We demonstrate the use of compact solar-tracking Fourier transform spectrometers (Bruker EM27/SUN) for differential measurements of the column-averaged dry-air mole fractions of CH_4 and CO_2 within urban areas. Using Allan variance analysis, we show that the differential column measurement has a precision of 0.01 % for X_(CO_2) and X_(CH_4) with an optimum integration time of 10 min, corresponding to Allan deviations of 0.04 ppm and 0.2 ppb,respectively. The sensor system is very stable over time and after relocation across the continent. We report tests of the differential column measurement,and its sensitivity to emission sources, by measuring the downwind-minus-upwind column difference ΔX_(CH_4) across dairy farms in the Chino area, California, and using the data to verify emissions reported in the literature. Ratios of spatial column differences ΔX_(CH_4)∕ΔX_(CO_2) were observed across Pasadena within the Los Angeles basin, indicating values consistent with regional emission ratios from the literature. Our precise, rapid measurements allow us to determine significant short-term variations (5–10 min) of X_(CO_2) and X_(CH_4) and to show that they represent atmospheric phenomena.
Overall, this study helps establish a range of new applicationsfor compact solar-viewing Fourier transform spectrometers. Byaccurately measuring the small differences in integrated column amounts acrosslocal and regional sources, we directly observe the mass loadingof the atmosphere due to the influence of emissions in theintervening locale. The inference of the source strength is muchmore direct than inversion modeling using only surface concentrationsand less subject to errors associated with small-scale transportphenomena
