Air Quality Trends, Bureau of Air Quality, Department of Environmental Protection

Ozone Historical Trend

DESIGN VALUE

The design value is an important number because this is what EPA and Maine DEP use to determine whether an area is attaining (meeting) the National Ambient Air Quality Standard (NAAQS) for Ozone. (An explanation of how the design value is calculated can be found below the chart.)

Over the years various pollution control measures have been enacted. One can readily see from the chart below that ozone levels in Maine have dropped significantly since the 1980's. Compliance to EPA's recent rule regarding lower NOx emissions, known as the NOx SIP Call, began in 2003, 2004 or 2007 depending on the state. This rule limits NOx emissions during the summer months to reduce the amount available for the chemical reactions that form ozone. EPA reports there are significant reductions in ozone levels as a result of the NOx SIP Call. For more information on regional control programs including the NOx SIP Call (aka NOx Budget Program) you can view the 2007 Progress Report.

As more is learned about how ozone impacts the body scientists realize that impacts occur at lower levels than previously thought. EPA reviews the ozone NAAQS from time to time (approximately every 5-7 years) and issues a lower standard when it deems it necessary to do so.

EPA published the new Ozone NAAQS. It is an 8-hr average of 75 ppb and became effective on June 12, 2008.

EPA approved Maine's Ozone Redesignation Request for the former standard effective January 10, 2007. Therefore, Maine was designated as being in attainment of that standard.

On September 16, 2009 EPA announced that it will reconsider the ozone NAAQS and issue a proposal by the end of the year. As a result EPA also announced that it has stayed the nonattainment designations on the 2008 NAAQS. After the new ozone NAAQS is final, in August 2010, the State of Maine will use the most recent quality assured data to propose attainment/nonattainment areas for the new ozone NAAQS.

The design value for each monitoring site is the average of the fourth highest daily 8-hr ozone concentration for each of 3 consecutive years as long as data recovery rates meet requirements. The graph above displays the maximum design value in the state and is identified by the last year of the three year average.

Technically, the design value of a site for 2008 would be the average of the fourth high 8-hr ozone concentrations from 2006 and 2007 and 2008 [as long as the monitor was operating and accurately reporting at least 75% of the time for each season (or the fourth high was >75 ppb) and a three year data collection rate of at least 90% (or the design value was >75 ppb)].

COUNT BY AQI CATEGORY

In addition to the design value we can also look at annual ozone data using the most current Air Quality Index (AQI). In the chart below, the 8-hr ozone maximum value for each day in Maine is counted by AQI category (as established in June 2008). Yellow is Moderate, Orange is Unhealthy for Sensitive Groups, Red is Unhealthy and Purple is Very Unhealthy.

When looking at the maximum 8-hr value in the state one must realize that some days one region will record high values while the others do not and other times a different region will record high values so one cannot take this data and compare it to regional data.

By looking at the data this way you can see the differences between individual years. It quickly becomes apparent that over the last 10 years ozone values have only reached the Very Unhealthy category once. One can also see that while the number of moderate days was often lower in the early 80's there were more days at the higher categories. So whether one looks at the total number of days above the Good range or the fact that there are now fewer days that fall within the higher categories it is clearly demonstrated that great strides have been made in reducing ozone values in Maine.

NOx SIP CALL ANALYSIS

A regional pollution control strategy, often referred to as the NOx SIP Call, has been implemented in recent years. Maine DEP has analyzed the data to see if the NOx SIP Call has had an impact on ozone levels in Maine.

Since meteorology's impacts on air quality vary from year to year it is important to look at five year spans of data to reduce the impact of meteorology on the data. Therefore, the analysis focused on five year spans. Since the bulk of the NOx emissions reductions went into effect on May 1 of 2004, the five year span of 1999 to 2003 is compared with 2004 to 2008 for the 8-hour maximum ozone value within each of Maine's forecast regions. To ensure that we are comparing apples to apples only data from sites that operated for the entire 10 year span were included in the analysis. The Downeast Coastal region did not have any sites in operation for the entire 10 year span so it is not represented in the chart below. Maine's ozone monitoring strategy has been to have the seasonal monitors in operation by April 1st whenever possible and to keep them running through September 30th.

One important ozone statistic is the number of days when the 8-hour ozone maximum is above the 'Good' range on the AQI. Therefore, the total number of days when 8-hour ozone levels exceeded the 'Good' range during each 5 year span was calculated by month. In the chart below, the difference between the two 5 year spans is displayed. Positive values indicate ozone levels are lower during the second 5 year span than the first. The NOx SIP Call is in effect for the traditional ozone season, which runs from May through September.

Indeed ozone values in Maine have dropped during ozone season as a result of this regional pollution control strategy. However since the NOx SIP call went into effect there are now MORE days when the 8-hour ozone maximum in Maine is above the 'Good' range during the month of April .

As a consequence of the new, lower, Ozone NAAQS Maine's new ozone monitoring strategy is to have the seasonal monitors in operation by mid-March whenever possible and keep them running through mid to late October. In addition, Maine DEP has determined that an ozone monitoring site in southwestern Maine should remain in operation year-round to augement the previously existing year-round ozone monitoring sites.

Maine is working with other states and EPA to determine whether the official 'ozone season'' should now begin earlier and end later. Maine would likely benefit from an earlier start since the NOx emission reductions under the NOx SIP Call are only in effect during the 'ozone season'.