Routine observations of atmospheric NO2 at concentrations ranging from 0.1 to 100 parts per billion are needed for air quality monitoring and for the evaluation of photochemical models. We have designed, constructed, and field tested a relatively inexpensive and specific NO2 sensor using laser-induced fluorescence. The instrument combines a commercial cw external-cavity tunable diode laser
640 nm and a continuous supersonic expansion. The total package is completely automated, has a modest size of 0.5 m3 and 118 kg, and could be manufactured at competitive prices with the current generation of instruments. The sensitivity of the instrument is 145 parts per trillion by volume min1
signal-to-noise ratio of 2, which is more than adequate for monitoring purposes.