The isotopic record of Northern Hemisphere atmospheric carbon monoxide since 1950, implications for the CO budget

Abstract

Abstract. We present a 60-yr record of atmospheric CO concentration and stable isotopic ratios at high northern latitude based on firn air samples collected in the frame of the North Greenland Eemian Ice Drilling (NEEM) project. Concentration, δ13C, and δ18O of CO from trapped gases in the firn were measured by gas chromatography coupled with isotope ratio mass spectrometry (gc-IRMS). Using models of trace gas transport in firn, the long-term trend of atmospheric CO and its stable isotopic composition at high northern latitudes since the 1950s were reconstructed. Our best firn air scenarios suggest that δ13C decreased slightly from −25.8‰ in 1950 to −26.4‰ in 2000, then dropped to −27.2‰ in 2008. δ18O decreased more regularly from 9.8‰ in 1950 to 7.1‰ in 2008. The best firn air scenarios also suggest that CO concentration increased gradually from 1950 and peaked likely in the late-1970s, followed by a gradual decrease by present day (Petrenko et al., 2011). An isotope mass balance model is applied to quantify the temporal evolution of CO source partitioning able to explain the combined mixing ratio and isotopic ratio changes. It suggests that a slight increase followed by a large reduction in CO derived from fossil fuel combustion occurred since 1950. The increase of CO concentration from 1950 to the mid-1970s is the result of a combined increase of multiple sources. The reduction of CO emission from fossil fuel combustion after the mid-1970s is the most plausible mechanism for the drop of CO concentration during this time. The mitigation policy for CO emission from vehicle exhaust such as application of catalytic converters and the growth of diesel engine vehicles market share are the main expected reasons for the CO source strength change from fossil fuel combustion.