Assessment of tropical OH seasonality using atmospheric 14CO measurements from Barbados

Abstract

14CO can be used to assess the mean OH concentration if the atmospheric inventory is determined. However, atmospheric transport must be well represented. The dependence on transport has previously been predicted in model studies to be much smaller in the tropics than in the middle latitudes. To test this, we made weekly measurements of 14CO from a tropical station in the Atlantic (Barbados, West Indies, 13°10′N, 53°26′W) from July 1996 to July 1997. These data are compared with the 14CO seasonal cycle predicted from a zero dimensional chemistry model. Results show that the observed seasonal cycle correlates well with predicted 14CO from the chemistry model during the summer and fall, however significant deviation from the chemistry model becomes apparent in the wintertime, which is probably largely a result of 14CO being transported to the tropics from the middle latitudes, and is in contrast to earlier modeling studies. Transport has a stronger effect in the winter months because the latitudinal gradient in 14CO is large and the lifetime is long, since OH concentrations are at their seasonal low.