Deposition of brown carbon onto snow: changes of snow optical and radiative properties

Abstract

<p><strong>Abstract.</strong> Light-absorbing organic carbon aerosol – colloquially known as brown carbon (BrC) – is emitted from combustion processes and has a brownish or yellowish visual appearance, caused by enhanced light absorption at shorter visible and ultraviolet wavelengths (0.3 μm ≲ <i>λ</i> ≲ 0.5 μm). Recently, optical properties of atmospheric BrC aerosols have become the topic of intense research, but little is known about how BrC deposition onto snow surfaces affects the spectral snow albedo, which can alter the resulting radiative forcing and in-snow photochemistry. Wildland fires in close proximity to the cryosphere, such as peatland fires that emit large quantities of BrC, are becoming more common at high latitudes, potentially affecting nearby snow and ice surfaces.</p> <p>In this study, we describe the artificial deposition of BrC aerosol with known optical, chemical, and physical properties onto the snow surface and we monitor its spectral radiative impact and compare it directly to modeled values. First, using small-scale combustion of Alaskan peat, BrC aerosols were artificially deposited onto the snow surface. UV-vis absorbance and total organic carbon (TOC) concentration of snow samples were measured for samples with and without artificial BrC deposition. These measurements were used to estimate the imaginary part of the refractive index of deposited BrC aerosol with a volume mixing rule. Single particle optical properties were calculated using Mie theory, and these values were used to show that the measured spectral snow albedo of snow with deposited BrC was in general agreement with modeled spectral snow albedo using calculated BrC optical properties.</p> <p>The instantaneous radiative forcing by impurities present in the snow before the deposition experiments was found to increase the instantaneous radiative forcing at the surface of the natural snow at our site by 1.23 (+0.14/−0.11) W m<sup>−2</sup> per ppm of BrC deposited. However, we estimate that deposition onto a clean snowpack without light-absorbing impurities would have resulted in a more than twice as large instantaneous radiative forcing of 2.68 (+0.27/−0.22) W m<sup>−2</sup> per ppm of BrC deposited.

Lifecycle of light-absorbing carbonaceous aerosols in the atmosphere

“…This might be a relatively accurate way to measure BrC absorption but requires reliable BrC optical properties to convert it to BrC content in snow. Recently, Beres et al 215 measured spectral snow albedo reductions due to artificial BrC deposition, and found an instantaneous radiative forcing of about 2.7 W m −2 per ppm of BrC in snow. In addition to darkening the snow surface, BrC's optical impacts in the UV and short-visible wavelengths can alter photochemistry at the snow surface, such as nitrate/nitrite photolysis 216 and mercury and bromine chemistry 217 .…”

Spatial and Temporal Variability of Carbonaceous Aerosol Absorption in the Po Valley

“…atmospheric processing of aerosol precursors increases the probability of light absorbing aerosol formation in the upper part of the troposphere, including above clouds, potentially leading to a positive radiative forcing (Zhang et al, 2017). Deposition of BrC on snow and ice can promote their melting and albedo change (Beres et al 2020. In addition, BrC absorption at short wavelength (below 400 nm) reduces atmospheric actinic flux, impacting atmospheric photochemistry and ozone production (Mok et al 2016).…”

Brown Carbon Fuel and Emission Source Attributions to Global Snow Darkening Effect

“…There is also evidence that LAPs influence the SDE of one another by changing the snow grain sizes and surface albedo over which the SDE is calculated. For example, the presence of dust can reduce BC SDE by 10%–40% (Bond et al., 2013; Flanner et al., 2009) and the presence of BrC on clean snowpack can have twice the radiative impact of BrC deposited on a snow surface with other impurities (Beres et al., 2020).…”