The seasonal cycle of cosmogenic 14CO at surface level: A solar cycle adjusted, zonal-average climatology based on observations

Patrick Jöckel, and Carl A.M. Brenninkmeijer

Max Planck Institute for Chemistry, Atmospheric Chemistry Division, Mainz, Germany

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. D22, 4656, doi:10.1029/2001JD001104, 2002

Abstract. Mainly three processes determine the 14CO content of the troposphere: the cosmogenic production of 14C throughout the atmosphere, the removal (oxidation) by the hydroxyl radical (OH), and the transport of 14CO from the stratosphere into the troposphere. These characteristics make 14CO an interesting tracer for application in atmospheric dynamics and chemistry research. If 14CO observations from different times have to be compared for changes in OH or the stratosphere–troposphere exchange (STE), the varying global source strength of 14CO caused by variations in solar activity has to be taken into account. Three approaches for parameterizing solar activity with respect to the effect on the global average atmospheric 14CO production rate are developed and compared. These methods involve the sunspot number, heliospheric potential, and atmospheric neutron monitor data. Applying further results from previous studies about the atmospheric response, a rescaling procedure for 14CO observations of different epochs to the same solar activity conditions is derived. All three approaches yield consistent results, whereby small differences provide an uncertainty estimate. The rescaling is used to compile the first solar cycle adjusted, zonal-average 14CO climatology (cosmogenic contribution) from all currently available 14CO observations at the surface level. For this, in a first step, the smaller contribution of 14CO of noncosmogenic, that is, biogenic origin (secondary source) is estimated from CO observations and subtracted from the measurements. The resulting climatology can be used to trace changes if compared to future observations, and especially for the evaluation of OH distributions and the stratosphere–troposphere exchange in three-dimensional global atmospheric models.

Received 29 June 2001;revised 28 February 2002; accepted 28 February 2002; published 29 November 2002.

Keywords: carbon-14-monoxide, radiocarbon, climatology, solar cycle, atmospheric neutrons, heliospheric potential.

Index Terms: 1610 Global Change: Atmosphere (0315, 0325); 0330 Atmospheric Composition and Structure: Geochemical cycles; 2162 Interplanetary Physics: Solar cycle variations (7536); 0325 Atmospheric Composition and Structure: Evolution of the atmosphere.

Citation: Jöckel, P., and C. A. M. Brenninkmeijer, The seasonal cycle of cosmogenic 14CO at the surface level: A solar cycle adjusted, zonal-average climatology based on observations, J. Geophys. Res., 107(D22), 4656, doi:10.1029/2001JD001104, 2002.

Copyright 2002 by the American Geophysical Union.