A method for in-situ monitoring of an emission product includes transmitting a light, tuning the light to a first wavelength, receiving the light at a second location, varying the light from the first wavelength to a second wavelength during a first period, measuring a first absorption line and a first non-absorbing baseline signal during the first period, switching the light to a third wavelength, varying the light from the third wavelength to a fourth wavelength over a second period, and measuring a second absorption line and a second non-absorbing baseline signal during the second period. The light is transmitted from a first location by a tunable light source and received at a second location. The light passes along an optical path through the emission product between the first and second locations. The first wavelength corresponds to the first absorption line of the emission product. The third wavelength is different from the first and second wavelengths.