The CU Mobile Solar Occultation Flux instrument: structure functions and emission rates of NH<sub>3</sub>, NO<sub>2</sub> and C<sub>2</sub>H<sub>6</sub> Journal Article uri icon



  • 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&amp;thinsp;cm<sup>&amp;minus;1</sup> resolution), and a UV-Visible Spectrometer (UV-Vis, 0.55&amp;thinsp;nm resolution). The instrument is used to simultaneously measure the absorption of ammonia (NH<sub>3</sub>), ethane (C<sub>2</sub>H<sub>6</sub>), and nitrogen dioxide (NO<sub>2</sub>) 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&amp;thinsp;sec) and spatial (5&amp;ndash;19&amp;thinsp;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 NH<sub>3</sub> and C<sub>2</sub>H<sub>6</sub> are precise and accurate; the VCD error at high signal to noise is 2&amp;ndash;7&amp;thinsp;%. During the Front Range Air Pollution and Photochemistry Experiment (FRAPPE, 21 July&amp;ndash;3 September 2014) in Colorado, the CU mobile SOF instrument measured median (maximum, minimum) VCDs of 4.3 (45, 0.5)&amp;thinsp;x&amp;thinsp;10<sup>16</sup>&amp;thinsp;molecules/cm<sup>2</sup> NH<sub>3</sub>, 0.30 (2.23, 0.06)&amp;thinsp;x&amp;thinsp;10<sup>16</sup>&amp;thinsp;molecules/cm<sup>2</sup> NO<sub>2</sub>, and 3.5 (7.7, 1.5)&amp;thinsp;x&amp;thinsp;10<sup>16</sup>&amp;thinsp;molecules/cm<sup>2</sup> C<sub>2</sub>H<sub>6</sub>. All gases were detected in larger 95&amp;thinsp;% 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 NH<sub>3</sub>. The structure functions suggest that currently available satellites resolve about 10&amp;thinsp;% of the observed NH<sub>3</sub> and NO<sub>2</sub> VCD variability in the study area. We further quantify the trace gas emission fluxes of NH<sub>3</sub> and C<sub>2</sub>H<sub>6</sub> and production rates of NO<sub>2</sub> 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 NH<sub>3</sub> fluxes, and to a lesser extent also NO<sub>2</sub> production rates on two consecutive days; for C<sub>2</sub>H<sub>6</sub> the fluxes are affected by variable upwind conditions. Average emission factors were 12.0 and 11.4&amp;thinsp;gNH<sub>3</sub>/hr/head at 30&amp;thinsp;°C for feedlots with a combined capacity for ~&amp;thinsp;54,000 cattle, and a dairy farm of ~&amp;thinsp;7400 cattle; the pooled rate of 11.8&amp;thinsp;±&amp;thinsp;2.0&amp;thinsp;gNH<sub>3</sub>/hr/head is compatible with the upper range of literature values. At this emission rate the NH<sub>3</sub> source from cattle in Weld County, CO (535,766 cattle) could be underestimated by a factor of 2&amp;ndash;10. CAFO soils are found to be a significant source of NO<sub>x</sub>. The NO<sub>x</sub> source accounts for ~&amp;thinsp;1.2&amp;thinsp;% of the N-flux in NH<sub>3</sub>, and has the potential to add ~&amp;thinsp;10&amp;thinsp;% to the overall NO<sub>x</sub> emissions in Weld County, and double the NO<sub>x</sub> source in remote areas. This potential of CAFO to influence ambient NO<sub>x</sub> concentrations on the regional scale is relevant because O<sub>3</sub> formation is NO<sub>x</sub> sensitive in the Colorado Front Range. Emissions of NH<sub>3</sub> and NO<sub>x</sub> are relevant for the photochemical O<sub>3</sub> and secondary aerosol formation.;

publication date

  • August 24, 2016

Full Author List

  • Kille N; Baidar S; Handley P; Ortega I; Sinreich R; Cooper OR; Hase F; Hannigan JW; Pfister G; Volkamer R

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