The TRAX Light-Rail Train Air Quality Observation Project

Abstract

Observing air quality from sensors onboard light rail cars in Salt Lake County, Utah began as a pilot study in 2014 and has now evolved into a five-year, state-funded program. This metropolitan region suffers from both elevated ozone levels during summer and high PM 2.5 events during winter. Pollution episodes result predominantly from local anthropogenic emissions but are also impacted by regional transport of dust, chemical precursors to ozone, and wildfire smoke, as well as being exacerbated by the topographical features surrounding the city. Two electric light-rail train cars from the Utah Transit Authority light-rail Transit Express ("TRAX") system were outfitted with PM 2.5 and ozone sensors to measure air quality at high spatial and temporal resolutions in this region. Pollutant concentration data underwent quality control procedures to determine whether the train motion affected the readings and how the sensors compared against regulatory sensors. Quality assurance results from data obtained over the past year show that TRAX Observation Project sensors are reliable, which corroborates earlier preliminary validation work. Three case studies from summer 2019 are presented to illustrate the strength of the finely-resolved air quality observations: (1) an elevated ozone event, (2) elevated particulate pollution resulting from 4th of July fireworks, and (3) elevated particle pollution during a winter time inversion event. The mobile observations were able to capture spatial gradients, as well as pollutant hotspots, during both of these episodes. Sensors have been recently added to a third light rail train car, which travels on a north-south oriented rail line, where air quality was unable to be monitored previously. The TRAX Observation

Evaluating Wildfire Smoke Transport Within a Coupled Fire‐Atmosphere Model Using a High‐Density Observation Network for an Episodic Smoke Event Along Utah's Wasatch Front

“…Mobile PM 2.5 measurements from TRAX were evaluated using nearby UDAQ FEM sites (Mendoza et al, 2019;Mitchell et al, 2018). When TRAX PM 2.5 concentrations were compared with nearby UDAQ stations, good agreement was found between the TRAX instrument and the FEM stations.…”

“…Similar to other optical aerosol analyzers, the ES-642 Dust Monitors are sensitive to high humidity as a result of hygroscopic swelling of aerosol particles, which can artificially elevate PM 2.5 concentrations. To mitigate this, the ES-642 Dust Monitors incorporate onboard heaters to dry the air prior to being sampled (Mendoza et al, 2019;Mitchell et al, 2018). During this wildfire event, the RH values across the SLV were low (10-40%) so the onboard heaters were not needed.…”

“…Research-grade air quality sensors have also been deployed on mobile measurement platforms like Salt Lake City (SLC)'s public transit light-rail train (TRAX), which makes continuous real-time measurements of PM 2.5 , carbon dioxide (CO 2 ), methane (CH 4 ), and ozone (O 3 ) (Hasenfratz et al, 2015;Lin et al, 2018;Mendoza et al, 2019;Mitchell et al, 2018). While traditional air quality networks may be adequate for depicting regional-scale air quality events, the measurements within these networks often have difficulty resolving localized PM 2.5 gradients (Mitchell et al, 2018;Steinle et al, 2013).…”

The Association of Media and Environmental Variables with Transit Ridership

“…UTA provides public transportation to a seven-county area, primarily within Utah's Wasatch Front (Figure 1). This area faces significant air quality challenges both in winter, due to elevated fine particulate matter (PM 2.5 ) [2][3][4], and summer due to elevated ozone levels [5,6]. Other pollutants, including carbon monoxide (CO), sulfur dioxide (SO 2 ), and coarse particulate matter (PM 10 ), have also been found to be in excess of national ambient air quality standards (NAAQS) by the Utah Division of Air Quality (DAQ) ( Figure 1) [7].…”

Land Use Differences Across Socioeconomic Boundaries and Implications for Air Quality

“…We used data from the Utah Division of Air Quality (UDAQ) regulatory observational network, and the University of Utah (UofU) stationary and mobile platform network. The mobile network consists of air quality sensors that measure fine particulate matter (PM2.5) and ozone [33][34] mounted on top of electric Utah Transit Authority (UTA) light-rail trains. We estimated mean PM2.5 concentrations for three years (2017-2019) at each zip code's centroid using the inverse distance square weighting (IDW) method [35][36].…”