October 11, 2017

Technology Update: Optical Remote Sensing for Fenceline Monitoring

Optical remote sensing (ORS) technologies are well suited for monitoring emissions along plant boundaries and beyond. At this type of location, pollutants are well mixed and dispersed and therefore require monitoring techniques that can detect low levels (parts per billion or parts per trillion concentrations) of hydrocarbons and/or VOC.

The dispersed emission plumes at a property boundary are often transient in nature and highly dependent on ambient wind speeds and directions. To assess these plumes, continuous monitoring techniques that provide high time resolution (i.e., near real-time) are required. Options for continuous monitoring include passive monitors, open-path monitoring, and single-point extractive optical monitoring.

When used along the fenceline, passive monitors (such as VOC sorbent tubes) do not offer measurement time resolution over multi-day sampling periods. As a result, the facility is not able to identify emissions in real-time to make operational adjustments. In contrast, open-path or single-point optical monitors can provide near real-time data and when utilized in conjunction with meteorological towers, can provide plume characterization in near real-time.

The implementation of ORS technologies along or outside plant fencelines often employ the following technologies:

  • Open Path Fourier-Transform Infrared (OP-FTIR) at property boundaries
  • Ultraviolet Differential Optical Absorption Spectroscopy (UV-DOAS) at property boundaries
  • Open Path Tunable Diode Laser (OP-TDL) at property boundaries
  • Extractive FTIR at property boundaries or within community sites (ambient air monitoring stations)
  • Extractive UV Absorption Spectroscopy at property boundaries or community sites (ambient air monitoring stations)

One or a combination of these technologies utilized in conjunction with local meteorological data enables real-time streaming of air emissions data. Geosyntec practitioners have provided real-time emission data through deployment of an ambient air monitoring station application for a community site in Houston, Texas. The emission monitors operated continuously for a multi-year project to provide real-time VOC (including highly reactive VOC (HRVOC)) and BTEX concentration data that was then streamed to a public website. The system utilized two extractive air sampling cells, with one coupled to an FTIR spectrometer, for low level detection of VOCs and HRVOCs, and the other fiber optically coupled to a UV light source and spectrometer, for low-level detection of BTEX. The optical bench and support equipment was housed in a shelter located in a parking lot that serves a government building near refineries at the ship channel; a portion of the assembly is illustrated in Figure 1.

Figure 1. Extractive ORS Equipment Assembly at an Ambient Air Monitoring Station

Figure 1

The hardware and quantitative analysis method was configured to provide VOC/HRVOC detection limits at levels on the order of 5-25 ppbv for most VOCs and BTEX detection limits on the order of 1-5 ppbv. Besides providing publicly available real-time reporting of pollutant emissions, the system was instrumental in tracking ground-level ozone plumes (detection limit ~10 ppbv) which correlated with concurrent HRVOC emission excursions. These data were modeled with meteorological and other weather data to gauge the contribution to urban ground-level ozone formation and ambient concentrations.

Geosyntec personnel has conducted fenceline emission assessments using combined open path FTIR, UV, and TDL spectrometers with long range telescopes that direct collimated light beams hundreds of meters along plant boundary lines. The combined system allows for the speciation and quantification of facility emission plumes that originate in the plant and cross into adjoining communities. Low quantification limits can be achieved for compounds such as H2S and BTEX because OP-TDL and UV-DOAS systems, respectively, are employed along with OP-FTIR. A picture of an OP-FTIR assembly is illustrated in Figure 2 along with the representative field detection limits for a pollutant list specific to a refinery in California.

Figure 2. An OP-FTIR Assembly and Some Representative Detection Limits of a Total ORS Network

Figure 2

The hardware and quantitative analysis method was configured to provide VOC/HRVOC detection limits at levels on the order of 5-25 ppbv for most VOCs and BTEX detection limits on the order of 1-5 ppbv. Besides providing publicly available real-time reporting of pollutant emissions, the system was instrumental in tracking ground-level ozone plumes (detection limit ~10 ppbv) which correlated with concurrent HRVOC emission excursions. These data were modeled with meteorological and other weather data to gauge the contribution to urban ground-level ozone formation and ambient concentrations.

Geosyntec personnel have conducted fenceline emission assessments using combined open path FTIR, UV, and TDL spectrometers with long range telescopes that direct collimated light beams hundreds of meters along plant boundary lines. The combined system allows for the speciation and quantification of facility emission plumes that originate in the plant and cross into adjoining communities. Low quantification limits can be achieved for compounds such as H2S and BTEX because OP-TDL and UV-DOAS systems, respectively, are employed along with OP-FTIR. A picture of an OP-FTIR assembly is illustrated in Figure 2 along with the representative field detection limits for a pollutant list specific to a refinery in California.

Fugitive Chemical Detection Limit (ppbv)*
1,3-Butadiene 5
Ethylene 4
MTBE 3
Carbonyl Sulfide 2
Carbon Monoxide 40
Total Hydrocarbons 5
Methyl Mercaptan 11
Ammonia 16
H2S 25**
BTEX <1***

*as monitored by OP-FTIR
**as monitored by OP-TDL
***as monitored by UV-DOAS

The ORS systems described here are typically automated, with continuous audit procedures, data processing/validation/flagging algorithms and remote alarm/access/internet archival features. However, these systems are sophisticated devices that require standard operating procedures and maintenance to be carried out by trained personnel for effective utilization.

More Information

For more information on this article, please contact Curtis Laush, Ph.D. at This email address is being protected from spambots. You need JavaScript enabled to view it. or Yu Jung Leong, Ph.D., at This email address is being protected from spambots. You need JavaScript enabled to view it..

Geosyntec Consultants
Geosyntec is a specialized consulting and engineering firm that works with private and public sector clients to address new ventures and complex problems involving our environment, natural resources, and civil infrastructure.

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