Particle Pollution Annual Trend

An important statistic for particle pollution is the number of days when the 24-hour average is above the 'Good' range on the Air Quality Index. Since it is important to reduce yearly meteorological variances in analyses a 5 year monthly average was calculated. In the annual trend chart below, it becomes obvious that for most sites there are two peaks of fine particle pollution during the year. There is a winter peak and a summer peak. Notice that the rural elevated site only has one peak during the year and that is during the summer. This is because this site is not impacted by the factors that contribute to a winter peak.

Below the graph is an explanation of what causes the summer and winter peaks in particle pollution levels.

WINTER PEAK:

We've learned that winter levels of particle pollution are often a result of local emissions combined with topography and meteorology.

First, let's talk about the meteorological impacts on winter-time fine particle levels. Strong nocturnal inversions occur on clear, cold winter nights. The surface cools during the overnight hours and chills the air near the surface. The air a few hundred feet above the ground is then warmer than the air at the surface. This is an inversion. Inversions block air from rising thus trapping pollution close to the ground.

Next let's look at how topography plays a role in winter-time fine particle levels. There are many river valleys in Maine . Depending on the depth of the valley, how wide or narrow it is, how steep the walls of the valley are and sometimes even the direction the valley runs a valley may be good at contributing to the trapping of pollution. Think of the inversion as the ceiling and the valley sides as walls in a room.

Finally, let's look at local emissions. During the winter months we like to keep warm so we use our furnaces and woodstoves, especially at night. Many cars and trucks idle longer to ‘warm up' before being driven. This is in addition to the ‘normal' daily levels of emissions. These emissions also occur close to the ground, well below both the inversion level and the height of the surrounding hills.

So now we have a ‘room' into which we're adding more pollutants. If there is little or no wind the following day to clear things out fine particle levels remain higher longer and reach 24-hour averages in the Moderate or Unhealthy for Sensitive Groups ranges.

Occasionally, about once every two to three years, a major winter-time regional event will occur and is caused by the same factors that cause the summer peak discussed below.

SUMMER PEAK:

Summer particle pollution events are regionally driven (meaning that weather patterns cause pollution levels to build in a broad area and are then transported into Maine).

During the summer the sun is shining earlier in the day so the air starts to mix at an earlier hour and continues mixing longer into the evening. Since the air is well mixed local emissions are diluted and therefore don't contribute to higher particle levels.

It takes large slow moving high pressure systems to increase the particle levels high enough for a 24-hour average to rise above the Good range during the summer. The high pressure traps pollutants closer to the surface. When they move slowly across the U.S. they pick up more and more pollution. By the time they arrive in New England these high pressure systems are a 'dirty air mass' and particle levels are high.

Air Quality Trends

Particle Pollution Annual Trend