The isotopic characterization of carbon monoxide in the troposphere

Abstract

The distributions of stable isotopes in trace atmospheric species are controlled mainly by the isotopic compositions of precursor molecules and isotope fractionation effects during production and removal of the species. Distributions of radioactive isotopes are controlled mainly by the isotopic compositions of precursor molecules and radioactive decay processes. As a result, through their isotopic compositions, atmospheric species are traceable to sources and sinks. Thus, isotopic compositions provide useful information for estimating source strengths and for understanding the importance of removal processes in the cycling of the species. The use of radioactive 14C and the stable isotopes ( 13C and 18O) are reviewed here for understanding production and removal processes of CO in the troposphere. Carbon monoxide is a critical component in atmospheric chemistry because of its large effect on levels of OH, the principal oxidant in the atmosphere. In the troposphere, this is due to relatively high concentrations of CO and a short lifetime (2–4 months). Initially, 14CO measurements were instrumental in estimating accurately the tropospheric lifetime. Since seasonal 14CO variation is controlled largely by OH, 14CO serves as an important surrogate measure of tropospheric OH. Global 14CO measurements have also been used to estimate the biogenic component of the global CO budget, specifically contributions from biomass burning, oxidized non-methane hydrocarbon (NMHC) emissions, oceans and plants. Research using 14CO measurements is also active in quantifying fossil and non-fossil urban emissions. Kinetic isotopic fractionation during production of 13CO and C 18O from reduced precursors allows one to distinguish, at least qualitatively, different varieties of CO based on seasonal tropospheric isotopic measurements. Difficulties in interpreting the stable isotopic record arise from large fractionation effects that obscure source isotopic signatures (in particular the oxygen kinetic isotope effect for the CO+OH reaction), and large seasonal/latitudinal variability in source fluxes. While dual-isotopic CO source signatures do not allow a direct mathematical determination of contributions from several sources, plots of δ 18O vs δ 13C may help estimate relative proportions of dominant sources. In addition to understanding CO sources, the pressure dependence of the carbon kinetic isotope effect (KIE) has helped elucidate pathways for the CO+OH reaction. Since formaldehyde (HCHO) is an important intermediate in hydrocarbon oxidation to CO and CO 2, the stable carbon and oxygen isotopes in atmospheric HCHO may help distinguish different hydrocarbon sources to regional CO budgets and elucidate the importance of different oxidative reaction pathways.