Urban air pollution monitoring: laser-based procedure for the detection of NOx gases

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 NO_x(x= 1,2) in the urban environment. The average concentrations of NO_x 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.