Reactive nitrogen (N
Ozone (O
These wintertime high O
Atmospheric measurements in the Uintah Basin during UBWOS2012, UBWOS2013, and
UBWOS2014 reveal that the total reactive nitrogen abundances
(NO
In addition to aiding in the formation and maintenance of a stable air mass
with enhanced UV radiation, snow may also recycle reactive nitrogen oxides
(N
The photolysis of nitrate occurs in the liquid-like region (LLR) in or on ice
grains (Domine et al., 2013) in the top snow layer where UV radiation is
present, which is known as the snow photic zone. Snow nitrate photolyzes at
wavelengths (
The measured quantum yields (
Nitrate nitrogen isotopes (
In this study, we investigate the importance of snow photochemistry as a
source of reactive nitrogen oxides to the boundary layer in the Uintah Basin
using chemical, isotopic, and optical measurements from the snow collected
during the UBWOS 2014 campaign. In Sect. 2 we describe the field, laboratory,
and modeling techniques used in this study. In Sect. 3 we present the
chemical and optical measurements made during UBWOS 2014 and model-calculated
fluxes of snow-sourced N
UBWOS 2014 occurred from 17 January to 13 February 2014 at the Horsepool
field-intensive site (40.1
Snow covered the ground throughout the duration of the campaign and ranged
in depth from 10 to 30 cm, depending on how snow was redistributed by wind
after deposition. The snow was deep enough to cover some of the lowest-lying
vegetation, but branches from bushes were still visible. Three snow events
occurred before the campaign, one event on 4 December, which deposited most
of the snow (19 cm), and two smaller events on 8 and 19 December, which
deposited roughly 3 and 1 cm of snow, respectively. There was a distinct
crust layer roughly 4 cm below the snow surface, providing evidence of
surface melting between the later two snowfall events. The temperature
difference between the soil and the air was at least 15 K for several weeks,
allowing vapor to redistribute through the snow, leading to the formation of
large hoar crystals (radiation equivalent mean ice grain radii (Hansen and
Travis, 1974) (
Twelve snow pits were dug approximately every 2 to 3 days during the campaign.
Snow pits were dug from the snow surface to about 1 cm above the subniveal
ground and ranged in depth from 9 to 24 cm. The snow pits were dug in a
variety of directions roughly 150 m from the main Horsepool site, except for
snow pit 5 (24 January), which was dug roughly 800 m away from Horsepool. The
snow pits were dug wearing clean, nitrate-free gloves using a stainless steel
spatula. For each snow pit, vertical profiles (1 cm depth resolution) of snow
density (
The snow from each plastic bag was spooned into a clean glass beaker and
melted in a microwave oven at USU. The meltwater was transferred to a
stainless steel funnel and passed through a 0.4
The absorption spectrum of each Nuclepore filter was measured using an ISSW
spectrophotometer (Grenfell et al., 2011) in the Arctic Snow Laboratory at
UW. The Nuclepore filter is placed between two integrating spheres lined with
Spectralon material to create a fully diffuse medium. An Ocean Optics USB-650
spectrophotometer is used to measure the absorption spectrum in units of
optical depth,
Surface upwelling and downwelling irradiance was measured using a commercial
spectral radiometer equipped with a photodiode array (Metcon GmbH & Co. KG,
In a laboratory on the USU campus in Vernal, UT, a 50
The denitrifier method (Casciotti et al., 2002; Kaiser et al., 2007;
Sigman et al., 2001) was used to determine the nitrogen isotopic signature
(
Aerosol nitrate was collected throughout the campaign in 12 h intervals.
Aerosol nitrate was sampled from an inlet 13 m above ground and drawn
through a heated (283 K) pipe, where it was then collected on a two-stage,
multi-jet cascade impactor. The impactor Tedlar films separates aerosols with
diameters less than 2.5
A four-stream, plane-parallel radiative transfer model using the discrete
ordinates method with a
The modeled vertical profiles of actinic flux and observed snow nitrate
concentrations are used to calculate daily-average fluxes of snow-sourced
N
The flux of snow-sourced N
In TRANSITS, nitrate is deposited to the snow surface via dry deposition.
Nitrate dry deposition is calculated using the campaign-averaged observed
boundary layer mixing ratios for HNO
We include only the major channel for the production of N
In this study, TRANSITS is run at hourly resolution and is spun up beginning
27 days before the start of the campaign using available atmospheric chemical
(boundary layer, gas-phase, and aerosol-phase nitrate) and meteorological data
(air, temperature, and pressure). A constant model boundary layer height of
50 m is assumed, which is a rough estimate of daily averaged boundary layer
heights based on sodar facsimile data from NOAA. The campaign-averaged
observed boundary layer total nitrate (HNO
Figure 1a shows mean surface snow
Figure 1b shows surface snow nitrate concentration measurements for each
snow pit, which range from 800 to 18 000 ng g
Generally, the surface-snow
In this section and the following sections, we focus on three snow pits (22, 31 January, and 4 February) as being representative of the time period before, during, and after the largest snow event. The other nine snow pits will not be discussed in detail, but observed and modeled vertical profiles of chemical and optical measurements for all 12 snow pits can be found in the Supplement Sect. A.
Figure 2a and b show vertical profiles of snow optical properties from an
18 cm deep snow pit dug on 22 January, which represents typical profiles from
the beginning of the field campaign until before the first snow event. Black
carbon concentrations (
Snow optical properties measured on 22 January (left),
31 January (middle), and 4 February (right). (top) Vertical profiles of mean
snow black carbon (
Figure 2c and d show vertical profiles of snow optical properties from a
14 cm deep snow pit dug on 31 January. It snowed 5 cm between the afternoon
of 30 January and the morning of 31 January, and this new snow layer is evident
in Fig. 2c and d because the dusty layer is now located roughly 5 cm below
the snow surface. Figure 2c shows that
Figure 3a–c show observed vertical profiles of nitrate in snow from
snow pits dug on 22, 31 January, and 4 February. Prior to the fresh snowfall
event, snow nitrate concentrations were highest at the surface
(13 900 ng g
Measured (black) and modeled (
Figure 3d–f show measured snow
The last snowfall event prior to the start of the campaign occurred on
19 December and resulted in roughly 1 cm of snow accumulation (Supplement
Fig. S5a). The high concentrations of LAI and nitrate in surface snow on
22 January, combined with the prolonged lack of snowfall, suggest continual
dry-deposition of LAI to the surface snow. We speculate that the major source
of LAI originates from truck traffic on the dirt roads in the area of the
field site due to high values of
The
Figure 4a–c show calculated vertical profiles of UV actinic flux normalized
to surface downwelling irradiance for the three snow pits. On 22 January, the
normalized actinic flux ratio is nearly 4 at the snow surface because actinic
flux is calculated by integrating irradiance over a sphere (surface area of
In the snow pits following the fresh snowfall event, the existence of the
dusty layer deeper in the snow influences the vertical actinic flux profile
and increases the photic zone depth from 5 to 7 cm. The fresh snow at the
surface contains less LAI compared to the dusty layer. Therefore, actinic flux
values are higher in the top several centimeters of snow compared to actinic
flux values measured before the snowfall event, even though
The presence of a new dusty layer on the snow surface five days after the fresh snowfall event does not significantly alter the vertical profile of normalized UV actinic flux, likely because UV absorption by LAI in the surface layer is at least five times lower than UV absorption by LAI in the original dusty layer (surface snow from 22 January snow pit). Surface snow UV albedo is strongly influenced by the presence of LAI, and Fig. S2b in the Supplement shows that snow UV albedo is lowest right before the snowfall event on 30–31 January and highest immediately afterwards.
Snow photic zone depth and daily averaged modeled
We use these actinic flux profiles and the observed snow nitrate
concentrations (Fig. 3a–c) to calculate daily averaged fluxes of
snow-sourced N
Modeled diurnal profiles of snow-sourced N
The snow chemistry column model is used to calculate the time-dependent flux
of snow-sourced N
Figure 6 shows hourly
Modeled snow-sourced N
Figure 3 shows modeled snow nitrate concentrations and
Modeled
To examine the sensitivity of snow nitrate to photolysis, we turn off
photolysis of snow nitrate in the model by setting
In another sensitivity study, we calculate the maximum possible
We first assume that all N
Only the major channel for snow nitrate photolysis (Reaction R1) is simulated in
the TRANSITS model, although nitrate can also photolyze via Reaction (R2) and form
both NO
This study estimates the influence of snow nitrate photolysis on the boundary
layer reactive nitrogen (N
The daily averaged flux snow-sourced N
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