1. Stratospheric Aerosol Size Distribution Retrievals Using SAGE III Mark Hervig GATS Inc. Terry Deshler University of Wyoming

2. 1) Introduction  Goal of this work: Retrieve stratospheric aerosol size distributions using SAGE III data Main ingredients: aerosol measurements & aerosol model  SAGE III measurements: Aerosol extinction profiles (0.5 km vertical spacing) 9 wavelengths: 384, 449, 520, 601, 676, 755, 869, 1022, and 1545 nm Latitude coverage from roughly 46 – 80° (N & S)  Modeled aerosol extinctions: Consider stratospheric sulfate aerosols

3. 2) Modeled Sulfate Aerosol Extinctions Lognormal size distribution (3 parameters) total concentration (N) median radius (r m ) distribution width (  ) Sulfate refractive indices from Palmer & Williams [1975] 25 - 95 wt. % H 2 SO 4 300 K, use Lorentz-Lorenz theory to adjust in temperature Mie theory to compute extinction (scattering + absorption) Absorption matters only for = 1545 nm & radii < 0.1  m

4. 3) Modeled Sulfate Aerosol Extinctions, cont’d Sulfate refractive indices depend on temperature and composition. Extinction magnitude changes with temperature and composition, but the wavelength dependence is relatively unaffected. Effects of varying T & wt. % H 2 SO 4, each curve represents a different SAGE 3 channel

5. 4) Size Distribution Retrievals Determine unimodal lognormal size distributions (3 parameters) Requires measured extinction at 3 or more wavelengths Model calculations account for temperature and composition dependence of the sulfate refractive index Algorithm uses look-up tables and can retrieve a single profile in a few seconds.

6. 5) Size Distribution Retrievals, cont’d Two step inversion: 1) Determine distribution shape from extinction ratios One extinction ratio yields a range of solutions (r m vs.  ) The solutions from many ratios tend to converge at the target 2) Determine concentration N = (measured extinction) / (simulated extinction with N=1)

7. 6) Information Content of SAGE III Spectra Extinction ratios help visualize sensitivity to size Single particles: Distinguish radii from  0.08 to 1  m Lognormal particle populations: Distinguish median radii from  0.02 to 0.4  m (calculations used  = 1.5)

8. 7) Information Content of SAGE III Spectra, cont’d SAGE III should be able to distinguish median radii between roughly 0.02 and 0.4 microns Vertical bars: range of size distributions where r m can be determined to within 25% assuming measurement errors of 5% Blue dots: range of size distributions observed over using OPCs over Laramie during 1987-2000 Fraction of useable extinction ratios and observable distribution widths vs. median radius

9. 8) Simulated Retrievals Under Background Conditions SAGE III extinctions were simulated using in situ OPC measurements over Laramie on 28 July 2003. The simulated SAGE III extinctions were used in size distribution retrievals. The retrievals match the original size distributions. SAGE III should be able to infer size distributions under the current background state

10. 9) Retrievals Using SAGE III Data  Best results use only the 449, 520, 675, and 869 nm channels 601 & 755 nm channels are known to be unreliable Model errors could be at fault (refractive index is one suspect) Some reduction in range of r m that can be distinguished  Best results use averaged SAGE III data as input Average at least 50 sequential profiles (4 + days) Vertical smoothing of +/- 1 or 2 points (1 or 2 km “boxcar”) helps Realizing 1/root(N) improvements

11. 10) SAGE 3 vs. Optical Particle Counters (OPCs) U. Wyoming balloon-borne OPCs measure size distribution profiles OPC PI: Terry Deshler 2 comparisons near Laramie, Wyoming (41.2°N) 2 coincidences over Kiruna, Sweden (67.5°N)

12. 11) SAGE 3 vs. OPC, Laramie, July 2003 OPC flight over Laramie (41.2°N) on 28 July 2003 SAGE III retrieval used a 150 profile average with a 2 km vertical boxcar

13. 12) SAGE 3 vs. OPC, Kiruna, January 2004 OPC flight over Kiruna, Sweden (67.5°N) on 10 January 2004 SAGE III retrieval used a 150 profile average with a 2 km vertical boxcar

14. 13) SAGE 3 vs. OPC Time Series Northern hemisphere time series compared to OPC data from both Laramie and Kiruna May 2002 – January 2004 18 km altitude SAGE III data are retrievals of 10-day averages

15. 14) Summary Retrievals perform best using only the 448, 520, 675, and 869 nm channels The other channels are: Too noisy? (601 & 755) Not well modeled? Retrievals using averaged extinction spectra give the best results Reduce noise (by root N) Good agreement with in situ OPC size distributions A work in progress Explore measurement and model errors Investigate retrieval robustness (is it overly sensitive to errors?) Publication planned, release results via web Contact: Mark Hervig, m.e.hervig@gats-inc.com