JANUARY 2003: Global 3-D Model Evaluation of Atmospheric Budgets of HCN and CH₃CN

This figure shows GEOS-CHEM simulated monthly mean concentrations of HCN and CH₃CN in surface air and at 500 hPa for April 2001. Surface concentrations are maximum over biomass burning regions in SE Asia, central America, and central Africa (> 1000 pptv). Surface concentrations are very low (< 50 pptv) at high southern latitudes, reflecting the remoteness from sources and the sink from ocean uptake under the assumption of uniform saturation ratio. Observations in this part of the world are evidently needed. Concentrations in the middle troposphere at 500 hPa are highest over SE Asia and downwind, with a band of relatively high concentrations (> 200 pptv for HCN) at 5-40°N, due to biomass burning influence. Global atmospheric budgets of HCN and CH₃CN through a GEOS-CHEM 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. Ocean uptake is the dominant sink for both gases, resulting in tropospheric lifetimes of 5.3 months for HCN and 5.8 months for CH₃CN. This work was led by Qinbin Li and a full account is given in Li et al. [2002].