ausblenden:
Schlagwörter:
-
Zusammenfassung:
Traditional techniques for measuring the mole
fractions of greenhouse gases in the well-mixed atmosphere
have required dry sample gas streams (dew point <−25 C)
to achieve the inter-laboratory compatibility goals set forth
by the Global Atmosphere Watch programme of the World
Meteorological Organisation (WMO/GAW) for carbon dioxide
(±0.1 ppm in the Northern Hemisphere and ±0.05 ppm
in the Southern Hemisphere) and methane (±2 ppb). Drying
the sample gas to low levels of water vapour can be expensive,
time-consuming, and/or problematic, especially at remote
sites where access is difficult. Recent advances in optical
measurement techniques, in particular cavity ring down
spectroscopy, have led to the development of greenhouse gas
analysers capable of simultaneous measurements of carbon
dioxide, methane and water vapour. Unlike many older technologies,
which can suffer from significant uncorrected interference
from water vapour, these instruments permit accurate
and precise greenhouse gas measurements that can
meet the WMO/GAW inter-laboratory compatibility goals
(WMO, 2011a) without drying the sample gas. In this paper,
we present laboratory methodology for empirically deriving
the water vapour correction factors, and we summarise a series
of in-situ validation experiments comparing the measurements
in humid gas streams to well-characterised dry-gas
measurements. By using the manufacturer-supplied correction
factors, the dry-mole fraction measurements have been
demonstrated to be well within the GAWcompatibility goals
up to a water vapour concentration of at least 1 %. By determining
the correction factors for individual instruments once
at the start of life, this water vapour concentration range can
be extended to at least 2% over the life of the instrument,
and if the correction factors are determined periodically over
time, the evidence suggests that this range can be extended up to and even above 4% water vapour concentrations.
