Using a ground-based mm-wave spectrometer, we have observed stratospheric N2O over Thule, Greenland (76.3 N, 68.4 W) during late February and March, 1992. Vertical profiles of mixing ratio ranging from 16 to 50 km were recovered from molecular emission spectra. The profiles of early March show an abrupt increase in the lower-stratosphere N2O mixing ratio similar to the spring-to-summer change associated with the break up of the Antarctic polar vortex. This increase is correlated with changes in potential vorticity, air temperature, and ozone mixing ratio

Dezafra, Robert L.

Emmons, Louisa K.

Reeves, John M.

Shindell, Drew T.

English

NASA Technical Reports Server

N95- 11032
OBSERVED CHANGES IN THE VERTICAL PROFILE OF STRATOSPHERIC
NITROUS OXIDE AT THULE, GREENLAND, FEBRUARY-MARCH, 1992
Louisa K. Emmons, John M. Reeves, Drew T. Shindel£ and Robert L. de Zafra
Physics Department, State University of New York
Stony Brook, NY 11794 USA
Abstract
Using a ground-based mm-wave
spectrometer, we have observed
stratospheric NgO over Thule, Greenland
(76.3 N, 68.4 W)during late February and
March, 1992. Vertical profiles of mixing
ratio ranging from 16 to 50 km were
recovered from molecular emission
spectra. The profiles of early March show
an abrupt increase in the lower-
stratosphere N20 mixing ratio similar to
the spring-to-summer change associated
with the break up of the Antarctic polar
vortex. This increase is correlated with
changes in potential vorticity, air
temperature, andozone mixing ratio.
Introduction
Nitrous oxide is an important tracer of transport.
and in some cases an indicator of chemical processing, I
in the stratosphere. Observations of N20 distribution in
the polar regions could further the understanding of
polar atmospheric dynamics, yet it has not been
aggressively studied. While aircraft and satellite
measurements are vastly improving the body of polar
N20 data, complementary ground-based measurements
can provide continuous, long-term monitoring of the
vertical profile at virtually any latitude. As part of the
/UlPePer Atmosphere Research Satellite Correlative
asurements campaign and the European Arctic
Stratospheric Ozone Experiment, we have measured the
vertical profile of nitrous oxide at Thule, Greenland
during February and March, 1992.
Briefly, the experimental method was the
following: The emission intensity of the molecular
rotation line at 276.328 GHz (1.08 mm) in N20 was
measured with a mm-wave heterodyne
receiver/spectrometer whose mixing element is a
superconducting (SIS) niobium tunnel junction. Details
of the apparatus and data acquisition are given
elsewhere.ZPressure-broadened emission spectra were
measured in a bandwidth of 512 MHz at 1 MHz
resolution, with emission intensity calibrated against
black-body sources of known temperature. The pressure-
broadened emission spectrum is not a simple Lorentzian
543
lineshape; rather, it is the sum of emission spectra from
altitudes of differing ambient pressure, and pressure
depends upon altitude. In the case of N_O, the half-
width at half maximum of molecules at 20-km is -140
MHz, but fails to -15 MHz by 35 km. The measured
composite lineshape thus contains information on the
verttcal distribution of radiating molecules which is
recoverable through numerical deconvolution. We
extracted N_O mixing ratios as functions of altitude_ with
a modified -Chahine-Twomey inversion algorithm, _ and
used the N.,O oressure-broadening coefficient 2.38
MHz/mb rep_rteffby Lacome et al. 4
Observations and Discussion
The results of eleven separate NgO observations
spanning February 22 through March 2"5 are shown in
Figure 1. Each curve, labeled at right by month and day
in the format todd, is the result of two hours of
integration. Spectra are displayed as zenith-looking
intensit),, without atmospheric attenuation, expressed as
an equivalent black-body radiation temperature. Each
trace is offset 200 mK from the one below. The
important feature is a marked increase in intensity,
particularly near line center, following March 6 and
persisting through March 25. The more pronounced
peaking of these spectra indicate that the N20
concentration increased in the middle and upper ranges
of its distribution rather than everywhere uniformly in the
stratosphere. This change is demonstrated in the vertical
profiles of Figure 2. Lower and upper solid curves are
typical mixing ratios recovered for the periods before and
after March 7, respectively. Alsoplotted for comparison
are a 1984 summer balloonsonde measurement from
Antarctica 5 (upper dashed line), and a vertical profile
from McMurdo Station, Antarctica, obtained by mm-
wave spectroscopy in the spring of 1986° (lower dashed
line). The data before March 7 yield a vertical profile
much like that from the Antarctic spring, now established
as indicatin_ downward transport m a polar vortex6
region, wl_ile the late-March profile from Thule is
similar to that of the Antarctic summer.
Our spectrometer bandwidth places a practical
low-altitude limit on the recovery of vertical profiles, as
highly pressure-broadened emission from lower in the
atmosphere appears flatter and broader with decreasing
altituoe in the instrument s spectral window. Thus we
have assumed a constant N20 mixing ratio -300 ppbv
https://ntrs.nasa.gov/search.jsp?R=19950004619 2020-06-16T11:31:53+00:00Z
x.-
*._.,
>, 2
r-
iD
-4-
¢.-
t-
O 1
O')
Oq
E
LO
0
N20 Emission Spectra
f i
,0
0 200 400
Channel No. [MHz)
3_.3
310
3OO
224
6OO
FIG. 1. N20 emission spectra at 276 GHz, recorded on
11 separate days in February and March. Each trace,
labeled at right by month and day (mdd), was averaged
for two hours beginning about 23:00 local time. The
vertical scale is relative emission intensity from the zenith
direction expressed as an equivalent black-body radiation
temperature, and 1 channel = 1 MHz on the horizontal
axis. Curves are offset vertically by 200 mK.
60
40
03
E_ 20
N20 Thule, Greenland Feb.-Mar. 1992
.... , .... i .... i .... i .... i .... , ....
0 50 100 150 200 250 300 350
Mixing Ratio (ppbv)
FIG. 2. N_O vertical mixing ratio profiles. The lower
(upper) solid curve is a profile recovered from emission
intensity data before (after) March 7; shaded regions
indicate the uncertainties in the distributions. The upper
dashed curve is a balloonsonde measurement from the
Antarctic summer, and the lower dashed curve is our own
ground-based measurement from the Antarctic spring.
Potentiol Vorticity
over Thule, Creenlond
below 10 kin, where the atmosghere is thoroughly mixed,
for the deconvolutions shown. _ The accuracy of these
profiles is limited by both noise and residual baseline
deviations in the data; this uncertainty is indicated by
shaded areas about each profile.
The increase in mixing ratio shown in Figure 2
occurs during a period around March 8 when
stratospheric circulauon over Thule switched from the
riphery of the polar vortex (typically centered over
ndanavia and the Barents Sea) to flow across Canada
from the Aleutian high. A transition can be clearly seen
in plots of potential vorticity. In Figure 3, we show
potential vorticity at three altitudes (potential
temperature levels)for the days March 4 through March
11. The steepest potential vorticity gradient, marking the
circulation edge of the polar vortex, is seen to have
passed over Thule during March 7 to March 9. The
increase in N20 was also accompanied by changes in
temperature and ozone distribution. "Danish
Meteorological Institute data for the same period show
an increase from about 5 ppmv to 6.5 ppmv m the ozone
profile from 20-35 km, and a 10 to 20 K increase in
temperature from 10-30 km. While the ozone change
was most pronounced between March 4 and March 11,
the overalltrend is one of fluctuation, so the increase is
noted simply as further evidence of stratospheric change.
Another indication of stratospheric change in early
March is discussed in a companion paper on observed
chlorine monoxide concentraffon over'Thule. 8
2OO
I
CT_
Z_
E 1 50
I
o
-_ lO0
.__
O
_ 50
c
Z
C2
o
6_, 72
I I l I I r I i
I i I
64 65 616 67 618 619 7J0 7Jl
Doy
FIG. 3. Potential vorticity at three potential temperature
levels over Thule, Greenland in early March. 700K
approximately corresponds to 25 kin, 550K to 22 km, and
475K to 19 km. The horizontal axis begins with March 3
= day 63. Data were provided by the Danish
Meteorological Institute (DMI) through the Norwegian
Institute for Air Research (NlLU).
544
Conclusions
In addition to its use as a tracer of stratospheric
transport and chemicalprocessin$, we believe N20
concentration can be useful in defining the tropopause in
the polar regions. There, the traditional tropopause
marker of minimum temperature, or the sometimes-
favored altitude at which ozone concentration reaches a
minimum, may fail. Temperature profiles may not show
a clear boundary, and Ox may be too depleted at the
altitude of ir_terest. N20 _s uniformly distributed in the
troposphere,' but its mi3dng ratio drops with height above
roughly 10 kin due to photolysis and, at higher altitudes,
reaction with 1D atomic oxygen. This destruction occurs
slowly in the polar stratosphere, however, so the N20
profile is determined mainly by air movement and hence
Is a.good indicator of transport. The altitude at which the
mlxmg ratio drops from its tropospheric value may
therefore be the most readily-defined beginning of the
polar stratosphere. Measurement of nitrous oxide
concentration could provide a consistent definition of the
tropopause which is largely independent of abnormal
polar chemistry.
Acknowledgements
We would like to thank the Danish
Meteorological Institute, particularly Bjorn Knudsen, and
the Norwegian Institute for Air Research for the
correlative 0 3 and meteorological data used in this
paper. We are grateful to the U.S. Air Force for
permission to use the facilities at Thule Air Base and
their cooperation during our stay. This work is supported
by Grant NAGW-2182 through NASA's Upper
Atmosphere Research Program and Grant NAG-l-1354
from the Upper Atmosphere Research Satellite
Correlative Measurements Program.
1.
.
3.
4.
.
.
7.
8.
References
S.E. Strahan, M. Loewenstein, J.R. Podolske, W.
Starr, M.H. Proffitt, K.K. Kelly, and K.R. Chart,
Correlation of N_O and Ozone in the Southern
Polar Vortex IYuring the Airborne Antarctic
Ozone Experiment, Y. Geophys. Res. 94, 413-423,
1980.
R.L. de Zafra, W.H. Mallison, M. Jaramillo, J.M.
Reeves, LK. Emmons, and D.T. Shindell, A New
High-Sensitivity Superconducting Receiver for
ram-Wave Remote-Sensing Spectroscopy of the
Stratosphere, these proceedings.
S. Twomey, B. Herman, and R. Rabinoff, An
Extension of the Chahine Method of Inverting the
Radiative Transfer Equation, J. Atmos. Sc_ 34,
1085-1090, 1977.
N. Lacome, A. Levy, and G. Guelachvili, Fourier
Transform Measurement of Self-, N2-, and 0 2-
Broadening of N_O Lines: lemperature
Dependence of Lin_widths, AppL Optica 23, 425,
1984.
P.D. Goldan, W.C. Kuster, D.L. Albritton, and
A.L. Schmeltekopf, Stratospheric CFCI_, CF_CI_,
and N_O Height Profile Measurements _t Se_eraq
Latitudes, J. Geophys. Res. 85, 413-423, 1980.
A. Parfish, R.L. de Zafra, M. Jaramillo, B.
Connor, P.M. Solomon, and J.W. Barrett,
Extremely Low N_O Concentrations in the
Springtime Stratosphere at McMurdo Station,
Antarctica, Nature 332, 53-55, 1988.
R.F. Weiss, The Temporal and Spatial
Distribution of Tropospheric Nitrous Oxide, J.
Geophys. Res. 86, 7185-7195, 1981.
R.L. de Zafra, L.]C Emmons, J.M. Reeves, and
D.T. Shindell, Measurements of the Vertical
Profile, Diurnal Variation, and Secular Change of
CIO in the Stratosphere over Thule, Greenland,
February-March 1992, these proceedings.
545


Observed changes in the vertical profile of stratopheric nitrous oxide at Thule, Greenland, February - March 1992

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server