Carbon monoxide short term measurements at Amsterdam island: estimations of biomass burning emission rates

Abstract

A fully automated gas chromatograph instrument for atmospheric carbon monoxide (CO) measurements based upon mercuric oxide reduction/UV absorbance has been operating at Amsterdam Island (37°S, 77°E) in the Indian ocean since 1996. This technique allows measurements every 10 min and enables the survey of short term variations of CO. In parallel, Radon-222, a well known tracer of continental air masses, was simultaneously monitored and used for the understanding of the CO variations.The measurements undertaken since March 1996 allow to draw a seasonal variation of CO with a typical summer minimum of about 35 ppbv, and a spring maximum of the order of 60 ppbv. This seasonal cycle is coherent with the observations in other stations of the Southern Hemisphere, particularly Crozet Islands (46°S, 51°E), and Cape Grim (41°S, 145°E, Tasmania). It is consistent with both the seasonal variation of the main CO sink, (i.e. reaction with OH radicals) and of one main CO source since the monthly CO maximum is phased with the seasonality of biomass burning in Africa which mainly occurs from June to November.A statistical analysis of the short term variations of CO is made for the 1996 and 1997 data. Episodes of elevated CO are observed between June and October, and most (65%) are associated with events of high Radon-222. A careful examination of the relative amplitudes of CO and Radon-222 peaks backed up with results of the back trajectories analysis is made for 22 particular events. The largest variations of CO correspond to rapid advection of continental air across the Indian Ocean from Southern Africa. On the basis of the ratio CO/Rn observed at Amsterdam Island for these 22 events, we estimate the CO flux from biomass burning in the range of 0.4–3.3 mg cm−2 h−1. Furthermore, it is pointed out that these events do not significantly influence the monthly means, and thus our measurements at Amsterdam Island can be considered as representative of the Southern Hemisphere CO background.