 On March 3rd, 2015, the South Coast Air Quality Management District (SCAQMD) initiated three projects utilizing optical remote sensing (ORS) technologies to monitor volatile organic compound (VOC) emissions. The projects include:
What is Optical Remote Sensing? In a broad sense, remote sensing is defined as the measurement or acquisition of information of an object or phenomenon by a recording device that is not in physical contact with or close proximity to the object. Within the field of emissions monitoring, ORS is technique that uses optical systems (lasers, infrared cameras, etc.) to conduct measurement of air emissions away from the point or area where the pollutant is released. For example, ORS technologies can be used to measure and characterize emissions from a flare using devices located outside of the facility boundary. Common ORS technologies are summarized below. DIAL: Differential Absorption LIDAR (LIDAR stands for Light Detection and Ranging) uses lasers directed into the atmosphere to measure species concentrations of target aerosols, dust, and gases in the lower few kilometers of the atmosphere. Using the DIAL approach, spatial concentrations are obtained from the reflected or backscattered light from two wavelengths of light. Key strengths include device simplicity and the ability to measure an average concentration over a predetermined path length or area. The average concentration can then be multiplied by wind speed to determine the mass rate of emissions through the path line or area. DIAL is the most common ORS used in the emissions monitoring field. Species Measured: VOCs, alkanes, hydrogen chloride, mercury, nitric oxide, nitrogen dioxide, nitrous oxide, ozone, and sulfur dioxide. Concentration Range: 0.5-1000 ppb depending on target compound FTIR: Fourier Transform Infrared Spectroscopy is a technology that performs real-time monitoring of gaseous and volatile organic compounds in air. The technique is capable of detecting and quantifying multiple compounds simultaneously, even in harsh industrial environments, using the characteristic spectral features of the individual compounds. Key strengths include instrument durability and a wide range of measureable compounds. Species Measured: VOCs, ammonia, carbon monoxide, hydrogen chloride, hydrogen cyanide, hydrogen sulfide, ozone, sulfur dioxide, sulfur hexafluoride, and more. Concentration Range: 1-50 ppm TDL: Turntable Diode Lasers measure gas concentrations by shining a laser beam through a sample of gas and measuring the amount of laser light absorbed. Key strengths include lightweight design, unattended measurements, and interference minimization. A key negative is that these devices only detect one compound per laser. Species Measured: Acetylene, ammonia, carbon monoxide, carbon dioxide, formaldehyde, hydrogen chloride, hydrogen cyanide, hydrogen fluoride, hydrogen sulfide, methane, nitric oxide, nitrogen dioxide, oxygen, and water. Concentration Range: 15-800 ppm UV-DOAS: Ultraviolet Differential Optical Absorption Spectroscopy (UV-DOAS) quantifies concentrations of gaseous compounds by measuring the absorption of UV light by chemical compounds in the air and applying the Beer-Lambert law. A significant strength of the UV-DOAS is its extremely long path-length capability – typically 500 meters with some research applications up to 10 kilometers. Species Measured: Ammonia, benzene, carbon disulfide, formaldehyde, nitrous acid, nitrogen dioxide, ozone, sulfur dioxide, toluene, xylenes. Concentration Range: 1-1000 ppb Infrared Camera: Thermal infrared (IR) cameras use IR radiation to form an image similar to the way photographic cameras use visible light. IR cameras are commonly used to monitor for VOC leaks from storage tanks, pipelines, and other storage devices. The major downside to IR cameras is that they cannot speciate the emissions, and the measurements are qualitative only. Potential Impacts for Your Facility: The AQMD ORS projects are still in progress, and results have not been released at this time. Similar studies have shown that fence line emissions measured using ORS technologies typically exceed reported emissions. Although ORS is still an emerging technology, the majority of studies regarding ORS have shown it to be a reliable technology that yields fairly accurate results. A potential goal of regulatory agencies is to use ORS technology as enforceable emissions monitoring. This would reduce the amount of time and effort required to conduct facility-wide emissions monitoring, as well as allow the agencies to perform monitoring without gaining access to a facility. Enforceable monitoring using ORS technologies would result in many compliance challenges as ORS equipment is expensive and complex. Performing a two week study using ORS technology can cost up to $500,000. Stay tuned to the Alta Environmental Newsletter for updates on SCAQMD’s ORS projects and any resulting policy changes. Blog Post authored by Chris Waller, Chris.Waller@altaenviron.com. Comments are closed.
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