WOODSMOKE IN MAINE

In Maine , with cold winters and a forested landscape, wood is an obvious choice for home heating. First, wood continues to be plentiful and second wood has been used to heat dwellings since the first people began to live here. Because of its long history of use, there is a myth that burning wood is less harmful than other home heating fuels. The information on other pages demonstrates why this is NOT the case. This page deals with the potential for wood smoke to impact public health in Maine .

Winter typically brings long cold nights. When these nights are dry, windless and cloudless the ground cools more rapidly than the air aloft. Given these conditions an inversion is created. Inversions act like a cap or a ceiling preventing local emissions from rising. In view of the fact that this type of inversion is created during the night it is often called a nocturnal inversion. There is more information about inversions at the bottom of this page.

Nocturnal inversions during the winter occur at a time when there is likely to be more emissions from home heating fuels; so pollution, including wood smoke, builds up. Particle pollution levels are usually highest in the morning hours. If the rest of the day is marked by little wind and/or little vertical mixing pollution levels may not decrease. Depending on how high the levels built up overnight and whether or not they drop off during the day, particle pollution may be classified as 'Moderate'* for that day.

Maine experiences a number of days with Moderate levels of particle pollution during the winter months, even when much of New England might have lower levels of particle pollution.

 

In the image to the left, you will notice that the 24-hour average particle pollution levels in New England were mostly 'Good'* except those areas affected by inversions on this winter night. This is a sign of a local, rather than a regional event because a regional event would have caused more uniformly Moderate levels of particle pollution. The night of December 5/6, 2006 was marked by clear skies and little or no wind.

 

DEP Air Quality Meteorologists forecast air quality year-round.

 

In many areas of the state, one must also take into account the influence of terrain on particle pollution concentrations. Valley walls hinder the atmosphere's ability to dilute locally emitted pollutants, including wood smoke.

 

Based on information gleaned from the 2000 census, suburban areas have the greatest density of households including those that heat with wood while rural areas have a greater percentage of wood burning households. Terrain combined with density means that suburban valleys have the greatest potential for problems stemming from wood smoke. Nevertheless, rural areas, even with a lower density of households, can still encounter problems stemming from wood smoke because of the inversion with or without terrain influence.

Hence, it is important for everyone using a wood burning device of any kind to follow manufacturer, state and federal recommendations for placement, operation and maintenance of the unit. It is in the public's best interest to promote the use of efficient wood burning devices that improve combustion and reduce emissions, thus reducing the impacts of wood smoke. More information is available on the 'Reducing Impacts of Wood Smoke' page.

* 'Moderate' and 'Good are categories of the Air Quality Index (AQI).

More About Inversions

Since 'inversion' simply means the temperature increases with height there are other types of inversions but they are not the focus of this discussion. The greater the difference between the temperature near the ground and the air aloft, the stronger the inversion. Inversions can be shallow or deep depending on the location and other circumstances.

The sun always radiates energy to the ground and the ground always radiates energy away. The ground cools at night because the energy continues to radiate away from the ground with little coming in since that part of the earth is turned away from the sun. Snow cover contributes to the cooling in two ways: 1) it acts like a blanket over the ground keeping the soil from warming the air above it by conduction (touching), and 2) snow is a good emitter of infrared radiation.

Strong nocturnal inversions develop during long, dry, windless and cloudless nights because:

• Longer nights have a better chance for a strong inversion to form. The surface radiates infrared energy better than air. The longer the night, the more likely it is for a large difference between the air near the surface and air aloft.
• Dry conditions help inversion formation. Moisture at the surface or in the air hinders night-time cooling because as the air cools water vapor condenses and releases energy which 'heats' the air around it.
• Calm conditions allow inversions to form because the surface and the air above it don't interact with the air aloft. Wind mechanically mixes the air which decreases the difference in temperature so inversions are hindered.
• Clear nights also allow inversions to form because the energy continues to radiate away from the surface. Clouds absorb and radiate the energy back to the ground reducing how much it cools which hinders the development of an inversion.

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