Abstract

We construct global atmospheric budgets of HCN and CH₃CN through a 3-D simulation of the HCN-CH₃CN-CO system constrained and evaluated with aircraft observations from the TRACE-P mission over the NW Pacific in February-April 2001. Observed background vertical gradients of HCN and CH₃CN imply a dominant ocean sink for both gases, with deposition velocity of 0.13 cm s^-1 for both and saturation ratios of 0.79 for HCN and 0.88 for CH₃CN. Observations for both gases in the free troposphere implied a dominant source from biomass burning. Enhancement of HCN observed in Chinese urban plumes is attributed tentatively to residential coal burning. Biomass burning and residential coal burning emission ratios relative to CO of 0.27% and 1.6% respectively for HCN, and of 0.20% and 0.25% respectively for CH₃CN, are consistent with observations in biomass burning and Chinese urban plumes, and provide the best fit in the model for simulation of observed TRACE-P vertical profiles, HCN-CH₃CN-CO correlations, as well as long-term records of HCN columns and CH₃CN observations over the northern Indian Ocean. Biomass burning and residential coal burning contribute 0.63 and 0.2 Tg N yr^-1 respectively to global HCN and 0.47 and 0.03 Tg N yr^-1 respectively to CH₃CN. Ocean uptake is the dominant sink for both gases, with oxidation by OH representing an additional minor sink. The resulting tropospheric lifetimes are 5.3 months for HCN and 5.8 months for CH₃CN. The model predicts very low HCN and CH₃CN concentrations at high southern latitudes, reflecting the assumption of uniform saturation ratio; observations in that region are needed. In the free troposphere, the dominance of biomass burning sources (70-85% for HCN and 90-95% for CH₃CN) implies that both gases can be used as biomass burning tracers. More work is needed to identify the urban source apparent in the Chinese plume observations.