Modelling the oxidation of seventeen volatile organic compounds to track yields of CO and CO2

A box model simulating tropospheric conditions was used to trace the oxidation pathways of 17 volatile organic compounds (VOCs) covering saturated, olefinic, oxygenated and aromatic species. Yields of carbon monoxide (CO), carbon dioxide (CO2) and other oxidation products were calculated over the duration of the model simulation. Conversion factors to CO and CO2 were determined for individual VOCs at various simulated levels of NOx (an indicator of pollution level). Global budget estimates for CO and CO2 formed from the oxidation of reduced carbon were then calculated based on these conversion factors. At medium levels of NOx (160 pptv) we calculate that isoprene degradation forms 626 ± 250 Tg CO yr−1, whilst the oxidation of non-methane VOCs produces 955 ± 362 Tg CO yr−1 and VOC oxidation accounts for 1804 ± 660 Tg CO yr−1, all higher than previous literature estimates. To maintain the oxidation balance within global chemical transport models which do not include a detailed suite of non-methane VOCs (NMVOCs) it may be vital to include this CO source from NMVOC oxidation. We found the oxidation of NMVOCs to produce 545 ± 211 Tg C yr−1 as CO2, whilst overall VOC oxidation accounted for 916 ± 338 Tg C yr−1 of CO2, at medium NOx.