Abstract. When computing climatological averages of atmospheric trace-gas mixing ratios
obtained from satellite-based measurements, sampling biases arise if data
coverage is not uniform in space and time. Homogeneous spatiotemporal
coverage is essentially impossible to achieve. Solar occultation
measurements, by virtue of satellite orbit and the requirement of direct
observation of the sun through the atmosphere, result in particularly sparse
spatial coverage. In this proof-of-concept study, a method is presented to
adjust for such sampling biases when calculating climatological means. The
method is demonstrated using carbonyl sulfide (OCS) measurements at 16 km
altitude from the ACE-FTS (Atmospheric Chemistry Experiment Fourier Transform
Spectrometer). At this altitude, OCS mixing ratios show a steep gradient
between the poles and Equator. ACE-FTS measurements, which are provided as
vertically resolved profiles, and integrated stratospheric OCS columns are
used in this study. The bias adjustment procedure requires no additional
information other than the satellite data product itself. In particular, the
method does not rely on atmospheric models with potentially unreliable
transport or chemistry parameterizations, and the results can be used
uncompromised to test and validate such models. It is expected to be
generally applicable when constructing climatologies of long-lived tracers
from sparsely and heterogeneously sampled satellite measurements. In the
first step of the adjustment procedure, a regression model is used to fit a
2-D surface to all available ACE-FTS OCS measurements as a function of
day-of-year and latitude. The regression model fit is used to calculate an
adjustment factor that is then used to adjust each measurement individually.
The mean of the adjusted measurement points of a chosen latitude range and
season is then used as the bias-free climatological value. When applying the
adjustment factor to seasonal averages in 30∘ zones, the maximum
spatiotemporal sampling bias adjustment was 11 % for OCS mixing ratios at
16 km and 5 % for the stratospheric OCS column. The adjustments were
validated against the much denser and more homogeneous OCS data product from
the limb-sounding MIPAS (Michelson Interferometer for Passive Atmospheric
Sounding) instrument, and both the direction and magnitude of the adjustments were in agreement with the adjustment of
the ACE-FTS data.
An empirical model for estimating the concentration of carbonyl sulfide in surface seawater from satellite measurements