Deposition of light-absorbing aerosols on snow: method development and changes to snow optical and radiative properties

Abstract

Snow and ice cover a significant portion of the Earth’s surface, and not only provide fresh water to billions of people, but also contribute to the Earth’s energy balance through a high surface albedo, reflecting most incident solar radiation back towards space. The surface albedo and reflected radiation can be greatly reduced by small amounts of light-absorbing impurities within the snowpack, such as black carbon (BC), mineral dust aerosol, and microbial growth. A wealth of observations and modelling of snow optical and radiative properties recognize BC and mineral dust aerosol as dominating the reduction of snow albedo after deposition, but growing research into atmospheric, light-absorbing organic carbon (OC) aerosol, known as brown carbon (BrC), encourages a greater understanding of the impacts of this impurity in the snowpack. Little is known about how BrC deposition onto snow surfaces affects the spectral snow albedo and what radiative forcing results from BrC deposition. This is of special importance for areas of the Arctic where wildfires burn fuels, such as peat, that emit BrC aerosol in close proximity to snow and ice. The goal of this dissertation is to explore this snow-BrC aerosol relationship. A simple apparatus was developed to generate and deposit aerosols onto a snow surface to study aerosol deposition and its effect on snow albedo experimentally and to compare with theory. This portable apparatus was used to deposit combustion and mineral dust aerosol – including black carbon, brown carbon, and hematite – in situ onto snow surfaces. Aerosols were generated and continuously transported into a deposition chamber placed on the snow surface, where there were evenly deposited, thereby modifying the snowpack’s optical and radiative properties. Field operation of this apparatus was demonstrated and changes in snow surface reflectivity were monitored by measuring the spectral directional reflectance of the deposited areas and of adjacent natural snowpack. The apparatus was further used for the artificial deposition of emissions from Alaskan peat combustion onto a natural snow surface. This combustion occurs mostly in the smoldering phase, emitting mainly BrC aerosol designated by strongly enhanced light absorption at short-visible and ultraviolet wavelengths. This wavelength dependence was validated by observing a strong reduction in measured spectral snow albedo after deposition, mainly at these same wavelength regions predicted from previous studies of the optical properties of these aerosols generated under similar conditions of combustion. The imaginary refractive index spectrum of deposited BrC was derived from UV-vis absorption spectra of melted snow samples and total organic carbon (TOC) concentrations in the snow before and after deposition, which agreed well with results of other investigations into the refractive indices of smoldering peat combustion emissions. The optical properties of deposited BrC aerosol were calculated with Mie theory and incorporated into a snow/impurity radiative transfer model calculating surface albedo spectra of snow with BrC impurities. Measured and modelled surface albedo spectra were shown to be in good agreement – to within approximately 5% – across the UV-visible wavelength region. Finally, the amount of energy absorbed by the snowpack due to the presence of impurities before deposition to be about 40 W/m2. After depositing BrC, the instantaneous radiative forcing was increased by over 10 W/m2, yielding a direct estimate of the mass weighted radiative forcing, or forcing efficiency, for the conditions during the experiment of 1.2 ± 0.2 W/m2 per ppm of deposited BrC. In addition to estimating the impact from the artificial deposition of BrC, the snow surface spectral albedo, TOC concentration and absorption spectra of melted snow at three depths, and instantaneous radiative forcing was measured for the natural snowpack at the field site in the Sierra Nevada; these results can provide this much needed information for a spatial understanding of snow and impurity properties both locally and worldwide.In summary, this work has developed and characterized a novel apparatus for artificial deposition of aerosols onto snow and has used it, together with other measurements and radiative transfer theory, to determine the changes to snow optical properties and radiative forcing due to the deposition of BrC aerosol produced from smoldering peat combustion.