Using dynamic aerosol optical properties from a chemical transport model (CTM) to retrieve aerosol optical depths from MODIS reflectances over land Fall AGU 2007 Abstract # A33C-1417 Easan Drury (drury@fas.harvard.edu)1, Daniel J. Jacob1, Jun Wang2, Robert J. D. Spurr3, Kelly Chance4 1 Harvard University 2 University of Nebraska, Lincoln, 3RT Solutions, Inc., 4Harvard-Smithsonian Center for Astrophysics A paper draft of this research can be downloaded at: http://www-as.harvard.edu/chemistry/trop/publications/drury_2007.pdf SUMMARY 2. Modeling TOA reflectance 4. AOD comparison Quantitative evaluation of chemical transport models (CTMs) with aerosol optical depth (AOD) products retrieved from satellite backscattered reflectances can be compromised by inconsistent assumptions of aerosol optical properties and errors in surface reflectance estimates. We present an improved AOD retrieval algorithm for the MODIS satellite instrument using locally derived surface reflectances and CTM aerosol optical properties for the 0.47, 0.65, and 2.13 μm MODIS channels. Assuming negligible atmospheric reflectance at 2.13 μm in cloud-free conditions, we derive 0.65/2.13 surface reflectance ratios at 1˚x1.25˚ horizontal resolution for the continental United States in summer 2004 from the subset of top-of-atmosphere (TOA) reflectance data with minimal aerosol reflectance. We obtain a mean ratio of 0.57 ± 0.10 for the continental United States, with high values over arid regions and low values over the Midwest prairies. The higher surface reflectance ratios explain the high AOD bias over arid regions found in previous MODIS retrievals. We calculate TOA reflectances for each MODIS scene using local aerosol optical properties from the GEOS-Chem CTM, and fit these reflectances to the observed MODIS TOA reflectances for a best estimate of AODs for that scene. Comparison with mean ground-based (AERONET) AOD observations at 15 sites in the western and central United States in summer 2004 show strong correlations ( R0.47μm = 0.83, R0.65μm = 0.54 ) and a 20% low bias, representing considerable improvement over the operational MODIS AOD products. Multi-day averaging significantly improves the quality of the retrieved MODIS AOD product. GEOS-Chem AODs GEOS-Chem Single Scattering Albedos (SSAs) Aerosol optical depths (AODs), single scattering albedos (SSAs) and scattering phase functions (P) are derived from the GEOS-Chem CTM. These aerosol optical properties are simulated from mass concentrations of dust, sulfate, nitrate, black carbon, organic carbon and sea salt, by assuming externally mixed spherical aerosols with type-dependent size parameters, hygroscopic growth factors and refractive indices taken from the Global Aerosol Data Set [Koepke et al., 1997]. TOA reflectances are simulated for each MODIS scene using the LIDORT radiative transfer model with GEOS-Chem aerosols [Spurr et al., 2001] and a reflective surface in the sun / satellite geometry of the satellite instrument. Plotted above are the average AODs retrieved from MODIS reflectances over North America at 0.47 and 0.65 μm for the period Jul 1 – Aug 15, 2004. Our retrieval (top row) is compared to the collection 5 (C005-middle row) and collection 4 (C004 – bottom row) MODIS AOD products. Mean AERONET AODs collected within ± 1 hour of MODIS retrievals are plotted as filled circles for stations with at least 9 days of coincident measurements. Total TOA reflectance Inferred TOA aerosol reflectance 1. Estimating Land Surface Reflectance Comparison with AERONET Effects of Multi-Day Averaging The first step in retrieving AODs from MODIS reflectances requires separating the aerosol reflectance from the surface and Rayleigh reflectances. [1] [2] TOA reflectance measured by MODIS (top row) and modeled (bottom row) using LIDORT with aerosol optical properties derived from GEOS-Chem. Inferred TOA aerosol reflectance from MODIS (top row) and model (bottom row). These values are derived by removing Rayleigh and surface reflectances from the total TOA reflectances. Surface reflectance at 0.65 μm is estimated by scaling 2.13 μm reflectance (where we assume the atmosphere is nearly transparent [Kaufman and Tanre, 1997]). We derive local 0.65/2.13 μm surface reflectance ratios (ξ0.65) at 1˚x1.25˚ horizontal resolution for the continental United States in summer 2004 from a subset of the TOA reflectance data with minimal aerosol reflectance. We define minimal aerosol reflectance cases as the bottom quartile of the 0.65 to 2.13 μm reflectance relationship (plotted as blue squares on the left below). Surface reflectance at 0.47 μm is defined as ξ0.47 μm = ξ0.65 μm / 2, following Remer et al. [2006]. 3. Retrieving AODs from MODIS Reflectances Scatterplots of mean MODIS and AERONET AODs for stations in the western and central US with at least 9 days of coincident measurements. The correlations and RMS errors of the MODIS AODs we retrieve compared to AERONET AODs are improved by averaging over multiple days of coincident measurements. Sensitivity of retrieved AODs to the 0.65/2.13 surface reflectance ratio (ξ0.65) Sensitivity of retrieved AODs to single scattering albedos (ω) Scatter Plot of 0.65 / 2.13 μm reflectances 0.65 / 2.13 μm surface reflectance ratios (ξ0.65) Retrieved AODs are most sensitive to errors in the 0.65 / 2.13 μm surface reflectance ratio in the western US. Retrieved AODs are most sensitive to errors in the assumed single scattering albedos (SSAs) in the eastern US. CONCLUSIONS We retrieve MODIS AODs from the comparison of modeled (ρ*m,λ) to observed (ρ*λ) TOA reflectances by iteratively fitting the modeled AODs to match the observed reflectances. We define a match threshold (Δρλ) of Δρ0.47 μm = ±0.005 and Δρ0.65 μm =±0.001 corresponding to the uncertainty in modeling Rayleigh reflectance using scalar radiative transfer code [Levy et al., 2004]. Aerosol single scattering albedos (ω), and scattering phase functions (P) are held fixed in the retrieval. Mean 2.13 μm surface reflectance The use of locally derived 0.65 / 2.13 μm surface reflectance ratios (ξ0.65) eliminates the high bias in AODs retrieved over the arid southwest. The AODs we retrieve compare well with AERONET AODs in the western and central US, but are biased low in the eastern US. The AOD retrieval is significantly better at 0.47 μm than 0.65 μm because of the higher signal to noise ratio. Multiple day averaging of retrieved AODs improves the comparison with AERONET AODs. The use of CTM aerosol optical properties to derive AODs from MODIS reflectances ensures consistency in the AOD comparison. REFERENCES ACKNOWLEDGEMENTS Koepke et al., [1997] Kaufman and Tanre [1997] Levy et al., [2004] Remer et al., [2006] This work was supported by the NASA Atmospheric Composition Modeling and Analysis Program and an Earth and Space Science graduate fellowship awarded to Easan Drury.