Ozone production from Canadian wildfires during June and July of 1995

[1] During the summer of 1995, especially between June and mid July, extensive wildfires occurred throughout Canada, primarily north of 55°N latitude. A previous report used aircraft and surface observations and tracer simulations to show these fires strongly influenced CO concentrations as far south as 35°N in the central and eastern United States [Wotawa and Trainer, 2000]. This study extends those results by incorporating wildfire emissions estimates for CO, NOx, and nonmethane hydrocarbons into a three-dimensional photochemical transport model specifically designed to simulate ozone photochemistry in the continental United States. The results of the model are compared to observations from four measurement platforms deployed during the time period of interest: National Oceanic and Atmospheric Administration WP-3 aircraft observations collected during the 1995 Southern Oxidants Study (SOS-95) field campaign; 12 eastern U.S. surface stations that measured ozone, CO, and NOy; rural ozone measurements from the Aerometric Information Retrieval System network collected by the U.S. Environmental Protection Agency; and daily ozonesondes obtained near Nashville, Tennessee, during SOS-95. Model performance, as determined by correlation and bias with observations from these four platforms, is significantly improved for both O3 and CO when the Canadian fires are considered. Both observations and model results show enhanced O3 from air transported from the Northwest Territory. The model results imply that during the period of strongest fire influence 10 to 30 ppbv enhancement of O3 throughout a large region of the central and eastern United States was due to these fires. Modeled O3 increases are sensitive to the NOx/CO emission ratio assumed for the fires, which is highly uncertain and variable. A molar NOx/CO ratio of 0.007 yields model comparisons that are most consistent for O3 and ΔO3/ΔCO observations within aged fire plumes during SOS-95, and is also consistent with previously observed NOx/CO ratios from boreal fires. For this NOx/CO emission ratio, and considering the entire eastern United States, most of the O3 increase is associated with the NOx emitted directly by the fires and the photochemical O3 formation that occurs before the plumes actually reach the United States. However, the in situ oxidation of CO from the Canadian fires with NOx emitted locally leads to significantly higher O3 increases for high-NOx-emitting regions that are limited by hydrocarbon availability. Thus O3 in urban areas, or any other region modified by nearby NOx sources, is more sensitive to long-range fires compared to less populated or polluted regions.

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