Optical Properties of Secondary Organic Aerosol Particles and their Internal Mixtures with Black Carbon

Secondary organic aerosol (SOA) particles cover a significant fraction of the ambient tropospheric aerosol and can affect atmosphere by interfering with radiative transfer and by reducing visibility. To estimate the radiative effects of SOA aerosol, detailed knowledge of its optical properties is needed. These properties depend among others on the SOA particle size, the shape and the refractive index or chemical composition. Recently, it has been shown that SOA particles can be found in atmosphere in amorphous solid or semi-solid states. The amorphous phase state can affect the optical properties of SOA particles compared to the liquid phase state but, until now, optical studies of amorphous SOA particles are scarce. Furthermore, coating of other atmospheric particles with SOA mass in atmospheric aging processes can change the optical behavior of these aerosol particles. For example, it has been reported that coating of black carbon (BC) with non-absorbing organic material leads to an enhancement of the BC absorption cross section.

In radiative transfer calculations, the optical properties of atmospheric particles are usually estimated using particle models with spherical symmetry. These spherical symmetric particle models, however, predict zero depolarization properties, which makes them unsuitable for remote sensing applications. More realistic particle models are been developed but the validation of these models needs information of the optical properties of the real atmospheric particles. In this contribution the optical properties of amorphous SOA particles and BC particles coated with SOA were studied in aerosol chamber simulations studies. SOA particles and SOA coatings were produced through the ozonolysis of alfa-pinene precursors. In the first part of this contribution it will be shown that amorphous SOA particles will induce depolarization, unlike SOA particles in the liquid phase. This information can be used to detect the amorphous phase state of SOA with a non-invasive optical method. We will show the results of direct measurements of the SOA phase transition under different atmospheric conditions. In the second part of this contribution the change in the depolarization and in the absorption properties of BC particles as a function of SOA coating thickness are shown. The results are discussed in the light of developing and validating optical particle models.