¹⁴CO and its application in studies of atmospheric chemistry and transport: A brief history

The primary origin of atmospheric ¹⁴CO is production by cosmic radiation. High energy cosmic rays (mainly protons) induce large nucleonic particle cascades in the atmosphere and produce atmospheric neutrons. Most of them diffuse and thermalize before they are captured by nitrogen nuclei forming ¹⁴C (¹⁴N(n,p)¹⁴C). The recoil ¹⁴C atom rapidly oxidizes to ¹⁴CO, with a yield that has been determined to be approximately 95% [ Pandow et al., 1960; MacKay et al., 1963]. In this way, a natural tracer is produced throughout the atmosphere, almost equally partitioned between the stratosphere and the troposphere, however with its maximum in polar regions caused by the influence of the geomagnetic field on the primary cosmic ray particles. The average source strength is 1.6 - 2 molecules per second and per square-centimeter of the Earth's surface, corresponding to a total production of approximately 13-16 kg ¹⁴CO per year. Since the cosmic ray flux reaching the atmosphere is modulated by the solar wind intensity, the cosmogenic ¹⁴CO production rate oscillates with a phase of 11 years (solar cycle) with higher production rates during times of low solar activity.

The secondary (``biogenic'') contribution, comprising 20-25% of the total source, consists of recycled ¹⁴CO from the biosphere, entering or evolving in the atmosphere by oxidation of natural methane and higher hydrocarbons, and by biomass burning. The use of fossil fuel does not contribute to atmospheric ¹⁴CO as geological production times vastly exceed the ¹⁴C half life of about 5730 years.

The significance of ¹⁴CO is that it constitutes a natural tracer that can be used to assess the hydroxyl radical (OH) abundance, because ¹⁴ CO + OH is its main sink reaction, with an average tropospheric lifetime of ¹⁴CO of about 2-3 months.

Already before the discovery of the important role of OH in the troposphere [Levy, 1971], Weinstock [1969] estimated the residence time of CO, using ¹⁴CO measurements, or more precisely the specific activity of CO measured by MacKay et al. [1963]. The implicit assumptions of the approach of Weinstock [1969] and the implications for CO budget calculations have been discussed by Junge et al. [1971]. Volz et al. [1981] applied the ¹⁴CO concept in a systematic manner and concluded that the abundance and seasonality of ¹⁴CO is in accordance with that of OH used in a two-dimensional (2-D) atmospheric chemistry model. ¹⁴CO measurements had been exclusively performed using proportional gas counters requiring large amounts of air (~ 200 m³) to be processed.

Routine measurements of ¹⁴CO in smaller air samples with increased precision became possible with the arising technique of the accelerator mass spectrometry (AMS). Air sampling techniques suitable for isotopic analysis and extraction procedures for isolating CO from the air samples are described for instance by Brenninkmeijer and Roberts [1994], Mak and Brenninkmeijer [1994], and Brenninkmeijer [1993]. Aspects of the AMS measurements are discussed in Rom et al. [2000b]. Brenninkmeijer et al. [1992] observed lower ¹⁴CO levels in the southern hemisphere, which were attributed to higher southern hemisphere (SH) OH levels, in contradiction to ideas about higher northern hemisphere (NH) OH values due to the importance of NO in recycling OH [Crutzen and Zimmermann, 1991]. Mak et al. [1992]; Mak et al. [1994] measured ¹⁴CO in air sampled in the free troposphere, applied two different 2-D models and concluded that apart from the NH-SH asymmetry, generally atmospheric levels seemed lower than inferred by the models employed. Quay et al. [2000] investigated various ¹⁴CO measurements with a 2-D model and concluded that either a higher horizontal mixing or a higher OH concentration in the SH is responsible for the observed inter-hemispheric asymmetry of ¹⁴CO. In the meantime, more and more ¹⁴CO measurements at surface level and in the free troposphere have become available [Mak and Southon, 1998; Tyler et al., 1999; Röckmann and Brenninkmeijer, 1997; Röckmann et al., 1999; Kato et al., 2000; Rom et al., 2000a; Quay et al., 2000]. The first ¹⁴CO analysis of lower stratospheric air samples is reported by Brenninkmeijer et al. [1995].

Three independent estimates of the primary cosmogenic ¹⁴CO source distribution exist [Lingenfelter, 1963; O'Brien et al., 1991; Masarik and Beer, 1999] which differ mostly according to the vertical gradient of the production rate. However, Jöckel et al. [1999] (and Jöckel [2000]) showed that this uncertainty is not a principle problem for the ¹⁴CO methodology. Also the effect of the solar variation is well understood and can be taken into account when ¹⁴CO observations of different epochs are to be compared [Jöckel et al., 2000]. This sets the fundament for compiling a ¹⁴CO climatology, i.e., a zonally averaged seasonal cycle at the surface comprising 1088 ¹⁴CO observations from 4 institutes [Jöckel and Brenninkmeijer, 2002]. Jöckel et al. [2002] (Jöckel [2000]) used this climatology for the evaluation of two 3-dimensional atmospheric models and revisited the observed inter-hemispheric asymmetry of atmospheric ¹⁴CO. Evidence for a higher OH abundance in the SH is not longer supported, since the asymmetry can consistently be explained by inter-hemispheric differences of the exchange rate between the stratosphere and the troposphere.

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