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This proof‐of‐concept study demonstrates that methane (CH4) emissions from natural gas (NG) and agriculture can be disentangled using the concept of excess column observations. A network of cost‐effective sensors measured excess column‐averaged dry‐air mole fractions for CH4 (ΔXCH4), ethane (ΔXC2H6 as NG tracer), and ammonia (ΔXNH3 from agriculture) in the Denver‐Julesburg Basin during March 2015. ΔXCH4 varied up to 17 ppb and was >3 times higher with winds from directions where NG is produced. The ΔXCH4 variance is explained by variations in the C2H6‐NH3 tracer pair, attributing 63 ± 17% to NG, 25 ± 10% to agriculture, and 12 ± 12% to other sources. The ratios ΔXC2H6/ΔXCH4 (16 ± 2%; indicates wet NG) and ΔXNH3/ΔXCH4 (43 ± 12%) were compatible with in situ measured ratios. Excess columns are independent of boundary layer height, characterize gases in the open atmosphere, are inherently calibrated, average over extended spatial scales, and provide a complementary perspective to quantify and attribute CH4 emissions on regional scales.

Abstract. We describe the University of Colorado mobile Solar Occultation Flux instrument (CU mobile SOF). The instrument consists of a digital mobile solar tracker that is coupled to a Fourier transform spectrometer (FTS) of 0.5 cm−1 resolution and a UV–visible spectrometer (UV–vis) of 0.55 nm resolution. The instrument is used to simultaneously measure the absorption of ammonia (NH3), ethane (C2H6) and nitrogen dioxide (NO2) along the direct solar beam from a moving laboratory. These direct-sun observations provide high photon flux and enable measurements of vertical column densities (VCDs) with geometric air mass factors, high temporal resolution of 2 s and spatial resolution of 5–19 m. It is shown that the instrument line shape (ILS) of the FTS is independent of the azimuth and elevation angle pointing of the solar tracker. Further, collocated measurements next to a high-resolution FTS at the National Center for Atmospheric Research (HR-NCAR-FTS) show that the CU mobile SOF measurements of NH3 and C2H6 are precise and accurate; the VCD error at high signal to noise ratio is 2–7 %. During the Front Range Air Pollution and Photochemistry Experiment (FRAPPE) from 21 July to 3 September 2014 in Colorado, the CU mobile SOF instrument measured median (minimum, maximum) VCDs of 4.3 (0.5, 45)  ×  1016 molecules cm−2 NH3, 0.30 (0.06, 2.23)  ×  1016 molecules cm−2 NO2 and 3.5 (1.5, 7.7)  ×  1016 molecules cm−2 C2H6. All gases were detected in larger 95 % of the spectra recorded in urban, semi-polluted rural and remote rural areas of the Colorado Front Range. We calculate structure functions based on VCDs, which describe the variability of a gas column over distance, and find the largest variability for NH3. The structure functions suggest that currently available satellites resolve about 10 % of the observed NH3 and NO2 VCD variability in the study area. We further quantify the trace gas emission fluxes of NH3 and C2H6 and production rates of NO2 from concentrated animal feeding operations (CAFO) using the mass balance method, i.e., the closed-loop vector integral of the VCD times wind speed along the drive track. Excellent reproducibility is found for NH3 fluxes and also, to a lesser extent, NO2 production rates on 2 consecutive days; for C2H6 the fluxes are affected by variable upwind conditions. Average emission factors were 12.0 and 11.4 gNH3 h−1 head−1 at 30 °C for feedlots with a combined capacity for  ∼  54 000 cattle and a dairy farm of  ∼  7400 cattle; the pooled rate of 11.8 ± 2.0 gNH3 h−1 head−1 is compatible with the upper range of literature values. At this emission rate the NH3 source from cattle in Weld County, CO (535 766 cattle), could be underestimated by a factor of 2–10. CAFO soils are found to be a significant source of NOx. The NOx source accounts for  ∼  1.2 % of the N flux in NH3 and has the potential to add  ∼  10 % to the overall NOx emissions in Weld County and double the NOx source in remote areas. This potential of CAFO to influence ambient NOx concentrations on the regional scale is relevant because O3 formation is NOx sensitive in the Colorado Front Range. Emissions of NH3 and NOx are relevant for the photochemical O3 and secondary aerosol formation.