Elevated ozone in the boundary layer at South Pole - Doug Davis
Elevated ozone in the boundary layer at South Pole - Doug Davis
Elevated ozone in the boundary layer at South Pole - Doug Davis
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ARTICLE IN PRESS<br />
D. Helmig et al. / Atmospheric Environment 42 (2008) 2788–2803 2801<br />
Solomon, 2002). Lower surface w<strong>in</strong>ds and temper<strong>at</strong>ures<br />
were observed <strong>at</strong> SP, follow<strong>in</strong>g a long-term<br />
trend towards <strong>in</strong>creased <strong>in</strong>version strength <strong>in</strong> <strong>the</strong><br />
1990s (Neff, 1999), a period when <strong>the</strong> AAO was <strong>in</strong><br />
its positive <strong>in</strong>dex st<strong>at</strong>e. Thus, <strong>in</strong>creases <strong>in</strong> <strong>the</strong> AAO<br />
as reported by Thompson and Solomon (2002), if<br />
<strong>the</strong>y cont<strong>in</strong>ue, should lead to more frequent<br />
episodes of light w<strong>in</strong>ds and stagn<strong>at</strong>ion <strong>in</strong> <strong>the</strong> SP<br />
region. Our d<strong>at</strong>a show <strong>the</strong> strong dependency of<br />
<strong>ozone</strong> production on <strong>boundary</strong> <strong>layer</strong> stability. It is<br />
noteworthy th<strong>at</strong> <strong>the</strong> <strong>in</strong>creas<strong>in</strong>g surface <strong>ozone</strong> trend<br />
dur<strong>in</strong>g 1990–2004 has exclusively resulted from<br />
an <strong>in</strong>crease <strong>in</strong> <strong>ozone</strong> dur<strong>in</strong>g November–January<br />
(Oltmans et al., 2006; Helmig et al., 2007a), when<br />
surface <strong>layer</strong> photochemical <strong>ozone</strong> production<br />
chemistry is expected to be most important. Therefore,<br />
we hypo<strong>the</strong>size th<strong>at</strong> a stronger AAO, by<br />
foster<strong>in</strong>g more stable <strong>boundary</strong> <strong>layer</strong> conditions,<br />
may have <strong>in</strong>fluenced <strong>ozone</strong> production <strong>in</strong> <strong>the</strong><br />
surface <strong>layer</strong> and has contributed to <strong>the</strong> observed<br />
recent <strong>in</strong>creases <strong>in</strong> <strong>the</strong> SP surface <strong>ozone</strong> record.<br />
3.9. Comparison of SP with o<strong>the</strong>r polar sites<br />
The <strong>ozone</strong> enhancements <strong>in</strong> <strong>the</strong> SP surface <strong>layer</strong><br />
are unique compared to o<strong>the</strong>r polar research sites.<br />
For <strong>in</strong>stance, <strong>at</strong> Summit, Greenland, <strong>ozone</strong> chemistry<br />
has been noted to be much different. Summit is<br />
<strong>at</strong> similar elev<strong>at</strong>ion and with similar year-round<br />
snowpack. However, be<strong>in</strong>g <strong>at</strong> 721N Summit experiences<br />
significant diurnal radi<strong>at</strong>ion cycles. The<br />
snowpack rema<strong>in</strong>s <strong>at</strong> sub-freez<strong>in</strong>g temper<strong>at</strong>ures<br />
year-round, although is some 10–151 warmer dur<strong>in</strong>g<br />
<strong>the</strong> summer than <strong>at</strong> SP, with daytime snow surface<br />
temper<strong>at</strong>ures regularly warm<strong>in</strong>g up to 10 to 5 1C<br />
(Helmig et al., 2007c). Episodes with <strong>in</strong>creased<br />
<strong>ozone</strong> <strong>at</strong> Summit are rel<strong>at</strong>ed to transport events<br />
with a frequent occurrence of transport from <strong>the</strong><br />
higher troposphere/lower str<strong>at</strong>osphere as well as<br />
occasional upslope flow with polluted air from<br />
lower l<strong>at</strong>itudes (Helmig et al., 2007e). Our ANTCI<br />
d<strong>at</strong>a and earlier studies (Oltmans and Komhyr,<br />
1976; Crawford et al., 2001) have shown th<strong>at</strong> high<br />
<strong>ozone</strong> <strong>at</strong> SP orig<strong>in</strong><strong>at</strong>es near <strong>the</strong> surface and is not<br />
transported from higher altitudes. Fur<strong>the</strong>rmore,<br />
<strong>the</strong>re is no <strong>in</strong>dic<strong>at</strong>ion for polluted, anthropogenically<br />
<strong>in</strong>fluenced air reach<strong>in</strong>g SP. Summit, <strong>in</strong> contrast<br />
to SP, dur<strong>in</strong>g summer is subject to substantial<br />
diurnal radi<strong>at</strong>ion and temper<strong>at</strong>ure cycles and<br />
<strong>the</strong> MBL is much more dynamic; e.g. stability<br />
regimes change frequently and are <strong>in</strong>homogeneous<br />
with altitude (Cohen et al., 2007). Snowpack<br />
temper<strong>at</strong>ures <strong>at</strong> Summit are higher and surface<br />
he<strong>at</strong><strong>in</strong>g dur<strong>in</strong>g sunny daytime conditions results <strong>in</strong><br />
convective he<strong>at</strong><strong>in</strong>g, which contributes to <strong>boundary</strong><br />
<strong>layer</strong> growth and <strong>in</strong>creased vertical mix<strong>in</strong>g. Stable<br />
<strong>at</strong>mospheric conditions <strong>at</strong> Summit mostly occur<br />
dur<strong>in</strong>g night, when <strong>the</strong>re is very little sunlight to<br />
drive photochemistry. Air reach<strong>in</strong>g Summit is<br />
mostly represent<strong>at</strong>ive of NH, lower tropospheric<br />
composition, ra<strong>the</strong>r than be<strong>in</strong>g transported upslope<br />
over <strong>the</strong> Greenland glacial ice shield. Consequently,<br />
<strong>the</strong> effective footpr<strong>in</strong>t and residence time of air <strong>in</strong><br />
contact with <strong>the</strong> snow surface on average is much<br />
shorter and susta<strong>in</strong>ed residence of air <strong>in</strong> a shallow<br />
surface <strong>layer</strong>, as <strong>at</strong> SP, is not encountered <strong>at</strong><br />
Summit. Under <strong>the</strong>se conditions, NO concentr<strong>at</strong>ions<br />
and <strong>ozone</strong> production <strong>in</strong> <strong>the</strong> surface <strong>layer</strong> do<br />
not build up to <strong>the</strong> high levels observed <strong>at</strong> SP (<strong>Davis</strong><br />
et al., 2004).<br />
4. Conclusions<br />
Enhanced <strong>ozone</strong> concentr<strong>at</strong>ions are a frequent<br />
phenomenon <strong>in</strong> <strong>the</strong> summertime surface and lower<br />
<strong>boundary</strong> <strong>layer</strong> <strong>at</strong> SP. Ozone is predom<strong>in</strong>antly<br />
produced and transported from <strong>the</strong> high altitude<br />
Antarctic pl<strong>at</strong>eau <strong>in</strong> <strong>the</strong> area surround<strong>in</strong>g SP from<br />
N to SE. Ozone production occurs by photochemical<br />
processes <strong>in</strong> a shallow surface <strong>layer</strong>, dur<strong>in</strong>g<br />
stable, light w<strong>in</strong>d, strongly str<strong>at</strong>ified <strong>boundary</strong> <strong>layer</strong><br />
conditions.<br />
These experiments show th<strong>at</strong> strong vertical<br />
<strong>ozone</strong> gradients, which result from a buildup of<br />
<strong>ozone</strong> <strong>in</strong> <strong>the</strong> surface <strong>layer</strong>, are a common, summertime<br />
condition <strong>at</strong> SP. Our d<strong>at</strong>a fur<strong>the</strong>r illustr<strong>at</strong>e th<strong>at</strong><br />
even between <strong>the</strong> surface and <strong>the</strong> 17 m-high <strong>in</strong>let of<br />
<strong>the</strong> ARO observ<strong>at</strong>ory up to 5 ppbv <strong>ozone</strong> gradients<br />
can be encountered. Hence, <strong>ozone</strong> mix<strong>in</strong>g r<strong>at</strong>ios will<br />
depend on <strong>the</strong> sampl<strong>in</strong>g height and consider<strong>at</strong>ion of<br />
<strong>the</strong> <strong>in</strong>let loc<strong>at</strong>ion will be of importance <strong>in</strong> compar<strong>in</strong>g<br />
<strong>the</strong> SP <strong>ozone</strong> record with d<strong>at</strong>a from o<strong>the</strong>r sites.<br />
Previously reported upwards <strong>ozone</strong> fluxes out of<br />
snow <strong>in</strong> o<strong>the</strong>r environments may have resulted from<br />
similar conditions of photochemical <strong>ozone</strong> production<br />
<strong>in</strong> a shallow <strong>at</strong>mospheric <strong>layer</strong> above <strong>the</strong> snow<br />
surface.<br />
These new observ<strong>at</strong>ions solidify <strong>the</strong> previous<br />
analyses and estim<strong>at</strong>es of summertime <strong>ozone</strong><br />
production chemistry <strong>at</strong> SP. Our measurements<br />
po<strong>in</strong>t towards <strong>the</strong> occurrences of <strong>ozone</strong> production<br />
r<strong>at</strong>es th<strong>at</strong> are <strong>in</strong> <strong>the</strong> upper range of previous<br />
calcul<strong>at</strong>ions. These d<strong>at</strong>a provide new evidence th<strong>at</strong><br />
polar surface <strong>ozone</strong> concentr<strong>at</strong>ions are tied to