The need for the detection of chemical warfare agents (CWAs) is no longer confined to battlefield environments because of at least one confirmed terrorist attack, the Tokyo Subway [Emerg. Infect. Dis. 5, 513 (1999)] in 1995, and a suspected, i.e., a false-alarm of a CWA in the Russell Senate Office Building [Washington Post, 9 February 2006, p. B01]. Therefore, detection of CWAs with high sensitivity and low false-alarm rates is considered an important priority for ensuring public safety. We report a minimum detection level for a CWA simulant, dimethyl methyl phosphonate (DMMP), of $<0.5\mathrm{ppb}$ (parts in ${10}^9$ ) by use of a widely tunable external grating cavity quantum cascade laser and photoacoustic spectroscopy. With interferents present in Santa Monica, California street air, we demonstrate a false-alarm rate of $1:{10}^6$ at a detection threshold of $1.6\mathrm{ppb}$ .