CO₂ and radon 222 as tracers for atmospheric transport

Abstract

Time variations of the CO 2 concentration and the radon 222 daughter activity in the atmosphere are observed 16 m above the ground in the outskirts of Heidelberg (Germany), situated in the rather densely populated and intensively cultivated upper Rhine valley. These parallel recorded data series are used to estimate the CO 2 respiration rate from the soil and from plant leaves. Atmospheric concentration variations of both gases appear to be merely due to variations in atmospheric mixing. From direct measurements of concentration profiles in the soil air we deduce the 222 Rn exhalation rate, which is nearly constant with time and about 4000 dpm m −2 h −1 . This flux is consistent with the one derived from observed pileup of the 222 Rn activity of an air mass during passage over the continent. The constant 222 Rn exhalation rate quoted serves as a reference for atmospheric dilution of the soil‐borne CO 2 . The comparison of the two concentrations leads to a total CO 2 respiration flux to the atmosphere of 21 mmol CO 2 m −2 h −1 for a short time period in August 1980. Direct measurements derived from CO 2 concentration profiles in the soil yield an average regional CO 2 flux from the soil alone of 12 mmol m −2 h −1 with a maximum of 20 mmol m −2 h −1 in June and July and a minimum of 3 mmol m −2 h −1 or less in winter. The different 222 Rn exhalation rates from the continent and ocean (where it is smaller by 2 orders of magnitude) are further used to identify the continental or the maritime origin of air masses.