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<p>Open-path Fourier transform infrared spectroscopy (OP-FTIR) has often been used to measure hazardous or trace gases from hot point sources (e.g. volcano, industrial, or agricultural facilities) but seldom used to measure greenhouse gases (GHGs) from field-scale sources (e.g. agricultural soils). Closed-path mid-IR laser-based <span class="inline-formula">N₂O</span>, nondispersive-IR <span class="inline-formula">CO₂</span> analysers, and OP-FTIR were used to measure concentrations of <span class="inline-formula">N₂O</span> and <span class="inline-formula">CO₂</span> at a maize cropping system during 9–19 June 2014. To measure <span class="inline-formula">N₂O</span> and <span class="inline-formula">CO₂</span> concentrations accurately, we developed a quantitative method of <span class="inline-formula">N₂O∕CO₂</span> analysis that minimized interferences from diurnal changes of humidity and temperature. Two chemometric multivariate models, classical least squares (CLS) and partial least squares (PLS), were developed. This study evaluated various methods to generate the single-beam background spectra and different spectral regions for determining <span class="inline-formula">N₂O</span> and <span class="inline-formula">CO₂</span> concentrations from OP-FTIR spectra. A standard extractive method was used to measure the actual path-averaged concentrations along an OP-FTIR optical path in situ, as a benchmark to assess the feasibilities of these quantitative methods. Within an absolute humidity range of 5000–20&thinsp;000&thinsp;ppmv and a temperature range of 10–35&thinsp;<span class="inline-formula">^∘</span>C, we found that the CLS model underestimated <span class="inline-formula">N₂O</span> concentrations (bias&thinsp;<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M12" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>=</mo><mo>-</mo><mn mathvariant="normal">4.9</mn><mo>±</mo><mn mathvariant="normal">3.1</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="62pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="1199c60d5da8ecf5da7795e506317b9d"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-12-3403-2019-ie00001.svg" width="62pt" height="10pt" src="amt-12-3403-2019-ie00001.png"/></svg:svg></span></span>&thinsp;%) calculated from OP-FTIR spectra, and the PLS model improved the accuracy of calculated <span class="inline-formula">N₂O</span> concentrations (bias&thinsp;<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M14" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>=</mo><mn mathvariant="normal">1.4</mn><mo>±</mo><mn mathvariant="normal">2.3</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="54pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="4cf55b45aaf6347f81021eebb53ce659"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-12-3403-2019-ie00002.svg" width="54pt" height="10pt" src="amt-12-3403-2019-ie00002.png"/></svg:svg></span></span>&thinsp;%). The bias of calculated <span class="inline-formula">CO₂</span> concentrations was <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M16" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>-</mo><mn mathvariant="normal">1.0</mn><mo>±</mo><mn mathvariant="normal">2.8</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="52pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="80143b66c290d40986e3425df7c2a702"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-12-3403-2019-ie00003.svg" width="52pt" height="10pt" src="amt-12-3403-2019-ie00003.png"/></svg:svg></span></span>&thinsp;% using the CLS model. These methods suggested that environmental variables potentially lead to biases in <span class="inline-formula">N₂O</span> and <span class="inline-formula">CO₂</span> estimations from OP-FTIR spectra and may help OP-FTIR users avoid dependency on extractive methods of calibrations.</p>