Ozone Regional Trend, Bureau of Air Quality, Department of Environmental Protection

Ozone Annual Trend

Ozone season is from April through September in the northeast. Since ozone is a ‘summer' phenomena many monitors in the northeast shutdown during the winter months. Maine DEP also shuts down many ozone monitors during the winter months but works to keep a few operating year-round.

Ozone levels are lowest during the coldest months and begin climbing in February. In 2006, ozone levels exceeded the current standard in late March on Cadillac Mountain!

With the new standard in place it is important to begin thinking about ozone earlier in the spring and later in the autumn than we have in the past. Spring is a particularly vulnerable time because the leaves are not out on the trees to help reduce ozone levels and industrial sources have not yet begun to reduce their emissions to summer levels.

Here is a graph of the 5 year average of days when ozone levels are above the good range for each region by month. It is apparent that ozone is not just a summer phenomena in Maine. Spring ozone levels are not to be taken lightly. In fact, for some regions of the state, ozone levels are at thier highest in April.

The graph below is from a site that operates year-round. It is quickly evident that ozone levels are good for most of the year. 2002 was chosen because it was a very active summer for ozone. Weather patterns during the summer of 2002 were very favorable for ozone and its pre-cursors to be transported to Maine.

Notice that there is more variability in daily ozone values during the summer. It is also interesting to note that even during a year with high ozone values there are many days during the summer when ozone is as low as the rest of the year. Notice also that during February background levels of ozone begin climbing and ozone can actually be higher than some summer days.

Air Quality Trends

Ozone Annual Trend