『Abstract
The effect of snow cover on surface-atmosphere exchanges of nitrogen
oxides (nitrogen oxide (NO) + nitrogen dioxide (NO2);
note, here ‘NO2’ is used as surrogate for
a series of oxidized nitrogen gases that were detected by the
used monitor in this analysis mode) was investigated at the high
elevation, subalpine (3,340 m asl) Soddie site, at Niwot Ridge,
Colorado. Vertical (NO + NO2) concentration
gradient measurements in interstitial air in the deep (up to 〜2.5
m) snowpack were conducted with an automated sampling and analysis
system that allowed for continuous observations throughout the
snow-covered season. These measurements revealed sustained, highly
elevated (NO + NO2) mixing ratios inside
the snow. Nitrogen oxide concentrations were highest at the bottom
of the snowpack, reaching levels of up to 15 ppbv during mid-winter.
Decreasing mixing ratios with increasing distance from the soil-snow
interface were indicative of an upwards flux of NO from the soil
through the snowpack, and out of the snow into the atmosphere,
and imply that biogeochemical processes in the subnival soil are
the predominant NO source. Nitrogen dioxide reached maximum levels
of 〜3 ppbv in the upper layers of the snowpack, i.e., 〜20-40 cm
below the surface. This behavior suggests that a significant fraction
of NO is converted to NO2 during its diffusive
transport through the snowpack. Ozone showed the opposite behavior,
with rapidly declining levels below the snow surface. The mirroring
of vertical profiles of ozone and the NO2/(NO
+ NO2) ratio suggest that titration of ozone
by NO in the snowpack contributes to the ozone reaction in the
snow and to the ozone surface deposition flux. However, this surface
efflux of (NO + NO2) can only account for
a minor fraction of ozone deposition flux over snow that has been
reported at other mid-latitude sites. Neither (NO + NO2)
nor ozone levels in the interstitial air showed a clear dependence
on incident solar irradiance, much in contrast to observations
in polar snow. Comparisons with findings from polar snow studies
reveal a much different (NO + NO2) and ozone
snow chemistry in this alpine environment. Snowpack concentration
gradients and diffusion theory were applied to estimate an average,
wintertime (NO + NO2) flux of 0.005-0.008
nmol m-2 s-1, which is of similar magnitude
as reported (NO + NO2) fluxes from polar
snow. While fluxes are similar, there is strong evidence that
processes controlling (NO + NO2) fluxes in
these environments are very different, as subnivial soil at Niwot
Ridge appears to be the main source of the (NO + NO2)
efflux, whereas in polar snow (NO + NO2)
has been found to be primarily produced from photochemical de-nitrification
of snow nitrate.
Keywords: Snow; Soil; Winter; Gas fluxes; Nitrogen oxides; Ozone』
Introduction
Experimental
Results and discussion
Snowpack levels of (NO + NO2) and ozone
Source of (NO + NO2)
Are (NO + NO2) soil emissions a source of
snow nitrate?
Fluxes of (NO + NO2)
Relationship between (NO + NO2) and ozone
Conclusions
Acknowledgment
References