The observed seasonal amplitude of atmospheric HCN concentrations implies an atmospheric lifetime of only a few months for HCN, much shorter than is commonly assumed from oxidation by OH (a few years). We propose that ocean uptake provides the missing sink, and show with a global 3-D model simulation that the observations of atmospheric HCN can be roughly reproduced in a scenario where biomass burning provides the main source (1.4--2.9 Tg N yr-1) and ocean uptake provides the main sink (HCN atmospheric lifetime of 2--4 months). Such a budget implies that HCN is a sensitive tracer of biomass burning on large scales, of particular value because it is readily observed from space. The ocean sink hypothesis can be tested with measurements of HCN concentrations in marine air and seawater. ¿ 2000 American Geophysical Union