Solar Occultation Flux (SOF)

The SOF instrument couples a mid-IR spectrometer to the CU Digital Solar Tracker (Figure 1). SOF measurements use direct sunlight as source of energy to determine vertically integrated concentrations of trace gases. Simultaneously, UV-Vis spectrometer can be coupled to the solar tracker.





Figure 1. 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.

sketch of doas
Figure 1.


Ground-Based SOF

SOF has been utilized for ground-based campaigns both from mobile laboratory and stationary as part of a small network of cost-effective sensors. During the mobile deployment during FRAPPE 2014 (Figure 2), SOF was used to quantify emissions from large cattle feedlots. Ammonia (NH3) emission fluxes were determined by driving a closed box around a feedlot to measure the vertical column density upwind and downwind of the source, then combining the measurements along the drive track with the wind field (Figure 3). During the stationary deployment during the KIT field campaign 2015 the objective was to disentangle methane (CH4) emissions from the sources in the Colorado Front Range. KIT instruments similar to SOF measured CH4, SOF measurements of tracers for agricultural sources (NH3) and natural gas sources (C2H6; ethane) were used to source apportion CH4 (Figure 4).




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.


Figure 4: Measurement location map

Figure 4: Measurement setup in context of feedlots, NG permits, and fracking wells in the Colorado Front Range. The SOF and a KIT instrument were measuring methane and its tracers at the Eaton location.


Airborne SOF

Airborne SOF applies the mobile SOF method from an aircraft. The solar tracker is able to quickly measure the movement of the platform and compensate for any changes in pitch and roll in real-time to provide direct sunlight to SOF. Airborne SOF has been demonstrated and ruggedized during ARISTO 2016 on the NCAR C-130 and Pre-BB-FLUX 2017 on the University of Wyoming King Air. During the airborne deployment during BB-FLUX 2018, SOF was used to quantify emissions from biomass burning events. Carbon monoxide (CO) emission fluxes are determined as well as fluxes of many other gases, both from SOF spectra and through synergy with in-situ measurements of emission ratios with respect to CO (Figure 5).

Figure 5. Santa Rosa fires

Figure 5: Satellite image of the Santa Rosa fires on 10 October 2017 overlaid with the SOF flight track colored by NH3, and timeseries of the flight showing measured CO and NH3, as well as the vertical cross section of the smoke plume.