The CU mobile Solar Occultation Flux instrument: structure functions and emission rates of NH3, NO2 and C2H6

摘要

We describe the University of Colorado mobile Solar Occultation Fluxinstrument (CU mobile SOF). The instrument consists of a digital mobilesolar tracker that is coupled to a Fourier transform spectrometer (FTS) of0.5 cm resolution and a UV–visible spectrometer (UV–vis) of 0.55 nmresolution. The instrument is used to simultaneously measure the absorptionof ammonia (NH), ethane (CH) and nitrogen dioxide(NO) along the direct solar beam from a moving laboratory. Thesedirect-sun observations provide high photon flux and enable measurements ofvertical column densities (VCDs) with geometric air mass factors, hightemporal resolution of 2 s and spatial resolution of 5–19 m. It is shownthat the instrument line shape (ILS) of the FTS is independent of theazimuth and elevation angle pointing of the solar tracker. Further,collocated measurements next to a high-resolution FTS at the National Centerfor Atmospheric Research (HR-NCAR-FTS) show that the CU mobile SOFmeasurements of NH and CH are precise and accurate; theVCD error at high signal to noise ratio is 2–7 %. During the Front Range AirPollution and Photochemistry Experiment (FRAPPE) from 21 July to 3 September2014 in Colorado, the CU mobile SOF instrument measured median (minimum,maximum) VCDs of 4.3 (0.5, 45)  ×  10 molecules cm NH, 0.30(0.06, 2.23)  ×  10 molecules cm NO and 3.5 (1.5, 7.7)  ×  10 molecules cmCH. All gases were detected inlarger 95 % of the spectra recorded in urban, semi-polluted rural andremote rural areas of the Colorado Front Range. We calculate structurefunctions based on VCDs, which describe the variability of a gas column overdistance, and find the largest variability for NH. The structurefunctions suggest that currently available satellites resolve about 10 %of the observed NH and NO VCD variability in the study area.We further quantify the trace gas emission fluxes of NH andCH and production rates of NO from concentrated animalfeeding operations (CAFO) using the mass balance method, i.e., theclosed-loop vector integral of the VCD times wind speed along the drivetrack. Excellent reproducibility is found for NH fluxes and also, to alesser extent, NO production rates on 2 consecutive days; forCH the fluxes are affected by variable upwind conditions.Average emission factors were 12.0 and 11.4 gNH h head at 30 °C for feedlots with a combined capacityfor  ∼  54 000 cattle and a dairy farm of  ∼  7400 cattle; the pooledrate of 11.8 ± 2.0 gNH h head is compatible with the upperrange of literature values. At this emission rate the NH source fromcattle in Weld County, CO (535 766 cattle), could be underestimated by afactor of 2–10. CAFO soils are found to be a significant source ofNO. The NO source accounts for  ∼  1.2 % of theN flux in NH and has the potential to add  ∼  10 % tothe overall NO emissions in Weld County and double the NOsource in remote areas. This potential of CAFO to influence ambient NOconcentrations on the regional scale is relevant because O formationis NO sensitive in the Colorado Front Range. Emissions of NH andNO are relevant for the photochemical O and secondary aerosolformation.