Ask TreeHugger: Emissions vs. Concentrations

Question: I read with interest your great response to a question about open burning on and I'm just curious if you may have a moment to answer another question about particulate matter.

I live in Nevada County California where open burning remains a persistent nuisance in winter months. At, Nevada County, California (zip code 95959) is listed as ranking (according to EPA statistics) in the bottom 10th percentile for particulate matter (pm2.5 and pm10) emissions but in the top 10th to 20th percentile for 24 hour averages of particulate matter concentrations.

Can you by chance tell me something about the possible reason for this discrepancy?

Response: Open burning does contribute to particle emissions and through this contribution, to particle concentrations as well. This contribution is particularly great for fine particles (PM2.5) that can be inhaled deep into people’s lungs and for inhalable particles (PM10) that can be inhaled but not always deep into the lungs. Particle emissions from open burning are determined by the amount of particles that are emitted from the fire in a certain amount of time (for example, in one second or in one hour). Particle emissions can be direct, where particles are emitted directly by the fire, or they can be indirect, occurring as a result of reactions that occur between other fire-emitted chemicals in the air. Emissions of particles and other pollutants are quantified by the US Environmental Protection Agency (EPA) and published by EPA and groups such as

Once in the air, the emitted particles will travel with the winds and will disperse (or otherwise scatter), possibly change, and eventually deposit on the ground, trees, and other surfaces. All of these factors are taken into account by particle concentrations, which provide a measure of the amount of particles that are present in a certain volume of air. In a sense, concentrations are a measure of the amount of emitted particles that exist in a certain location.

Since there are many sources of PM2.5, measured PM2.5 concentrations will include particles emitted not only from open burning but also from a variety of other sources as well. For pollutants such as PM2.5, it is the concentrations (and not emissions) that are regulated by EPA under the National Ambient Air Quality Standards of the Clean Air Act. To achieve these standards, however, particle emissions from particle sources may be regulated, which for open burning is generally through regulations on when and what can be burned.

The EPA website provides useful, general ifnormation on fine particles, their sources, their concentrations, and health effects.

Previous Ask Treehugger columns can be found here.

Helen Suh MacIntosh is a professor in environmental health at Harvard University and studies how pollution behaves in the environment and how it affects people's health. Please keep in mind that her answers are just her interpretation of available information and should not be taken as the only viewpoint or solution to a problem. Use this column at your own risk. Having said this, please feel free to post any of your environmental health questions to (Please use a descriptive email subject line and mention if you want to remain anonymous or not).

Tags: Air Pollution | Ask TreeHugger | California

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