The measurement of the important atmospheric molecules O3, NO2, CH2O, H2O, NO3, and HONO can be measured using long path differential absorption spectroscopy. The experiment is located at the Fritz Peak Observatory, 17 km west of Boulder, Colorado. This site permits both the measurement of clean continental air during times of westerly air flow, as well as polluted urban air during easterly up-slope episodes. The spectrograph used in this study is a low resolution double crossed Czerny-Turner, with a well matched receiving telescope. This spectrograph uses a 1 x 1024 element Reticon diode array detector to measure the molecular absorption spectra of these molecules in 40 nm bands in the near ultraviolet-visible region. The total optical path for this experiment is 20.6 km, and the path is folded by a 121 element retroreflector array thereby allowing the light source and spectrograph to be in the same location. Deduction of atmospheric concentrations these molecules over this path is accomplished by using a least squares procedure that employs the method of singular-value decomposition.