An apparatus for monitoring air pollution sources, such as, for example nitrous oxide (NO), sulphur dioxide (SO2), or sulphur trioxide (SO3) is disclosed. The apparatus includes a tunable diode laser as a transmitter of infrared energy. The diode laser is tuned in a periodic fashion such that the laser light emission wavelength is scanned through an absorption line of a pollutant gas. The tuning effect is produced by driving the diode laser with a current which is the sum of a d.c. and an a.c. current. The laser energy is transmitted to a beamsplitter. The beam-splitter divides the energy between a sample path and a reference path. The sample path may be an open air path or include a sample cell; the reference path contains a cell having a known concentration of the pollutant gas being monitored. A chopper is provided to alternately sample the radiation passing the sample and reference cells to a detector which produces an a.c. signal whose peak to peak difference value contains the information of the difference in concentration levels of the contaminant in the reference and sample paths. A feedback circuit is provided to feed-back a d.c. current, equivalent to the time difference between the offset in the center point of the diode emission and the center of the contaminant absorption line, to the laser drive circuit to provide automatic lock onto the center of the contaminant absorption line.
