Aircraft measurements of spectral surface albedo and its consistency with ground-based and space-borne observations

Spectral surface albedo, a boundary condition which needs to be accurately known for aerosol remote sensing, surface aerosol forcing, and radiative transfer calculations, also strongly affects Earth's radiation balance. The difficulty in deriving surface albedo from space and aircraft observations lies mainly in the atmospheric correction, especially in aerosol-burdened regions. Because of the different scales, comparing satellite retrievals with airborne or ground-based observations is not straightforward. We use Solar Spectral Flux Radiometer (SSFR) measurements of upward and downward irradiance from aircraft altitude during Megacity Initiative: Local and Global Research Observations (MILAGRO) to determine spectral surface albedo at ground stations and along the flight track (over the wavelength range 350 to 2100 nm), thereby linking flight-level retrieved measurements to larger-scale satellite observations in the polluted Mexico City environment. Our approach involves iteratively adjusting the surface albedo input of a SSFR specific radiative transfer model until the modeled upward irradiance matches the SSFR measurements at flight level. A sensitivity analysis of surface albedo to aerosol optical properties provides a retrieval uncertainty, which can outweigh the SSFR instrument uncertainty under highly variable conditions (or uncertain measurements) of aerosol optical depth and asymmetry parameter. Comparisons between spectral surface albedo derived from the SSFR, Multi-Filter Rotating Shadowband Radiometer, and the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument onboard the NASA-EOS Terra and Aqua satellites are shown with differences of 6-10% and 0.025-0.05 units, respectively. Along-track comparisons between the SSFR and MODIS show that two instruments (aircraft and satellite) can capture inhomogeneous surface albedo scene changes.