In January 2018, a panel with expertise in optical remote sensing and real- world source measurements (“Panel”) was assembled with the purpose of providing an independent critical review of the FluxSense method and its ability to quantify facility-wide emissions (i .e ., fluxes) of methane (CH4), non- methane volatile organic compounds (VOCs), benzene, BTEX (i .e ., benzene, toluene, ethylbenzene and xylenes), nitrogen oxides (NOX), and sulfur dioxide (SO2) . This report is a product of that review . The FluxSense method employs optical remote sensing (ORS) approaches for ambient and fenceline detection and measurement of emissions flux from various stationary sources of atmospheric pollutants. Four techniques are used to measure fluxes of various chemical constituents, with all of the instrumentation utilized to employ these techniques housed in one mobile platform, i .e . a van . This enables the measurement team to drive around and collect measurements at sites with several spread-out sources of emissions . The four techniques are: 1 . Solar occultation flux (SOF), which applies Fourier transform infrared spectroscopy (FTIR) to measure attenuation of direct solar infrared (IR) radiation (in the 2-16 µm region) as it passes through the atmosphere according to its telescope field of view . SOF relates the measured absorption spectra to those of selected VOCs, particularly alkanes and alkenes . The SOF instrument is always pointed directly at the sun; 2 . Mobile differential optical absorption spectroscopy (DOAS), which FluxSense refers to as “SkyDOAS” . This technique relies on scattered ultraviolet (UV) and visible sunlight as its radiation source . It is always pointed towards the zenith viewing nitrogen dioxide (NO2), SO2, and formaldehyde (HCHO) absorption spectra in the scattered solar radiation that reaches the ground; 3 . Mobile-extractive FTIR, which is a fixed pathlength sensing tool that provides further spectral analyses of chemical constituents in the near ground-level emission plume; and 4 . Mobile white-cell DOAS, which is also a fixed pathlength sensing tool that analyzes emission plumes near ground level.
FluxSense Inc. and the SCAQMD has been deploying this combination of sensing techniques in the field for several years, mainly in Europe and the United States . This report provides an assessment of the inherent uncertainties of the FluxSense approach in estimating emission mass fluxes . It also discusses the ancillary data required for estimation and evaluation of FluxSense’s emission flux calculation results . A summary of the Panel’s findings and recommendations for the most appropriate uses of the FluxSense method will be discussed.
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