Density and Homogeneous Internal Composition of Primary Brown Carbon Aerosol

The presence of atmospheric brown carbon (BrC) has been the focus of many recent studies. These particles, predominantly emitted from smoldering biomass burning, absorb light in the near-ultraviolet and short visible wavelengths and offset the radiative cooling effects associated with organic aerosols. Particle density dictates their transport properties and is an important parameter in climate models and aerosol instrumentation algorithms, but our knowledge of this particle property is limited, especially as functions of combustion temperature and fuel type. We measured the effective density (ρ_eff) and optical properties of primary BrC aerosol emitted from smoldering combustion of Boreal peatlands. Energy transfer into the fuel was controlled by selectively altering the combustion ignition temperature, and we find that the particle ρ_eff ranged from 0.85 to 1.19 g cm^-3 corresponding to ignition temperatures from 180 to 360 °C. BrC particles exhibited spherical morphology and a constant 3.0 mass-mobility exponent, indicating no internal microstructure or void spaces. Upon partial thermal volatilization, ρ_eff of the remaining particle mass was confined to a narrow range between 0.9 and 1.1 g cm^-3. These findings lead us to conclude that primary BrC aerosols from biomass burning have homogeneous internal composition, and their ρ_eff is in fact their actual density.