Wildland Fire Impacts on Surface Ozone Concentrations

1. Wildland Fire Impacts on Surface Ozone Concentrations Literature Review of the Science State-of-Art Ned Nikolov, Ph.D. Rocky Mountain Center USDA FS Rocky Mountain Research Station

2. 1. Introduction Vegetation fires influence chemically Earth’s atmosphere on a global scale Biomass burning releases NOx and volatile organic compounds (VOCs), which react photo-chemically causing elevated ozone concentrations in the troposphere Globally, vegetation fires contributes about 10% to the observed tropospheric ozone level Most ozone formation due to wildfires occurs in the tropics, (e.g. over Africa, nearly 24% of the observed O3 in the planetary boundary layer is a result of biomass burning).

3. 2. Photochemistry of Ozone Production in Smoke Plumes Ozone is a secondary product of biomass burning formed through the oxidation of VOCs driven by sunlight and temperature and catalyzed by elevated levels of NOx Specifically, tropospheric ozone is formed through the photolysis of NO2. However, most NOx emissions into the atmosphere are in the form of NO. Radical species (generated from VOCs) convert NO into NO2 Thus, ozone production from biomass burning can be limited by either VOCs or NOx emissions

4. 2. Photochemistry of Ozone Production in Smoke Plumes (cont.) In the VOCs limited state, O3 concentration decreases with increasing NOx levels In the NOx limited state, O3 formation increases with higher NOx concentration because radical species are readily available to convert NO to NO2 VOCs can increase radical species production and rates of photochemistry within a smoke plume, therefore increasing O3 production VOCs can also accelerate the titration of NOx out of the smoke plume resulting in the slowing of O3 production. Thus, the net impact of emitted VOCs on O3depends on the complex balance between these two effects and may be either positive or negative

5. 2. Photochemistry of Ozone Production in Smoke Plumes (cont.) Predicting O3 production due to biomass burning at the regional scale need to include both direct emissions of VOCs and a realistic estimate of initial NOx concentrations

6. 3. Importance of Scale Ozone production from biomass burning occurs at the scale of individual plumes due to the oxidation of short-lived chemical species such as alkenes and aldehydes, and at the regional scale due to the oxidation of longer-lived emissions such as CO and alkanes At larger scales, the effect of fires on tropospheric ozone formation results from complex interaction between photochemical mechanisms and long-range atmospheric transport. Entrainment of a gradually subsiding plume into the PBL is an important mechanism for transporting pollutants from elevated layers to the surface

7. 3. Importance of Scale (cont.) Photochemical O3 production also takes place in very young smoke plumes, within a few tens of minutes after the fire emissions are released. Ozone production in individual smoke plumes can be significant for the regional-scale ozone budget Close to the fire, ozone in the plume maybe depleted compared to background concentrations, due to the reaction of ozone with the emitted NO. Further from the fire, ozone can be produced at a rate of about 25 ppb h-1 increasing downwind ozone concentration well above the background

8. 3. Importance of Scale (cont.) Primary VOC emissions are critical for the formation of ozone in young smoke plumes Unfortunately, only about 70% of the VOCs present within a smoke plume can be identified with current technology Production of radicals through the photolysis is crucial for the formation of ozone and nitrogen reservoir species, and the oxidation of alkenes in fresh plumes Enhanced radical production leads to enhanced ozone production, enhanced formation of nitrogen species, and higher oxidation efficiency in young plumes

9. 4. Modeling Ozone Production due to Biomass Burning Approaches used depending on the scale of interest: - Meso-scale Simulation Systems - Individual Smoke Plume Models

10. 4. Modeling Ozone Production: Meso-scale Approach Couple a 3-D atmospheric chemistry model (CMAQ) with a 3-D meso-scale atmospheric model (WRF), and an Emission Inventory Estimator (GloBEIS) McKeen et al. (2002) applied such a modeling system to study impact of 1995 Canadian fires on ozone levels in Eastern USA: 10 to 30 ppbv ozone enhancement AIRPACT-3 (Vaughan et al. 2004) – predicts ozone, PM, and other pollutants over the Pacific Northwest Lamb et al. (2007) enhanced AIRPACT-3 to include fire emissions predicted by the BlueSky modeling system Lamb et al. found that wildfires in PMW increased regional ozone levels by more than 30 ppbv

11. 4. Modeling Ozone Production: Individual Smoke-Plume Models Simulate the photochemical evolution of rising individual smoke plumes and are designed for applications at small temporal and spatial scales. Plume models can be: - 3D models, e.g. Active Tracer High Resolution Atmospheric Model (ATHAM), Trentmann et al. (2003); - Zero dimensional (box) models - designed to study the detailed photochemistry of very young fast evolving smoke plumes; have very detailed photochemical mechanisms; data intensive. Examples - Fedonia (Mason et al. 2001), Mainz (Jost et al. 2003)

12. 5. Conclusions Ozone production occurs at the scale of individual smoke plumes due to the oxidation of short-lived chemical species such as alkenes and aldehydes, and at the regional scale due to the oxidation of longer-lived emissions such as CO and alkanes. Ozone production from fires depends on many factors: - amounts of chemical reactants and catalysts - radiation loading and air temperature, - persistence and size of the source, - chemical composition of the burning biomass, - intensity of the fire, - weather-controlled dispersion of the plume. Ozone precursors formed in smoke plumes can be transported by weather systems over large distances spanning continental scales.

13. 5. Conclusions (cont.) Significant fraction of the enhanced ozone concentration at the regional scale is due to photochemical production of ozone in local smoke plumes before dispersion to the regional scale Chemical evolution of young smoke plumes is an important and complex phenomenon with VOC emissions playing a critical role in the formation of ozone Photochemistry of tropospheric ozone formation due to biomass burning has been studied at the regional level through gridded mesoscale simulation systems, and at the scale of individual fires though smoke-plume models.

14. 5. Conclusions (cont.) Regional photochemical models are limited in their ability to simulate important local processes controlling the variance and time evolution of ozone. Individual smoke-plume models are limited by the availability of detailed fire emission inventories and high-resolution data on plume dispersion and/or the meteorological environment in the immediate vicinity of the fire. __________________________________________

15. US FS Rocky Mountain Center Website: www. FireWeather.info

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