A sensor for NO2 with ultrahigh sensitivity of detection is created by combining off-axis integrated cavity output spectroscopy (OA-ICOS) (which can provide large path lengths of the order of several km in a small volume cell) with multiple line integrated absorption spectroscopy (MLIAS) (where the absorption spectra is integrated over a large number of rotational-vibrational transitions of the molecular species to further improve the sensitivity). Employing an external cavity tunable quantum cascade laser operating in the 1601-1670 cm−1 range and a high finesse optical cavity, the absorption spectra of NO2 over 100 transitions in the R-band have been recorded. Based on the observed linear relationship between the integrated absorption vs. concentration of NO2, the sensor has an effective sensitivity of detection of 28 ppt for NO2, which is among the most sensitive levels of detection of NO2 to date. This technique can be applied using any tunable laser source, such as diode lasers, capable of tuning over multiple lines of target species including complex molecules and explosive compounds.