Oliver Wild* and Michael J. Prather
University of California, Irvine
* Now at the Frontier Research System for Global Change, Tokyo.
Coupling of local chemical processes over the globe by atmospheric transport leads to the existence of chemical modes that are a fundamental characterization of global atmospheric chemistry and provide a true description of the atmospheric response to small changes in trace-gas emissions. Such coupled chemistry-transport modes in global tropospheric chemistry are an inherent feature of 3-D chemical transport models (CTMs). In CTMs, these modes cannot be solved for explicitly, as they have been for the case of low-order, fully linearized systems, but they are investigated here through a series of perturbation experiments. When using meteorological fields that re-cycle every year, the long-lived modes are readily seen as seasonal decay patterns that e-fold each year. An important application of chemical modes is the study of how emissions of CO and NO excite perturbations to the CH4-like mode, the longest-lived (primary) mode found in tropospheric chemistry (i.e., with fixed stratospheric composition). Perturbation experiments are conducted with the UCI GISS II' 3-D tropospheric CTM to identify this primary tropospheric mode and to determine its 5-dimensional structure. The previous demonstrations of a long-lived chemical mode with 1.5 times the lifetime of CH4 are corroborated. The ability of emissions of CO and NO to excite this mode is then demonstrated, and a quantitative evaluation of the indirect effect of CO emissions on the greenhouse gases CH4 and tropospheric O3 is made, showing that 100 kg of CO is equivalent to 5--6 kg of CH4 emissions.
J. Geophys. Res., 105, 24647-24660, 2000.
(Submitted: March 2000; Revised: June 2000; Accepted: August 2000)