AAAR 34th Annual Conference
October 12 - October 16, 2015
Hyatt Regency
Minneapolis, Minnesota, USA
Abstract View
Physical State of Secondary Organic Material Affects the Production of Brown Carbon
PENGFEI LIU, Yong Jie Li, Yan Wang, Adam Bateman, Yue Zhang, Zhaoheng Gong, Mary Gilles, Scot Martin, Harvard University
Abstract Number: 76 Working Group: The Role of Water in Aerosol Chemistry
Abstract Secondary organic material (SOM) can become light-absorbing (i.e. brown carbon) via multiphase reactions. The physical states of SOM, however, potentially slow the diffusion of reactant molecules in an organic matrix under conditions where semisolids or solids prevail, thus inhibiting the browning reaction pathways. In this study, the kinetic limitations of in-particle diffusion were investigated by measuring the evaporation of water from several types of SOM using a quartz-crystal-microbalance (QCM). At sufficiently low relative humidities (< 20% RH) at 293 K, the diffusion of water was significantly impeded in SOMs derived from aromatic precursors toluene and m-xylene, suggesting that these materials became solid (glassy). For < 5% RH, water diffusivity in the solid aromatic-derived SOMs ranged from 10^-17 to 10^-16 m^2 s^-1, which was 100 to 1000 times lower than that of the semi-solid SOM derived from α-pinene. Optical properties were measured for toluene-derived SOM after ammonia exposure at varied RHs. The results suggest that multiphase reactions with ammonia were kinetically limited in the glassy organic matrix, which otherwise produce brown carbon. These results have significant implications for understanding the production pathways and optical properties of brown carbon in urban atmospheres, and ultimately their influences on regional climate and tropospheric photochemistry.