1. Relating Aerosol Mass and Optical Depth in the Southeastern U.S. C. A. Brock, N. L. Wagner, A. M. Middlebrook, T. D. Gordon, and D. M. Murphy Earth System Research Laboratory, NOAA, Boulder, Colorado, and Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, Colorado Many thanks to: B. Anderson, A. Beyersdorf, P. Bui, P. Campuzano-Jost, A. Carlton, D. Day, G. Diskin, J.-L. Jimenez, D. Lack, J. Liao, M.Richardson, R.Washenfelder, A. Welti, and L. Ziemba

2. AOD is dependent upon Aerosol mass concentration Aerosol hygroscopicity (f(composition)) Aerosol size distribution Aerosol refractive index (f(composition)) Atmospheric RH Vertical distribution (especially w.r.t. RH profile) (ignoring aerosol shape (e.g., dust) for now) Need to better quantify how aerosol mass relates to aerosol optical properties Compare modeled with measured AOD (model validation) Aeronet Satellites (MODIS, MISR, etc.) Direct radiative forcing Heating rates modeled aerosol mass remotely sensed AOD in situ measurements

3. Mixing between FT & mixed layer "PBL" Well mixed Short chemical lifetimes Long chemical lifetimes Testing the sensitivity of AOD to aerosol and meteorological parameters A "simple" system with many replicates—the planetary boundary layer in the SE U.S. fair-weather cloud-topped fully-developed (afternoon) distinct vertical structure Wagner et al., ACPD, 2015

4. ID profiles with this structure in the SE U.S. Normalize heights Average 37 profiles from SENEX and SEAC 4 RS Calculate statistics Use simple mixing model AMS massDry Extinction Aerosol volume

5. a b Calculating AOD from these profiles need: extinction at ambient RH have: extinction at <25%, ~70%, and ~90% RH at 532nm also have: AMS composition, aerosol composition model, UHSAS size distributions, UHSAS response model, κ-Köhler, Mie theory

6. Calculating AOD calculate extinction at ambient RH Large increase in extinction due to aerosol water Factor of ~3 increase for moistest profiles integrate vertically to top of profile to get AOD AOD ranges from ~0.1 to 0.35

7. Calculated AOD consistent with available Aeronet sunphotometer AODs Atlanta Centreville (SOAS site) Maximum in seasonal AOD cycle

8. Testing the sensitivity of AOD to Aerosol mass Aerosol hygroscopicity (f(composition)) Aerosol size distribution Aerosol refractive index (f(composition)) Vertical distribution (especially w.r.t. RH profile) Atmospheric RH Aerosol mass hygroscopicity size distribution refractive index ambient RH height of PBL

9. Aerosol mass hygroscopicity size distribution refractive index ambient RH height of PBL Hold other parameters constant using their median profiles

10. Very sensitive to RH! Size distribution matters

11. Summary useful way to evaluate sensitivites get the RH profile right! choose parameters (e.g., standard deviation) with care modeled aerosol mass remotely sensed AOD in situ measurements

13. We want to calculate AOD from these profiles need: extinction at ambient RH have: extinction at <25%, ~70%, and ~90% RH at 532nm also have: aerosol composition, size distribution

14. Wagner et al., ACPD, 2015 Organic Sulfate Ammonium Nitrate

17. Transition layer mixing model mixed layer free troposphere local production or loss. Concentration at height h Concentration in mixing layer (constant) fraction of mixing layer air Concentration in free troposphere (constant) Production or loss

18. Use observed CO mixing ratio to determine the mixing fraction. Set P(h) = 0 to determine the concentration expected from vertical mixing alone (dashed line).

19. Three take-home messages: 1) Only small (~10%) enhancement in aerosol mass aloft above the PBL does not support suggestion by Goldstein et al. and Ford & Heald of significant layer aloft (presumably by cloud processing) 2) AOD is highly sensitive to RH and PBL height. Even if models calculate aerosol mass concentration right, they will not get the right AOD if the RH fields and PBL (= well-mixed layer + cloud layer) are not well simulated 3) Parameterization of f(rh) matters, especially at high (>90%) RH.