original Coagulation New Coagulation

14. Improved Agreement with Alert Effective Diameter from SMPS Revised coagulation and scavenging Revised scavenging only No nucleation Original Simulation Effective Diameter [nm] Month 300 140

15. Revised Model Agrees Better with Alert Integrated SMPS Revised coagulation and scavenging Revised scavenging Number [cm -3 ] Month 300

16. Summary and Outlook Wet scavenging efficiency has a strong seasonal cycle in the Arctic. Efficient wet scavenging in summer reduces accumulation mode and allows for new particle formation (NPF: >75% of total number). Aerosol-aerosol coagulation in clouds in seasons of low efficiency wet scavenging may be an important sink for the Aitken mode. Future work: Evaluation of number and size simulation with NETCARE aircraft and ship measurements.  Accurate simulation of aerosol number and size is important for prediction of CCN and IN.