ABSTRACT
The frequency and intensity of forest fires are amplified by climate change. Substantial quantities of PM1 emitted from forest fires can undergo gradual atmospheric dispersion and long-range transport, thus impacting air quality far from the source. However, the chemical composition and physical properties of PM emitted from forest fires and its changes during atmospheric transport remain uncertain. In this study, the evolution of organic carbon (OC), elemental carbon (EC), water-soluble ions, and water-soluble metals in the particulate phase of smoke emitted from the typical forest vegetation combustion in Southwest China before and after photo-oxidation was investigated in the laboratory. Two aging periods of 5 and 9 days were selected. The OC and TC mass concentrations tended to decrease after 9-days aged compared to fresh emissions. OP, OC2, and OC3 in PM1 are expected to be potential indicators of fresh smoke, while OC3 and OC4 may serve as suitable markers for identifying aged carbon sources from the typical forest vegetation combustion in Southwest China. K+ exhibited the highest abundant water-soluble ion in fresh PM1, whereas NO3- became the most abundant water-soluble ion in aged PM1. NH4NO3 emerged as the primary secondary inorganic aerosol emitted from typical forest vegetation combustion in Southwest China. Notably, a 5-day aging period proved insufficient for the complete formation of the secondary inorganic aerosols NH4NO3 and (NH4)2SO4. After aging, the mass concentration of the water-soluble metal Ni in PM1 from typical forest vegetation combustion in Southwest China decreased, while the mean mass concentrations of all other water-soluble metals increased in varying degrees. These findings provide valuable data support and theoretical guidance for studying the atmospheric evolution of forest fire aerosols, as well as contribute to policy formulation and management of atmospheric environment safety and human health.
ABSTRACT
The monomer of soot fractal aggregate is usually considered to be sphere, but the monomer shapes are cube and hexagon by some transmission electron microscope (TEM) and scanning electron microscope (SEM) observation. In this paper, the fractal soot models of different monomer shapes (sphere, cube, ellipsoid, hexagonal prism) were established. And the optical properties of models are calculated by discrete dipole approximation (DDA). After systematically comparing the Muller matrix and optical cross section properties between the models, we find that monomer deviation from sphericity does not necessarily lead to further decline of F22(π)/F11(π) even at shorter wavelengths. In other words, the non-sphericity of monomers does not necessarily affect the non-sphericity of whole soot particle. This can provide some implication for lidar remote sensing observation. However, other light scattering matrix elements can keep good consistency. The maximum deviation of extinction cross section of hexagonal prism model is 11.2%. The more the monomer shape deviates from the sphere, the more the optical integral properties of the non-spherical monomer model deviates from the optical integral properties of sphere monomer model. Hence, the difference in optical properties caused by different monomer shapes cannot be neglected when the monomer deviates significantly from a spherical shape. This work is helpful to evaluate the optical properties of soot aggregates more precisely.
