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

Abstract

[1] Mainly three processes determine the 14 CO content of the troposphere: the cosmogenic production of 14 C throughout the atmosphere, the removal (oxidation) by the hydroxyl radical (OH), and the transport of 14 CO from the stratosphere into the troposphere. These characteristics make 14 CO an interesting tracer for application in atmospheric dynamics and chemistry research. If 14 CO observations from different times have to be compared for changes in OH or the stratosphere-troposphere exchange (STE), the varying global source strength of 14 CO 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 14 CO 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 14 CO 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, zonalaverage 14 CO climatology (cosmogenic contribution) from all currently available 14 CO observations at the surface level. For this, in a first step, the smaller contribution of 14 CO 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.

Precise Comparison of 14C Ages from Choukai Jindai Cedar with IntCal04 Raw Data

“…The 14 C production rate due to cosmic rays at higher latitudes-where the European trees originate-is greater than at the mid-latitude of the Choukai Jindai cedar because of geomagnetic cut-off rigidities: 1-3 GV in Ireland and 3-5 GV in Germany compared to 10 GV in Japan. However, as the 14 C is quickly distributed as 14 CO 2 and/or 14 CO (Jöckel and Brenninkmeijer 2002) in the atmosphere, the latitude gradient of 14 CO 2 is diluted. Using a three-dimensional global tracer transport model simulation, Braziunas et al (1995) indicated that the preindustrial atmospheric 14 C latitudinal gradient was rather modest at latitudes lower than 60°N in the Northern Hemisphere.…”

Evaluation of stratosphere–troposphere exchange and the hydroxyl radical distribution in three‐dimensional global atmospheric models using observations of cosmogenic 14CO

“…Especially in the SH, only a limited amount of aircraft data is available. Construction of a 2‐D climatology in the same way as for the surface observations [ Jöckel and Brenninkmeijer , 2002] is not yet feasible. Such an approach is further complicated by the fact that the aircraft measurements are not all sampled at the same pressure level.…”

“…The effects of the changes in solar activity are better documented and can be corrected [ Jöckel , 2000] (available at http://www.ub.uni-heidelberg.de/archiv/1426). A climatology for cosmogenic 14 CO is now available [ Jöckel and Brenninkmeijer , 2002]. The next move is to improve the sophistication of the modeling of 14 CO, and this paper analyzes in detail how successful or unsuccessful two current three‐dimensional (3‐D) chemistry transport models (CTMs) are in describing the surface and vertical distribution of 14 CO.…”

Radiocarbon in the Atmosphere

“…The short lifetime and high-altitude source of 14 CO makes it more sensitive to OH concentrations at high altitudes and high latitudes than to OH concentrations at low altitudes and low latitudes (Krol et al 2008). The distribution of 14 CO can additionally be used to test stratosphere-troposphere exchange in atmospheric models (Jöckel and Brenninkmeijer 2002).…”

“…In addition, very strong vertical gradients observed within the stratosphere indicate vertical mixing of stratospheric air is slow. Investigation of stratosphere-tropospheric dynamics can also utilize 14 CO, the dominant form of newly produced 14 C atoms which oxidizes to 14 CO 2 over a timescale of weeks to months (Jöckel and Brenninkmeijer 2002;Sect. 4.9.1).…”