Investigating the Impacts of Roadway Activities on Coarse Particle Concentrations Using Emerging Sensor Networks

Abstract:

As manufacturers have reduced vehicle tailpipe emissions, roadway sources of PM₁₀ and PM_2.5 (particulate matter with diameters ≤ 10 and 2.5 µm, respectively) generated by tires, brakes, and the roadway itself play a larger role in near-roadway exposures. Tire and brake wear are of concern because they contain high concentrations of transitional metals. In this study we utilized the spatially dense network of low-cost Purple Air PM sensors (PA; https://www2.purpleair.com/). Approximately 500 sensors are deployed in the Los Angeles basin, with 63 sensors located within 300 meters of a major roadway— a higher number of sensors very close to the roadways than in other locations. Plotting the PM₁₀ and PM_2.5 concentrations for all the PA sensors located within 300 meters of a freeway or access ramp between January of 2019 and February of 2020 revealed no distance-decay trend. We then used the NOAA HYSPLIT model (https://www.arl.noaa.gov/hysplit/hysplit) to determine the hourly wind direction at each sensor and separated PM values according to whether they were downwind or upwind of their respective nearby roadways. The downwind data were then divided into 20 groups: five times of day for each of the four seasons. Using these groupings, almost half of the groups did not have sufficient data to determine if there was a distance-decay trend. For those with sufficient data, results show that the coarse particles (PM₁₀ - _2.5) were usually elevated at the sensors closest to the roadways and decayed to the baseline concentration by ~60–80 m. The concentrations near the roadways were most elevated in the mornings (5:00–10:00 a.m.) and overnight (11:00 p.m.–5:00 a.m.) and strongest in the winter and spring, coinciding with periods of higher atmospheric stability and lower wind speeds. The concentration of coarse particles between 0–20 m from the edge of the freeway was 30–100% higher than the concentration between 40–80 m. For PM_2.5, the trends were much weaker, and in many cases clearly absent, although when present they coincided with the periods with strongest trends for coarse PM. This study indicates that coarse PM from tire and brake wear and road dust particles results in elevated exposures in near-roadway environments and thus requires thorough further investigation.