Photochemical Aging Induces Changes in the Effective Densities, Morphologies, and Optical Properties of Combustion Aerosol Particles

Abstract

Effective density (ρ_eff) is an important property describing particle transportation in the atmosphere and in the human respiratory tract. In this study, the particle size dependency of ρ_eff was determined for fresh and photochemically aged particles from residential combustion of wood logs and brown coal, as well as from an aerosol standard (CAST) burner. ρ_eff increased considerably due to photochemical aging, especially for soot agglomerates larger than 100 nm in mobility diameter. The increase depends on the presence of condensable vapors and agglomerate size and can be explained by collapsing of chain-like agglomerates and filling of their voids and formation of secondary coating. The measured and modeled particle optical properties suggest that while light absorption, scattering, and the single-scattering albedo of soot particle increase during photochemical processing, their radiative forcing remains positive until the amount of nonabsorbing coating exceeds approximately 90% of the particle mass.

Keywords: aerosol optics; black carbon; brown coal; combustion aerosol; morphology; photochemical aging; residential combustion; soot; wood.

Figure 1

Morphology and chemical composition of the sampled combustion aerosol particles. (a) Effective densities of the studied aerosols as a function of mobility diameter (K and D_fm values are not presented when the effective density function does not follow the power law), example micrographs of (b) freshly emitted and (c) aged particles, and (d) diagrams of the particulate chemical composition measured with SP-AMS for sample cases.

Figure 2

Aerosol optical properties calculated with the core–shell N-Mie model at seven aethalometer wavelengths. (a) SSA, (b) MAC, (c) RFE, and (d) RFE of green light (wavelength 520 nm) as a function of f(rBC). The dashed lines in (d) are empirical fits through the data points.