*Article* **Fast Quantification of Air Pollutants by Mid-Infrared Hyperspectral Imaging and Principal Component Analysis**

**Juan Meléndez and Guillermo Guarnizo \***

LIR–Infrared Laboratory, Department of Physics, Universidad Carlos III de Madrid, 28911 Leganés, Spain; juan.melendez@uc3m.es

**\*** Correspondence: guillermoandres.guarnizo@uc3m.es

**Abstract:** An imaging Fourier-transform spectrometer in the mid-infrared (1850–6667 cm−1) has been used to acquire transmittance spectra at a resolution of 1 cm−<sup>1</sup> of three atmospheric pollutants with known column densities (Q): methane (258 ppm·m), nitrous oxide (107.5 ppm·m) and propane (215 ppm·m). Values of Q and T have been retrieved by fitting them with theoretical spectra generated with parameters from the HITRAN database, based on a radiometric model that takes into account gas absorption and emission, and the instrument lineshape function. A principal component analysis (PCA) of experimental data has found that two principal components are enough to reconstruct gas spectra with high fidelity. PCA-processed spectra have better signal-to-noise ratio without loss of spatial resolution, improving the uniformity of retrieval. PCA has been used also to speed up retrieval, by pre-calculating simulated spectra for a range of expected Q and T values, applying PCA to them and then comparing the principal components of experimental spectra with those of the simulated ones to find the gas Q and T values. A reduction in calculation time by a factor larger than one thousand is achieved with improved accuracy. Retrieval can be further simplified by obtaining T and Q as quadratic functions of the two first principal components.

**Keywords:** infrared imaging; multispectral and hyperspectral imaging; air pollution monitoring; remote sensing and sensors; spectroscopy; fourier transform; image processing
