An ozone exceedance occurs when monitored ozone concentrations exceed the National Ambient Air Quality Standards (NAAQS).  Ozone is collected as an hourly average of continuous data and is then used to determine the 8-hour average value for the day. An exceedance of the 8-hour standard is an 8-hour averaged value that is greater than 0.075 ppm.

The Difference between an Exceedance and a Violation

An ozone exceedance occurs when a monitor records ambient levels of ozone above a standard. A violation of an ozone standard (as opposed to an exceedance) is based on 3-year averages of data, so monitoring an exceedance does not necessarily mean that a violation of the standard has occurred.

Violations of the 8-hour standard are determined using the annual 4th-highest daily maximum 8-hour ozone value at each monitor.  A violation requires a 3-year average of the annual 4th-highest daily maximum 8-hour value that is greater than 0.075 ppm.  In other words, the 8-hour values for each day during a year for a specific monitor are ranked from highest to lowest.  Then, the 4th-highest value for 3 consecutive years is averaged.  If the 3-year average is greater than 0.075 ppm, a violation of the 8-hour standard has occurred at that specific monitoring site.

Ozone Exceedances During 2008

The Table below shows the 2008 ozone exceedances in Massachusetts. There were 18 days when the 8-hour ozone standard was exceeded (exceedance days). There were 54 total exceedances (49 at MassDEP monitors and 5 at the Wampanoag Tribal monitor) during those 18 days.

Ozone Violations as of 2007

Violations of the ozone standard are based on 3-year averages.  Using data from 2005 - 2007, three sites out of 14 violated the 8-hour standard.  These sites were located in Chicopee, Ware and Milton at the Blue Hill Observatory.  Violations have not yet been calculated using 2008 data.

Exceedance Days and Total Exceedance Trends

Figures 1 and 2 show the trends in number of 1-hour and 8-hour exceedance days and the total number of exceedances.  These charts do not include the Wampanoag Tribal monitor on Martha's Vineyard. 

Note: although the 1-hour standard was revoked on June 15, 2005, Figure 1 reflects the total number of 1-hour exceedances through 2008. 

Figure 1
1-hr Ozone Exceedance Days and Total Exceedances 1987-2008
1-hour standard = 0.12 ppm (revoked June 15, 2005)
 

Figure 2
8-hr Ozone Exceedance Days and Total Exceedances 1987-2008
8-hour standard = 0.08 ppm 1987-2008; 0.075 ppm for 2008

2008 Ozone Exceedances (ppm)

NOTE:  Martha's Vineyard monitor is operated by the Wampanoag Tribe of Gay Head (Aquinnah) as part of the Tribe's environmental laboratory.

2007 Ozone Exceedances (ppm)