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| Downtown Salt Lake City from the Avenues Foothills, 5:24 PM October 24, 2013 |
Over the past few days, PM2.5 levels in Salt Lake City have been gradually climbing. They reached a maximum hourly value of 27.3 ug/m3 yesterday afternoon and the 24-h average reached at 16.2 ug/m3 this morning. That's below the National Ambient Air Quality Standard, but still high enough to put us into the "Moderate" air quality category.
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| 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.
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.
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