A modelling study of tropospheric distributions of the trace gases CFCl3 and CH3CCl3 in the 1980s
Interhemispheric transport is a key process aecting the accuracy of source quanti®cation for species such as methane by inverse modelling, and is a source of dierence among global three-dimensional chemistry transport models (CTMs). Here we use longterm observations of the atmospheric concentration of long-lived species such as CH 3 CCl 3 and CFCl 3 for testing three-dimensional chemistry transport models (CTMs); notably their ability to model the interhemispheric transport, distribution,
... distribution, trend, and variability of trace gases in the troposphere. The very striking contrast between the inhomogeneous source distribution and the nearly homogeneous trend, observed in the global ALE/ GAGE experiments for both CH 3 CCl 3 and CFCl 3 illustrates an ecient interhemispheric transport of atmospherically long-lived chemical species. Analysis of the modelling data at two tropical stations, Barbados (13 N, 59 W) and Samoa (14 S, 124 W), show the close relationship between inter-hemispheric transport and cross-equator Hadley circulations. We found that crossequator Hadley circulations play a key role in producing the globally homogeneous observed trends. Chemically, the most rapid interaction between CH 3 CCl 3 and OH occurs in the northern summer troposphere; while the most rapid photolysis of CH 3 CCl 3 and CFCl 3 , and the chemical reactions between CFCl 3 and O( 1 D), take place in the southern summer stratosphere. Therefore, the cross-equator Hadley circulation plays a key role which regulates the southward¯ux of chemical species. The regulation by the Hadley circulations hence determines the amount of air to be processed by OH, O( 1 D), and ultraviolet photolysis, in both hemispheres. In summary, the dynamic regulation of the Hadley circulations, and the chemical processing (which crucially depends on the concentration of OH, O( 1 D), and on the intensity of solar insolation) of the air contribute to the seasonal variability and homogeneous growth rate of observed CH 3 CCl 3 and CFCl 3 .