Urban air pollution monitoring: laser-based procedure for the detection of NO x gases

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Abstract
Urban air quality is at present a matter of considerable importance in many cities throughout Europe and the USA. The combustion of fossil fuels and, in particular, automobile emissions have lead to the increase of NOx(x= 1,2) in the urban environment. The average concentrations of NOx in the atmosphere vary considerably from city to city in America and Europe. They are also a function of weather conditions and time of day but an EC (now EU) directive suggests that they should not exceed 135 µg m–3(72 ppb by volume). Many other urban pollutants are also required to be measured and current legislation is forcing scientists to develop better detection techniques which can be applied to monitoring a number of different pollutants with the same instrumentation. To this end, a laser-based procedure, using simple ionization chambers, has been developed in Glasgow which will detect NO in N2 down to sub-parts per billion levels and NO2 in air to a few parts per billion. A number of different laser wavelengths can be used but a two-photon process at 226 nm or a three-photon process at 384 nm are likely to be the most sensitive and selective wavelengths for this purpose. This procedure can also be used for a number of other urban pollutants using a single laser operating in the wavelength range 200–400 nm.

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