PARTICULATE MATTER


depending on weather and plume volume, finer fractions move farther westward for up to 10 days (Goudie and Middleton, 2001). Saharan dust transported to the U.S. usually peaks from late June through mid-August (CDC, 2023). Bozlaker et al. (2013) showed that average PM concentrations, and the


percentage contribution of dust to PM2.5 in Houston, Texas, U.S., more than doubled with Saharan dust episodes.


Methods The EPA Air Quality System (AQS) (EPA,


2023) was used to compile and plot PM2.5 concentrations (averaged over 24 hours)


for the five-year period from 2018 through


2022. From the EPA database, PM2.5 concentration data were downloaded for monitoring station 481210034 in Denton, Texas (latitude 33.2190759, longitude -97.1962841). The Denton station occupies rural pasture surrounded by open fields and sparse residential land cover. Open surroundings reduce obstruction from trees, buildings, and other obstacles (TCEQ, 2023a). TCEQ maintains quality control measures to ensure proper operation of monitoring equipment, following federal sampling and analytical requirements (TCEQ, 2023a).


attenuation method (BAM 1022) (TCEQ, 2023b). Station 481210034 is a Special Purpose Monitor, for which testing procedures follow quality assurance and control measures outlined in the U.S. Code


PM2.5 was measured with the beta


of Federal Regulations (CFR, 2023). PM2.5 concentrations above 20 μg/m3 were considered high in this study; only 23 of 1,667 (1.38%) of samples met this criterion. By comparison, the primary 24-hour standard for PM2.5 is 35 μg/m3 (EPA, 2022).


NOAA’s Hybrid Single-Particle Lagrangian


Integrated Trajectory (HYSPLIT) (Stein et al., 2015) establishes back trajectories, calculated using archived Global Data Assimilation System (GDAS) meteorological data, and NASA Worldview (NASA, 2023) were used to identify likely external sources


for days with high PM2.5 concentrations. Modern-Era Retrospective Analysis for Research and Applications, Version 2 (MERRA-2) and the Events tab were used in NASA Worldview. MERRA-2 uses space-based observations and models to produce long-term global information and was accessed to display dust surface mass concentration (kg/m3) over the earth (NASA, 2023).


μg/m3 to 52.0 μg/m3, with a mean concen- tration of 7.4 μg/m3, median of 6.7 μg/m3,


Results and Discussion PM2.5 concentrations ranged from 0.2


www.aipg.org Figure 1. PM2.5 concentrations for calendar years 2018 through 2022 (EPA, 2023).


and standard deviation 4.1 μg/m3. Most samples were less than 10 μg/m3, or half the threshold used in this study (Figure 1). Only 23 of 1,667 samples exceeded 20 μg/ m3, and only two samples exceeded 35 μg/ m3. Overall, the concentration plots show


a tendency for higher PM2.5 concentrations in June, July, and August, with somewhat lower concentrations in December, Janu-


ary, and February (Figure 1). Most samples (19 of 23) exceeding 20 μg/m3 were in June, July, or August. Calendar years 2020 and 2022 had the most samples, eight each, above 20 μg/m3, whereas 2019 had none above the threshold (Table 1).


HYSPLIT back trajectories for 21 of the 23 samples above 20 μg/m3, including the


Table 1. Dates with PM2.5 Concentrations above 20 μg/m3 Date


June 30, 2018 July 1, 2018 July 2, 2018


August 14, 2018 June 26, 2020 June 27, 2020 June 28, 2020 June 30, 2020 July 1, 2020 July 3, 2020 July 16, 2020


September 19, 2020 July 24, 2021 July 25, 2021


December 16, 2021 May 9, 2022 May 20, 2022 June 13, 2022 June 14, 2022 June 16, 2022 June 17, 2022 July 17, 2022 July 18, 2022


Concentration (ug/m3)Event 30.3 27.3 25.1 26.8 20.6 52.0 22.4 21.5 20.1 23.5 24.1 21.1 21.8 22.0 22.5 25.3 25.0 30.8 32.3 36.2 31.8 23.1 26.5


Saharan dust Saharan dust Saharan dust Saharan dust Saharan dust Saharan dust Saharan dust Saharan dust Saharan dust Saharan dust Saharan dust


Pacific Northwest wildfires Saharan dust Saharan dust


Oklahoma, Kansas wildfires Saharan dust Saharan dust Saharan dust Saharan dust Saharan dust Saharan dust Saharan dust Saharan dust


Jul.Aug.Sep 2024 • TPG 35


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