On April 12, 2018, a PM₁₀ monitor in Pinal County, Arizona (specifically the Stanfield Monitor) reported an average daily concentration of 1,100 μg/m³. This number still technically has not been finalized, but it has been provided by Pinal County as a draft PM exceedance. For perspective, that concentration is more than 7 times more than the National Ambient Air Quality Standard (NAAQS) for 24-hour PM₁₀ (150 μg/m³)! This is the highest monitored 24-hour PM₁₀ concentration I've ever seen and as such, I figured I'd do a bit of investigating to see what led to this atypically high concentration.
From looking at the 2018 Draft PM Exceedance summary, it's not atypical for this monitor to have the highest monitored concentration among all PM₁₀ monitors in Pinal County, but to have a concentration this high is quite atypical. The 24-hour average PM₁₀ concentration at Stanfield has been above 500 μg/m³ six times so far in 2018 (as of the end of September). Interestingly, the Stanfield PM₁₀ monitor did not monitor any 24-hour average PM₁₀ concentrations above 500 μg/m³ in 2017, 2016, 2015, or 2014 (which is as far back as is archived on the Pinal County Monitoring Network website).
Here's the AQI map (airnow.gov) for 5:00 PM on April 12. The Stanfield monitor is located off the southern edge of the left map, but is south of Phoenix and west of I-10, presumably in the dark purple "Hazardous" area. Keep in mind this is the combined AQI, but on this particular day, the AQI is dominated by particulate.
This widespread and severe event is corroborated by the other nearby monitors which range from 244 to 780 μg/m³ 24-hour PM₁₀. I made a KMZ of the Pinal County monitors above the NAAQS from that day which can be downloaded here.
The draft exceedance summary provides some information regarding what caused these elevated concentrations. It indicates there was a fast-moving springtime cold front that led to very high winds (47 mph max) and elevated PM concentrations. The daily weather maps for that day and the following day corroborate the passage of a cold front in the region. A map of the wind fields in the area (shown below and accessible through mesowest.utah.edu) indicates similar maximum wind speeds with winds blowing from the southwest and west throughout the day.
At this point, I've established that on April 12, 2018, Arizona was windy. The wind was caused by a speedy cold front and it kicked up quite a lot of dust. What I was still curious about is why this day was so dusty for so long, to the point where two Pinal County PM₁₀ monitors recorded concentrations over 500 μg/m³. I'd also like to know why the Hidden Valley PM₁₀ monitor, which is the same equipment as the Stanfield monitor and located less than 5 miles west, only monitored a concentration of 244 μg/m³. Cue the Hybrid Single Particle Lagrangian Integrated Trajectory model (HYSPLIT) in reverse.
HYSPLIT is a model developed by NOAA that utilizes gridded meteorological data to estimate trajectories of atmospheric pollutants. This web-accessible model can be configured to run in reverse to help determine whether the concentration at a particular location (i.e., a particulate monitor) was influenced by the transport of a pollutant from another location (i.e., a known source of dust).
I configured HYSPLIT to run in several ways for both monitors and due to the consistency and magnitude of the wind present during the time in question, there was no configuration that showed anything other than very defined trajectories coming from the west-southwest for the hours leading up to and during the high monitored concentrations. Here is the HYSPLIT configuration for the trajectories below. All parameters are the same for both monitors except coordinates. The trajectories continue across southern Arizona and California.
Earlier this week I spent an hour or so in the middle of a cornfield in northern Colorado during a relatively windy day. It just so happens the week I decide to write a blog post about dust in an agricultural area in Arizona correlates with a week that I visit an oil and gas facility in the middle of a cornfield on a windy day north of Denver. While I was there, a tractor was spreading manure in the field with quite a plume of dust coming from the back of the tractor. It gave me the idea to look more into the agriculture near the Stanfield PM₁₀ monitor. Time to call in the experts.
I discussed my theory with an agricultural expert from the Pinal County Office of the University of Arizona's Cooperative Extension and unfortunately, there was likely little to no agricultural activity in this region in mid-April. The crops that exist there (in the rectangular fields) are likely either corn, alfalfa, or cotton. All three of which would have already been planted and left to grow by April. My conversation did, however, point me to another possible source of the dust, an empty lot. The monitor in Stanfield is located behind the Sheriff's office surrounded by empty lots, which by the looks of Google Street View, is frequently disturbed, and therefore susceptible to blowing dust during high-wind events.
I can't say for sure that a freshly-disturbed dirt lot during a day of exceptionally high winds caused 24-hour concentrations of PM₁₀ over 1,000 μg/m³, but in order for there to be such a difference between two monitors in relatively close proximity there either had to be a very localized activity which caused airborne particulate or an equipment issue (e.g., malfunction, audit). Assuming both monitors were functioning correctly, that leaves the option for a very localized source of dust that only affects one monitor. I can't say with 100% certainty, but if there's anything to Occam's Razor, I'd wager the origin of much of the PM was simply an empty lot downwind of the Stanfield monitor on a windy day during the third driest month of the year.
Have other ideas? Let me know!