Time Dependence of Aerosol Light Scattering Downwind of Forest Fires

 

Authors

Larry Kleinman — Brookhaven National Laboratory
Arthur J Sedlacek — Brookhaven National Laboratory
Jian Wang — Washington University in St. Louis
Ernie R. Lewis — Brookhaven National Laboratory
Stephen R. Springston — Brookhaven National Laboratory
Duli Chand — Pacific Northwest National Laboratory
John E Shilling — Pacific Northwest National Laboratory
William Patrick Arnott — University of Nevada Reno
Andrew Freedman — Aerodyne Research, Inc.
Timothy B Onasch — Aerodyne Research, Inc.
Ed Fortner — Aerodyne Research, Inc.
Qi Zhang — University of California, Davis
Robert James Yokelson — University of Montana
Kouji Adachi — Meteorological Research Institute
Peter R. Buseck — Arizona State University

Category

Absorbing aerosol

Description

In the first phase of BBOP (Biomass Burn Observation Project), a Department of Energy (DOE)-sponsored study, wildland fires in the Pacific Northwest were sampled from the ARM Facility G-1 aircraft via sequences of transects that encountered emission whose age (time since emission) ranged from approximately 15 minutes to two to four hours. This allowed us to determine the near-field-time evolution of trace gases, aerosol, and optical properties. Effects of plume dilution were taken into account by normalizing extensive variables to the mixing ratio of CO, a conservative tracer of fire emissions. Over a few hours of transport, the photochemical age ratio, NOx/NOy, decreased by as much as an order of magnitude, indicating an active chemical environment, though as shown by PTR-MS hydrocarbon measurements, not having unusually high OH concentration. There are variations between the sampled fires, but typically organic aerosol concentrations (which is > 90% of total aerosol mass) increased by amounts that were significantly less than the observed increase in light scattering, which for some fires exceeded a factor of two. Two possible causes for the discrepancy between scattering and mass are i) the downwind formation of refractory tar balls that are not detected by the AMS and therefore contribute to scattering but not to aerosol mass and ii) changes to the aerosol size distribution. Tar balls are the subject of a poster by Sedlacek et al. This poster investigates aerosol size distributions, a task complicated by the extremely high aerosol number concentration resulting in coincidence problems for the PCASP and UHSAS probes. We instead use extrapolated data from the FIMS as well as scattering measurements at multiple wavelengths that yield Angstrom exponents and backscatter ratios.