I began this series of posts with photographs taken in Bryce Canyon National Park on a clear day and a hazy day, shown again at right. Given that the data I had reviewed previously suggested that air quality had markedly improved, I asked what could account for such haze.
(Click on photo for larger view.)
The previous several posts provide part of the answer. Fortunately, air quality data is also collected at Bryce Canyon, so we can also look at pollutant levels right there. Bryce Canyon is 80 miles from the nearest city (St. George, UT), so pollution there truly represent background levels.
The site at Bryce Canyon focuses on small particulate matter – tiny particles that float freely in the air. They are too small to be seen individually with the naked eye, but collectively they cause haze. They also get into your lungs when you breathe, where they cause lung disease and other problems. The smallest ones (PM2.5) get most deeply into your lungs and are the greatest health hazard.
I downloaded PM2.5 data from the Bryce Canyon IMPROVE Site. For each year, I selected the 10 highest readings and I averaged them. Then I selected the 10 lowest readings and I averaged them. The results are shown in the graph at right. The blue line represents the high readings, the red line the low readings. The black lines show the trends.
(Click on chart for larger view.)
First, notice that in 1983 the bad days already had 5 times as much particulate matter in the air as the good days.
In addition, however, the level of particulates on good days seems to be trending slightly down. But on bad days, it is trending up. That is the opposite of what we saw with ozone at the Cherokee Nation Site (previous post). The chart covers a longer period (25 years vs. 10), but the slope of the trend line is significantly more steep. In 2009, the level of particulate matter on bad days was approximately 20 times the level on low days.
This would seem to answer the question posed by the two pictures. On bad days, particulate pollution at Bryce Canyon is much worse, and it is causing the haze.
IMPROVE Aerosol RHR (New Equation) Dataset, Database Query Wizard, Federal Land Manager Database, Interagency Monitoring of Protected Visual Environments (IMPROVE). http://views.cira.colostate.edu/web/DataWizard.