AAAR 33rd Annual Conference
October 20 - October 24, 2014
Rosen Shingle Creek
Orlando, Florida, USA
Abstract View
Ultraviolet and Visible Complex Refractive Indices of Brown Carbon Formed via Photooxidation of Aromatic Toluene and m-Xylene
PENGFEI LIU, Scot Martin, Harvard University
Abstract Number: 445 Working Group: Aerosol Chemistry
Abstract Recent field and laboratory studies have suggested that the secondary organic materials (SOM) formed from anthropogenic volatile organic compounds act as brown carbon. However, the spectral data of their optical properties are still scarce. In the present study, spectroscopic data of real and imaginary refractive indices (m = n - ik) were obtained in the ultraviolet (UV)-visible region, using the spectroscopic ellipsometry and the UV-visible spectrometry, respectively. The SOMs were produced in an oxidation flow reactor by photooxidation of toluene and m-xylene at various levels of nitrogen oxides (NOx). The results show that k values of the studied anthropogenic SOMs were more than ten times higher than that of the biogenic SOMs reported in our previous study. The presence of NOx can form organo-nitrogenic compounds, such as nitro-aromatics and organic nitrates, further enhancing the light absorption of studied SOMs. Compared with the SOM derived from m-xylene, the SOM derived from toluene had larger k values, as well as a greater NOx-induced enhancement of k, suggesting different brown-carbon-formation potentials of different aromatic precursors. Bulk composition analyses using the infrared spectrometry suggest a major fraction of SOMs formed via particle-phase heterogeneous reactions, which can result in a larger SOM yield than model prediction. Our results imply that fresh anthropogenic SOM formed in urban environments can be a large source of atmospheric brown carbon. This type of brown carbon can potentially play an important role in affecting UV irradiance in the troposphere, which might consequently exert influences on the concentration of tropospheric oxidants and the lifetime of greenhouse gases.