A modelling study of tropospheric distributions of the trace gases CFCl3 and CH3CCl3 in the 1980s
Abstract. Interhemispheric transport is a key process affecting the accuracy of source quantification for species such as methane by inverse modelling, and is a source of difference among global three-dimensional chemistry transport models (CTMs). Here we use long-term observations of the atmospheric concentration of long-lived species such as CH3CCl3 and CFCl3 for testing three-dimensional chemistry transport models (CTMs); notably their ability to model the interhemispheric transport, 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 CH3CCl3 and CFCl3 illustrates an efficient 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 cross-equator Hadley circulations play a key role in producing the globally homogeneous observed trends. Chemically, the most rapid interaction between CH3CCl3 and OH occurs in the northern summer troposphere; while the most rapid photolysis of CH3CCl3 and CFCl3, and the chemical reactions between CFCl3 and O(1D), take place in the southern summer stratosphere. Therefore, the cross-equator Hadley circulation plays a key role which regulates the southward flux of chemical species. The regulation by the Hadley circulations hence determines the amount of air to be processed by OH, O(1D), 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(1D), and on the intensity of solar insolation) of the air contribute to the seasonal variability and homogeneous growth rate of observed CH3CCl3 and CFCl3.
Key words: Atmospheric composition and structure (middle atmosphere - composition and chemistry; pollution - urban and regional) - Meteorology and atmospheric dynamics (convective processes)