ACADIA NAT’L PARK — If it seems like alerts about unhealthy ozone levels on summer days are more common now than they used to be, they are. The number of days on which ozone in the air at ground level exceeded federal standards was up in 2017 from the previous few years, but the change is partly due to more stringent standards.
On six days this year, ground-level ozone in Acadia National Park reached unhealthy levels, according to the monitoring station on Cadillac Mountain. Unhealthy levels were detected on two days at the monitoring station on McFarland Hill.
The U.S. Environmental Protection Agency (EPA) considers air unhealthy when the average reading of ground-level ozone over an eight-hour period exceeds 70 parts per billion (ppb).
On June 12, the reading on Cadillac was 112 ppb at 7 p.m. and 113 ppb at 8 p.m.; on McFarland Hill, readings ranged from 73-97 ppb between 7 p.m. and midnight, said air monitoring specialist Bill Gawley.
“This high hourly value was considerably higher than we’ve had in a number of years,” he said.
This year’s ozone numbers also were unusual in that the high readings came early in the season rather than later in summer, he said.
Poor air quality affects different people at different levels. Ozone levels in the range of 71-85 ppb are considered unhealthy for the very young, the elderly or those with asthma, respiratory problems or heart disease, while ozone readings of 86-105 ppb are unhealthy for the general population. Averages above the 105 ppb level are considered very unhealthy. When those levels are reached, the EPA recommends limiting strenuous exercise and moving activities indoors.
Ozone levels first exceeded 85 ppb on Cadillac Mountain on April 11, and June 12 was the last date that ozone levels exceeded the standard. Although the Maine Department of Environmental Protection issued advisory alerts on a number of days later in the summer, ozone levels did not exceed the standard again.
The current standard of 70 ppb over eight hours is considerably more stringent than when the EPA began its monitoring program in 1981. From 2001 to 2008, the standard for unhealthy air was 101 ppb. In 2008, the standard was lowered from 84 ppb averaged over eight hours to an eight-hour average of 75 ppb.
The EPA’s implementation of more stringent standards based on data like that from the park, said Gawley, is partly responsible for improvements in air quality over the last 36 years. The improvements also can be attributed to provisions of the 1973 Clean Air Act and substantive amendments issued in 1990.
According to historical data compiled by the EPA office in New England in 1988, Maine would have had 40 days that exceeded the 75 ppb standard and 49 days that would have exceeded the new standard of 70 ppb.
Historically, 1988 is the year with the highest number of exceedances in Maine, but the table listing the number of historical exceedances does not break out the numbers by region, and National Park Service data does not include data for 1988.
In Acadia, ground-level ozone forms in the presence of high temperatures, wind from the southwest and a high pressure system that stalls the polluted air mass. The majority of ground-level ozone coming into Maine arrives on plumes of air from areas south and west of here, areas that have coal-fired power plants.
According to an EPA fact sheet, the 2015 standards are based on extensive scientific evidence about ozone’s effects on public health. In addition to protecting public health, the new standards are expected to protect trees, plants and ecosystems.
The EPA’s fact sheet indicates that reducing air pollution for children and adults to 70 ppb will result in public health benefits worth an estimated $2.9-$5.9 billion. These benefits outweigh the estimated $1.4 billion cost of compliance with the new standard. The Clean Air Act requires the EPA to review the standards every five years.