Constraining Aerosol Properties Using H2O Retrievals from the California Laboratory for Atmospheric Remote Sensing (CLARS)

H2O has absorption features across the electromagnetic spectrum, from the ultraviolet to the infrared. The California Laboratory for Atmospheric Remote Sensing (CLARS) on the top of Mt Wilson, California, offers continuous high-resolution spectral measurements from 4000 to 8000 cm-1. We retrieve H2O slant column densities (SCDs) at different wavelengths using CLARS data. In particular, we compare retrievals from the spectralon, which is above the planetary boundary layer and relatively immune to aerosol scattering, with those from West Pasadena, a location in the Los Angeles basin that is influenced by aerosol scattering. SCD retrievals for West Pasadena show significantly larger variance across different wavelengths. The retrieval error in West Pasadena is much larger than can be attributed to spectroscopic uncertainties, and reflects the wavelength dependence of aerosol scattering. Using a two-stream enhanced single scattering (2S-ESS) radiative transfer (RT) model, we simulated the effect of aerosol scattering on H2O SCD retrievals at different wavelengths. We found the effects are sensitive to the surface albedo, aerosol phase function and single scattering albedo. Using an empirical relationship derived from the radiative transfer model simulations, we relate the H2O retrieval variance to the aerosol optical depth Angstrom coefficient and compare the results with AERONET observations. The additional information gained from H2O retrieval variance within a large range of wavelengths could be used to improve OCO-2 type CO2 retrievals in the presence of aerosols.