Atmospheric and Plume Chemistry

Traditionally, ozone and secondary aerosol precursors have been discussed within the context of urban smog caused by auto exhaust and reactive organic compounds emitted from industrial facilities. But the same pollutant and tropospheric chemical reactions occur in both urban settings and in rural areas where wildfire smoke may be an important if not dominant source of ozone precursor emissions. In these situations, emissions from fire may play an important role in ozone formation as well as nitrate and, indirectly, sulfate aerosol formation, which results in visibility impairment and increased PM2.5 concentrations.

At present, there is an urgent need to understand the impact of fire emissions on emerging visibility and ambient air standards as they relate to fire planning at the strategic, programmatic, and operational scales (Fox and Riebau 2000; Sandberg and others 1999). Chemical processes that occur in plumes from fires, directly or indirectly, touch on a number of these issues and are critical to the development of a regional model that will be used to assess the impact of fire on air quality.

Because of the Environmental Protection Agencys (EPA) pressing regulatory need to assess inter-State ozone transport and sources of precursor emissions, a new regional-scale mechanistic model called Models- 3/CMAQ (Byun and Ching 1999) is being used by the Ozone Transport Commission (OTC) region of Northeastern and Mid-Western States, and the Western Regional Air Partnership (WRAP). Future applications will likely involve regional haze modeling in other areas of the country. Oxides of nitrogen (NO_x) and volatile organic compounds (VOCs) emissions from fire in the OTC region have not previously been considered significant, but the new model photochemistry module requires that precursor emissions be included for all sources. As Models-3/CMAQ develops, NO_x and VOC emissions from fire will be included in ozone and secondary modeling.

For detailed information on atmospheric and plume chemistry, see:

• Ozone Formation in Plumes
• Factors Affecting Plume Chemistry
• Emission Factors for Reactive Species
• Particle Formation in Plumes