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

Field observations of ozone formation in smoke plumes from fires date back nearly 25 years when aircraft measurements detected elevated ozone at the edge of forest fire smoke plumes far downwind (Stith and others 1981). More recent observations (Wotawa and Trainer 2000) suggest that high concentrations of ozone are found in forest fire plumes that are transported great distances and across international boundaries. Measurements made during EPAs 1995 Southern Oxidant Study indicate that Canadian forest fires changed the photochemical properties of air masses over Tennessee on days with strong fire influence. Regional background ozone levels were elevated by 10 to 20 ppb on fire impact days as compared with nonimpact days during the study. Aircraft measurements found that, although forest fire plumes were always well defined with respect to carbon monoxide, they gradually lost their definition with respect to ozone after being mixed into the boundary layer. The amount of ozone transported to the surface measurement sites was found to depend upon where and when the plumes reached the ground. Elevated plumes were always marked by enhanced ozone concentrations, at times reaching values of 80 to 100 parts per billion (ppb) above tropospheric background.

Stith and others (1981) mapped ozone mixing ratios in an isolated, fresh, biomass-burning plume. At the source, or near the bottom, of the horizontally drifting plume they measured low or negative changes in ozone values, which they attributed to titration by NO and low ultraviolet (UV) intensity. Near the top of the plume, 10 km downwind, and in smoke less than 1 hour old, they measured change in ozone values as high as 44 parts per billion by volume (ppbv). Greater changes in ozone were positively correlated with high UV. Thus the initial destruction of ozone by reactive species in the plume followed by its gradual formation was documented.

A new and potentially useful tool for assessing impacts of long-range plume transport is based on the concept of using ΔO₃/ΔCO (excess O₃ over excess CO) as a "photochemical clock" to denote the degree of photochemical processing in a polluted air mass by using carbon monoxide as a stable plume signature. As the plume disperses, its volume expands and absolute values of ozone can drop even though net production of ozone is still occurring. The ΔO₃/ΔCO normalizes for plume expansion and is a useful measure of net ozone production. In the course of atmospheric chemistry research, numerous observations of ΔO₃/ΔCO ratios have been made in biomass burning haze layers. Unfortunately, the observations represent haze of various ages and uncertain origin. In haze layers 1 to 2 days old, changes in the ΔO₃/ΔCO ratios of 0.04 to 0.18 were measured over Alaska (Wofsy and others 1992) and ratios of 0.1 to 0.2 were measured over Eastern Canada (Mauzerall and others 1996). High ratios, up to 0.88, were measured at the top of haze layers that had aged about 10 days in the tropics (Andreae and others 1994).

In 1997, airborne Fourier transform infrared spectroscopy (FTIR) measurements in large isolated biomass burning plumes in Alaska revealed new details of downwind chemistry. Downwind smoke samples that had aged in the upper part of one plume for 2.2 ± 1 hours had ΔO₃/ΔCO ratios of 7.9 ± 2.4 percent, resulting from initial, absolute ozone formation rates of about 50 ppb/hr. Downwind samples obtained well inside another plume, and of similar age, did not have detectable ΔO₃, but did have ΔNH₃/ΔCO ratios about one-third of the initial value. ΔHCOOH/ΔCO (formic acid) and ΔCH₃COOH/ΔCO (acetic acid) usually increased about a factor of 2 over the same time scale in samples from both plumes. NO_x was below the detection limit in all the downwind samples. These data provided the first precise in-plume measurements of the rate of O₃/CO increase and suggested that this rate depended on relative position in the plume. The apparently rapid disappearance of NO_x is consistent with the similar early observation, and the drop in NH₃ was consistent with a reaction with HNO₃ to form ammonium nitrate, which is a NO_x sink. Secondary sources of formic acid relevant to polluted air have been described (Finlayson-Pitts and Pitts 1986). Jacob and others (1992, 1996) discussed several gas-phase sources of acetic acid that could occur in biomass burning plumes. These experiments provide the first experimental indication of the approximate time scale of secondary organic acid production in actual plumes.

A large number of photochemical modeling studies of biomass burning plumes have been published (Chatfield and Delaney 1990; Chatfield and others 1996; Crutzen and Carmichael 1993; Fishman and others 1991; Jacob and others 1992, 1996; Koppmann and others 1997; Lee and others 1998; Lelieveld and others 1997; Mauzerall and others 1998; Olson and others 1997; Richardson and others 1991; Thompson and others 1996). Nearly all these studies conclude that the net production of ozone occurs either in the original plume, or as a result of the plume mixing with the regional atmosphere. Several studies have shown a strong dependence of the final modeled results on the details of the post-emission-processing scenario such as the timing between production of the emissions and their convection to the free troposphere (Chatfield and Delaney 1990; Jacob and others 1996; Lelieveld and others 1997; Pickering and others 1992; Thompson and others 1996).

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.

Emission factors for hydrogen oxide (HO_x, a collective term for OH and HO₂) precursors, NH₃, and NO_x have been estimated with the Missoula, MT, open-path spectroscopic system (Yokelson and others 1997). These experiments reveal that smoke contains high levels of oxygenated organic compounds, methanol (CH₃OH), acetic acid (CH₃COOH), and formaldehyde (HCHO). These compounds typically oxidize or photolyze within hours in a smoke plume to release HO_x that is important in sulfate aerosol formation processes. Under clear-sky conditions typical for noon on July 1 at 40°N latitude, the formaldehyde photolysis lifetime is about 3.8 hours (Yokelson and others 1997). Since the HCHO/CO source ratio for fires is typically near 2 percent, this process clearly injects large quantities of HO₂ into fresh plumes (Yokelson and others 1997). HO_x emissions from fire may become a critical input to regional haze models that simulate secondary sulfate formation processes.

The H₂O₂ is soluble in cloud droplets where it would play a major role influencing reaction rates during aqueous-phase sulfate formation chemistry (NRC 1993).

A number of processes are important in plume particle formation and growth. Many of these processes involve interaction with the trace gases in a plume originating from nucleation in which two gases react to form a solid nucleus for subsequent particle growth. An example of nucleation is the reaction of ammonia and nitric acid. In addition, condensation can create new particles when gases cool or through particle growth when a trace gas collides with and condenses on an existing particle. The second condensation process is quite common because biomass burning aerosol is hydrated. Soluble nucleilike ammonium nitrate promotes this process. There is a little evidence that organic gases also condense on particles. Nucleation and condensation are both examples of trace-gas-to-particle conversion, which will increase the mass of particles in a plume, decrease the concentration of certain trace gases in the plume, and, in the case of condensation, contribute to an increase in average particle diameter. Andreae and others (1988) measured particle-NH₄⁺/CO₂ ratios of 0.7 to 1.5 percent in slightly aged biomass burning plumes. Measurements of NH₃/CO in fresh smoke are typically near 2 percent. Thus, there is probably rapid conversion of gas-phase NH₃ to particle NH₄⁺ either through nucleation or dissolution in the surface water of other hydrated particles.

Coagulation is when two particles collide and combine. This increases the average particle diameter, reduces particle number, and does not effect total particle mass. Coagulation probably contributes to the increase in average particle diameter that occurs downwind from fires (Reid and others 1998).

At any given point in its evolution a particle may impact the trace gas chemistry in a smoke plume. For instance, it is known that NO₂ reacts on the surface of soot particles to yield gas phase HONO. This and other heterogeneous reactions such as ozone destruction may occur on smoke aerosol. Some recent research suggests that oxygenated organic compounds emitted from fires could also be important in heterogeneous processes. Hobbs and Radke (1969), Desalmand and others (1985), Andreae and others (1988), and Roger and others (1991) found that a high percentage (25 to 100 percent) of fire aerosol particles from fires could be active as condensation nuclei (CCN). Radke and others (1990) observed that cumulus clouds greater than 2 km in depth scavenged 40 to 80 percent of smoke particles. The high concentrations of CCN in smoke plumes can contribute to the formation of clouds with smaller than "normal" cloud droplet size distributions. This type of cloud is more reflective to incoming solar radiation and less likely to form precipitation. Some work suggests that absorbing aerosol can reduce cloud formation. Finally, clouds can evaporate and leave behind chemically altered particles.

All of these mechanisms alter both the chemical nature and number of particles contained within smoke plumes from fires. In addition, reactive species emitted from fires may alter the conversion rate of gaseous precursors of secondary sulfate and nitrate particles, affecting regional haze modeling results.

Although the regulatory implications of reactive species emissions from fire are yet to be determined, much more attention to these issues will occur once fire is including in regional haze and ozone modeling efforts.