KnowitAll Project – Wallace and Hobbs #4 – Atmospheric Composition

by Joe Archive on February 13, 2008

1.3.2 Atmospheric composition as a function of heightbr /br /The distributions of various species vertically in the atmosphere is determined by the competing influences of molecular diffusion and fluid mixing.br /br /Diffusion – mixing by random molecular motions. Lighter particles tend to diffuse upwards faster than heavier particles, due to gravity increasing with particle mass while the forces involved in collisions remaining (approximately) constant. Thus the mean molecular weight decreases with height, and only the lightest gases are present at the highest levels.br /br /Mixing due to fluid motions doesn’t select any particular molecular weights. So, at heights where mixing dominates, the atmospheric composition is constant.br /br /In the troposphere and stratosphere, the mean free paths are so short that the diffusive separation by mass is negligible compared to the mixing processes, so the atmospheric composition is roughly constant. One important caveat (not in this section of WH) is water – because of the various phase changes of water, the mixing ratios for water vapor, liquid and ice are not constant throughout the tropo- and stratospheres. Further, the liquid and solid phases form very heavy condensates that fall out of the atmosphere faster than vertical mixing can lift them upwards.br /br /Above ~100km, the diffusive processes begin to dominatebr /br /1.3.3 Escape of light elementsbr /br /Above about 500km, the mean free path (average distance between collisions) is so long that parcels can travel a very long, ballistic, trajectory between collisions. In this region also, a fraction of atoms will possess speeds in excess of the escape velocity (approx 11km/s) and will escape to space.br /br /The speed distribution is given by a Boltzmann distribution. The mean velocity is proportional to m^{-1/2}, so lighter species have a higher mean velocity. Due to the shape of the Boltzmann distribution, the fraction of molecules with speeds higher than the escape velocity is decreases exponentially with the ratio of the escape velocity to the mean velocity.br /br /eg, for Hydrogen in the exosphere, 1/10^6 molecules possess escape velocity, whereas, for O_2, only 1 in 10^84 molecules can escape. Thus, the escape of hydrogen from the atmosphere has been considerable over the lifetime of the planet, whereas the escape of oxygen has been negligibe.br /br /1.3.4 Variable Constituentsbr /Water vapor (ok, so here is where they mention this…) and ozone vary considerably in space and time. This is due to the various phase and chemical changes these species are involved in (which are significant when compared to their small concentrations in the atmosphere.

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