Figure 1. Air Pollution Affects Visibility at Bryce Canyon National Park. Source: National Park Service, 2017.

I began my last post with photographs taken in Bryce Canyon National Park on three days ranging from clear to hazy, shown again at right. Because it is one of the most remote locations in the continental USA, it is a good place to observe background air pollution.

(Click on photo for larger view.)

The haze in Bryce Canyon is caused by pollutants that have dispersed widely throughout the atmosphere. The previous post reviewed data on two pollutants that contribute the most to acid rain: sulfur dioxide and nitrogen dioxide.

Bryce Canyon, however, is most impacted by particulates, 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. How small are they? They are 2.5 microns or less in diameter, while the average human hair is 50-70 microns in diameter.

Figure 2. Source: IMPROVE Aerosol RHR (New Equation) Dataset.

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.

Since 1983 the level of particulates on good days has trended slightly down.

In 1983 the bad days had roughly 5 times as much particulate matter in the air as the good days. The level of particulates on bad days trended up and peaked in 2009, about 20 years after data collection started. By then, the level had almost doubled, and the level of particulate matter on bad days was approximately 20 times the level on low days. Since then, the PM2.5 level has declined, and is now slightly lower than the level at which it started.

I don’t know what accounts for the reversal. My first guess would be the retirement of one or more coal-burning power plants, but searching the web does not seem to indicate it. The Navajo Generating Station, the largest in the West and the closest to Bryce Canyon, has been scheduled for retirement, but it has not occurred yet. It has been required to upgrade its pollution control equipment over the years, and perhaps that plays a role. It is also possible, however, that pollution from as far away as Las Vegas, Phoenix, or Southern California may have been involved. I just don’t know. If somebody out there does, please leave a comment and let us all know.

It was an issue of concern that the PM2.5 level on bad days continued to trend upward for so many years, and, whatever the cause, it is a relief to see that it has declined significantly. Hopefully it will decline even further from here.

Sources:

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.

Source: Federal Land Manager Environmental Database. Database Query Wizard. Data downloaded 12/8/2017 from http://views.cira.colostate.edu/fed/DataWizard/Default.aspx.