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Impacts of atmospheric brown carbon on surface UV and ozone in the Amazon Basin

Mok, J., N. Krotkov, A. Arola, O. Torres, H. Jethva, M. Andrade, G. Labow, T. F. Eck, Z. Li, and R. Dickerson (2016), Impacts of atmospheric brown carbon on surface UV and ozone in the Amazon Basin, Sci. Rep., 6, 36940, doi:10.1038/srep36940.

The spectral dependence of light absorption by atmospheric particulate matter has major implications
for air quality and climate forcing, but remains uncertain especially in tropical areas with extensive
biomass burning. In the September-October 2007 biomass-burning season in Santa Cruz, Bolivia, we
studied light absorbing (chromophoric) organic or “brown” carbon (BrC) with surface and space-based
remote sensing. We 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 in the range 305–368 nm, we quantified a strong spectral
dependence of absorption by BrC in the UV and diminished ultraviolet B (UV-B) radiation reaching
the surface. Reduced UV-B means less erythema, plant damage, and slower photolysis rates. We use
a photochemical box model 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

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Radiation Science Program (RSP)