Mixing state of black-carbon particles from wildfires remains dynamic throughout their lifetime
Sedlacek, Arthur J — Brookhaven National Laboratory
Lewis, Ernie R. — Brookhaven National Laboratory
Area of research:
The evolution of the mixing state of black carbon-containing particles formed by wildfires is characterized using field measurements. These particles rapidly (within an hour or so) attain a thick coating of organic material, but the majority of the lifetime of these particles is characterized by a slow loss of this coating.
Most model treatments of biomass burn aerosol are based on measurements in the near-field region where the particles have thick coatings, yet the vast majority of the lifetime of these particles is characterized by loss of coating and needs to be taken into account.
The life cycle of black carbon (BC)-containing particles from biomass burns is examined using aircraft and surface observations of the BC mixing state for plume ages from ∼15 min to 10 days. Because BC is nonvolatile and chemically inert, changes in the mixing state of BC-containing particles are driven solely by changes in particle coating, which is mainly secondary organic aerosol (SOA). The coating mass initially increases rapidly (kgrowth = 0.84 h−1), then remains relatively constant for 1−2 days as plume dilution no longer supports further growth, then decreases slowly until only ∼30% of the maximum coating mass remains after 10 days (kloss = 0.011 h−1). The mass ratio of coating to core for a BC-containing particle with a 100 nm mass-equivalent diameter BC core reaches a maximum of ∼20 after a few hours and drops to ∼5 after 10 days of aging. The initial increase in coating mass can be used to determine SOA formation rates. The slow loss of coating material, not captured in global models, comprises the dominant fraction of the life cycle of these particles.