Concentration of CO₂ in the Upper Troposphere and Lower Stratosphere

Abstract

DURING a number of flights of commercial aircraft over the polar route from Frankfurt/Main to Tokyo via Copenhagen the concentration of CO₂ in the atmosphere was continuously recorded using a non-dispersive infrared gas analyser. The flights took place between April 1967 and February 1968. The recorded concentration of CO₂ from all flights shows a remarkable change when the aircraft enters and leaves the stratosphere. A quite sudden change of about 3–5 p.p.m. CO₂ is usually observed at the transition from tropospheric into stratospheric air and vice versa. The striking difference between the CO₂ content of the upper troposphere and the stratosphere was clear enough on the record for the crew of the aircraft to use our CO₂ analyser as an indicator of the position of the aircraft relative to the tropopause. Fig. 1 shows the record obtained during the flight from Tokyo to Copenhagen on May 29–30, 1967, together with the position of the aircraft and the tropopause. Sections of the flight within the stratosphere are marked. They clearly show a drop of the CO₂ content in the stratosphere. The sudden peak at 70° N 145° W during the flight through the stratosphere may be explained by influx of tropospheric air into the stratosphere. The relative accuracy of our measurements is about ±0.3 p.p.m. CO₂, which gives confidence in the reality of the observed fluctuations. The accuracy of the absolute values of the CO₂ concentration amounts to ±1.5 p.p.m. due to difficulty in calibrating the reference gases. The marked decrease of the CO₂ concentration in the lower stratosphere compared with the upper troposphere suggests that, contrary to previous practice, it is wrong to assume a constant mixing-ratio of CO₂ in the troposphere and stratosphere. Exact knowledge of the CO₂ concentration with altitude is needed for many calculations of the radiation budget of the atmosphere.