Smoky haze which occurs during large-scale wildfires essentially transforms the radiative regime of the atmosphere over large territories. The variability of shortwave radiation fluxes in a smoke-laden atmosphere is driven by variations in the optical and microphysical properties of smoke aerosols, including the spectral dependences of the imaginary part of the refractive index. These dependences are determined by the presence of black carbon, brown carbon, and radiation-selective absorbing organic compounds in aerosol particles. This study analyzes the aforementioned spectral dependences based on AERONET data during large-scale wildfires in Alaska in 2019 and Canada in 2023. The analysis includes the cases of extreme radiation absorption by black and brown carbon, where the imaginary part of the refractive index at a wavelength of 440 nm attained 0.50 and 0.27, respectively. Variations in the spectral dependence of the imaginary part of the refractive index under moderate manifestations of selective absorption of smoke aerosol during massive fires in Alaska and Canada are analyzed. Approximations for the spectral dependence of the imaginary part of the refractive index are suggested. The aerosol radiative forcing at the top of the atmosphere is estimated for the cases of extreme radiation absorption by black carbon and brown carbon in the visible and near-infrared spectral regions and of anomalous selective absorption. The results can be useful in monitoring of the radiative regime of the atmosphere and for the development of atmospheric remote sounding techniques.

Organic aerosol (OA) can absorb solar radiation in the low-visible and ultra-violet wavelengths thereby modifying radiative forcing. Agricultural waste burning emits a large quantity of organic carbon in many developing countries. In this work, we improved the extraction and analysis method developed by Chen and Bond, and extended the spectral range of OC absorption. We examined light absorbing properties of primary OA from pyrolysis of corn stalk, which is a major type of agricultural wastes. Light absorption of bulk liquid extracts of OA was measured using a UV vis recording spectrophotometer. OA can be extracted by methanol at 95%, close to full extent, and shows polar character. Light absorption of organic aerosol has strong spectral dependence (Absorption Angstrom exponent = 7.7) and is not negligible at ultra-violet and low-visible regions. Higher pyrolysis temperature produced OA with higher absorption. Imaginary refractive index of organic aerosol (k(OA)) is 0.041 at 400 nm wavelength and 0.005 at 550 nm wavelength, respectively. (C) 2016 Elsevier Ltd. All rights reserved.