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 three-dimensional 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 recycle 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 University of California, Irvine, three-dimensional tropospheric CTM to identify this primary tropospheric mode and to determine its five-dimensional structure. The previous demonstrations of a long-lived chemical mode with 1.5 times the lifetime of CH₄are 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.

Anthropogenic emissions of short-lived, chemically reactive gases, such as NO_x and CO, are known to influence climate by altering the chemistry of the global troposphere and thereby the abundance of the greenhouse gases O₃, CH₄ and the HFCs. This study uses the characteristics of the natural modes of the tropospheric chemical system to decompose the greenhouse effect of NO_x and CO emissions into (i) short-lived modes involving predominantly tropospheric O₃ and (ii) the long-lived mode involving a global coupled CH₄-CO-O₃perturbation. Combining these two classes of greenhouse perturbations—large, short-lived, regional O₃ increases and smaller, long-lived, global decreases in CH₄ and O₃—we find that most types of anthropogenic NO_x emissions lead to a negative radiative forcing and an overall cooling of the earth.

Photolysis rates in the troposphere are greatly affected by the presenceof cloud and aerosol layers. Yet, the spatial variability of theselayers along with the difficulty of multiple-scattering calculationsfor large particles makes their inclusion in 3-D chemical transportmodels computationally very expensive.This study presents a flexible and accurate photolysis scheme, Fast-J,which calculates photolysis rates in the presence of an arbitrary mix ofcloud and aerosol layers. The algorithm is sufficiently fast to allow thescheme to be incorporated into 3-D global chemical transport models andhave photolysis rates updated hourly. It enables tropospheric chemistrysimulations to include directly the physical properties of the scatteringand absorbing particles in the column, including the full, untruncatedscattering phase function and the total, uncorrected optical depth.The Fast-J scheme is compared with earlier methods that have been usedin 3-D models to parameterize the effects of clouds on photolysis rates.The impact of Fast-J on tropospheric ozone chemistry is demonstratedwith the UCI tropospheric CTM.