Broadband optical properties of biomass-burning aerosol and identification of brown carbon chromophores

The radiative effects of biomass-burning aerosols on regional and global scales can be substantial. Accurate modeling of the radiative effects of smoke aerosols requires wavelength-dependent measurements and parameterizations of their optical properties in the UV and visible spectral ranges along with improved description of their chemical composition. To address this issue, we used a recently developed approach to retrieve the time- and spectral-dependent optical properties of ambient biomass-burning aerosols from 300 to 650 nm wavelengths during a regional nighttime bonfire festival in Israel. During the biomass burning event, the overall absorption at 400 nm increased by about 2 orders of magnitude, changing the single scattering albedo from a background level of 0.95 to 0.7. Based on the new retrieval method, we provide parameterizations of the wavelength-dependent effective complex refractive index from 350 to 650 nm for freshly emitted and slightly aged biomass-burning aerosols. In addition, PM_2.5 filter samples were collected for detailed offline chemical analysis of the water-soluble organics that contribute to light absorption. Nitroaromatics were identified as major organic species responsible for the increased absorption at 400 to 500 nm. Typical chromophores include 4-nitrocatechol, 4-nitrophenol, nitrosyringol, and nitroguaiacol; oxidation-nitration products of methoxyphenols; and known products of lignin pyrolysis. Our findings emphasize the importance of both primary and secondary organic aerosols from biomass burning in absorption of solar radiation and in effective radiative forcing. Plain Language Summary The radiative effects of biomass-burning aerosols on regional and global scales are substantial. Accurate modeling of the radiative effects of smoke aerosols requires wavelength-dependent measurements and parameterizations of their optical properties in the UV and visible spectral ranges along with improved description of their chemical composition. To address this issue we used a recently developed approach to retrieve the time- and spectral-dependent optical properties of the ambient aerosol from 300 to 650 nm wavelengths and a high-resolution mass spectrometry analysis of fine particulate matter. We found a significant increase in aerosol light absorption in the UV-Vis spectral range which is correlated to high levels of nitroaromatic compounds identified in the water-soluble extracts of the filter samples. Additionally, for further applications of our results in radiative transfer models, we provide parameterizations of the wavelength-dependent effective complex refractive index from 350 to 650 nm for freshly emitted and aged biomass-burning aerosols.