Factors Affecting Plume Chemistry

The specific chemical composition of a smoke plume depends on many factors: the details of post-emission atmospheric reactions including dilution rates, photolysis rates, position within the plume, altitude, and smoke temperature, which varies by time of day and combustion stage. Equally important is the chemistry of the downwind air that mixes with the plume, which could be clean air or contain aged plumes from urban areas or other fires. In addition, the physical aspects of the plume mixing are important. For example, at the relatively low temperatures typical of higher altitudes in the troposphere, peroxyacetyl nitrate (PAN) is a stable molecule, which can be transported. At lower altitudes, PAN can thermally decompose and rerelease NO_x. Nitric acid (HNO₃) can also be an important, transportable reservoir species for NO_x at high altitudes but for a different reason. HNO₃ has a narrower absorption cross-section at lower temperatures and therefore is less susceptible to photolysis. The rate of bimolecular reactions among smoke components usually decreases with temperature (thus typically with altitude or at night). Reaction rates depend even more strongly on the dilution rate, at least initially. Dilution by a factor of 2 will decrease a bimolecular reaction rate by a factor of 4.