Mixing State Determination with the Single-Particle Soot Photometer

The composition of an aerosol particle (denoted as the chemical mixing state) and the way in which the substances composing the particle are arranged (the morphological mixing state) both are important factors affecting the optical and microphysical properties of the particle, including its ability to activate to form a cloud drop. An aerosol–a suspension of particles in the atmosphere–can likewise be characterized by a population mixing state, which describes the extent to which the individual particles are similar. Particles that contain refractory black carbon (rBC) can have a large array of chemical and morphological mixing states, depending on their sources and atmospheric residences. Such particles are often formed from incomplete combustion engines and biomass burning (BB) and can accumulate other substances ("coatings") via condensation and coagulation processes during transport in the atmosphere, which can alter the properties of the original particles in several ways, including structural rearrangements, enhancement of light absorption, and so forth. The single-particle soot photometer (SP2) is an instrument that detects individual BC-containing particles and for each determines the mass of BC contained within the particle, and also provides some information on the light scattering from the particle, from which the amount of coating can be inferred, although crudely. The extent to which the SP2 can determine the chemical and morphological mixing states for individual particles, and the population mixing state, are discussed and results are presented.