The Impact of Aerosols on Near-IR Radiances: Implications for CO Retrievals

The presence of dust and other atmospheric aerosols, both of natural and anthropogenic origin, significantly affects the accuracy of the retrieval of key environmental parameters from spaceborne instruments. Considerable efforts have been invested in developing the methods to account for the effects of aerosols in the UV/VIS spectral region. Similarly, efforts are currently underway for the thermal infrared region. However, the impact of aerosols on top of atmosphere radiances measured in the near-IR (3- 6 micron) spectral region has been largely overlooked. We present the results from forward modeling studies that explore the sensitivity of high-resolution near-IR spectral radiances to various aerosol characteristics, including aerosol composition, size spectra, vertical distribution and loading. High resolution near-IR spectral aerosol optical properties for individual aerosol species, including minerals, carbonaceous, sulfates, sea-salt, nitrates and their mixtures, were computed by utilizing the Library of Atmospheric Aerosol Refractive Indices (LAARI) and Mie code. These optical models were then incorporated into the DISORT radiative transfer code linked with kCARTA (kCompressed Atmospheric Radiative Transfer Algorithm developed at the Atmospheric Spectroscopy Laboratory of the University of Maryland). Linked kCARTA and DISORT codes enable calculations of high spectral resolution radiances, accounting for gaseous absorption and for absorption and multiple scattering by aerosol particles. We contrast the measurement sensitivity (i.e., weighting function) obtained in clear sky conditions with those for several cases of aerosol loading, including mineral dust, biomass burning and carbonaceous aerosols and consider the impact on CO retrievals, focusing on the 4.7 micron absorption band. The variability in radiances due to these aerosol loadings is compared with the variability estimated given different CO atmospheric profiles, and temperature and water vapor profiles.