Low concentration pollutants in the atmosphere can be detected by their infrared absorption spectra. We use a diode laser spectrometer in a dual-beam configuration for this purpose. The laser source is frequency modulated to provide the sensitivity enhancement associated with derivative spectroscopy. One of the laser beams is passed through a reference cell containing the gas to be detected in order to lock the laser frequency to the center of the absorption line. The other beam passes through a White cell with 64 m absorption path length. Sample air is sucked through this cell at a pressure of about 100 mbar. Although the pressure reduction reduces the density of absorbing molecules by a factor of ten, the increase in absorption cross section due to the narrowing linewidth nearly compensates this effect and drastically reduces interference from other gases. The absorption is observed as a modulation of the laser intensity at twice the modulation frequency. The intensity modulation is proportional to the second derivative of the absorption line. The spectrometer was used in a field experiment on board a research vessel in the North Sea for the measurement of HCI in the plumes of incineration ships. A HCI detection sensitivity of 50 ppbii-Lz was achieved.