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Free keywords:
carbon-14-monoxide; radiocarbon; climatology; solar cycle; atmospheric neutrons; heliospheric potential
Abstract:
[1] Mainly three processes determine the (CO)-C-14 content of the troposphere: the cosmogenic production of C-14 throughout the atmosphere, the removal (oxidation) by the hydroxyl radical (OH), and the transport of (CO)-C-14 from the stratosphere into the troposphere. These characteristics make (CO)-C-14 an interesting tracer for application in atmospheric dynamics and chemistry research. If (CO)-C-14 observations from different times have to be compared for changes in OH or the stratosphere-troposphere exchange (STE), the varying global source strength of (CO)-C-14 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 (CO)-C-14 production rate are developed and compared. These methods involve the sunspot number, the heliospheric potential, and atmospheric neutron monitor data. Applying further results from previous studies about the atmospheric response, a rescaling procedure for (CO)- C-14 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 (CO)-C-14 climatology (cosmogenic contribution) from all currently available (CO)-C- 14 observations at the surface level. For this, in a first step, the smaller contribution of (CO)-C-14 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.
