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Absorption spectroscopy is an important method for determining gas properties in harsh environments. Several studies have described how absorption spectroscopy can be used to determine spatial temperature variations along the optical path by measuring the unique, nonlinear response to temperature of quantum absorption transitions of molecules along the path. New broadband laser sources, such as dual frequency comb spectroscopy, offer the potential to improve measurements with these approaches. However, systematic techniques to fit distorted spectra that arise from nonuniform environments, and systematic studies to determine the extent to which these sources could improve line-of-sight temperature distribution sensing are sparse. In this work, we demonstrate a new approach to use broadband, high-resolution absorption spectra to determine line-of-sight temperature nonuniformity. We develop a constrained spectral fitting technique called E"-binning to fit an absorption spectrum arising from a nonuniform environment. The information extracted from the fit can then be used with an inversion approach to determine the temperature distribution. We demonstrate this approach using 58,000-point dual frequency comb absorption spectra taken at three horizontal heights across a laboratory tube furnace up to 1000K. From these three spectra, we extract three different temperature distribution shapes that have good agreement with a natural convection model. We also study the effect of optical bandwidth on nonuniform temperature distribution sensing. In Part I, we found that in the absence of noise, only the first ~14 well-selected absorption features improve the temperature resolution. Here, we show that for real-world measurements with noise and absorption model error, increasing the bandwidth and the number of measured absorption transitions may provide improvements in precision and uncertainty.

Resolving nonuniform temperature distributions with single-beam absorption spectroscopy. Part II: Implementation from broadband spectra