Abstract. 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≲λ≲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. 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 first derive a BrC (mass) specific absorption (m2 g−1)
across the UV–Vis spectral range. We then estimate the imaginary part of the
refractive index of deposited BrC aerosol using 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. The instantaneous radiative forcing
per unit mass of total organic carbon deposited to the ambient snowpack was
found to be 1.23 (+0.14/-0.11) W m−2 per part per million (ppm). We estimate the same
deposition onto a pure snowpack without light-absorbing impurities would
have resulted in an instantaneous radiative forcing per unit mass of 2.68
(+0.27/-0.22) W m−2 per ppm of BrC deposited.