Multi-instrument approach for measuring spectral aerosol absorption properties in UV and VIS wavelengths

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The spectral dependence of light absorption by atmospheric particulate matter (PM) has major implications for air quality, surface ultraviolet (UV) radiation, and tropospheric oxidation capacity, but remains highly uncertain. Quantifying the spectral dependence of aerosol absorption at UV and visible wavelengths is important for the accurate air pollution characterization using current (e.g., Aura/OMI) and future (e.g., TROPOMI, TEMPO, GEMS) satellite measurements, photolysis rates calculations in chemical and aerosol transport models and surface radiation modeling. Measurements of column atmospheric absorption and its spectral dependence remain the most difficult part of atmospheric radiation measurements. Currently available ground measurements of spectral aerosol absorption properties (e.g., column effective imaginary refractive index (k), single scattering albedo, (SSA), and aerosol absorption optical depth (AAOD)) are limited to the cloud free conditions and few discrete wavelength bands in the visible spectral region by AERONET almucantar inversions. To address the lack of spectral aerosol and gaseous absorption measurements in the UV, a suite of complementary ground-based instruments, modified UV Multifilter Rotating Shadowband Radiometer (UV-MFRSR) was established in 2002 and is currently in use at NASA Goddard Space Flight Center (NASA/GSFC) in Greenbelt, Maryland. In addition, several field campaigns have been carried out to measure aerosol absorption properties in UV and VIS from different sources in different locations.

In September-October 2007 biomass-burning season in the Amazon basin (Santa Cruz, Bolivia), light absorbing (chromophoric) organic or “brown” carbon (BrC) is studied with surface and space-based remote sensing. It is found that BrC has negligible absorption at visible wavelengths, but significant absorption and strong spectral dependence at UV wavelengths. Using the ground-based inversion of column effective imaginary refractive index (k) at UV wavelengths down to 305 nm, a strong spectral dependence of specific BrC absorption is quantified in the UV implying more strongly reduced ultraviolet B (UV-B) radiation reaching the surface. Reduced UV-B means less erythema, plant damage, but also a slower ozone photolysis rate. A photochemical box model is used to show that relative to black carbon (BC) alone, the combined optical properties of BrC and BC slow the net rate of production of ozone by up to 18% and lead to reduced concentrations of radicals OH, HO2, and RO2 by up to 17%, 15%, and 14%, respectively. The optical properties of BrC aerosol change in subtle ways the generally adverse effects of smoke from biomass burning.

The objective of this thesis is to develop a new method to infer column effective spectral absorption properties (k, SSA, and AAOD) of PM using the ground-based measurements from AERONET in the visible wavelengths and UV-MFRSR in the UV and ozone and NO2 from ground-based (Pandora and Brewer) or satellite spectrometers, such as Ozone Monitoring Instrument (OMI) on NASA EOS Aura satellite. This represents the first effort to separate effects of gaseous (ozone and NO2) and aerosol absorption and partition black and brown (light-absorbing organic) carbon absorption in the short UV-B wavelengths. These measurements are essential to answer key science questions of the atmospheric composition and improve data products from the current and future satellite atmospheric composition missions.