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We present a multi-line flame thermometry technique based on mid-infrared direct absorption spectroscopy of carbon dioxide at its fundamental around 4.2 μm that is particularly suitable for sooting flames. Temperature and concentration profiles of gas phase molecules in a flame are important characteristics to understand its flame structure and combustion chemistry. One of the standard laboratory flames to analyze polycyclic aromatic hydrocarbons (PAH) and soot formation is laminar non-premixed co-flow flame, but PAH and soot introduce artifact to most non-contact optical measurements. Here we report an accurate diagnostic method of the temperature and concentration profiles of CO₂ in ethylene diffusion flames by measuring its vibrational fundamental. An interband cascade laser was used to probe the R-branch bandhead at 4.2 μm, which is highly sensitive to temperature change, free from …