Wednesday, February 10, 2016

Above My Pay Grade

Some of you have commented about the large difference between the DAQ sampler at Hawthorne and the observations collected along the Trax line as well as at Neil Armstrong Academy and the University of Utah.

For the most part, these sensors have generated pretty similar results for the first few days of the inversion.  PM2.5 at Hawthorne has risen fairly steadily during the period (with some ups and downs) with a range from midnight on the 9th to midnight on the 10th between about 38 and 54 ug/m3.  PM2.5 at Neil Armstrong Academy (sorry for change of scale) was generally close to Hawthorne through midnight on the 10th, with the range from midnight on the 9th to midnight on the 10th of about 39 to 65 ug/m3.

Source: MesoWest
Source: MesoWest
The divergence begins shortly after midnight on the 10th (last night), with NAA going to much higher values.  Similar, other samplers operated by the University of Utah at the University of Utah and along Trax are also reporting high values compared to Hawthorne.

It is above my pay grade to explain these differences.  I know little about the measurement of PM2.5, other than it is very difficult, or the instruments used, other than the fact that the DAQ and University of Utah samplers are different.  The discrepancy appears to have developed during a period when fog was in the area.  I don't think this is a coincidence, but I lack knowledge of the sensor characteristics and atmospheric chemistry to provide a reasonable hypothesis why.  I'd rather say I don't know than speculate.  

As such, in future posts during this event, I will be referencing both samplers and mentioning the uncertainty at play.  

Note, however, that the lower Hawthorne sampler was in the unhealthy category for several hours today and the 24-h average is now very close to the unhealthy threshold.  Thus, even though there are uncertainty in the measurements, this remains a serious event.


  1. This is interesting. A few thoughts:

    - The trends between NAA and QHW are basically identical.

    - Highest humidity should have been at night and early AM. The discrepancy shows up in the afternoon. This goes against and idea that maybe the high humidity caused an instrument problem.

    - The two observing sites are not exactly close to each other. NAA is in West Valley while QHW is 700 East 1700 South. A little surprising to explain the difference soley on their locations, but much larger differences have been observed in our Valley. Yesterday was a day with eroding stratus. Perhaps this has something to do with it.

    - Two different instruments. Trend was the same but magnitudes will be different simply because the two instruments are different. Big magnitude difference though.

    1. With regards to the humidity comments, the discrepancy begins to show up overnight and, at least yesterday, maximized around noon. The humidity in the afternoon on prior days was dipping down to 70% or lower, sometimes for a few hours. There was only a very brief drop to 70% yesterday afternoon (roughly during the PM2.5 drop in the NAA graph above - extreme right of the graph), but otherwise it was 90% or higher for most of the day. Thus, my take is this is still in play.

      I picked NAA, but this discrepancy is evident relative to all U of U sensors, including the one on the Trax line closer to Hawthorne. I picked NAA for convenience.

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  3. Today's Tribune article is way understating the PM 2.5 levels:

    Utah's bad air is the worst it's been in years -- and it's likely to stick around -

  4. Today's Tribune article is way understating the PM 2.5 levels:

    Utah's bad air is the worst it's been in years -- and it's likely to stick around -

  5. As someone who is not a Weather Weenie and knows nearly nothing about air monitoring I find it concerning that during this inversion the sole station (DAQ Hawthorne) reported on by the media and apparently used by schools for recess cancellation has consistently had values far below the Trax line, Neil Armstrong Academy, and the University of Utah monitors.

    The DAQ web site states that the "data is not quality assured." Should we reasonably assume the DAQ Hawthorne monitor is accurate or at least more accurate a than the others? Should we be relying on this one monitor?

    According to the DAQ the air quality never approached the 90 ug/m3 need to trigger public school recess cancellation. If their data is not robust, it may matter not just for the potential health impacts of area children but also because multiple days of indoor recess might be a direct impact to a larger number of households and might make for a big news event, which might get the attention of Utah legislatures. Would the differences in reported data have been large enough to have an effect on individual actions and future policy or legislation? Would the differences in reported levels affected federal/EPA oversight?

    According to the Hawthorne site this pollution event abruptly ended in the wee hours of the morning. Has our air been clean for the last 7 hours? I look out the window and find that hard to swallow. This reduces my faith in the Hawthorne site or at least in its exclusive use by media and policy makers.

    2016-02-13 07:00 Local
    0.10 ug/m3

    2016-02-13 07:50 Local
    93.00 ug/m3

    2016-02-13 07:55 Local
    93.00 ug/m3