One of the world’s most intensive studies of tropical air chemistry took place in 1996. The effects of biomass burning on the African rainforest were the focus of EXPRESSO, the Experiment for Regional Sources and Sinks of Oxidants, which measured the smog created by agricultural and other fires in the Central African Republic and Congo. Depicted here, an EXPRESSO trailer carrying laboratory equipment takes a treacherous drive across unpaved logging roads in eastern Cameroon en route to the experimental site.