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



  • &lt;p&gt;&lt;strong&gt;Abstract.&lt;/strong&gt; 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;#8201;cm&lt;sup&gt;&amp;#8722;1&lt;/sup&gt; resolution), and a UV-Visible Spectrometer (UV-Vis, 0.55&amp;#8201;nm resolution). The instrument is used to simultaneously measure the absorption of ammonia (NH&lt;sub&gt;3&lt;/sub&gt;), ethane (C&lt;sub&gt;2&lt;/sub&gt;H&lt;sub&gt;6&lt;/sub&gt;), and nitrogen dioxide (NO&lt;sub&gt;2&lt;/sub&gt;) 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;#8201;sec) and spatial (5&amp;#8211;19&amp;#8201;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&lt;sub&gt;3&lt;/sub&gt; and C&lt;sub&gt;2&lt;/sub&gt;H&lt;sub&gt;6&lt;/sub&gt; are precise and accurate; the VCD error at high signal to noise is 2&amp;#8211;7&amp;#8201;%. During the Front Range Air Pollution and Photochemistry Experiment (FRAPPE, 21 July&amp;#8211;3 September 2014) in Colorado, the CU mobile SOF instrument measured median (maximum, minimum) VCDs of 4.3 (45, 0.5)&amp;#8201;x&amp;#8201;10&lt;sup&gt;16&lt;/sup&gt;&amp;#8201;molecules/cm&lt;sup&gt;2&lt;/sup&gt; NH&lt;sub&gt;3&lt;/sub&gt;, 0.30 (2.23, 0.06)&amp;#8201;x&amp;#8201;10&lt;sup&gt;16&lt;/sup&gt;&amp;#8201;molecules/cm&lt;sup&gt;2&lt;/sup&gt; NO&lt;sub&gt;2&lt;/sub&gt;, and 3.5 (7.7, 1.5)&amp;#8201;x&amp;#8201;10&lt;sup&gt;16&lt;/sup&gt;&amp;#8201;molecules/cm&lt;sup&gt;2&lt;/sup&gt; C&lt;sub&gt;2&lt;/sub&gt;H&lt;sub&gt;6&lt;/sub&gt;. All gases were detected in larger 95&amp;#8201;% 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&lt;sub&gt;3&lt;/sub&gt;. The structure functions suggest that currently available satellites resolve about 10&amp;#8201;% of the observed NH&lt;sub&gt;3&lt;/sub&gt; and NO&lt;sub&gt;2&lt;/sub&gt; VCD variability in the study area. We further quantify the trace gas emission fluxes of NH&lt;sub&gt;3&lt;/sub&gt; and C&lt;sub&gt;2&lt;/sub&gt;H&lt;sub&gt;6&lt;/sub&gt; and production rates of NO&lt;sub&gt;2&lt;/sub&gt; 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&lt;sub&gt;3&lt;/sub&gt; fluxes, and to a lesser extent also NO&lt;sub&gt;2&lt;/sub&gt; production rates on two consecutive days; for C&lt;sub&gt;2&lt;/sub&gt;H&lt;sub&gt;6&lt;/sub&gt; the fluxes are affected by variable upwind conditions. Average emission factors were 12.0 and 11.4&amp;#8201;gNH&lt;sub&gt;3&lt;/sub&gt;/hr/head at 30&amp;#8201;&amp;#176;C for feedlots with a combined capacity for ~&amp;#8201;54,000 cattle, and a dairy farm of ~&amp;#8201;7400 cattle; the pooled rate of 11.8&amp;#8201;&amp;#177;&amp;#8201;2.0&amp;#8201;gNH&lt;sub&gt;3&lt;/sub&gt;/hr/head is compatible with the upper range of literature values. At this emission rate the NH&lt;sub&gt;3&lt;/sub&gt; source from cattle in Weld County, CO (535,766 cattle) could be underestimated by a factor of 2&amp;#8211;10. CAFO soils are found to be a significant source of NO&lt;sub&gt;x&lt;/sub&gt;. The NO&lt;sub&gt;x&lt;/sub&gt; source accounts for ~&amp;#8201;1.2&amp;#8201;% of the N-flux in NH&lt;sub&gt;3&lt;/sub&gt;, and has the potential to add ~&amp;#8201;10&amp;#8201;% to the overall NO&lt;sub&gt;x&lt;/sub&gt; emissions in Weld County, and double the NO&lt;sub&gt;x&lt;/sub&gt; source in remote areas. This potential of CAFO to influence ambient NO&lt;sub&gt;x&lt;/sub&gt; concentrations on the regional scale is relevant because O&lt;sub&gt;3&lt;/sub&gt; formation is NO&lt;sub&gt;x&lt;/sub&gt; sensitive in the Colorado Front Range. Emissions of NH&lt;sub&gt;3&lt;/sub&gt; and NO&lt;sub&gt;x&lt;/sub&gt; are relevant for the photochemical O&lt;sub&gt;3&lt;/sub&gt; and secondary aerosol formation.&lt;/p&gt;;

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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