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Optical Characterization of Fresh and Photochemically-Aged Aerosols Emitted from Laboratory Siberian Peat Burning
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Carbonaceous aerosols emitted from biomass burning into the atmosphere greatly influences radiative forcing and climate change on regional and global scales. Of particular interest are emissions from high latitude peat burning because of the large amount of terrestrial carbon stored in peatlands and becoming increasingly susceptible to wildfires due to amplified climate change at high latitude. Here, we have combusted small amounts of Siberian Peat in a laboratory biomass-burning facility. We have characterized the optical properties of freshly-emitted and photochemically-aged combustion aerosols, including absorption and scattering coefficients, at three wavelengths, 405, 532, and 781nm. The atmospheric aging of emitted aerosols was simulated using an Oxidation Flow Reactor (OFR) at timescales equivalent to three weeks to two months of atmospheric aging. Siberian peat was used as fuel because of its slow and smoldering combustion and its common importance in high-latitude peatland fires. Our results show that freshly emitted aerosols cause light extinction including scattering and absorption and while this extinction is modified during atmospheric aging, scattering and absorption can continue for weeks to months after the burn event until the aerosol is deposited on the earth surface; therefore emitted aerosols can continue to alter the radiation budget on these timescales.Our measurements on Siberian peat biomass burn emissions showed an increase in particle number density during aging. However, total particle volume either increased or decreased, depending on the aging duration. Measured scattering and absorption coefficients and their sum, the extinction coefficient, were used to calculate aerosol single-scattering albedo (SSA) as ratio of scattering and extinction coefficients, SSA is the dominant intensive aerosol optics parameter determining aerosol radiative forcing. Our data analysis showed that SSA of both fresh and aged aerosol increased with increasing wavelength; this is expected for brown carbon (BrC) aerosols. Changes in SSA between freshly emitted and aged aerosols were greatest at the 405 nm wavelength where SSA values increased by ~2% after aging. This SSA increase was likely dominated by a decrease in absorption coefficient instead of by an increase in scattering coefficient. SSA changes at the 532 and 781 nm wavelengths were less obvious and were easier to observe in terms of co-albedo. Single scattering co-albedo values showed aged emissions became more absorbent than fresh emissions at 532 nm by 32% and at 781 nm became less absorbent by ~16% indicating a shift in absorption toward longer wavelengths and a decrease in Absorption Ångström exponent (AAE). Although these percentages may seem significant, they refer to values whose magnitudes are of the order of 10-3. Absorption Ångström exponents ranged from ~6 to 9 denoting much stronger absorption at shorter wavelengths, both before and after aging. The extraction of the complex refractive index yielded real parts that increased with increasing wavelength which is unusual for wavelengths well above the wavelengths of the dominant absorption features. The imaginary part of the complex refractive index decreased with increasing wavelength as expected. For all three wavelengths (405, 532, and 781nm), the values of the real part of the complex refractive index ranged from 1.3 to 1.6 for aged emissions and 1.3 to 1.8 for fresh ones while imaginary parts of the complex refractive index were in the range of 0.0002 to 0.0096 for aged emissions and 0.0002 to 0.0175 for fresh ones. These values are typical for brown carbon aerosols from biomass combustion.