An apparatus includes a reactive species source, a spectral measurement volume, a light source to emit a light beam into the spectral measurement volume, a spectrometer to receive the light beam from the spectral measurement volume. The apparatus includes an a controller configured to, when a reactive species is present in the spectral measurement volume, control the light source to emit the light beam into the spectral measurement volume and the spectrometer to determine an environment spectrum using the light beam, and when the reactive species is not present in the spectral measurement volume, control the light source to emit the light beam into the spectral measurement volume and the spectrometer to determine a baseline spectrum using the light beam, calculate a net spectrum based on a difference between the environment spectrum and the baseline spectrum, and estimate a concentration of the reactive species based on the net spectrum.