Open Path Monitoring
Open path optical techniques measure gas concentrations in ambient air across large distances rather than transporting the gas to an analysis cell as with extractive sampling. Both techniques have advantages and disadvantages unique to each application.
Open path measurements produce path-averaged concentration data which is better suited for many applications and certainly for most modern atmospheric modeling tools that may be integrated into the measurement program. Fence-line monitoring projects are obviously good potential candidates for open path technology deployment, since industrial facilities can essential be surrounded by a border of long optical measurement paths located near the fence line. This kind of configuration significantly reduces the likelihood that a narrow chemical plume could traverse the fence line undetected. Even if many point monitors were placed around the same fence line, they may miss smaller emission events, and they would be of significantly greater cost than a few open path monitors.
The methods for ensuring high data quality when using extractive, single point measurements of trace gas species are well documented. The accuracy and overall quality of these data are easily established provided one of several proven methodologies are followed. Most experienced practitioners understand this and, to a lesser extent, the methods that ensure the measurement program’s validity.
On the other hand, open path concentration data truly only represents the average molecular density encountered by the beam over the entire light path, of which perhaps only a small portion actually contains plume gas. Averaging over a partially clean sample effectively dilutes the concentrations that would be measured if the plume traveled directly across an extractive point monitor. This apparent sensitivity loss is usually made up for, at least partially, by the relatively long path lengths used in open path configurations as compared with path lengths commonly utilized for “closed”, extractive, point monitoring systems. Furthermore, as previously alluded to, since open path lengths can reach one-way distances of more than 500 meters, fewer instruments are typically required to comparably monitor the same fence line geometry as would a deployment of point monitors.
However, it is important to realize that several drawbacks should be kept in mind whenever contemplating the use open of path techniques, even for fence line monitoring projects that seem otherwise ideally suited them. Open path ‘samples’ are not controllable. ‘Samples’ are ever-changing sections of ambient air which make open path configurations susceptible to a variety of issues that are not encountered or can be greatly mitigated when “closed” point monitors are used. Examples of such issues are: data loss due to precipitation and/or icing, uncertainty associated with fast temperature fluctuations and subsequent mismatch with spectral reference sets, non-uniform surface heating, problematic climate control of the instrumentation, flow and gas density fluctuation induced spectral distortion, and strong, long path length-induced carbon dioxide and water interferences (and no option to pre-treat or condition the samples).