Evolution of Biomass Burning Aerosol Optical Properties in the Near Field
Authors
Larry Kleinman — Brookhaven National Laboratory
William Patrick Arnott — University of Nevada Reno
Andrew Freedman — Aerodyne Research, Inc.
Stephen R. Springston — Brookhaven National Laboratory
Timothy B Onasch — Aerodyne Research, Inc.
Arthur J Sedlacek — Brookhaven National Laboratory
Duli Chand — Pacific Northwest National Laboratory
Category
Absorbing Aerosol
Description
Biomass burning (BB) events are known to produce both a chemically rich environment that can impact the evolution of primary aerosols and influence secondary aerosols production rates. With their increasing frequency, these events are expected to exert an ever-increasing impact aerosol radiative forcing. Recent literature suggests that BB aerosols undergo a rapid evolution near their source. During the summer of 2013, the Biomass Burning Observation Project (BBOP) specifically targeted the near field (< 2 hours) using the DOE Gulfstream-1 (G-1) aircraft.
To investigate the optical and microphysical evolution, the BBOP instrument suite included a Single Particle Soot Photometer (SP2) to probe the mixing state of refractory black carbon (rBC), the Soot Particle Aerosol Mass Spectrometer (SP-AMS) to investigate the composition of both non-refractory and rBC-containing particles, a 355 nm Photoacoustic spectrometer (PAS), a 532 nm photothermal interferometer (PTI), 628 nm cavity Attenuation Phase Shifted (CAPS) spectrometer, a 3-λ nephelometer, and a 3-λ PSAP to provide in situ measurement of aerosol optical properties. BBOP campaign represented the maiden aircraft deployment for the SP-AMS, the 355 nm PAS and 532 nm PTI.
Analysis of the wildfire known as Government Flats reveals that the single scattering albedo (SSA) at 532 nm increases from 0.79 to 0.93 within the first 1.5 hours of aging – consistent with increasing aerosol size due to condensation growth. The SSA at 355 nm is found to increase from 0.84 to 0.88 within this aging window. Similarly, the absorption Ångstrom exponent (AAE) for the 355/532 nm wavelength combination is found to increase from ~3 to ~4 with plume age while the 464/522 nm wavelength combination from the PSAP increases from ~2 to ~3 – consistent with brown carbon (BrC) production. Analysis of the compositional dependence of the aerosol mass absorption cross-section reveals a modest dependence of the 355 nm absorption on nitrate concentration. This finding parallels previous investigations where of BrC absorption has been attributed to the presence of nitrate-containing species. Finally it is found that the reported PSAP absorption coefficient measured at 522 nm is nominally 4x greater than that measured by the PTI. This difference is ascribed to filter-bias associated with the PSAP instrument. These findings and a contrasting agricultural burn will be presented.
