Mobile Solar Occultation Flux

Gases from anthropogenic sources have the potential to have a profound impact on air quality. Emissions from large cattle feedlots and oil and natural gas sites are comprised of ammonia (NH3) and ethane (C2H6) as pollutants, respectively. For the measurements of total column densities the Solar Occultation Flux (SOF) method is applied. SOF measurements use direct sunlight as source of energy to determine vertically integrated concentrations of trace gases. The measurement of total column abundance ultimately allows the emission flux and source strength to be determined when driving a closed box around or upwind and downwind of a source with the mobile laboratory and knowledge of wind fields. A commercial Bruker EM27 FTIR, covering a wide spectral range in the mid-IR, was coupled to the CU mobile solar tracker in Northern Colorado during FRAPPE to measure total column density of NH3 and C2H6.

sketch of doas
Figure 1. A) Conceptual sketch of the mobile SOF instrument components: The photons along the direct solar beam are reflected by a set of mirrors from the solar tracker. A dichroic optic separates the infrared from the UV-vis wavelengths and directs the beam towards the FTS. The UV-vis wavelengths continue through several optics before entering the optical fiber coupler directing the beam into the UV-vis grating spectrometer.


Figure 2: Conceptual sketch of the CU mobile laboratory. The solar tracker provides direct sunlight for the spectrometers deployed inside the trailer.


Google Earth vis
Figure 3: Google earth visualization of NH3 vertical columns from a research drive during FRAPPE. The zoomed area shows enhanced column abundance downwind of a cattle farm.