Why consider monitoring instead of modeling?

With lower standards, higher background concentrations, outdated and overly conservative emission factors, and more industrial sources popping up every day, it is becoming increasingly challenging to find ways to demonstrate compliance with ambient air quality thresholds using just air dispersion modeling to move forward with your projects. While updated modeling guidance from the Environmental Protection Agency (EPA) regarding nearby sources and background concentration has allowed for more accurate modeling demonstrations, at its core, air dispersion modeling is a very conservative exercise designed to provide the worst-case impacts.
More facilities are now choosing to demonstrate compliance with ambient air quality standards through air quality monitors rather than modeling. These are permanent units at a facility that can provide ground-truth monitoring data. However, monitoring units are expensive and require monthly and quarterly maintenance checks. In addition, facilities may be stuck monitoring indefinitely, as the monitoring off-ramp procedure in Minnesota has certain conditions that could be challenging to meet. Finally, there is always risk in committing to doing monitoring before knowing what the results may show.

This is where temporary monitors can be a useful tool for facilities.

Temporary monitors are non-regulatory air quality monitors that can provide representative monitoring data for your facility. There are several types of temporary air monitors that can serve a variety of purposes. One example are the small air quality sensors that the City of Minneapolis has installed throughout local neighborhoods. These sensors provide real-time data by using lasers to count the number of particulates in the air. This method allows for a much smaller monitoring unit, but the tradeoff is that the calculated air quality concentrations are generally higher than reality; thus, these are designed more for general public use, rather than industrial facilities.

There are other non-regulatory monitors that use the same methodology that regulatory sensors employ. These monitors are more expensive and require regular maintenance, but the calculated air quality concentrations from these monitors are similar to what a regulatory monitor would measure. These non-regulatory monitors are portable and can be completely off-grid by using batteries or solar panels to power the unit. They are easy to install and can also come with a portable meteorology station as well.

So how can temporary monitors help your facility?

Temporary monitors can help in the decision-making process when evaluating whether a facility wants to show compliance with ambient air quality standards using monitoring instead of air quality modeling. A facility may install upwind and downwind monitors to learn what the actual monitored concentrations in their area are. In addition, these monitors can also help identify whether there are external impacts affecting pollutant concentrations, such as wildfire smoke or other infrequent activities in the area that could cause a temporary increase in monitored concentrations. If there are challenges with showing compliance using a temporary monitor, then a facility may need to look at other methods to demonstrate compliance.

Temporary monitors can also help in refining emission rates. If a facility is using emission factors that are overly conservative, temporary monitors could be used to determine a more accurate emission factor for different emission units. Essentially, a facility can use temporary monitors as a pseudo stack test for emission units that do not have a stack. This exercise does require time and effort, and may disrupt facility operations for a time, but by using temporary monitors to measure air quality concentrations from certain emission units, a facility can back-calculate an emission factor. This allows for more accurate emission calculations and air quality modeling demonstrations. Trinity recommends consulting the regulatory agency in advance of pursuing this process.

Temporary monitors can also be used to determine a representative background concentration for the area for an air quality modeling demonstration. If a facility is in an area where there are no representative background monitors, conducting an exercise to measure background concentration may be a preferable alternative rather than using background concentrations for other monitoring sites that are not representative. These exercises for determining background concentration are more time-consuming and do require a regulatory monitor to be installed, as the facility would need monitoring data for at least a year. Following the demonstration, the facility would not be required to keep the monitor on-site and running if compliance can be demonstrated using air quality modeling.

Temporary monitors have a multitude of uses to assist a facility in showing compliance with ambient air quality standards. If you would like to discuss installing temporary monitors, please email John Ke in Trinity’s Minneapolis office or call 651.275.9900.