A global three‐dimensional model analysis of the atmospheric budgets of HCN and CH₃CN: Constraints from aircraft and ground measurements

We construct global atmospheric budgets of HCN and CH 3 CN through a three‐dimensional (3‐D) model simulation of the HCN‐CH 3 CN‐CO system constrained and evaluated with aircraft observations from the Transport and Chemical Evolution Over the Pacific (TRACE‐P) mission over the NW Pacific in February–April 2001. Observed background vertical gradients of HCN and CH 3 CN 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 CH 3 CN. Observations for both gases in the free troposphere imply 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 CH 3 CN, are consistent with observations in biomass burning and Chinese urban plumes. They provide the best model simulation of the ensemble of TRACE‐P observations including vertical profiles and HCN‐CH 3 CN‐CO correlations. They also allow successful simulation of the long‐term observations of HCN columns at sites in the Northern Hemisphere, and of the CH 3 CN vertical distribution observed over the northern Indian Ocean. Global biomass burning and Asian residential coal burning sources in the model are 0.63 and 0.2 Tg N yr −1 , respectively, for HCN and 0.47 and 0.03 Tg N yr −1 , respectively, for CH 3 CN. 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 CH 3 CN. The model predicts very low HCN and CH 3 CN concentrations at high southern latitudes, reflecting the assumption of a uniform saturation ratio for ocean uptake; observations in that region are needed. In the free troposphere, the dominance of biomass burning sources (70–85% for HCN and 90–95% for CH 3 CN) implies that both gases can be used as biomass burning tracers. In the boundary layer, CH 3 CN appears to be a better biomass burning tracer. More work is needed to identify the origin of the Chinese urban source of HCN.