Abstract
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.
Highlights
Aerosol light absorption in the Earth’s atmosphere lowers the planetary albedo, thereby causing radiative heating (Moosmüller et al, 2009; Satheesh and Krishna Moorthy, 2005; Stier et al, 2007; Stocker et al, 2013)
Both black carbon (BC) and organic carbon (OC) aerosols are formed by incomplete combustion of carbon-containing fuels (Andreae and Gelencsér, 2006; Bond et al, 2013), and their contribution to a source’s total emissions and their characteristics depends on many factors including fuel type, moisture content, packing density and source depth (Sumlin et al, 2018b), combustion phase (Bond et al, 2004; Patterson and McMahon, 1984; Reid et al, 2005), and other elements of the system (Chen et al, 2006)
Such experiments can deposit brown carbon (BrC) aerosol onto snow – from fuel sources that are likely to exist near snow surfaces, such as peat – and quantify changes in snow composition (e.g., OC concentration) and radiative properties
Summary
Aerosol light absorption in the Earth’s atmosphere lowers the planetary albedo, thereby causing radiative heating (Moosmüller et al, 2009; Satheesh and Krishna Moorthy, 2005; Stier et al, 2007; Stocker et al, 2013). With the exception of metals, all atmospheric aerosols with a nonzero imaginary part of their refractive index do contribute to atmospheric light absorption This absorption is distinct from other radiative forcing mechanisms such as aerosol scattering because light-absorbing aerosols continue to substantially contribute to radiative forcing after deposition onto high albedo surfaces such as snow and ice (Skiles and Painter, 2018; Warren, 1982). They contribute to climate forcing (Hansen and Nazarenko, 2004; Jacobson, 2004) by significantly lowering snow and ice albedo (Chýlek et al, 1983; McConnell et al, 2007; Warren and Wiscombe, 1980), thereby reducing snow cover duration and changing spring runoff timing (Déry and Brown, 2007; Painter et al, 2007; Strack et al, 2007). Primary BrC aerosol deposition onto snow and ice is of particular concern in northern latitudes, due to the proximity of peat fuels to snow and ice surfaces (e.g., Evangeliou et al, 2019)
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