Abstract
Abstract. Since 2002 in situ airborne measurements of atmospheric CO2 mixing ratios have been performed regularly aboard a rental aircraft near Bialystok (53°08´ N, 23°09´ E), a city in northeastern Poland. Since August 2008, the in situ CO2 measurements have been made by a modified commercially available and fully automated non-dispersive infrared (NDIR) analyzer system. The response of the analyzer has been characterized and the CO2 mixing ratio stability of the associated calibration system has been fully tested, which results in an optimal calibration strategy and allows for an accuracy of the CO2 measurements within 0.2 ppm. Besides the in situ measurements, air samples have been collected in glass flasks and analyzed in the laboratory for CO2 and other trace gases. To validate the in situ CO2 measurements against reliable discrete flask measurements, we developed weighting functions that mimic the temporal averaging of the flask sampling process. Comparisons between in situ and flask CO2 measurements demonstrate that these weighting functions can compensate for atmospheric variability, and provide an effective method for validating airborne in situ CO2 measurements. In addition, we show the nine-year records of flask CO2 measurements. The new system, automated since August 2008, has eliminated the need for manual in-flight calibrations, and thus enables an additional vertical profile, 20 km away, to be sampled at no additional cost in terms of flight hours. This sampling strategy provides an opportunity to investigate both temporal and spatial variability on a regular basis.
Highlights
The increase of CO2 mixing ratios in the atmosphere since pre-industrial times is the most important cause of climate change (IPCC, 2007), and this rise is due to human activities, mainly those involving fossil fuel burning and land use change (Le Quere et al, 2009)
Regular aircraft profiles are desirable in order to increase the coverage of atmospheric CO2 observations and to improve how the vertical mixing is represented in transport
Apart from the cylinder size, the variations of various parameters in different testing setups were investigated, and no influence on the trend of the CO2 mixing ratios was observed. These laboratory tests led to a strategy for the use of the calibration system of the non-dispersive infrared (NDIR) analyzer during flight: (1) calibrating the CO2 mixing ratio of air in the small cylinder after being filled instead of using the value of the filling tank; (2) using the cylinders only when the pressure is above 30 bar, a conservative level below which CO2 mixing ratios may significantly increase due to desorption of CO2 molecules from the walls of the cylinders; (3) flushing the dead volume in the pressure regulators before measurements are started during a flight; (4) calibrating the small cylinders before and after deployment in the field to characterize a potential long-term drift in CO2 mixing ratios due to the diffusion effect
Summary
The increase of CO2 mixing ratios in the atmosphere since pre-industrial times is the most important cause of climate change (IPCC, 2007), and this rise is due to human activities, mainly those involving fossil fuel burning and land use change (Le Quere et al, 2009). A quantitative determination of the distribution of carbon sources and sinks is paramount if climate studies are to be able to analyze the response of terrestrial ecosystems to climate change and monitor fossil fuel emissions reductions in the near future. To achieve these objectives, long term accurate monitoring of atmospheric CO2 is indispensable (Heimann, 2009). The main purpose of these aircraft measurements is to regularly obtain the vertical distribution of atmospheric CO2, which is essential to improve the representation of the vertical mixing in transport models These profiles are made up to 3 km above ground, and have been used in combination with model results to compare with FTS CO2 retrievals (Messerschmidt et al, 2011b).
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