Influence of Atmospheric Aging on Fire-Derived Carbonaceous Particles: Laboratory Studies and Modeling in Support of FIREX

We propose a joint measurement-modeling project that aims to better constrain the climate and air quality impacts of North American wildfires, via the detailed examination of the evolving optical, physical, and chemical properties of fire-derived particulate matter (PM). Such PM includes absorbing species such as black carbon (BC) and brown carbon (BrC), the properties of which are likely to change dramatically in the atmosphere subsequent to emission. Such effects are, however, poorly understood at present, representing major limitations in our ability to predict the amounts, properties, and impacts of wildfire emissions. Thus, we will carry out an extensive series of laboratory experiments involving the detailed measurement of the changes to fire-derived PM with atmospheric oxidation.

Central to this work is the implementation of a new suite of state-of-the-science analytical techniques, providing highly detailed measurements of the chemistry, hygroscopicity, and optics of fine particles in real time. Measurements will be made as a function of oxidant exposure, varied by sampling through a novel oxidation flow reactor. Experiments will be carried out as part of two planned FIREX laboratory intensives, first at the Missoula Fire Sciences Laboratory, and then at the CIRES environmental chamber facility; additional experiments will be carried out in a smaller chamber as well. These experiments will provide precise, multi-wavelength measurements of key optical parameters of fire-derived PM (e.g., absorption and scattering coefficients), as a function of fuel/burn type, oxidant exposure, and relative humidity. This will help address key uncertainties in the contribution of BrC to total light absorption, the extent to which fire-derived PM changes in absorption and scattering over its atmospheric lifetime, and the effect of water uptake on light absorption by BC-containing PM. Results from these experiments will be used to develop a simplified scheme to describe fire-derived carbonaceous aerosol aging. This scheme will be implemented in the coupled GEOS-Chem-RRTMG global chemical transport and radiation model and tested against a suite of airborne observations of smoke over the United States, including measurements made aboard the P3 aircraft during the FIREX intensive. Finally, this model will be used to estimate the climate impacts of U.S. fires, as well as the visibility and human health impacts, and the relative importance of representing carbonaceous aerosol aging in estimating these impacts.