Air Quality Sensors Scan Ozone and Particulate Matter in Salt Lake Valley

Air quality sensors developed by the University of Utah have traveled on TRAX light rail trains for over five years, checking for air pollution along the Red and Green Lines of the train.

A TRAX light rail train in Salt Lake City, Utah. Image Credit: University of Utah.

The research, which was once an exciting project of the University of Utah scientists, has currently become a long-term, state-funded observatory with another sensor added to the Blue Line into Sandy and Draper. The study offers an additional understanding of the events that affect air quality, including winter inversions and summer fireworks, of the Salt Lake Valley.

In the latest study published in the Urban Science journal, a research team that included Daniel Mendoza and Logan Mitchell has reported the latest results from the TRAX Observation Project. These include data validation studies that support the data’s value for other scientists and also three case studies from the latest events demonstrating the capacities of the mobile air quality sensors.

What’s New: Blue Line and Data Validation

The TRAX system of the University of Utah includes three light rail lines—blue, green, and red. Up until November 2019, the sensors that measure particulate matter and ozone were set up only on the Green and Red Line trains, because the same train cars were used by both lines.

Both these lines pass through downtown Salt Lake City, the central I-15 corridor, and the west side of the valley. However, with an extra sensor installed on the Blue Line, air quality measurements currently extend into the southeastern quadrant of the Salt Lake Valley.

That’s a really important area of the valley,” stated Mitchell. “There’s a lot of people down there.” In addition, the Blue Line goes up and down in elevation, the same way the Red Line does as it climbs from downtown Salt Lake City to the University of Utah’s campus.

Since elevation is such a key part of the air quality and understanding the depth of the inversion on different days, under different conditions, it’s going to be a really important piece of the dataset for us.

Logan Mitchell, Researcher, University of Utah

Moreover, extending into the south valley enables researchers to get a better understanding of the way air masses travel to and fro between Utah and Salt Lake counties, via the narrow Point of the Mountain passage.

That’s actually really critical because we sometimes have very different meteorological phenomenon going on between the two valleys,” stated Mendoza. “We can now examine in our basin an exchange of air masses.”

Validation of the data coming from the mobile sensors is the other significant development in the TRAX Observation Project. This is a crucial step in a revolutionary project like this and serves, along with quality control and quality assurance procedures, as a certificate on the archived data. This data is currently available to other scientists. It also reduces concerns that the readings would be distorted by the air turbulence induced by the moving train.

In the experiment, a stationary particulate matter sensor was positioned approximately 10 feet (3 m) from the rail line that would collect readings whenever the TRAX trains come within 500 feet (150 m) of the sensors. A comparison of both stationary and mobile readings demonstrated an accuracy of 96%, added Mendoza.

That really gives us a great deal of confidence that our TRAX sensors are actually performing really well compared to regulatory sensors and can be used for health studies, policy and so on.

Daniel Mendoza, Researcher, University of Utah

Watching the Fireworks

Many air quality events were captured by the TRAX Observation Project as a result of five years of continuous observations. Mendoza, Mitchell, and their collaborators recorded three specific events in their study—that is, the fireworks on July 4, 2019, an increased ozone event from August 2019, and a cold air pool inversion event in November 2019.

Among these, the fireworks event was considered unique because it was not a phenomenon brought about by the geography of the Salt Lake Valley or by an atmospheric event. Rather, it was an incidence of numerous point sources of particulate matter air pollution, enabling observation of the way those plumes of particulate matter traveled via the valley.

After good air quality in general, hotspots of increased pollution began to appear in the TRAX data between 10 p.m. to 11 p.m. on Independence Day. By midnight, moderate to unhealthy air quality was recorded in most of the valley.

According to Mendoza, the train data not only reveals the dispersion of the smoke but also shows the evening winds that come down Emigration Canyon on the east side of the valley, removing some of the air pollution. Dispersion of smoke is something that is not seen in wintertime inversions, which have reduced atmospheric energy.

These are examples of the kinds of things that we’re seeing that you couldn’t see with stationary monitors. It’s helping us understand where the gradients are in the valley, how they evolve through pollution events such as during the Fourth of July or an inversion or an ozone event. You can see the air masses moving around. You can see where the pollution is and how it moves from different parts of the valley.

Logan Mitchell, Researcher, University of Utah

Next Steps

Mitchell added that the researchers are now hoping to add sensors that quantify carbon monoxide and nitrogen oxides, which are both crucial components of atmospheric chemistry. The team would also like to apply their research to light rail trains in cities like Denver or Portland.

It would be really interesting for us to be able to compare the spatial patterns we’re seeing here with another city that has different topography around it and a different mix of emission sources,” added Mitchell, “so that we can understand how cities in general are being affected by these things and how that’s similar or different from what’s going on in Salt Lake City.”

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