Open-path Fourier transform infrared (OP-FTIR) spectroscopy is an accepted technology for measuring gaseous air contaminants. OP-FTIR absorbance spectra acquired during changing aerosols conditions reveal related changes in very broad baseline features. Usually, this shearing of a spectrum's baseline is viewed as undesirable because it interferes with quantifying gases. However, the paper shows that this wavelength-dependent absorbance can be used to measure particulate matter (PM). It describes a developed inversion algorithm for retrieving the size distribution of the PM in the beam path. It gives preliminary results from OP-FTIR inversion algorithm and measured optical properties, the calculated extinction spectra were fitted to the measured extinction, using the Mile theory for spherical particles. Time series data reveal significant rapid shifts in size distribution of the PM in the OP-FTIR. Results indicate that size distribution parameters may be retrieved from OP-FTIR spectra acquired over an open optical path. The suggested method may provide real-time concentrations of gaseous and PM contaminants simultaneously. Emission flux estimate of PM may be generated for near-ground-level (roads) and area sources when several beam paths are deployed downwind.