The CU Mobile Solar Occultation Flux instrument: structure functions and emission rates of NH3, NO2 and C2H6
Authors: Kille Natalie , Baidar Sunil , Handley Philip , Ortega Ivan , Sinreich Roman , Cooper Owen R. , Hase Frank , Hannigan James W. , Pfister Gabriele , Volkamer Rainer 
Published: August 2016

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, 0.5 cm−1 resolution), and a UV-Visible Spectrometer (UV-Vis, 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 (2 sec) and spatial (5–19 m) resolution. 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 is 2–7 %. During the Front Range Air Pollution and Photochemistry Experiment (FRAPPE, 21 July–3 September 2014) in Colorado, the CU mobile SOF instrument measured median (maximum, minimum) VCDs of 4.3 (45, 0.5) x 1016 molecules/cm2 NH3, 0.30 (2.23, 0.06) x 1016 molecules/cm2 NO2, and 3.5 (7.7, 1.5) x 1016 molecules/cm2 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 to a lesser extent also NO2 production rates on two consecutive days; for C2H6 the fluxes are affected by variable upwind conditions. Average emission factors were 12.0 and 11.4 gNH3/hr/head 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/hr/head 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.