Black carbon and organic carbon in aerosol particles from crown fires in the Canadian boreal forest

Abstract

In the boreal forest, high‐intensity crown fires account for an overwhelming proportion of the area burned yearly. Quantifying the amount of black carbon (BC) from boreal crown fires in Canada is essential for assessing the effect on regional climate from natural wildfire aerosol emissions versus that from anthropogenic activities. This is particularly relevant because climate change will likely lead to increased wildfire activity in northern Canada. During 4–5 July 1998, two controlled fires in Northwest Territories, Canada, were conducted as part of the International Crown Fire Modeling Experiment. We report here the BC and organic carbon (OC) compositions of aerosols produced during the flaming and smoldering stages of burning. Particles were collected on back‐to‐back quartz‐fiber filters by helicopter with a hi‐vol sampler and at ground level with a dichotomous sampler to separate the fine (≤2.5 μm diameter) and coarse (2.5–10 μm diameter) particle fractions. An analysis of the back filter in relation to the front filter from the dichot sampler for both the fine and coarse fractions provided a means to correct for the adsorption of gas‐phase organic compounds on filters (positive artifact) and for the loss of particulate carbon from filters by volatilization (negative artifact). BC and OC masses, which combine here to give total carbon (TC), were determined by the thermal‐optical method. The BC to TC ratio for the flaming stage was 0.085 ± 0.032 ( ± ksn−1/2, k = 2, n = 2), based on aerial sampling of the dark plume 300–500 m above the flame front. BC/TC for the smoldering stage was 0.0087 ± 0.0046 from ground‐based sampling. Uncertainties consist of the combined variances in measurement and sampling and in emissions from different fires. These averages and uncertainties serve as important aerosol data input for predictions of climate change on both global and regional scales.