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We use the GEOS-Chem global 3-D chemistry transport model to investigate therelative importance of chemical and physical processes that determineobserved variability of hydrogen cyanide (HCN) in the troposphere and lowerstratosphere. Consequently, we reconcile ground-based FTIR columnmeasurements of HCN, which show annual and semi-annual variations, withrecent space-borne measurements of HCN mixing ratio in the tropical lowerstratosphere, which show a large two-year variation. We find that theobserved column variability over the ground-based stations is determined by asuperposition of HCN from several regional burning sources, with GEOS-Chemreproducing these column data with a positive bias of 5%. GEOS-Chemreproduces the observed HCN mixing ratio from the Microwave Limb Sounder andthe Atmospheric Chemistry Experiment satellite instruments with a meannegative bias of 20%, and the observed HCN variability with a mean negativebias of 7%. We show that tropical biomass burning emissions explain most ofthe observed HCN variations in the upper troposphere and lower stratosphere(UTLS), with the remainder due to atmospheric transport and HCN chemistry. Inthe mid and upper stratosphere, atmospheric dynamics progressively exertsmore influence on HCN variations. The extent of temporal overlap betweenAfrican and other continental burning seasons is key in establishing theapparent bienniel cycle in the UTLS. Similar analysis of other, shorter-livedtrace gases have not observed the transition between annual and biennielcycles in the UTLS probably because the signal of inter-annual variationsfrom surface emission has been diluted before arriving at the lowerstratosphere (LS), due to shorter atmospheric lifetimes

What drives the observed variability of HCN in the troposphere and lower stratosphere?