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Open-path Fourier transform infrared spectroscopy (OP-FTIR) is susceptible to environmental variables which can become sources of errors for gas quantification. In this study, we assessed the effects of water vapour, temperature, path length, and wind speed on the uncertainty of nitrous oxide (N2O) and carbon dioxide (CO2) concentrations derived from OP-FTIR spectra. The presence of water vapour resulted in underestimating N2O in both lab (−3 %) and field (−12 %) experiments at 30 °C using a classical least squares (CLS) model. Differences in temperature between the sample and reference spectra also underestimated N2O concentrations due to temperature broadening and the increased interferences of water vapour in spectra of wet samples. Changes in path length resulted in a non-linear response of spectra and bias (e.g. N2O and CO2 concentrations were underestimated by 30 % and 7.5 %, respectively, at the optical path of 100-m using CLS models). For N2O quantification, partial least squares (PLS) models were less sensitive than CLS to the influence of water vapour, temperature, and path length, and provided more accurate estimations. Uncertainties in the path-averaged concentrations increased in low wind conditions (< 2 m s-1). This study identified the most common interferences that affect OP-FTIR measurements of N2O and CO2. The quantified errors can be used as references for current or future OP-FTIR users. [ABSTRACT FROM AUTHOR]