Ozone Regional Trend

By charting regional ozone levels over the course of several years one can discover the differences between regions well as the impact meteorology (weather patterns) has on the state as a whole.

The focus on this page is on recent ozone values. The Ozone Historical Trend page displays the long-range trend.

In the charts below, the height of the line is the total number of days for that year that ozone levels were above the Good range using the Air Quality Index (AQI). In the first chart, the regions are colored as close to the colors in the map as possible to make it easier to match the information to the appropriate forecast region.

Weather patterns, which differ from one year to another, need to be factored into a trends analysis.

For example: 2000, 2004, 2008 & 2009 were years in which the weather patterns were not as conducive to transporting ozone and its precursors to Maine. In fact, 2008 was the cleanest year since ozone began to be measured in the early 1980's! 2008's lower ozone values were due in part to a wetter summer combined with a lack of transport on days when ozone values climbed in Southern New England. Additionally, the vehicle miles traveled (and therefore emissions from automobiles) dropped significantly due to high gas prices.

In contrast, 1998, 2001, 2002 & 2007 were years in which the weather patterns brought ozone and it precursors into Maine and produced more frequent and higher levels of ozone.

Other years were between these extremes.

Ozone levels tend to be higher along the coast during the summer months than at other locations in Maine.

By looking at the number of days with poor air quality across several years and all the regions at a glance, one can also see that most years winds favor the Southwest Coast while in other years the winds favor the Mid-Coast.

However, looking at the total number of days above Good only tells part of the story. Since the coastal sites are the most 'active' for ozone during the summer months we'll look at the three most active regions in the state: Southwest Coast, Mid-Coast and High Elevation (Cadillac Mtn.). In the graphs below the three regions are presented by year and with the breakdown between the AQI categories. In the legends 'Mod' = Moderate, 'USG' = Unhealthy for Sensitive Groups and 'UNH' = Unhealthy and above. The colors correspond to the appropriate AQI category.

Ozone forms from chemicals emitted by large power plants in the Mid-West but also by metropolitan areas. Large metropolitan areas concentrate many smaller sources of emissions close together. These smaller sources include cars, trucks, aircraft and trains as well as various industries. The Northeast has many large metropolitan areas and the region from Washington DC including New York City and up through Boston qualifies as a 'megalopolis'. Maine experiences higher levels of ozone when the surface wind is from the southwest during the summer months because that lines up over the Northeast megalopolis.

Here are a few observations of ozone transport to Maine:

The Southwest Coast is geographically closer to the Northeast megalopolis, yet because of the curvature of the coast line and the way the wind blows it is sometimes spared higher levels of ozone.
The Mid-Coast is further away from the Northeast megalopolis, yet the Gulf of Maine greatly enhances surface transport of ozone in several ways.
The High Elevation region typically has more days with ozone above the Good range and often records higher values of ozone than the other regions. This is because the top of Cadillac Mountain is at a height at which ozone is transported efficiently both from the Mid-West but also from the Northeast megalopolis. Ozone levels at low elevation sites drop off after sunset, however at high elevations ozone often remains high late into the night primarily because:
- ozone does not get cleaned out by vegetation, and
- NOx emissions (from transportation and industrial sources) are less efficient at cleaning ozone from the air at high elevations. (Studies have shown that in areas of higher NOx emissions ozone levels are low in the immediate area then rise downwind due to the chemical reactions of NOx with VOCs in the presence of strong sunlight.)

Looking at the charts below one can see that 1999, 2001 and 2002 have nearly the same number of days above Good for both the Southwest Coast and the Mid-Coast regions, yet there are marked differences in the number of days within each category from year to year and from region to region. For the Southwest Coast region 2002 would be the worst year based on the number of Unhealthy days. While for the Mid-Coast region 2001 would be the worst year using the same criteria.

For the High Elevation region the number of days above the Good range for 1999, 2006 and 2007 are roughly equal. In this case, 1999 is the worst of these years because there were fewer Moderate days than either 2006 or 2007.

In recent years (1999 - 2009), 2002 is considered to be the worst year for ozone in Maine. From these graphs you can see that it is largely due to the fact that both the Southwest Coast and the High Elevation regions had more 'Unhealthy' days in 2002 than any other year in recent history. In fact, there was one day when the Southwest Coast reached the Very Unhealthy category on the AQI in 2002.

Air Quality Trends

Ozone Regional Trend