Recent advances have shown that dust interactions with sea spray aerosols can create atomic chlorine (Cl), a halogen which oxidizes CH₄ 20 times faster than the dominant atmospheric sink, OH (van Herpen, 2023; Hossani, 2016). However, we are unaware of any studies using real-world measurements to quantify changes in ambient CH₄ concentrations due to oxidation by halogens like Cl. This is primarily due to the lack of systems that can reliably measure regional CH₄ signals, particularly over the Cl-rich ocean where satellite/aircraft retrievals are limited by surface characteristics (Ayasse, 2018) and in-situ networks cannot be easily deployed. Furthermore, halogen sources located in feasible study areas that are large enough to exhibit measurable CH₄ depletion are rare. Therefore, there has been very little scientific interest in quantifying these mechanisms by existing expensive sensor networks (i.e. TCCON, GOSAT, TROPOMI).

Butterfly’s goal is to measure a chlorine-mediated CH₄ sink via direct observations of total column CH₄ at strategic locations in Salt Lake City (SLC), Utah by developing a novel network of low-cost solar spectrometers (Meyer, 2023). Womack et. al. (2023) have shown that the U.S. Magnesium facility in SLC exhibits “the highest levels of ... Cl₂ , Br₂, and BrCl ever measured in ambient air, outside volcanic plumes” (Figure 1). By deploying sensors upwind and downwind of this halogen source, we expect to monitor CH₄ depletion at regional scales. SLC is also located downwind of the largest region of exposed saline playa in the United States. Recent studies show that dust from these playas can generate high levels of ClNO₂, another common source of Cl (Royer, 2021; Mitroo, 2019). Hence, Butterfly’s proposed system could also measure the effects of episodic dust plumes from the Great Salt Lake, one of the largest natural inland sources of CH₄ oxidizing sinks.

Butterfly’s proposed system will provide an essential monitoring, reporting and verification (MRV) capability that can be deployed easily, cheaply and autonomously anywhere that sunlight is available to assist future studies on natural and engineered methane sinks. The quantification of CH₄ fluxes due to a large engineered sink and an episodic natural sink will allow us to gain process-level knowledge to develop and scale up inorganic chloride (MgCl₂ , NaCl) particle-based engineered facilities to draw down atmospheric CH₄, as is being done for CO₂ commercially. As models of atmospheric CH₄ photochemistry improve, a system which can reliably quantify regional CH₄ signals is crucial for MRV, particularly over the ocean where existing methods are lacking.