Estimation of Effective Density and Fractal Dimension of Secondarily Produced Brown Carbon Particles

ALEXANDER B. MACDONALD, Nilofar Raeofy, Kunpeng Chen, Haofei Zhang, Ying-Hsuan Lin, Roya Bahreini, University of California, Riverside

     Abstract Number: 137
     Working Group: Aerosol Physics

Abstract
Brown carbon (BrC) aerosols are light-absorbing organic particles that have a potentially large but uncertain impact on Earth’s radiative forcing. Examining BrC morphology (size and shape) and density will help reduce this uncertainty since these parameters are important in calculating aerosol radiative and transport properties. Primary BrC particles are emitted by biomass burnings and secondary BrC particles are produced via oxidation reactions of precursor volatile organic compounds (VOCs). Primary BrC particles are reported to have a fractal-like branched structure. This study examines how the VOC precursor influences secondary BrC particle density and morphology. BrC aerosols are generated in an environmental chamber from nitrate radical oxidation of three different heterocyclic VOCs (furan, pyrrole, and thiophene) under different NOx conditions. An Aerodyne mini aerosol mass spectrometer (mAMS) and a Brechtel scanning electron mobility sizer (SEMS) are used to measure size-resolved distributions of mass concentration and number concentration, respectively. Transmission electron microscopy (TEM) is used to obtain magnified images of aerosol samples from a subset of the experiments. Effective particle density (ρeff) is derived using the mAMS’s vacuum aerodynamic diameter (Dva) and the SEMS’s electrical mobility diameter (Dm). Particle morphology is approximated with the fractal dimension (Df), a mathematical parameter that describes the non-spherical branched structure of a particle. Df is approximated by applying the self-preserving size distribution theory to SEMS size distribution data when agglomeration is the dominant mechanism driving particle growth. Our findings suggest that BrC aerosols with a fractal-liked structure are formed in some systems due to the agglomeration of significantly smaller primary particles (spherules) and that the size distribution of these fractal-like aerosols is adequately described using the self-preserving size distribution theory. The ρeff and Df appear to be influenced by the VOC precursor.