Intra‐annual cycles of NMVOC in the tropical marine boundary layer and their use for interpreting seasonal variability in CO

Carbon monoxide and nonmethane volatile organic compounds (NMVOC) have been measured for the first time on a continuous basis in the tropical marine boundary layer of the Atlantic Ocean. CO and ethane, which have similar lifetimes with respect to OH degradation, vary intra‐annually with the sinusoidal variability expected due to a primary hydroxyl radical (OH) sink, but with CO showing a smaller cycle amplitude. The ethane measurements were used to derive the seasonal variation in the nominal OH concentration (n[OH]) experienced along the air mass trajectories arriving at Cape Verde. The n[OH] represents the variability in both the true OH concentration and any intra‐annual changes in ethane emissions, and was subsequently used to simulate the seasonal cycle of CO, allowing identification of potentially differing sources and sinks from those of ethane. Deviation of the observed CO from the n[OH] fit indicated that summer sources of CO are approximately 60% (30–68%) higher than winter, assuming values of ±8% and ±49% seasonal variability, respectively, in fossil fuel and biomass burning emissions of CO and ethane, and including −29% uncertainty due to the losses of ethane through reaction with chlorine atoms. The evidence suggests that secondary production of CO from the oxidation of methane (CH 4 ) and NMVOC and in particular from methanol, acetone and acetaldehyde may be dominant summer CO sources in this region.