by 
Danny
ANSWERS: 2
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Probably by using a mass spectrometer, where the sample mixture of molecules is ionized so it can be pulled by an electrostatic field. The ions might be something like O-, N-, etc. As the ions fly toward a charge-sensitive target a perpendicular magnetic field bends their trajectories. A heavier particle will bend less than a light particle. There will be a big spike at masses corresponding to nitrogen, oxygen, etc. These can be measured quantitatively to infer composition percentages. Another possible method is gas chromatography, where the different molecules in the sample mixture are separated according to their solubility or rate of diffusion. In addition, oxygen concentration is readily measured with devices that rely either on oxygen's chemical properties (Clark electrode) or on it's paramagnetic properties. Both methods give a quantitative oxygen reading but cannot measure nitrogen or other gases. I defer to Wikipedia for details on any of these methods.
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"During the last decade, predictive models have been developed to simulate the transport and dispersion of power station emissions in the Melboume area. A study of air composition was performed in, and close to, the westem suburbs to provide data for verification of these models. Non-methanic hydrocarbon concentrations were measured at ground level and from an aircraft. In order to monitor continuously concentrations at an unattended site, the Division has developed an automatic, high-precision gas chromatograph which will operate for up to seven days at a time. It is self-calibrating and can monitor individual hydrocarbons of one to ten carbon atoms in composition." Source and further information: http://www.cmar.csiro.au/e-print/open/annualreports/annualreport_1988-90.pdf Further information: http://en.wikipedia.org/wiki/Gas_chromatograph http://en.wikipedia.org/wiki/Gas_chromatography-mass_spectrometry
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