Troposphere mixing times

Recall that tropospheric mixing times are about 2 weeks from the ground to the tropopause, about 3 months from pole to tropics, and about 1 year between hemispheres. 

Carbon Monoxide. Methods for determining carbon monoxide include detection by conversion to mercury vapor, gas filter correlation spectrometry, TDLAS, and grab sampling followed by gas chromatograph (GC) analysis. The quantitative liberation of mercury vapor from mercury oxide by CO has been used to measure CO (73). The mercury vapor concentration is then measured by flameless atomic absorption spectrometry. A detection limit of 0.1 ppbv was reported for a 30-s response time. Accuracy was reported to be 3% at tropospheric mixing ratios. A commercial instrument providing similar performance is available. 

Because of this growth, which is observed in the abundance of other halocarbons as well, "average" reference profiles as required for model atmospheres can only be determined for a specified time [24], Likewise, averaging over a time span of several years is not appropriate when absolute numbers of the mixing ratios are taken. We have, therefore, calculated relative abundances for every individual balloon profile with respect to tropospheric mixing ratios as measured at the same time. This use of relative data with respective tropospheric mixing ratios fixed at 100% has another advantage, as it eliminates the errors related to the absolute calibration. 

In general, the troposphere has a vertical mixing time of a month or less, and the altitude profiles for species with photochemical lifetimes of one month or more are each well described by a single mixing ratio. 

Levy (155) calculated an mixing ratio of 0.7 ppm and a tropospheric residence time of 2 yr. From this agreement it appears that both the major source and the major sink for H2 are homogeneous gas-phase reactions in the troposphere. 

Within the steady-state approximation, the total column loss rate for CH and the total column production and loss rates for CO [Wofsy et al. (256) and Levy (155)] and for H2 [Levy (155)] have been calculated. The resulting residence times and tropospheric mixing ratios are in harmony with recent experimental results, with the exception of the n(C0) mixing ratio. This ratio is... 

Chemicals become well mixed throughout large portions of the troposphere in a matter of weeks. Exchange of atmospheric chemicals across the equator, however, is relatively slow compared with the mixing time within either hemisphere alone. Thus, for some purposes, the troposphere may be thought of as containing two boxes, the Northern and Southern Hemispheres. Within each box, chemicals mix in a few weeks to mix between them, however, requires 1 to 2 years. 

Carbonyl sulfide is the most abundant sulfur gas in the global background atmosphere because of its low reactivity in the troposphere and its correspondingly long residence time. It is the only sulfur compound that survives to enter the stratosphere. (An exception is the direct injection of SO2 into the stratosphere in volcanic eruptions.) In fact, the input of OCS into the stratosphere is considered to be responsible for the maintenance of the normal stratospheric sulfate aerosol layer. Measurements of atmospheric OCS mixing ratios and surface fluxes have been reviewed by Chin and Davis (1995). OCS exhibits an average tropospheric mixing ratio of about 500 ppt. 

This process does not lead to net ozone depletion because it is rapidly followed by reaction 2, which regenerates the ozone. Reactions 2 and 3 have, however, another important function, namely the absorption of solar energy as a result, the temperature increases with altitude, and this inverted temperature profile gives rise to the stratosphere (see Figure 1). In the lower layer, the troposphere, the temperature decreases with altitude and vertical mixing occurs on a relatively short time scale. In contrast, the stratosphere is very stable towards vertical mixing because of its inverted temperature profile. 

As shown in Fig. 3, CHEMGL considers 10 major well-mixed compartments air boundary layer, free troposphere, stratosphere, surface water, surface soil, vadose soil, sediment, ground water zone, plant foliage and plant route. In each compartment, several phases are included, for example, air, water and solids (organic matter, mineral matter). A volume fraction is used to express the ratio of the phase volume to the bulk compartment volume. Furthermore, each compartment is assumed to be a completely mixed box, which means all environmental properties and the chemical concentrations are uniform in a compartment. In addition, the environmental properties are assumed to not change with time. Other assumptions made in the model include continuous emissions to the compartments, equilibrium between different phases within each compartment and first-order irreversible loss rate within each compartment [38]. 

As discussed in Chapter 2, the troposphere is characterized by decreasing temperature with altitude, whereas the opposite is true for the stratosphere. In the troposphere, vertical mixing occurs on a time scale... 

The rhodium-102 and cadmium-109 high altitude tracer experiments provide the best information on the pattern and time-scale of downward mixing from the mesosphere to the lower stratosphere and troposphere (20, 21). The 102Rh tracer injected into the lower thermosphere on Aug. 11, 1958, mixed down into the lower stratosphere selectively at high lati-... 

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