Biogeochemistry of Seasonally Snow-Covered Catchments (Proceedings of a Boulder Symposium,
July 1995). IAHS Publ. no. 228, 1995.
23
Variation in ambient air nitrogen concentration and
total annual atmospheric deposition at Niwot Ridge,
Colorado
DAWN RUSCH & HERMAN SIEVERING
Center for Environmental Sciences, CB 136, University of Colorado at Denver,
Colorado 80217, USA
Abstract Sampling of ambient air concentrations of particulate nitrate
(pN03~) and ammonium (pNH4+) has been ongoing at the subalpine site
of the University of Colorado's Mountain Research Station (MRS),
known as C-l (3018 m), for the summer seasons of 1993 and 1994.
Year-round sampling of pN03", pNH 4 + , and nitric acid (HN03) have
also been undertaken at the tundra site of the MRS, the Saddle site (3520
m), since January 1993. Additionally, analysis for particulate sulfate
(S042~) and trial sampling of gaseous ammonia (NH3) was begun at C-l
the summer of 1994. The comparison of the data from the C-l and
Saddle sites has shown variations in ambient concentrations of these
nitrogen species, leading to the conclusion that they are influenced
differently by wind and weather patterns. While the C-l site is often
subject to upslope contamination by the Front Range metropolitan areas,
the Saddle remains a relatively pristine environment. A further comparison of our data with those obtained at C-l over a decade ago show an
increase of ambient concentrations and pollution inputs over time.
INTRODUCTION
In unpolluted terrestrial environments, the limit to primary productivity is usually
dictated by the ability of the soil microbial system to fix N2 and mineralize organic N;
thus most terrestrial ecosystems are considered nitrogen limited (Friedland et al., 1991 ;
Bowman et al., 1993; Tietema, 1993). Anthropogenic increases in atmospheric
deposition can have profound and adverse effects on terrestrial and aquatic ecosystems
(Ollinger et al., 1993), and of particular concern are increases in the deposition of Ncontaining species. These species may act as fertilizer N, which is utilized by the plants
with little or no energy expenditure on their part. In the tundra, increased N inputs may
alter community structure and potentially have deleterious effects on the fragile plants
by increasing sensitivity to water stress, frost, and herbivory (Bowman etal., 1993). In
forests, high rates of N deposition can possibly alter foliar balances of N, Ca, or Mg,
which can lead to a loss of forest hardiness (McNulty etal., 1991 ). A reliable estimation
of the magnitude of N dry deposition to the alpine environment is thus integral to
understanding the nutrient cycling within these systems and the identification of subtle,
long-term anthropogenically derived changes to ecosystem health and productivity.
24
Dawn Rusch & Herman Sievering
METHODS AND MATERIALS
Niwot Ridge is an east-west oriented ridge in the Colorado Rocky Mountains, located
about 6 km east of the Continental Divide and 40 km northwest of Boulder, Colorado
(Fig. 1). It is the location of a considerable amount of research conducted through the
Long-Term Ecological Research (LTER) program of the National Science Foundation,
as well as being a UNESCO Biosphere Reserve site. Sampling was done at two Niwot
Ridge sites operated by the University of Colorado Mountain Research Station (MRS):
C-l, the subalpine site at 3018 m, and at Saddle, the tundra site at 3520 m.
The summer 1993-1994 sampling at C-l was done using a Sierra Instruments
Dichotomous Sampler. This is a virtual impactor system, which splits the suspended
particles into two size fractions, with an effective cut-point for the altitude at Niwot
Ridge of 2.2 /xm. Particulate N03" and NH4+ were sampled weekly using a 37-mm,
2-fxm pore size Gelman Sciences Teflon ring filter. Sample duration averaged 4 h in
length and samples were taken at mid-morning to obtain measurements that approximate
the daily mean of diurnal variation (Fahey et al., 1986). Occasional 3 to 4-h samples
were taken in the mid-to-late afternoon in an attempt to measure variation in ambient
concentrations during local upslope conditions and input from the urban areas to the
east. Extended 24-h samples were also taken weekly, with additional analysis for
particulate S042~.
Sampling at the Saddle site utilized an in-line pump system with a tower mounted
filter pack incorporating a 47-mm, 2-/xm Gelman Teflon prefilter to catch pNH4+ and
pN03", and a 47-mm, l-fxm Gelman nylon filter to capture HN03(g). Sampling took
place once weekly, usually mid-to-late morning, and have been restricted to 4-6 h due
to limited wind and solar power as well as access to the remote site. A similar in-line
system was designed and implemented at the C-l site the summer of 1994 for trial
sampling of ammonia. The same Teflon prefilter was used for catching pNH4+ and
pN03~, combined with two 47-mm acetic acid impregnated Whatman 41 paper filters to
capture NH3(g).
All filters were brought to the Kiowa Lab of the MRS and frozen for batch analysis.
The filters were diluted with deionized water and mixed ultrasonically for 45 min. The
pNH 4 + , NH3, and pN03" concentrations were analyzed using a Lachat flow-injection
colorimetric analyzer. A Dionex ion chromatograph was used to determine the 24 h
pS042" and pN03" concentrations. Field blanks were also frozen and analyzed, and a
uniform blank subtraction value applied to sample results in determining a final
measurement of nitrogen or sulfur concentration. The sample-to-blank ratios were
variable, ranging from 2.5:1 for fine pN03" (the smallest contributor to total ambient
concentrations of N) to 16:1 for fine pNH4+(the largest contributor). For all species
analyzed, the sample to blank ratios were acceptable, allowing for an assumption of
minimal filter contamination from exposure in handing and sufficient filter loading to
be above the averaged blank value, the detection limit, for the majority of cases.
RESULTS
Atmospheric chemical interactions
Preliminary plots of 1994 concentrations of the particulates at C-l are highly variable
Variation in ambient air nitrogen concentration at Niwot Ridge, Colorado
25
26
Dawn Rusch & Herman Sievering
with a generally increasing trend into late summer (late July to early August) (Figs 2 and
3). The division into coarse and fine size fractions is fairly uniform, with an average of
95 % of the pNH4+ and 91 % of the pS042~ in the fine fraction and 80% of the pN03" in
the coarse.
Calculations of the fine fraction sample molar ratios {pNH4+/(pN03~ + pS042")} all
fell between 1 and 2 (average 1.67) leading to the conclusion that the majority, if not all,
of these fine particulates are associated as ammonium nitrate, ammonium bisulfate, and
ammonium sulfate. Less than 10% of pNH4+ and pS042~ are found in the coarse
fraction, and their molar ratios support the assumption that they are also an associated
ammonium salt. The 80% of the pN03" remaining in the coarse mode is likely a product
of chemical reactions associated with airborne soil particles (Wolff, 1984).
Ambient atmospheric conditions
Hourly meteorological data are available for both the C-l and Saddle sites and can be
directly compared with the N-species concentrations. The prevailing wind pattern is
generally from west to east, delivering relatively clean air to the area (Parrish et al.,
1986). However, easterly upslope flows (Brazel & Brazel, 1983) may regularly bring
air containing aged anthropogenic pollutants from the Front Range urban areas (Fahey
et al., 1986; Parrish et al., 1990). At C-l, 43 % of the 1994 sampling occurred during
upslope conditions, compared to only 9% at Saddle since January 1993, and 11 % during
the 1994 summer. Ten samples were taken simultaneously at both sites, 6 of which had
easterly flow at C-l, only 2 at Saddle. Thus, even though the two sites are within a few
kilometers of one another, the differences in altitude and topography result in marked
variations in atmospheric patterns and regional pollutant input.
Fig. 2 Plot of individual sample total ammonium concentration, identifying coarse (c)
and fine (f) particle fractions.
Variation in ambient air nitrogen concentration at Niwot Ridge, Colorado
1000 |
27
1
[Êâ p[N03]f S p[N03]c ]
100
-
10
-
(0
CO
o
O
z
CO
E
1
I gfisar
A
M
J
J
A
Fig. 3 Plot of individual sample total nitrate concentration, identifying coarse (c) and
fine (f) particles.
A plot of C-l data without the upslope cases shows a decreased variation in Nspecies concentrations; the plot of the upslope cases is highly variable (Figs 4 and 5).
The standard deviation of the non-upslope concentrations is about 35% of the average,
compared to about 75% of the upslope average. While the mean concentrations of
pNH4+and pN03" at C-1 increase with upslope conditions, 40% and 71 % respectively,
the averages at the Saddle remain constant. This indicates that the pollutants from the
Front Range urban areas have a predominant load of NOx emissions that are being
chemically altered to nitrates but not scrubbed from the atmosphere in the time and
distance of transport to the subalpine forest. With the upslope influence removed, no
other meteorological variable examined (temperature, relative humidity, or wind speed)
has any significant influence on N-species concentration variation at C-l.
Comparison of 1994 C-l data to Saddle data
Comparison of growing season (May-August) 1994 C-l data with that from Saddle
shows that concentrations of ammonium and nitrate at C-l are approximately 30% and
40% higher without upslope influence, 58% and 95% higher for the entire season. A
plot of the average measurements of the last 20 mo of Saddle data shows similar
pNH4+/pN03~ ratios as seen at C-l, and a general increase in particulate concentrations
with increasing temperatures into the growing season, especially pronounced for pNH4+
(Fig. 6). The growing season average ambient HN0 3 concentrations at Saddle are
comparable in magnitude to the pNH4+ values, and show the same seasonal trend. It can
be assumed that similar dynamics are taking place at C-l, thus temperature and
seasonality are certainly important aspects to consider when speculating on year-round
N-species concentrations at the lower altitude. HN0 3 has not been measured at the C-l
site since 1984, but if the ratios HN03/N03" and/or HN0 3 /NH 4 + have remained
Dawn Rusch & Herman Sievering
28
O Upslope
• Non-upslope 1
o
800
600
o
°
400
o
• •o
200
•
n •
•
o
•
°
•
"
_•
_•
°
M
"r
i
i
i
V
i
i
Q
t
Q
i
i
Fig. 4 Plot of individual sample total ammonium concentration (coarse + fine modes),
identifying sample days when upslope conditions occurred.
O Upslope
2
250
-
200
-
• Non-upslope 1
o
to
CO
°
150
O
CO
o
£
C
100
50
o
- •
•
°
*
•
_
• • • • •
O
n
i
I
'
1
1
1
>
'
'
O
1
1
•
• m
'
f
•
o
_
r
•
O
|
Q
1
1
, ,f
o
i
° •
r
i
t
Fig. 5 Plot of individual sample total nitrogen concentration (coarse + fine modes),
identifying sample days when upslope conditions occurred.
relatively constant, present-day concentrations of nitric acid are likely to be equal to or
greater than what is being currently measured at Saddle.
Comparison to previous research
Concentrations of pN03" for the month of July from three different research efforts at
C-l (July was the only month in which all the data sets had complete measurements) and
July 1994 are shown in Fig. 7 (Fahey et al., 1986;Parrishef al., 1986; Marquez, 1994).
Variation in ambient air nitrogen concentration at Niwot Ridge, Colorado
29
250
HNH4+
GN03-
*HN03
200 --
A
150
100
4 \
I \
f"
\H
-
I
50
o
I
/ \
\ \l
/i
/
\
s
// ^ \ i /
,à
-0
\
\ /
Jan
9
,
I
I
1
1
I
,
D
9
r>-\ / Q
Je"
,
/
t
/\\
i
i
f
,
\'
pif'*''
9'
,
,
6^
1
Y
A
1
1
A
1
!
1
Mar
May
July
Sep
Nov
Jan
Mar
May
July
Fig. 6 Plot of monthly averaged concentrations of ammonium, nitrate, and nitric acid
from weekly samples gathered January 1993 through August 1994 at the Saddle site.
ŒD Best Guess Estimates/July 1
80
e
60
-
40
-
20
--
•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:
:::::
«•
:::::
Ï S Ï m i ï i S »
:-y.
<1981
1984
1993
1994
Fig. 7 Estimates of total nitrate concentrations for the month of July from four different
research efforts. The pre-1981 data are from Parrish et al. (1986); the 1984 data from
Fahey et al. (1986). The 1993 data from Marquez (1994) incorporated the same
dichotomous sampling technique as used in 1994.
These data show an approximate doubling of ambient nitrate concentrations at the C-l
site in the last decade. Since various sampling techniques were used, and 1 mo cannot
be considered representative of the growing season, there is a large uncertainty
Dawn Rusch & Herman Sievering
30
associated with the estimate of doubling in the last 10 years. It is curious to note that the
averaged preliminary values gathered in the trial NH3 sampling at C-l were 2-5 X higher
than previously measured (Langford&Fehsenfeld, 1992; Langford et al., 1992), though
these data may have been influenced by the limited sampling season and unpredictable
local factors such as forest fires.
Estimation of atmosphere to surface flux
The dry deposition flux is determined by calculating the product of the particular species
concentration and a deposition velocity (Vd), which is highly dependent upon particle
size and wind speed (Sievering et al., 1992). The pNH4+ and pN03" concentrations at
C-l for summer 1994 were separated by size fraction (cut point ~ 2.2 /xm); the
estimated mass median diameter (MMD) for the coarse particles = 3-4 fim, and the fine
particles » 0.3-0.5 ,txm. A model incorporating the unique properties of the laminar
needle surfaces of a spruce forest and the effect of turbulent wind conditions (Peters &
Eiden, 1992) was used to estimate Vd for the estimated MMD's and sample time average
wind speeds. The resultant mean flux values are comparable to flux values estimated
from previous data gathered at the C-l site (Sievering et al., 1992) (Fig. 8).
Present estimates of Saddle dry flux for pN03" (Sievering, unpublished data) average
somewhat lower than estimated for C-l; those of pNH4+ are within the same range,
despite the lower average concentrations. This may be partially due to the difference in
mean wind speeds and vegetation type as applied to the Vd value, and/or the inability to
identify the coarse and fine particle fractions by the filter pack sampling method used
at the Saddle. Estimates of HN0 3 contribution to dry flux at Saddle average 25 X greater
than that of pN03", and constitute 90% of the total dry N deposition. This updated
0.08
E3 1994 C-1
11994 Saddle
•1992Niwot
0.06
0.04
N03
Fig. 8 Estimates of summer 1994 dry depositional flux of ammonium and nitrate at C-l
and Saddle compared to estimates by Sievering (1992) which incorporated various data
gained at Niwot Ridge over a period of 10 years.
Variation in ambient air nitrogen concentration at Niwot Ridge, Colorado
31
estimate of total dry N deposition is roughly equal in magnitude to the total wet
deposition at the Saddle during the growing season (Sievering, unpublished data). It can
only be speculated that the same is true for C-l.
Interestingly, while coarse pN03" contributes only 14-16% to the total average
particulate N concentration in ambient air at C-l, it makes up 56-60% of the total
average dry particulate flux. Since pN03" is also the species most increased during
upslope conditions, it follows that local anthropogenic pollutants in the atmosphere can
quickly and directly influence the actual N loading to the subalpine ecosystem. While
the average concentrations of all the N-species decrease naturally during the cold winter
months, the anthropogenic supply of nitrate-producing NO^ aerosols would be
maintained, becoming an even more important contributor to the total nitrogen load to
the forest during winter upslope events. Additionally, the pN03" averages at the Saddle
site had unexpected peaks during the early winter months, which could also affect the
winter dry flux deposited directly to the snowpack and stored until the spring melt, and
thus its contribution to the annual ionic pulse could be important for both environments.
CONCLUSION
This analysis of N-particulates at C-l does not attempt to draw specific conclusions
concerning the increase in total N dry deposition since NH3, HN0 3 , and other N-species
are not represented, and it has been limited to the growing season, only a third of the
annual cycle. It does suggest that the atmospheric contribution to ecosystem N-cycling
is increasing relative to soil and biotic processes and thus becoming more significant.
This analysis has also shown that atmospheric factors, species concentrations, and
nitrogen loading do differ between the two research sites at Niwot Ridge, and the C-l
site has the potential to be much more severely impacted by local anthropogenic
pollution inputs. However, both the tundra and subalpine ecosystems have the potential
to be severely altered by this increased nitrogen loading, in that they may have evolved
a greater nitrogen use efficiency and dependence upon atmospheric inputs than more
temperate systems (Bowman et al., 1993). Thus, even minor depositional increases
could be a large deviation from the natural cycle. Further, any alteration of the evolved
natural balance to the harsh and nutrient-limited conditions could have profound impacts
on biota. Long-term research is necessary to further quantify the magnitude of the
atmospheric nutrient input and its effects on these ecosystems.
Acknowledgments I would like to thank Chris Seibold and Tim Bardsley at the
University of Colorado, Mountain Research Station for their invaluable assistance in
data collection and analysis, and Lori Marquez for her instruction on the sampling
technique and preliminary interpretation. I also appreciate the helpful discussions with
Larry Anderson of the University of Colorado at Denver and the field support of Duane
Kitz of the National Oceanic and Atmospheric Administration in Boulder. This research
was supported by the Long-Term Ecological Research (LTER) program of the National
Science Foundation.
32
Dawn Rusch & Herman Sievering
REFERENCES
Bowman, W., Theodose.T., Schardt, J. &Conant, R. (1993) Constraints of nutrient availability on primary production in
two alpine tundra communities. Ecology 74, 2085-2097.
Brazel, A. & Brazel, S. (1983) Summer diurnal wind patterns at 3000 m surface level, Front Range, Colorado, USA. Phys.
Geogr. 4,53-61.
Fahey, D., Hubler, G., Parrish, D., Williams, E., Norton, R., Ridley, B., Singh, H., Liu, S. & Fehsenfeld, F. (1986)
Reactive nitrogen species in the troposphere: measurements of NO, N0 2 , HN03, particulate nitrate, peroxyacetyl
nitrate (PAN), 0 3 , and total reactive odd nitrogen (NOy) at Niwot Ridge, Colorado. Geophys. Res. 91, 9781-9793.
Friedland, A., Miller, E., Battles, J. & Thome, J. (1992) Nitrogen deposition, distributionand cycling in a subalpine sprucefir forest in the Adirondacks, New York, USA. Biogeochemistry 14, 31-55.
Langford, A., Fehsenfeld, F., Zachariassen, J. & Schimel, D. (1992) Gaseous ammonia fluxes and background
concentrations in terrestrial ecosystems of the United States. Glob. Biogeochem. Cyc. 6, 459-483.
Langford, A. & Fehsenfeld, F. (1992) Natural vegetation as a source or sink for atmospheric ammonia, a case study. Science
255, 581-583.
Marquez, L. (1994) Atmospheric loading of nitrogen to alpine tundra at Niwot Ridge, Colorado. Masters Project, Center
for Environmental Sciences, University of Colorado at Denver.
McNulty, S., Aber, J. & Boone, R. (1991) Spatial changes in forest floor and foliar chemistry of spruce-fir forests across
New England. Biogeochemistry 14, 13-29.
Ollinger, S., Aber, J., Lovett, G., Millham, S., Lathrop, R. & Ellis, J. (1993) A spatial model of atmospheric deposition
for the northeastern U.S. Ecol. Applic. 3, 459-472.
Parrish, D.,Hahn,C, Fahey, D., Williams, E., Bollinger, M., Hubler, G., Buhr, M., Murphy, P., Trainer, M., Hsie, E.,
Liu, S. & Fehsenfeld, F. (1990) Systemic variation in the concentration of NO^ (NO plus N02) at Niwot Ridge,
Colorado./. Geophys. Res. 95, 1817-1836.
Parrish, D., Norton,R., Bollinger, M., Liu, S., Murphy, P., Albritton.D. & Fehsenfeld,F. (1986)Measurementsof'HN03
and N03" particulates at a rural site in the Colorado mountains. J. Geophys. Res. 91, 5379-5393.
Peters, K. &Eiden, R. (1992) Modeling the dry deposition velocity of aerosol particles to a spruce forest. Atmos. Environ.
26(A), 2555-2564.
Sievering, H., Burton, D. & Caine, N. (1992) Atmospheric loading of nitrogen to alpine tundra in the Colorado Front
Range. Glob. Biogeochem. Cyc. 6, 339-346.
Tietema, A. (1993) Mass loss and nitrogen dynamics in decomposing acid forest litter in the Netherlands at increased
nitrogen deposition. Biogeochemistry 20, 45-62.
Wolff, G. (1984) On the nature of nitrate in coarse continental aerosols. Atmos. Environ. 18, 977-981.