We report observations of stratospheric chlorine monoxide over the altitude range approx. 16 to 50 km at Thule, Greenland from Feb. 8 to Mar. 24, 1992. A new, more sensitive ground-based mm-wave spectrometer was employed for these measurements, similar in principle to that used earlier for the discovery of low altitude ClO in the Antarctic springtime. In this report, we discuss different aspects of vertical distribution, secular trends, and diurnal variation of ClO in the Arctic stratosphere, based on a preliminary analysis of our Thule data. We see no evidence for large (approx. 1.2-1.5 ppb) amounts of ClO in the lower stratosphere at any time during February or March, in agreement with UARS-MLS findings for this period, and in marked contrast to findings reported for the Arctic in January. We have some evidence for small enhancements (approx. 0.2-0.5 ppb) in the 18-30 km range in late February-early March, which might be associated with volcanic aerosol, rather than PSC, processing

Dezafra, Robert L.

Emmons, Louisa K.

Reeves, John M.

Shindell, Drew T.

English

NASA Technical Reports Server

N95- 11031
MEASUREMENTS OF THE VERTICAL PROFILE, DIURNAL
VARIATION, AND SECULAR CHANGE OF CIO IN THE
STRATOSPHERE OVER THULE, GREENLAND, FEBRUARY-
MARCH, 1992
I_ L. de Zafra, L.I_ Emmons, J.M. Reeves, and D. T. Shindell
Physics Dept., State Univ. of N.Y.
Stony Brook, N.Y., 11794
Abstract. We report observations of stratospheric
chlorine monoxide over the altitude range -16 to 50 km
at Thule, Greenland from Feb. 8 to Mar. 24, 1992. A new,
more sensitive ground-based mm-wave spectrometer was
employed for t_ese measurements I11,similar in principle
to that used earlier for the discovery of low altitude CIO in
the Antarctic springtime 12)l. In this report, we discuss
different aspects of vertical distribution, secular trends,
and diurnal variation of CIO in the Arctic stratosphere,
based on a preliminary analysis of our Thule data. we see
no evidence for large (-1.2-1.5 ppb) amounts of CIO in
the lower stratosphere at any time during February or
March, in agreement with UARS-MLS findings for this
period, and in marked contrast to findings reported for the
Arctic in January. We have some evidence for small
enhancements (-0.2-0.5 ppb) in the 18-30 km ran_ge in
late February-early March, which might be associated with
volcanic aerosol, rather than PSC, processing.
Introduction: Low altitude CIO, occurring in the
15-25 km range, is the particular signature as well as the
causal agent of the massive ozone destruction which has
been characteristic of the Antarctic springtime in recent
ears. A limited number of previous C10 measurements
ave been made in the early Arctic spring in an attempt to
understand similarities and differences l'n northern polar
chemical p.rocessin_, but until 1992, none at a time
corresponding to me equivalent photochemical period
when major ozone destruction occurs over Antarctica.
Free CIO is not normally found in the 15-25 km
range of the stratosphere, due to rapid reaction with NO_
to form chlorine nitrate (CIONO_). Short wavelength
absorption by ozone above 25 km hmits the photolysis of
chlorine nitrate to very small values at 15-25 kin, severely
limiting CIO formation in this re_gion. Furthermore, the
very low, concentration of atomic oxygen renders the
'standard catalytic chlorine cycle ineffective in depleting
ozone, even though small amounts of CIO may be formed
by "dead-end" photochemical processes. Only m situations
where NO 2 has been effectively removed from the lower
stratosphere by heterogeneous chemical processing is C10
formed in large enough concentrations to promote rapid
CIO dimer formation (which depends on the square of
CIO concentration). The alternate catalytic cycle proposed
by Molina and Molina [41then becomes effective in ozone
destruction without the need of atomic O. These
conditions now regularly occur in the Antarctic springtime
stratosphere.
Above -30 km, CIONO 2 photolyses rapidly after
daybreak and destruction of ozone occurs during dayl!ght
hours through the standard catalytic chlorine reaction.
54(1
This yields a diurnal cycle of CIO formation and ozone
destruction centered around 40 km altitude at non-polar
latitudes 151.
Our 1992 March observations at Thule covered a
period of similar solar exposure at very nearly the same
polar latitude (76.3 N) as our various measurements
during September at McMurdo Station (77.5 S) 12'3'61. NO.
column measurements showed very low values within th_
Arctic region during the winter of 1991-92 I7], yet at no
time during the entire February-March observing interval
did we see evidence for low altitude formation of CIO
approaching the -1.5 ppb of CIO we have consistently
found overMcMurdo at -20 km. This is in sharp contrast
to a -20 km altitude layer of CIO observed over much of
northern Europe in the first half of January by the MLS
ram-wave spectrometer onboard UARS, and by more
limited ER-2 aircraft sampling over eastern Canada and
the northeastern U.S. 181
Thule lies at 76.3 N, 68.4 W, on the northwest coast
of Greenland. Defining the polar vortex region in terms of
temperature contours in the vicinity of 20 km (30 mb),
Thule was generally well outside the vortex core, which
remained over northern Scandinavia and the Barents Sea
during most of February and March 191. Northwest
Greenland did remain in the rim of circulation around this
core, as defined by potential vorticity contours, through
the first week in March, but after March 8-10, both
potential vorticity contours and maps of wind speed and
direction at -20 and 25 km show a pronounced shift to
circulation from outer portions of the Aleutian high.
During February and March, the stratosphere near Thule
was never much colder than 215 K between 12 and 25 km.
For most of the period covered here, in fact, temperatures
throughout the Arctic region were typically_ > 205 K, well
above that associated with either Type I or II polar
stratospheric cloud (PSC) formation. The comprehensive
meteorological coverage of the European Arctic
Stratospheric Ozone Experiment (EASOE) campaign
reported no significant PSC activity anywhere in the Arctic
after late January.
On the other hand, the massive aerosol cloud
injected into the lower stratosphere by the eruption of Mt.
Pinatubo in the early summer of 1991 exerted a strong
perturbing influence on Arctic stratospheric chemistry,
photolysis, and transport during the winter of 1991-92, as
determined by extensive measurements from EASOE and
NASA's second Arctic aircraft campaign. The Arctic
winter-spring stratosphere of 1992 was thus far from
normal, and presents many puzzles and challenges for
correct interpretation.
In view of this, we have begun a careful analysis of
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DAY NUMBER
Fig.l. Integrated C10 signal intensity vs time during
February and March, 1992. Feb 9=Day 40, Mar 24=Day
84. Mid-day data have been grouped over 3 to 5 day
intervals for most of the _oints shown. See Fig.2 for
details. Results for three different integration limits are
_.iven: +230, 180, and 130 MHz each side of the emission
line ce_er. Integrated intensity values plotted here for the
widest limits will give approximate column density in
molecules per cm 2 above 17 km when multiplied by
3.4x1013. Estimated uncertainties are +15 to 20 %.
a nearly two month record of CIO observations. This
record contains information on diurnal and secular
changes in the column density and vertical profile of C10
in the Thule region.
Secular changes in column density: We start with
the variation of integrated column density. Our present
ram-wave spectrometer has a bandwidth of 512 MHz,
sufficient to allow analysis of pressure-broadened line
shapes down to an altitude of -16 km. In Fig.l, the mid-
day integrated line intensity for the 278 GHz CIO
emission line is shown as a function of day number. (Day
61 = March 1.) Typically, three adjacent days are grouped
together to improve signal/noise ratio and to help average
out residual baseline artifacts. During some periods, poor
weather prevented daily observations, and a longer time
span is covered: e.g., days 61-66 for early March. Within
tllese intervals, whenever conditions were favorable,
observations were made continuously in integration
intervals of 20 minutes each.
There is a steady increase by more than 4-fold in
the total column of CIO from our initial average over Feb
8-10 to that over Mar 1-6, after which the total column
amount drops again. Exposure to sunlight at 16 km varied
from about 4.3 hrs on Feb 9 (day 40) to 14.5 hrs on Mar 20
(day 80), and the respective values for 40 km altitude are
about 6.5 hrs and 17 hrs. The noon solar zenith angle at 16
km also decreased from 92 to 77.5 degrees during this
period. The increase in total column amount is thus
primarily driven by the daily increase in solar exposure
time. In contrast, we believe that the drop-off in the total
column of C10 after early March is directly related to a
"re-noxification" of the lower and middle stratosphere
occurring about this time. We discuss this further below.
Another significant feature is present in Fig.1.
Integration has been carried out over three different
windows spanning intervals of +130, 180, and 230 MHz
around the line center (with the same zero baseline for
each). It is clear that integration over the two wider
intervals gives essentially identical values for each data
point, while integrated intensities for the narrowest
window fall consistently below these between -days 56
and 70 (Feb 25 to Mar 10), and are in good agreement
otherwise. This indicates that the +130 Ml-Iz limits are
too narrow to encompass the whole lineshape during the
Feb 25 - Mar 10 interval, i.e., there is evidence for atleast
a small enhancement of C10 in the mid-to-lower
stratosphere during this interval. We look fttrther at this in
the next section.
Evolution of vertical profiles: The mid-day spectral
lineshapes that were inte_ated to obtain the data for
Fig.1 are shown in Fig.2. In Fig.3, the mid-day average
over Feb 27-Mar 6 (the interval showing the greatest
evidence of low-altitude C10 enhancement), as well as the
spectrum for Mar 17-24, are contrasted vath the mid-day
lineshape recorded at McMurdo Station, Antarctica, as an
average over Sept 20-24, 1987. The contrast in pressure-
broadened (hence low altitude) line wing intensity is very
pronounced. Note that the Mar 17-24, 1992 data and the
Sep 20-24, 1987 data each fall at equal times after the
respective winter solstice, and that each location is about
equidistant from the respective pole, so that diurnal
insolation was essentially the same for each. What is of
course different is the vortex size, duration, temperature
minima, and location relative to respective poles.
The low-to-vanishing intenstty of line wings in our
1992 Thule data pose a considerable test for recovery of
vertical profiles in the 16-25 km altitude range.
541
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31724
31416
31013
30709
30106
22729
22426
_21922
_20810
-0.25 , , , ' ' . . ,
0 250 500
Fig 2. Evolution of mid-day emission line spectra for the
278 GHz rotational transition of CIO, during Feb-Mar,
1992 over Thule, Greenland. Integrations produced the
data points in Fig.1. Identification numbers are in the
format M-DD-DD so that 30106 is a sum of available data
over the period Mar 1-6, etc. Days with usable data =Feb
8,!0,19,21,22,24-29, Mar 1,6-10,12-17,20, and 24. The
ordinate gives radiation intensity in equivalent black-body
temperature units. The abscissa is in spectrometer channel
number, at 1 MHz per channel.
Acknowledgements:This research was carried out under
NASA grant No. NAGW-2182 and NSF grants No.
ATM8419733 and DPP8922688.
References:
" B.N. Ellison and R.E. Miller, Int. J. Infrared and
eter Waves 8, 609, 1987
[2]. RMC-Cryosysterns Model LST 4.5-025; see also R.L.
de Zafra, W.H. Mallison, L.K. Emmons, and D. Koller,
Rev. Sc£ lnstrum. 62, 1309-1310, 1991.
]. A. Parrish, R.L. de Zafra, P.M. Solomon, and J.W.
arrett, Radio Scienc_ 23, 106-118, 1988.
" Berkshire Technologies, Inc., Oakland, CA.94609,
el L1.4-30H
. see P. Goldsmith, _p.306-309, in Infrared and
llimeter Waves, Vot_ IC Button, ed., Acad. Press, N.Y.,
1982.
,_. see, e.g., R.L. de Zafra, M. Jaramillo, A. Parrish, P.
lomon, and B. Connor, Nature, 328, 408-411, 1987; P.M.
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R.L. de Zafra, J.W. Barrrett, A. Parrish, and P.M.
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11428, 1989.
542


Measurements of the vertical profile, diurnal variation, and secular change of ClO in the stratosphere over Thule, Greenland, February-March, 1992

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