Saturday, October 26, 2013

Declining Air Quality

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.

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,
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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