Greenhouse gas observations from Cabauw Tall Tower (1992-2010)

Abstract

Since 1992 semi-continuous in-situ observations of greenhouse gas concentrations have been performed at the tall tower of Cabauw (4.927° E, 51.971° N, -0.7 m a.s.l.). Through 1992 up to now, the measurement system has been gradually extended and improved in precision, starting with CO₂ and CH₄ concentrations from 200 m a.g.l. in 1992 to vertical gradients at 4 levels of the gases CO₂ , CH₄ , SF₆ , N₂ O, H₂ , CO and gradients at 2 levels for ²²² Rn. In this paper the measurement systems and measurement results are described for the main greenhouse gases and CO, for the whole period. The automatic measurement system now provides half-hourly concentration gradients with a precision better than or close to the WMO recommendations.

The observations at Cabauw show a complex pattern caused by the influence of sources and sinks from a large area around the tower with significant contributions of sources and sinks at distances up to 500-700 km. The concentration footprint area of Cabauw is one the most intensive and complex source areas of greenhouse gases in the world. Despite this, annual mean trends for the most important greenhouse gases, compatible with the values derived using the global network, can be reproduced from the measured concentrations at Cabauw over the entire measurement period, with a measured increase in the period 2000-2009 for CO₂ of 1.90 ± 0.1 ppm yr^-1 , for CH₄ of 4.4 ± 0.6 ppb yr^-1 , for N₂ O of 0.86 ± 0.04 ppb yr^-1 , and for SF₆ of 0.27 ± 0.01 ppt yr^-1 ; for CO no significant trend could be detected.

The influences of strong local sources and sinks are reflected in the amplitude of the mean seasonal cycles observed at Cabauw, that are larger than the mean Northern Hemisphere average; Cabauw mean seasonal amplitude for CO₂ is 25-30 ppm (higher value for lower sampling levels). The observed CH₄ seasonal amplitude is 50-110 ppb. All gases except N₂ O show highest concentrations in winter and lower concentrations in summer, N₂ O observations show two additional concentration maxima in early summer and in autumn.

Seasonal cycles of the day-time mean concentrations show that surface concentrations or high elevation concentrations alone do not give a representative value for the boundary layer concentrations, especially in winter time, but that the vertical profile data along the mast can be used to construct a useful boundary layer mean value. The variability at Cabauw in the atmospheric concentrations of CO₂ on time scales of minutes to hours is several ppm and is much larger than the precision of the measurements (0.1 ppm). The diurnal and synoptical variability of the concentrations at Cabauw carry information on the sources and sinks in the footprint area of the mast, that will be useful in combination with inverse atmospheric transport model to verify emission estimates and improve ecosystem models. For this purpose a network of tall tower stations like Cabauw forms a very useful addition to the existing global observing network for greenhouse gases.

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