PARTICULATE MATTER
Figure 2. HYSPLIT back trajectories from June 27, 2020;
arrows denote 24-hour time steps over 13 days. Figure 3. MERRA-2 dust surface mass concentration (kg/m3) for June, 2020 (NASA, 2023).
two highest samples on June 27, 2020 and June 16, 2022, extend toward the Sahara in northern Africa. Moreover, MERRA-2 dust concentrations for months coinciding with these 21 samples show high concen- trations in the Saharan region, which ta- per westward, crossing the Caribbean and southeastern U.S. These concentration pat- terns are highly consistent with back trajec- tories computed by HYSPLIT.
Figure 4. HYSPLIT back trajectories from September 19, 2020;
arrows denote 24-hour time steps over six days. For example, Figure 2 shows a back tra-
jectory for the June 27, 2020 sample, and Figure 3 illustrates a MERRA-2 dust concen- tration layer for the coinciding month. The June 27, 2020 sample came from the larg- est dust plume moving across the Atlantic over the past 50 years (Imbler, 2020). Sa- haran dust incursions potentially increase
PM2.5 levels at the Denton monitoring sta- tion for several consecutive days, such as June 30 to July 2, 2018, June 26 to July 3, 2020, and June 13 to June 17, 2022 (Table 1).
In contrast, a HYSPLIT back trajectory for Figure 5. Wildfires from
September 9 to September 19, 2020 in NASA Worldview (NASA, 2023).
the September 19, 2020 sample extends through the Pacific Northwest (Figure 4). At that time, numerous wildfires were burn- ing in the Pacific Northwest and Califor- nia, as documented with the Events tab of NASA Worldview (Figure 5). Li et al. (2021) estimated that western wildfires contrib-
uted 23% of surface PM2.5 in the contigu- ous U.S. from August to October, 2020. At peak activity, on September 14, 2020, wildfires contributed an estimated 41%
of surface PM2.5 in the contiguous U.S. (Li et al., 2021). Moreover, smoke from wild- fires in Oklahoma and Kansas reached the Denton monitor on December 16, 2021, as reported by Pena (2021) and supported by HYSPLIT back trajectories (Figure 6).
Figure 6. HYSPLIT back trajectories from December 16, 2021;
arrows denote six-hour steps over 24 hours. 36 TPG •
Jul.Aug.Sep 2024
Results and Discussion The objective of this study was to identi-
fy external sources of days with high PM2.5
concentrations at a monitoring station in north-central Texas. Only 23 of 1,667 24- hour samples over a five-year period ex- ceeded 20 μg/m3. Of those 23 samples, 21 were affected by Saharan dust and two were affected by smoke from wildfires in the Pacific Northwest and Great Plains. HYSPLIT back trajectories and NASA World- view helped identify these likely external sources. Saharan dust often influences air
quality on days with PM2.5 concentrations above 20 μg/m3 in north-central Texas. Monitoring and forecasting global-scale dust and smoke events is essential for in- forming regional air quality advisories.
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CDC (Centers for Disease Control and Prevention) (2023). Air Quality. Available at: https://www.
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CFR (Code of Federal Regulations) (2023). U.S. Code of Federal Regulations, Title 40, Chapter 1, Subchapter C, Part 58. Available at: https://
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www.aipg.org
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