AAAR 36th Annual Conference October 16 - October 20, 2017 Raleigh Convention Center Raleigh, North Carolina, USA
Abstract View
Controlled Combustion Experiments for Constraining the Light-absorption Properties of Brown Carbon
ZEZHEN CHENG, Rawad Saleh, University of Georgia
Abstract Number: 717 Working Group: Carbonaceous Aerosols in the Atmosphere
Abstract The light-absorbing components of organic aerosol emitted by incomplete combustion, referred to as brown carbon (BrC), exhibit highly variable light-absorption properties usually quantified in terms of the mass-absorption cross-section (MAC) or imaginary part of the refractive index (k). MAC and k of combustion BrC reported in the literature vary over two orders of magnitudes. There are two major reasons for this variability: 1) BrC is a collection of poorly characterized organic compounds, thus the reported k (or MAC) values depend on which of these compounds are featured in a particular measurement, which in turn depends on the combustion conditions associated with the measurement; and 2) BrC is often co-emitted with black carbon (BC), complicating the retrieval of its k (or MAC) due to the overlap of light absorption by BrC and BC, thus leading to discrepancy between different approaches employed to perform the retrieval. To investigate the variability in BrC light-absorption properties, we designed experiments in which combustion can be controlled at different conditions (temperatures and air/fuel ratios) and carbonaceous aerosol products with a narrow distribution of optical properties can be achieved and characterized at each combustion condition. Here, we present results from model fuels (benzene and toluene). By varying the combustion conditions from relatively inefficient (low temperature and/or air/fuel ratio) to relatively efficient (high temperature and/or air/fuel ratio), we isolated BrC components with progressively increasing k (or MAC), spanning the wide range of values reported in the literature. This indicates that the discrepancy in reported BrC light-absorption properties stems largely from the discrepancy in combustion conditions between different measurements. Of particular interest is that bringing the combustion conditions close to the carbonization (BC-formation) regime produced highly absorptive BrC, the existence of which is hotly debated.