Guest Post “A Simple Tool To Detect CO2 Background Levels” By Francis Massen

GUEST POST BY FRANCIS MASSEN

Biography of Francis Massen here.

Atmospheric CO₂ mixing ratios vary with latitude, regional specificities (as sea-side, continental or urban location) and time. Daily values may differ by more than 40%, as can be seen in the first figure showing CO₂ and wind speed time series at the meteorological station “meteoLCD” in Diekirch, Luxembourg (semi-rural environment).

 

Figure 1: CO₂ and wind speed values from 14 to 21 March 2010 in Diekirch, Luxembourg.

The causes of these hefty variations are many: magnitude of wind speed, periods of boundary layer inversions, changing plant behavior (photosynthetic CO₂ absorption or CO₂ emissions by plant respiration) and human activity. When the boundary layer is well mixed up, CO₂ levels tend to a reproducible minimum, which represents the regional background level and may even be close to the published global mean CO₂ mixing ratio. The mixing up of the near ground layer is essentially caused by the wind: a plot of CO₂ versus wind speed often has a typical boomerang shape, as shown in the second figure.

Figure 2:  Plot of CO₂ versus wind-speed using the values of Fig.1.

The background level can be thought as being the CO₂ mixing ratio that would exist if wind speed was infinite. Simple visual inspection, or better, fitting the data to an exponential function of the type CO₂ = a + b*exp(-c*windspeed) delivers this asymptotic CO₂ level. In the example above, the model is statistically significant (R² = 0.64) and suggests a regional background of 390 ppm (to be compared for instance to the 2009 seasonal corrected mean Mauna Loa level of approx. 388 ppm).

The CO₂ versus wind speed plot can also be used as a first step to validate historic CO₂ measurements, made by chemical methods. One example is the very careful measurements done from 1939 to 1940 by W. Kreutz in the town of Giessen, Germany. Kreutz used a chemical gas analyzer having an accuracy better than 1.5% and also recorded the wind speed. The exponential fit points to a very high asymptotic level of 398 ppm, well in excess to the consensus value of 310 ppm derived from the ice-cores.

This CO₂ background problem is studied in a peer reviewed paper I presented with E. Beck as coauthor (Beck is a specialist of historical CO₂ measurements) at the online conference Klima2009 organized in Nov. 2009 by the University of Applied Sciences of Hamburg, Germany. Our paper “Accurate estimation of CO₂background level from near-ground measurements at non-mixed environments” was rewarded “Best Paper” among the 103 contributions. A slightly edited version will be published in an upcoming book “Social, Economic and Political Aspects of Climate Change” (editor: Prof. Walter Leal, Springer Verlag).

The text of the original Klima2009 paper can be found here