As part of a program to develop quality assurance (QA) and quality control (QC) procedures for open-path Fourier transform infrared (FT-IR) monitoring, ambient gas concentrations were measured daily over an 11-month period. Some of these data will be used to describe the seasonal diurnal variation of atmospheric gases. To validate these data, a daily protocol was established to verify the long-term performance of the FT-IR system. This protocol includes measuring the electronic noise, the intensity of the return signal, and the magnitude of the baseline noise; examining the features and profiles of the single-beam spectra; and determining the repeatability of the position and full width at half height of selected absorption bands. Several atmospheric gases, such as methane, nitrous oxide, and cabon monoxide, are present in virtually all spectra taken. The use of the ambient concentration measurements of these species for QA/QC purposes was investigated. Of these gases, the ambient concentrations of nitrous oxide are the most stable, which makes it the most promising gas for use as a surrogate standard. In addition to the tests that were used to determine the stability of the instrument, procedures were used to evaluate the validity of the analysis method.