|Downtown Salt Lake City from the Avenues Foothills, 5:24 PM October 24, 2013|
|Source: Utah Division of Air Quality|
Such elevated values are unusual but not unheard of. The plot below shows the percent of days that PM2.5 exceeded 17.5 ug/m3 and 35 ug/m3 versus the week of the year from 1999–2011. We currently sit in week -10 (negative implies weeks prior to the first week of January). From 1999–2011, days with PM2.5 (presumably 24-h average) exceeding 17.5 ug/m3 represent about 2% of the days (probably 2 of the 84 days that make up all the week -10s during the 12 year period). Note that as I write this we still haven't hit 17.5, but we are close and perhaps we will get there today.
|Source: Dave Whiteman, University of Utah, http://home.chpc.utah.edu/~u0453210/PM2.5/PM2.5.html|
An interesting aspect of the graph above are the occasional episodes with PM2.5>17.5 or > 35 outside of the inversion season. Those are likely related to either fires or blowing dust events. That's certainly not the case this week. We're simply seeing a buildup of primary and secondary pollution from emissions in the Salt Lake Valley. In this case we've met the enemy and it is us.