Diurnal and Seasonal Variations of Nitrogen Oxides Within Snowpack Air
and the Overlying Atmosphere at Summit, Greenland
C.
1
Toro ,
R.E.
1†
Honrath ,
L.J.
1
Kramer ,
1
Atmospheric Sciences Program,
Michigan Technological University
[email protected]
Overview
The Arctic is one of the Earth’s most sensitive regions to
climate change. Reductions in snow cover concomitant with a
warming atmosphere will affect cryosphere-atmosphere
exchange of trace gases; thus, improved understanding of
atmospheric and surface processes is required to predict
future impacts of climate change on the atmospheric
chemistry of the Arctic.
Our research focuses on snowpack chemistry and air-snow
exchange of nitrogen oxides and ozone. Specific goals of the
research are to improve representations of surface and
atmospheric processes that influence the oxidative capacity
of the Arctic atmosphere and to develop and evaluate global
chemistry-climate models that simulate effects of snowpack
processes on the overlying atmosphere.
Here, we present the first set of continuous measurements of
nitrogen oxides within and above snowpack that detail the
annual cycle of the trace gases at Summit, Greenland.
Seasonal and diurnal variations of the reactive species are
discussed.
Experimental Site Description
GeoSummit Flux Facility
The GEOSummit Flux Facility was designed for air-snow
exchange studies. Conduits for sampling tubes and cables run
from the 10 m tower to a heated, insulated facility that is located
under the snow (downwind from the tower during clean sector
flow), and which is accessed through the entryway visible on the
right of the photo.
To the right is a picture of the
snowtower with two, air inlets at levels
separated by 30 cm between them.
The total flow (~2 lpm) is divided
between the inlets to minimize
disturbance of interstitial air.
The experiment started in June 2008
with 8 levels on the snowtower. Six
new levels were added in August
2009, as the inlets were buried by
accumulating snow. As of February
2010, the snowtower has a vertical
profile of 4.2 meters with 2.5 meters Figure 1: Snow
Tower
into the snowpack.
Acknowledgements: This work is funded by National Science Foundation grants ARC0713992 and
ARC0713943. Michael Dziobak provided essential instrumentation and field support.
We especially thank the Summit Science Technicians and support staff and the Flux Facility design
and construction team for their assistance.
2
D.Helmig ,
2Institute
of Arctic and Alpine Research,
University of Colorado
[email protected]
Results
I.
B.A. Van
2
Dam ,
Seasonal behavior of NOx enhancements
(snowpack concentrations minus ambient mixing
ratios measured at 2 m above snow ).
B.
2
Seok ,
L.
3
Ganzeveld ,
P.V.
1
Doskey
3Department
II. Response of NO2 and NO to Wind
Speed, Snowpack Temperature
and Ozone.
of Earth and Environmental Sciences,
Wageningen University
[email protected]
III. Diurnal cycle of NOx : seasonal and vertical variations through the
snowpack, and emissions to the overlying atmosphere.
a.
a.
Late Summer
Early Spring
April 27, 2009-May 05, 2009
a.
NO2
NO
Aug 02, 2009-Aug 07, 2009
NO2
NO
b.
b.
Figure 2: a. Time series of daily NO2 and NO enhancements (left and
right, respectively) at the major activity levels for each specie, where
level 4 is at 110 cm, 5 at 80 cm, 6 at 50 cm and 7 at 20 cm from the
snowpack surface. b. Contour plot of daily NOx enhancements inside
and above the snowpack. The depth of the lowest inlet was used as
reference and expressed as 0 m. The surface of the snowpack is over
plotted with black dots. From August 21, 2009 the plot shows
additional data from 6 new inlets that were installed at that time.
Conclusions: Sections I & II of Results
c.
d.
 NO2 represents most of NOx in the snowpack at Summit,
with a maximum approximately 4-5 times higher compared to
the maximum period of NO production. NO2 profile is clearly
shifted to early spring, while NO follows the radiation profile
with a peak around June-July (see Figure 2a).
NO is produced at the upper levels of the snowpack where
NO2 photolysis is possible. At these levels, ventilation and NO
emission to the atmosphere can occur, explaining why
negative values of NO enhancements are observed at deep
levels during the peak period of NO production. Low values of
NO2 are observed in the upper layers, most likely to
ventilation and photolysis (see Figures 2a, 3a and 3b).
High wind speed events increase the transport and mixing
of gases through wind pumping mechanism. This is clearly
observed in NO2 profiles, mainly due to the high
concentrations of this specie in the snowpack (see Figure 3e).
No evidence of a strong relationship between NOx and
ozone. Minimum values of O3 are observed during maximum
of NO production, but the magnitude of this last specie does
not account for the decrease in ozone concentrations.
b.
Figure 4: a. Diurnal cycles for NO2 and NO enhancements at different depths, during two
periods: early spring and late summer. Data is 4 hr averaged and all times are expressed as local
solar time. b. Two-hourly averaged data for NOx at three different heights above the snowpack.
Values are expressed as NOx with daily median subtracted.
Conclusions: section III of Results
e.
Figure 3: a. Contour plot of daily NO2
enhancements. b. Contour plot of daily NO
enhancements. c. Contour plot for daily O3
mixing ratios. d. Snowpack temperature (°C,
daily averages). e. Major daily enhancements
events for NO2 compared with wind speed data,
averaged over 24 hr.
 Diurnal cycles for NO and NO2 are clear during both periods, with profiles varying
with availability of radiation and with depth.
NO enhancements observed right above the snow (see Figure 4a, first row) follows
perfectly the radiation profile in both periods. On the other hand, NO2 diurnal cycle at
the same level is slightly offset during early spring, but in late summer the peak is
observed at times of maximum isolation. This same behavior is also observed in the
first 50 cm of the snowpack (see Figure 4a, second row).
At deeper levels (see Figure 4a, third row), NO2 maximums are observed at night during
early spring, while in late summer all levels reach their maximum values earlier in the
day, though not coincident with maximum isolation hours. NO at 80 cm in the snowpack
peaks with radiation in both periods analyzed, with NO at 110 cm also peaking with
radiation in late summer.
Snowpack emissions can eventually be transported downwind at Summit.
Figure 4b shows NOx mixing ratios at three levels above the snowpack, and at all these
heights is possible to observe a diurnal cycle, indicative that emissions from the
snowpack are possible to observe as high as 9 m, especially in early spring, and thus
possibly transported downwind.