URBAN AIR-POLLUTION MONITORING - LASER-BASED PROCEDURE FOR THE DETECTION OF NOX GASES

Urban air quality is at present a matter of considerable importance in many cities throughout Europe and the USA. The combustion of fossil f uels and, in particular, automobile emissions have lead to the increas e of NOx (x = 1,2) in the urban environment. The average concentration s of NOx in the-atmosphere vary considerably from city to city in Amer ica and Europe. They are also a function of weather conditions and tim e of day but an EC (now EU) directive suggests that they Should not ex ceed 135 mu g m(-3) (72 ppb by volume). Many other urban pollutants ar e also required to be measured and current legislation is forcing scie ntists to develop better detection techniques which can be applied to monitoring a number of different pollutants with the Same instrumentat ion. To this end,a laser-based procedure, using simple ionization cham bers, has been developed in Glasgow which will detect NO in N-2 down t o sub-parts per billion levels and NO2 in air to a few parts per billi on. A number of different laser wavelengths can be used but a two-phot on 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. Thi s procedure can also be used for a number of other urban pollutants us ing a single laser operating in the wavelength, range 200-400 nm.