Elevated ozone in the boundary layer at South Pole - Doug Davis

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ARTICLE IN PRESS
D. Helmig et al. / Atmospheric Environment 42 (2008) 2788–2803
shown in Fig. 2. During the period of this experiment,
surface ozone at SP showed large variations,
between minima of 18 ppbv (DOY 354) and maxima
of 50 ppbv on DOY 358. Both measurements, even
though 130 m separated by distance and 15 m by
height show excellent agreement, typically within
1 ppbv during the first phase. A striking feature of
these observations is that during the later part of
DOY 354, a significant increase in surface ozone
(almost doubling) was observed and that thereafter
both measurements showed a 3–4 ppbv disagreement
until ozone levels dropped back to below 30 ppbv on
DOY 359. Upon closer inspection, it becomes
apparent that generally high agreement between
these two data series is seen at lower ozone levels
and that the disagreement scales with the absolute
ozone levels. The vertical balloon profile data, to be
discussed in the following paragraphs, show that the
differences in these two measurements do not stem
from an analytical bias, but instead represent vertical
ozone gradients in the shallow SP surface layer.
3.2. Vertical ozone profiles
The vertical ozone distribution at SP showed
strong variations during December 2003. Two
examples that illustrate the extremes of these
conditions are presented in Fig. 3. On December
24 a strong variability in ozone was seen in the
lowest 500 m of the atmosphere. Near the surface,
ozone levels were approaching 50 ppbv. Ozone
mixing ratios declined steeply with altitude, dropping
to 22 ppbv at 180 m. Several layers with
2–4 ppbv enhanced ozone were seen between 200
and 500 m height. Data from the balloon ascent and
descent show a high degree of agreement, indicative
that ozone profiles changed very little during the 58-
min flight duration. It should be noted that due to
the 25–30 s response time of the ECC sonde, the
ozone readings are somewhat delayed causing a
slight upwards/downwards shift of the ozone profile
during ascent and descent, respectively (by 10 m at
the 0.3 m s 1 ascent/descent rate). Correcting for
this effect would further improve the agreement
between the ascent and descent ozone profiles.
Ozone mixing ratios measured near the surface
generally agreed within 1–2 ppbv with the concurrent
ARO and tower observations (Fig. 2) (Johnson
et al., 2007). Much different conditions were
encountered two days later, as shown in the pair
of profiles on the right in Fig. 3. Ozone was
homogenously distributed in the surface and
boundary layer, showing less than a 2 ppbv gradient
between the surface and 500 m. Again, both ascent
and descent data follow each other closely and
ozone data from the balloon instruments near the
500
450
Ascent
Descent
500
450
Ascent
Descent
400
400
350
350
Height (m)
300
250
200
Height (m)
300
250
200
150
150
100
100
50
50
0
15 20 25 30 35 40 45 50
Ozone (ppbv)
0
15 20 25 30 35 40 45 50
Ozone (ppbv)
Fig. 3. Two examples of vertical ozone distribution at South Pole during December 2003. The profiles on the left were measured on
December 24 (launch time DOY 358.82, flight duration 57 min). The profiles on the right were obtained on December 26 (launch time
DOY 360.89, flight duration 47 min).