On September 12, 2011, the Phoenix region experienc ed typical late summertime weather conditions with a maximum temperature of 100°F, relative humidity of 20-40%, variable to southeasterly winds at 5-12 miles per hour (mph) through the afternoon, and visibilit y at 10 miles all day, as reported at the Phoenix Sky Harbor Airport. Despite the spikes in PM10 concentrations a t most of the PM10 monitoring sites in the region on September 11 th and a few hours of elevated PM10 concentrations in the very early morning h ours, PM10 concentrations on September 12 th were generally low during the day (<50 μg/m 3 ). Between 6 p.m. MST and 8 p.m. MST on the 12 th , thunderstorms developed over portions of Arizona to the south of the P hoenix region (as indicated in Figure 5-12) and by 7 p.m., outflows from the thunderstorms entrained dust from desert areas of Pinal County and progressed northwestward into Maricopa County by 8 p. m., transporting large amounts of PM10 into the south-central portion of the Phoenix non attainment area. As shown in Figure 5-13, the arrival of the dust front had the following impacts in the Phoenix area: south-southeasterly winds increased to 20-25 mph, with gusts up to 32 mph; visibility dropped fr om 10 miles to 6 miles; and hourly PM10 concentrati ons increased to nearly 3000 μg/m 3 at the Durango Complex monitoring site and exceeded 1000 μg/m 3 at three additional sites. However, once the winds subsided by the 10 p.m. hour, visibility returned to 10 mil es and PM10 concentrations rapidly decreased, resulting in exceedances of the 24-hour PM10 standard at only t hree monitoring sites on September 12. As a result, it is very clear that without the existence of the emissions generated by the thunderstorm outflows, there would not have been any exceedances of the 24-hour PM10

standard in the Phoenix PM10 nonattainment area.

12:00 p.m. 2:00 p.m. 4:00 p.m. 6:00 p.m.

1900z Sep 12 2100z Sep 12 2300z Sep 12 0100z Sep 13

8:00 p.m. 10:00 p.m. 12:00 a.m. 3:00 a.m.

0300z Sep 13 0500z Sep 13 0700z Sep 13 1000z Sep 13

  



  Satellite imagery showing thunderstorms developing much o f Arizona through the afternoon and evening and dissipating as they moved northeastward. Strong storms over southern/central Arizona are believed to have been responsible for the thunderst orm outflow boundary that caused PM10 exceedances i n Maricopa and Pinal counties on September 12.

-43--44- Figure 5-13 depicts the dust event that occurred in Ma ricopa County impacting the Phoenix area on September 12, 2011. Hourly averaged PM10 concentrations at monitors throughout Maricopa County are s hown along with visibility readings (blue diamonds) and wind speed s reported at the Phoenix Sky Harbor Airport. Measurements at Sky Harbor were used due to its clo se proximity to the monitors that experienced exceedances on September 12. As can be seen in the graph, visibility was reduced to 6 miles at the Sk y Harbor Airport during the windblown dust event, and this visibilit y reduction coincided with rapidly increasing PM10 concentrations and sustained wind speeds up to 25 mp h in the Phoenix area during the 8 p.m. hour. Maps of the PM10 concentrations, sustained wind speeds, and wind gusts in Figures 5 -15 to 5-27 also show the rapid increases in PM10 concentrations and wind speeds as the primary outflow boundary reached the Phoenix area during that hour.

As shown in the graph in Figure 5-14 and on the map in Figure 5-15, several monitoring sites in Pinal County measured increased 1-hour average PM10 concentratio ns during the 3 p.m. hour on September 12 th due to sustained thunderstorm outflow winds over 25 mph. However, the winds were primarily out of the westsouthwest, limiting dust impacts to the Pinal County monitoring si tes. During the next few hours, PM10 concentrations were generally low, but there were missing PM10 data at a few of the monitoring sites for multiple hours. By 7 p.m., a second spike in PM10 concentrat ions due to a thunderstorm outflow boundary with sustained southerly winds between 20-25 mph and gusts above 30 mph, can be seen. The peak monitoring site—Cowtown—had a maximum 1-hour average PM10 concentration of 613 μg/m 3 , and the Maricopa and Stanfield monitoring sites also measur ed increased PM10 concentrations of 243 μg/m 3 and 346 μg/m 3 , respectively. These spikes preceded the PM10 concentrat ion spikes at the monitoring sites in Maricopa by 1-1.5 hours, which is consistent with wind speed s and directions observed at the Phoenix Sky Harbor Airport in Maricopa County and the Casa Grande Airpor t in Pinal County for the propagation of the thunderstorm outflow boundary across the region. The incre ase in PM10 concentrations between the Pinal County sites and the Maricopa County sites indicates that the w inds were strong enough to entrain additional dust into the atmosphere as the winds traveled over the same northern desert areas of Pinal County as they did on the previous day. Only monitoring sites in Maric opa and Pinal counties exceeded the NAAQS due to thunderstorm outflows on this day, but the broad range of sites and the high hourly- and daily-averaged PM10 concentrations reflect the magnitude of the dust storm. Ad ditionally, the NWS airport meteorological data tables for the Phoenix Sky Harbor and Casa Grande airports show that the strongest winds associated with the outflow boundary were out of the south. These winds, combined with the high PM10 concentrations in Pinal County, indicate that PM10 was already entrained into the atmosphere outside of the Phoenix PM10 nonattainment area and then transported into the Pho enix region by the strong winds. Additional evidence of the transport of windblown dust into the region is presente d in the GIS time series maps in Figure 5-15 through Figure 5-27 below.

A visualization of this dust event was created using Phoeni x visibility camera images that can be viewed as an

animation here:

Looking south toward Phoenix, with South Mountain in th e distance: www.phoenixvis.net/videos/mpeg4/SOMT_09122011.mp4

This video from September 12 th is from the same camera view as the video from the 11 th and shows multiple moderate dust plumes blowing into the Phoenix area in the afternoon and then the main dust front moving into the region from the south around 8 p.m., exactly wh en the hourly average PM10 data showed sharp spikes in

the PM10 concentrations.

Videos showing other views across the Valley on this date are also available (see the Visibility Cameras section).  



 

 Phoenix nonattainment area PM10 monitor readings and Sky Harb or wind speed and visibility for September 12, 2011.

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 Pinal County PM10 monitor readings and Casa Grande v isibility and wind speed for September 12, 2011.

      

     ! " #

Figures 5–15 through 5–27 provide a time series GIS-bas ed visualization of the meteorology and PM10

concentrations associated with the thunderstorm outflows observed on September 12, 2012. The data displayed in the following maps were gathered from the same fiv e data sources listed in Table 5-1. All available meteorological and air quality data was u sed in order to present the most complete story of t he

event.

! " # $

 %

 # &  ' ( A description of each time series map is provided to highlight important data in each map and explain the progression of the meteorology and PM10 concentratio ns through time. Taken as a whole, the maps and associated explanatory text describe the clear causal connection between the windblown dust generated and transported by the thunderstorm outflow winds and th e PM10 exceedances at the nonattainment area

monitors.

-46--47-

September 12, 2011, 3:00 PM – 4:30 PM

Figures 5-15 through 5-17 show light to moderate southeaste rly winds and low PM10 concentrations throughout the Phoenix region and the strong wester ly winds previously mentioned in Pinal County that generated high PM10 concentrations during the 3 p.m. hour. By 4:30 p.m., the str ong winds had subsided

and PM10 concentrations decreased back to low levels again.

September 12, 2011, 5:30 PM – 7:00 PM

Figures 5-18 through 5-20 show gusty, east-southeasterly winds t hat developed across southeastern Maricopa County and central Pinal County due to out flows from thunderstorms that formed over the Coolidge area between 5 and 6 p.m. During the 6-7 p.m. hour, dust entrained into the atmosphere began to impact PM10 monitors on the southern edge of the Phoenix PM 10 nonattainment area, which had hourly concentrations shown in the graph in Figure 5-1 3 reaching 400 μg/m 3 during the 6-7 p.m. hour. The plumes of dust transported into the Phoenix are a can also be seen in the visibility camera video l inked

above.

September 12, 2011, 7:00 PM – 8:00 PM

Figures 5-21 through 5-22 show lingering elevated PM10 concentrations in the central Phoe nix area and the development of strong southerly winds at the Pinal County monitoring site s and airports due to thunderstorms to the south. PM10 concentrations also spik ed at the Pinal County monitors closest to the strong winds during this hour.

September 12, 2011, 8:00 PM – 9:30 PM

Figures 5-23 through 5-25 show the arrival of the winds that impacted the Pinal County sites in the previous hour and very large, abrupt increases in P M10 concentrations at nearly every monitoring site in the central Phoenix area, which coincides with the dust f ront that is clearly evident in the visibility came ra videos and is shown on the cover of this report.

September 12, 2011, 9:30 PM – 10:30 PM

Figures 5-26 and 5-27 show subsiding winds across the region and rapidly decr easing PM10 concentrations behind the outflow boundary and as depo sition of windblown dust occurred.GIS Time Series Map for September 12, 2011, 3:00 PM – 3:30 PM.-48-GIS Time Series Map for September 12, 2011, 3:30 PM – 4:00 PM.-49-GIS Time Series Map for September 12, 2011, 4:00 PM – 4:30 PM.-50-GIS Time Series Map for September 12, 2011, 4:00 PM – 4:30 PM.-51-GIS Time Series Map for September 12, 2011, 5:30 PM – 6:00 PM.-52-