Variation of atmospheric CO, δ13C, and δ18O at high northern latitude during 2004-2009: Observations and model simulations

Atmospheric CO mixing ratios and stable isotope ratios (delta C-13 and delta O-18) were measured at a high northern latitude site (Westman Islands, Iceland) from January 2004 to March 2010 in order to investigate recent multiyear trends of the sources of atmospheric carbon monoxide in the extratropical Northern Hemisphere. During this period, we observed a decrease of about 2% per year in CO mixing ratios with little significant interannual variability. The seasonal cycles for delta C-13 and delta O-18 in CO are similar to that in the CO mixing ratio, and there is a pronounced interannual variation in their seasonal extremes occurring in summer and fall, which is driven by changes in the relative contribution of different sources. Some of the sources of CO are anthropogenic in character (e.g., fossil fuel and biofuel combustion and agricultural waste burning), and some are primarily natural (e.g., oxidation atmospheric methane and other hydrocarbons and wildfires), and distinction among the various major sources can, more or less, be distinguished by the stable isotopic composition of CO. We compare our observations with simulations from a 3-D global chemical transport model (MOZART-4, Model for Ozone and Related Chemical Tracers, version 4). Our results indicate the observed trend of anthropogenic CO emissions is mostly responsible for the observed variation in delta C-13 and delta O-18 of CO during 2004-2009. Especially, the delta O-18 enriched sources such as fossil fuel and biofuel sources are controlling the variation. The modeling results indicate decreasing trends in the fossil fuel and biofuel source contributions at Iceland of -0.61 +/- 0.26 ppbv/yr and -0.38 +/- 0.10 ppbv/yr, respectively, during the observation period.