Analysis of AERONET Extended Wavelength Retrievals of Aerosol Absorption Parameters Including 380 nm and 500 nm for Detection of Brown Carbon in Biomass Burning and Iron Oxides in Desert Dust

Abstract:

We recently conducted research and analysis of AERONET retrievals of spectral imaginary refractive index and size distributions with the input of additional wavelength data. This includes adding both measured spectral AOD and sky radiances at 380 nm and 500 nm to the four channels (440, 675, 870 and 1020 nm) that have been input to the standard retrieval data product. The vector radiative transfer code in the AERONET Version 3 retrievals enabled more accurate computations in the ultraviolet region (380 nm). Calibration of the 380 nm channel radiances was done by the vicarious method since the integrating sphere radiance source that is used for all other wavelengths does not produce sufficient energy at 380 nm. Spectral single scattering albedo, imaginary refractive indices and aerosol size distributions from these six wavelength retrievals are compared to the standard four wavelength retrieval values. For various biomass burning regions in the mid-latitudes and tropics the retrievals of fine mode dominated aerosol show enhanced absorption at 380 nm and 440 nm relative to 500 nm, consistent with expected brown carbon absorption in these wavelengths from organic particles. The inclusion of the 500 nm input data and inversion results are important as this allows for better quantification of enhanced brown carbon absorption in the short wavelength visible and UV-A region, while black carbon alone dominates the absorption from the mid-visible through the near-infrared wavelengths. Some comparisons to other retrieved and measured aerosol spectral SSA and imaginary refractive indices from tropical biomass burning regions are shown. The size distribution retrievals for fine mode dominated cases show minimal differences between the standard four wavelength and the new six wavelength retrievals. For coarse mode dominated observations of desert dust aerosol, the 6 channel retrievals show enhanced absorption by iron oxides at 380 nm. Dynamics of the dust spectral absorption are examined, as a function of AOD levels, day of year (seasonal) and for some selected cases variation in source regions inferred by back trajectories. Comparisons to recently published spectral SSA of desert dust in various regions are shown.