Deposition of light-absorbing aerosol on snow can drastically change the albedo of the snow surface and the energy balance of the snowpack. To study these important effects experimentally and to compare them with theory, it is desirable to have an apparatus for such deposition experiments. Here, we describe a simple apparatus to generate and evenly deposit light-absorbing aerosols onto a flat snow surface. Aerosols are produced (combustion aerosols) or entrained (mineral dust aerosols) and continuously transported into a deposition chamber placed on the snow surface where they deposit onto and into the snowpack, thereby modifying its surface reflectance and albedo. We demonstrate field operation of this apparatus by generating black and brown carbon combustion aerosols and entraining hematite mineral dust aerosol and depositing them on the snowpack. Changes in spectral snow reflectance are demonstrated qualitatively through pictures of snow surfaces after aerosol deposition and quantitatively by measuring hemispherical-conical reflectance spectra for the deposited areas and for adjacent snowpack in its natural state. Additional potential applications for this apparatus are mentioned and briefly discussed.
Aerosols in the Earth–atmosphere system play a critical role in radiative forcing and climate change (IPCC, 2013). However, our understanding of how they affect the cryosphere upon deposition onto snow surfaces is still limited (Qian et al., 2015), particularly for aerosols other than black carbon (Skiles et al., 2018). Understanding aerosol–cryosphere interactions is important on several levels, including (1) the radiative properties of the snowpack modified by deposited aerosols (Warren and Wiscombe, 1980; Warren, 1982; Gardner and Sharp, 2010; Bond et al., 2013; Qian et al., 2015), which alter the seasonal timing of snowmelt, runoff, and water management (Painter et al., 2010; Dozier, 2011); (2) the relation between deposited nutrients and the snowpack biosphere (Thomas and Duval, 1995; Jones, 1999; Kuhn, 2001; Hodson et al., 2008); and (3) snowpack chemistry and photochemistry, which are influenced by the deposited chemical compounds and their interaction with solar radiation (Grannas et al., 2007). One limitation in our understanding is establishing links between theoretical and experimental results because it is difficult to experimentally characterize these interactions in a controlled manner; generally, aerosol deposition on snow is spatially and temporally very inhomogeneous, and often deposition and its immediate effects are minor. One scarcely used but effective experimental method is to artificially deposit aerosols of interest directly onto the snow surface.
Atmospheric deposition is an important process by which an exchange of nutrients, gasses, and particles takes place between the atmosphere and land and sea surfaces. Many deposition processes are irreversible; for example, once the deposition of particles occurs, the probability of re-entrainment is low. Atmospheric constituents are removed from the atmosphere through dry and wet deposition (Seinfeld and Pandis, 2016). Dry deposition is facilitated by gravitational settling, inertial impaction, and Brownian diffusion processes. Wet deposition, on the other hand, involves the scavenging of gasses and particles by clouds and precipitation through dissolution, cloud condensation nuclei (CCN) activity, and collision processes. Aerosol and gasses can enter into the ice-grain matrix of the snowpack through different means and alter chemical, physical, and radiative properties (Kuhn, 2001; Grannas et al., 2007). For snowpack radiative processes and energy balance, the deposition of light-absorbing aerosols (i.e., black carbon (BC), brown carbon (BrC), and mineral dust; Moosmüller et al., 2009) is of special interest because deposition of even minute quantities of strongly light-absorbing aerosols drastically increases the co-albedo of the snow surface in the visible and near-visible spectral regions, where pure snow is “snow white” with hardly any intrinsic absorption (Warren, 1982).
Carbonaceous aerosols in the atmosphere, including BC and BrC (Chakrabarty et
al., 2010; Lack et al., 2014), are dominantly generated by incomplete
combustion of fossil and biomass fuels with significant additional generation
of secondary organic aerosols through oxidation of volatile precursors in the
atmosphere (Bond et al., 2004; Lin et al., 2014). These aerosols are lofted
into the atmosphere, where, during transport of a few days to weeks, they
undergo secondary processing (Jimenez et al., 2009) and eventually are
removed from the atmosphere through wet or dry deposition (Bond et al.,
2013). BC is a ubiquitous light-absorbing aerosol in the atmosphere that
directly affects Earth's radiative budget (Jacobson, 2001; Bond et al., 2013)
and the cryosphere (Hegg et al., 2009; Flanner et al., 2009; Hadley and
Kirchstetter, 2012; Qian et al., 2015). Recently, BrC has become of interest
regarding its role in atmospheric light absorption (Laskin et al., 2015) and
even more recently a topic of concern for affecting snow albedo and energy
balance (Dang and Hegg, 2014; Doherty et al., 2014; Lin et al., 2014; Wu et
al., 2016). The fraction of BC versus BrC mass emitted by combustion sources
depends greatly on a number of factors, including fuel type, fuel moisture
content, and packing density (Sumlin et al., 2018), combustion phase
(Patterson and McMahon, 1984; Reid et al., 2005; Bond et al., 2004), and
other elements of the system (Chen and Moosmüller, 2006). For BC, the
imaginary part
In addition to the importance of carbonaceous aerosol deposition on snow,
mineral dust deposition has been shown to be an important driver of early
snowmelt in some mountains (e.g., Painter et al., 2007, 2012, 2018; Skiles et
al., 2012). The light absorption of mineral dust is mostly caused by iron
oxides, such as hematite (
Previous experiments have utilized aerosol artificial deposition techniques to test non-natural snow surface albedo perturbations, but with varying success. Conway et al. (1996) manually mixed soot and volcanic ash with loose snow in a bucket before spreading it over a large plot. These methods were useful in understanding the characteristics and vertical location of aerosol during conditions of melt, but they likely restructured the bulk aerosol into agglomerations larger than what can be seen from normal deposition. Brandt et al. (2011) measured the snow albedo resulting from mixing commercially available soot into tap water and spraying the mixture through a commercially available snow-making machine over a field of artificial snowpack, produced with the same means. More recently, Peltoniemi et al. (2015) distributed chimney soot, glaciated silt, and volcanic sand onto the snow surface using a “salt shaker” in an attempt to measure the bidirectional reflectance factor of the resulting, contaminated snow. In addition, the snow albedo response to absorbing impurities on snow has been characterized by Singh et al. (2010) “spraying … soil equally on the surface”.
Our work presented here describes a simple apparatus to evenly deposit aerosols in an artificial manner onto a flat snow surface through dry deposition for the study of snow–aerosol interactions. Tests during the snow season of 2015–2016 were conducted at the Cold Regions Research and Engineering Laboratory–UC Santa Barbara Energy Site (CUES; Bair et al., 2018) on Mammoth Mountain, California, USA. During the 2016–2017 and 2017–2018 snow seasons, experiments were conducted at Tamarack Lake in the Carson Range of the Sierra Nevada in Nevada, USA. Controlled deposition experiments using the apparatus for hematite mineral dust entrainment and combustion aerosol production of BC and BrC to modify snow surface reflectance are presented as examples.
The deposition apparatus presented here is composed of two primary components: the aerosol production or entrainment chamber and the deposition chamber. The materials used for the air source and production or entrainment chamber depend on the type of test aerosol generated or entrained and deposited onto the snow surface. The deposition chamber is the same for any type of aerosol used.
A battery-powered pump provided combustion and transport air into
the aerosol production chamber for combustion aerosol
A schematic diagram of the aerosol production chamber for generating combustion aerosols is
shown in Fig. 1a. It consists of a flat plywood base with an area of
The air source used for this configuration is a battery-powered, 12 V air
pump that provides inlet air into the entrainment chamber, with a flow rate
of
The user of this apparatus can vary the mass of combustion material deposited by limiting the amount of time the fuel is combusting. Here, we purposefully produced heavy depositions for optical inspection and verification of the proper operation of the apparatus.
For pulverized, dry solids such as mineral dusts, the entrainment chamber
consists of a 1 L volume glass Büchner flask (e.g., Jensen, 2006), but
it is used in an opposing manner, providing positive pressure rather than a vacuum.
Here, the flask is filled with a generous amount (
The deposition chamber for all aerosols is a near-cylindrical galvanized
steel volume that measures approximately 50 cm in diameter at its midpoint
and has a volume of
The deposition chamber for all aerosol types consists of a
near-cylindrical volume with aerosols pumped into the inlet at the top of the
chamber from the production or entrainment chamber. The deposited area measures approximately 0.20 m
Previous implementations of the apparatus included a 90 mm diameter, 12 V DC-powered fan that was thought to help facilitate the dispersion of particles within the deposition chamber. This battery-powered fan was mounted in the approximate centroid of the chamber and was tested in several different flow directions. Figure 4a and b represent depositions of hematite while using this fan in the deposition chamber to test for increased deposition uniformity. However, the presence of the fan created less uniform depositions, so the fan was removed. Figure 4c shows a deposition of hematite with the most up-to-date configuration of the deposition apparatus, without the fan.
Use of the apparatus at the Tamarack Lake site for depositing BC aerosol.
To demonstrate the effectiveness of this deposition apparatus, it was tested
during the spring of the 2016, 2017, and 2018. Deposition of three different
aerosol types was demonstrated: BC and BrC in the combustion aerosol
production configuration and a dry, sieved mineral dust, hematite
(
Images of three hematite depositions allow for visual inspection
of the deposition uniformity. Panels
To quantify the effect of aerosol deposition onto a natural snow surface, both subjective and objective measures were used. The uniformity of the aerosol deposition was inspected visually for aerosol types that are visually dark, or optically absorbing, as for the hematite deposition in Fig. 4. In addition, the hemispherical-conical reflectance factor (HCRF; Schaepman-Strub et al., 2006) was measured with an Analytical Spectral Devices (ASD) FieldSpec3 spectroradiometer to verify that aerosol was indeed altering the surface spectral reflectivity of the snow and to spectrally quantify that effect. Note that the measured HCRF is related to the surface albedo, controlling solar energy input to the snowpack (Schaepman-Strub et al., 2006). After deposition of the different aerosol species, HCRF was measured for the deposition area as well as for an adjacent, untreated, natural surface that mirrored the snow properties of the deposition area prior to the experiment. This allowed the characterization and verification of the snow reflectivity reduction due to aerosol deposition. A total of 10 measurements of HCRF were performed and averaged for each deposition area. This averaged value is presented throughout this paper with 1 standard deviation of the mean.
Overview of deposition and HCRF collection information.
Measurement information and environmental conditions are summarized in
Table 1. Solar zenith and azimuth angles have been obtained from the date and
time of the measurement using the NOAA ESRL Solar Position Calculator
(
Generation and deposition of BC aerosol took place during the spring of 2018 at Tamarack Lake. A kerosene lamp was used to produce BC aerosol (Arnott et al., 2000; Arnold et al., 2014) to be deposited onto the snow surface. First, the 12 V pump was started to begin moving air through the complete apparatus and provide enough air to sustain the combustion; without this forced air, the flame would quickly be extinguished due to lack of oxidant. The kerosene lamp was filled with fuel and lit before being placed onto the combustion stage and the production chamber being sealed with clamps. Then, the deposition chamber was lowered onto the desired snow surface, thereby initiating BC deposition on the snow. For this experiment, the kerosene lamp was ignited, and aerosol emissions were pumped into the deposition chamber for 45 min. After this time, the flame was extinguished manually, and air was pumped through the apparatus for an additional 15 min to facilitate further deposition of aerosol onto and into the snowpack. The deposition chamber was promptly removed, and spectral reflection properties of the deposited and nearby unsoiled snow surfaces were characterized with HCRF measurements.
This deposition of BC aerosol reduced the HCRF from
Image and HCRF spectra for a BC deposition and adjacent natural snow in April 2018 at the Tamarack Lake
site. This BC deposition drastically reduced the high natural snow
reflectivity in the visible and near-visible spectral regions. Solar zenith:
26.7
The fuel combusted for BrC aerosol generation consisted of boreal peat
samples collected from interior Alaska, USA. Details of this fuel –
including its collection and preparation – have been given by Chakrabarty et
al. (2016). The optical, physical, and chemical properties of aerosol
emissions from combustion of this fuel have been extensively studied to
evaluate the impact of its combustion emissions on air quality and radiative
forcing in the atmosphere through optical, physical, and chemical
characterization (Chakrabarty et al., 2016; Samburova et al., 2016; Sengupta
et al., 2018; Sumlin et al., 2017, 2018). Prior to combustion,
the fuel samples were placed into a round, insulated container to mimic
simple, real-world conditions in which there is little lateral heat flux due
to largely homogeneous horizontal conditions. The fuel samples were burning
with nearly exclusively smoldering phase combustion, producing OC-rich
biomass burning aerosols. The fuel samples were smoldering for
BrC deposited onto the snow surface greatly reduced the measured HCRF, but
only at the shorter visible and UV wavelengths. The effect presented here
reduced the reflectivity of a springtime snowpack from
Image and HCRF spectra for a BrC deposition and adjacent natural
snow in May 2017 at the Tamarack Lake site. This BrC deposition
drastically reduced the high natural snow reflectivity in the ultraviolet and
short-wavelength visible (< 500 nm) spectral regions. Solar zenith:
34.6
Mineral dust in the atmosphere – and that deposited into the cryosphere –
has varying optical properties depending on its chemical and mineralogical
composition. Here, synthetically made, pure hematite (
A generous amount – approximately 90 g – of hematite dust was placed into the Büchner flask and was entrained and pumped into the deposition chamber as described in Sect. 2. The dust was allowed to gravitationally settle onto the snow surface for approximately 30 min, at which time the deposition chamber was removed and HCRF measurements were made for the deposited snow surface and the adjacent natural snow. The resulting image and spectrum from this test are shown in Fig. 7. The resulting HCRF was reduced by approximately 35 % in the 350–575 nm wavelength range when compared to that of the nearby natural snowpack.
Image and HCRF spectra for
a hematite deposition and adjacent natural snow in May 2016 at the CUES
research site. This hematite deposition drastically reduced the high natural
snow reflectivity in the ultraviolet and short-wavelength visible
(< 600 nm) spectral regions. Solar zenith: 21.5
The apparatus described here is not perfect but a work in progress and will benefit from further development by us and others. Some of the limitations and potential biases are outlined below.
A substantial but not quantified fraction of the aerosols generated in the production or entrainment chamber is lost during transport and deposited on walls and tubing, as evidenced by the surfaces of the apparatus darkening and acquiring a typical smell for BrC depositions. Additionally, the authors made no effort to monitor the mass of deposited material or how deep the aerosol penetrated into the snowpack; instead, they leave this issue for future development. Particle deposition onto the snow is not perfectly homogenous and this homogeneity varies from deposition to deposition. The dominant factor controlling homogeneity of the deposit seems to be wind, which causes the deposition to favor the lee side of the deposition area due to the air's ability to permeate and travel through the snowpack (e.g., Waddington et al., 1996). Additionally, this apparatus may alter the grain size of snow located at the top of the snowpack due to heating of the air inside the deposition chamber. Monitoring the grain size of surrounding natural snow and comparing it to that of grains within the deposition area after the experiment can shed light on the induced effects. By conducting this analysis using the methods outlined in Nolin and Dozier (2000), we have concluded that, for this set of experiments, there is no consistent change in grain size. Temperature artifacts in the deposition chamber could partly be mitigated by painting the exterior surface white to better reflect incident solar radiation.
Ensuring identical conditions inside and outside of the deposition chamber would be very challenging. Additional instrumentation within the deposition chamber can help quantify the impact that outside air temperature, incoming solar radiation, outgoing thermal infrared radiation, and wind speed could have on experimentation. Perhaps the use of an identical deposition chamber, one with and one without introduced aerosols, could minimize this problem.
The apparatus described here provides a means to generate or entrain and to artificially deposit aerosols evenly onto a snow surface. The apparatus has been proven to efficiently deposit carbonaceous aerosols – BC and BrC – from two combustion sources as well as entrained dry mineral dust onto snow, thereby altering the surface reflectivity of snow. The reduction of spectral surface reflectivity was verified by measuring the directional surface reflectance within the area of deposition and comparing it to the reflectance of neighboring natural snow. To the best of the authors' knowledge, this study is the first to deposit primary aerosol from combustion sources in situ, which provides the user of the apparatus with a novel tool to investigate the impact that these prolific snow impurities have after deposition. This investigation has proven that future applications of this apparatus are numerous.
The type of aerosol being deposited, the total mass of that aerosol, and the environmental conditions surrounding the deposition area can be adjusted by the users to suit their needs. The methods outlined by Skiles et al. (2017) to retrieve the refractive index of deposited aerosols from directional reflectance measurements can be applied to the artificial deposition methods described here with some additional radiative transfer analysis. Similarly, one can apply this apparatus to the testing of snow radiative transfer codes (e.g., SNICAR, Flanner and Zender, 2005; TARTES, Libois et al., 2013) as to their treatment of the influence of impurities deposited onto the snowpack. Beres et al. (2019) deposit varying concentrations of BrC onto the snow surface and verify measured total organic carbon concentrations for their albedo-reducing effect in an aerosol–snow coupled radiative transfer model.
The growing importance of understanding the link between radiative forcing by a variety of aerosols found in the atmosphere and their relationship to the change in physical, chemical, and optical properties of snow and ice are a field that can utilize this apparatus in the future. BrC aerosols have an impact on the cryosphere that is not well understood but potentially important, based on the close proximity of BrC-rich fuel sources to snow and ice surfaces and the proclivity of wildfires in the boreal forests of the northern latitudes (Flannigan et al., 2009; Oris et al., 2014; Beres et al., 2019). Additionally, the increased emissions of dusts across the globe (Mahowald et al., 2010) and their impact on snow radiative forcing can benefit from this device. For example, one could entrain other globally important dusts, such as those found in Engelbrecht et al. (2016), using this apparatus. While the first implementation of any new apparatus is imperfect, its usefulness and importance for the cryosphere sciences is obvious.
The authors provide the numerical values of HCRF spectra shown in figures within the paper in the Supplement. Additionally, the authors have provided normalized, laboratory-measured size distributions that correspond to aerosols produced for this study under similar conditions.
The supplement related to this article is available online at:
NDB and HM contributed approximately equally to all parts of this publication.
The authors declare that they have no conflict of interest.
This material has been supported in part by NASA EPSCoR under Cooperative Agreement no. NNX14AN24A, NASA ROSES under grant no. NNX15AI48G, and by the National Science Foundation under grant no. AGS-1544425. It is a pleasure to acknowledge Deep Sengupta for help with field experiments, Adam Watts for supplying the peat samples, and Jeff Dozier, Ned Bair, and Mammoth Mountain Ski Resort for generously providing access to and assistance at CUES. Edited by: Manfred Wendisch Reviewed by: Jeff Dozier and one anonymous referee