Measurement of the abundance of 14CO in the atmosphere and the 13C/12C and 18O/16O ratio of atmospheric CO with applications in New Zealand and Antarctica

Abstract

Equipment and method for accurate and precise concentration, 14C, 13C, and 18O isotope analysis for CO in background air is presented along with new results for Antarctica and New Zealand. High flow rate cryogenic extraction systems which separate CO after its oxidation to CO2 are used, incorporating a novel ultraefficient cryogenic trap. Air in quantities from a few hundred liters to 2 m3 with CO concentrations from 20 part per billion by volume (ppbv) to 1000 ppbv can be analyzed. The absolute CO concentration is determined volumetrically. The 13C/12C and 18O/16O ratios are determined by mass spectrometry. For 18O a correction is applied for the oxygen in the CO2 derived from the oxidant. Carbon 14 is determined by accelerator mass spectrometry. Prior to this the very small CO‐derived samples with their high specific activity are diluted accurately. For polluted air the proportional decrease in specific activity with increasing CO levels is confirmed. The 14CO abundance and CO concentration in background air in New Zealand and Antarctica are not much different, and both follow a distinct seasonal pattern, in particular 14CO which is mainly forced by OH seasonality. The 14CO abundance swings between its February minimum of about 6 and its August maximum of about 13 molecules per cm3 air (STP). CO has a smaller seasonality and shows a larger scatter due to local CO sources. The impact of changes in solar activity on 14CO for the period considered has been small. Most of the short‐term variability in 14CO is due to the sampling of different air masses. It appears that interannual OH variations may be reflected in 14CO variations. Both 13C/12C and 18O/16O at Scott Base show large seasonal variation, and the impact of biomass burning and isotopic fractionation in CO destruction are used to try to explain the respective isotopic compositions.