Data from the past 24 hours at MTMET (courtesy mesowest.utah.edu) shows very nicely the gradual rise in temperature during the day yesterday, with a high just above 90ºF. Just before 2000 (8 PM) MDT, temperatures dropped abruptly to 75ºF with the onset of easterly canyon outflow, after which temperatures held steady until about midnight when the temperatures jumped abruptly when the outflow weakened. The strength of the outflow varied for the rest of the night, with temperatures bottoming out in the low 70s just before 0800 (8 AM) MDT.
Ozone for the period spiked briefly from 1300–1400 (1–2 PM) MDT yesterday, hitting ~80 parts per billion (ppb). Ozone overnight, even with the supposedly "clean" canyon outflow, stayed near about 50 ppm until about 0800 (8 AM) MDT when the outflow weakened and winds gradually shifted to southwest.
We can compare this data with that from Hawthorne, which thankfully is available online again. It shows a similar spike in ozone in the early afternoon, but lower ozone concentrations than observed at MTMET overnight. In other words, from the standpoint (solely) of ozone concentrations, the outflow from Red Butte was "dirtier" than the air at Hawthorne.
|Source: Utah Division of Air Quality|
|Ozone concentrations over the Los Angeles Basin from 05-07 local time, 25 June 1987. Source: Dayan and Koch (1996).|
This may seem strange since one expects the pollution to be worst near the ground where the emissions are, but ozone is a secondary pollutant and the complexities of ozone photochemistry and meteorological turbulence lead to the strange distribution. Further, this effect is most pronounced at night and in the early morning. The Los Angeles data above was collected from 5-7 AM local time when ozone concentrations were less than 50 ppb near the surface, but exceeded 250 ppb aloft.
So, one hypothesis for the higher ozone at MTMET last night is that the ozone levels dropped in the valley, but the outflow from Red Butte was tapping into the higher ozone air further aloft.
Of course, this is an are educated guess. Comprehensive observations of meteorology and atmospheric chemistry are needed to better understand these local characteristics of our pollution.