E-G. Brunke1, C.
Labuschagne1, B. Parker1 and H.E. Scheel2
1South
African Weather Service,
2Forschungszentrum Karlsruhe, IMK-IFU, 82467
Garmisch-Partenkirchen, Germany
ernst.brunke@weathersa.co.za
The long-term
records of CO2, CH4, CO and N2O at the southern
hemispheric coastal station Cape Point (CPT, 34 °S, 18 °E) are now spanning 16
complete years for CO2, 26 years for CH4, and 30 years in
the case of CO. From the N2O observations, the first seven years
(1989 – 1995) have been excluded from current analyses because of lower data
quality during this period. Although the predominant air advection at Cape
Point is from the marine sector, clean air data filtering is a prerequite for estimating
long-term trends representative of southern hemispheric background conditions. Data
selection is done by way of a statistical filter which rejects locally elevated
trace gas concentrations. Updated results for long-trerm trends and growth
rates are presented. In particular, recent developments of the CH4,
CO, and N2O records are discussed.
CO2
levels have steadily increased from 355.6 ppm at the start of the measurements
in 1993 to approximately 383 ppm in 2008. Growth rates were calculated as
derivatives of the trend curve obtained via 5-year smoothing. These fluctuated
between 1.5 and 2.2 ppm yr-1. A straight line fitted to the rates
has yielded an increase of the fit from 1.6 ppm yr-1 in early 1993
to 2.1 ppm yr-1 at the end of 2008.
In the case of CH4,
the high average growth rates during the 1980s decreased with marked
fluctuation over the years. Methane levels stabilized from 2003 onwards, and
during 2006 the growth rate even dropped to about -1 ppb yr-1. A
linear fit of the growth rates has yielded values of 13 ppb yr-1 for
the beginning of 1983 and zero growth for mid-2005. However, starting around
October 2007, an increase was observed again. This is in line with observations
made at several other southern and northern hemispheric locations. Possible causes,
such as an increase in emissions in both hemispheres or a reduction in hydroxyl
radical concentration are being discussed in the literature (e.g., Rigby et
al., GRL, 2008).
In addition,
trends of non-background CO2 and CH4 have been examined
for the past 14 years. Wind sector-dependent growth rates of the two gases (
periods 1999 - 2005 and 2006 – 2008) reveal maxima for the northerly directions,
where densely populated areas are located.
Over most of its
30-year measuring period, the CO time series has not displayed any significant
long-term trend, whereas some inter-annual variability is evident. However,
since 2003 an overall decline has been observed in the CO mole fractions with
an abnormally low annual minimum during February 2006 followed by a lower than
normal annual maximum in October 2006. Thereafter CO returned to previously
observed levels again, but decreased to an unprecedented low annual maximum in
2008. Observations during the first half of 2009, however, indicate a rise of
CO concentrations again. The causes of the decrease are still under
investigation. In spite of thorough instrumental checks, the possibility of
recent analytical artefacts cannot entirely be ruled out.
For N2O
a nearly linear increase at a rate of 0.72 ppb yr-1 was determined
for the period 1996 till 2006. During mid-2007 a new gaschromatic system was
installed. Despite the new analytical system, certain issues with regard to
instrumental sensitivity and overall data uncertainty still persist and are
discussed.