Residential wood heating patterns differ depending on the season and the weather conditions. The levels of wood smoke pollution vary accordingly. In some neighbourhoods the heating patterns during the winter months cause an increase in wood smoke production. Together with weather and local topography this affects the air quality in these neighbourhoods.
In winter the air can become loaded with the products of incomplete combustion such as particulate matter (PM), volatile organic compounds (VOCs), carbon monoxide (CO) and nitrogen oxides (NOx). The severity of the resulting winter smog depends on the degree of atmospheric dispersion. The lower the level of atmospheric dispersion, the higher the level of winter smog. Atmospheric dispersion is mainly determined by wind speed and mixing height.
1. Wind speed pushes and disperses the pollutants horizontally. No wind means stagnant air and allows levels of pollutants to build up in the air (smog).
2. The mixing height refers to the maximum height the pollutants can reach if dispersed vertically. In normal situations, the mixing height is enough to disperse the pollutants high into the atmosphere. The pollutants are carried up by the layer of warm rising air to the colder air higher up (see diagram).
In the case of temperature inversion, the pollutants are trapped at ground level where it causes most harm. This inversion occurs for example when ahead of a warm front or in broad surface ridge. Cold air becomes trapped under the layer of warm air that acts as a lid. The pollutants in the cooler layer cannot be dispersed and the pollutants stay concentrated at ground level (see diagram).
Topography also plays an important role in the concentration of pollutant levels. The physical "walls" of a valley for example restrict air movement in the valley. High levels of pollutants in the air cannot be sufficiently dispersed and communities located in the valley will be covered in smog. A community located on an open plain will not have this dispersion problem. Smog also arises in places with stretches of rolling terrain where cold air can get trapped in the terrain's many pockets and cause temperature inversion.
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