10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Investigating the Dependence of Light-absorption Properties of Combustion Carbonaceous Aerosols on Combustion Conditions
ZEZHEN CHENG, Khairallah Atwi, Daniel Tarquinio, Rawad Saleh, University of Georgia
Abstract Number: 217 Working Group: Combustion
Abstract The light-absorbing components of organic aerosols emitted by incomplete combustion, collectively referred to as brown carbon (BrC), exhibit highly variable light-absorption properties, usually quantified in term of the wavelength-dependent mass-absorption cross-section (MAC). The wavelength dependence of MAC is quantified in terms of the Absorption Ångström Exponent (AAE). There are two major reasons for the variability in BrC’s MAC and AAE: 1) BrC is a collection of poorly characterized compounds, and thus the reported MAC and AAE 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 MAC and AAE due to the overlap of light absorption by the two components, thus leading to discrepancies between different approaches employed to achieve this retrieval.
To address these challenges, we designed controlled combustion experiments that enabled us to isolate BrC components with varying light-absorption properties, as well mixtures of BrC and BC. Combustion was performed under steady-flow conditions in a temperature-controlled quartz chamber. Different combustion conditions were achieved by varying the chamber temperature and the relative flowrates of air and fuel. We also used an extra stream of nitrogen as a passive diluent to fine-tune the combustion conditions. Adding a passive diluent has a similar effect on the products as making the combustion more fuel rich. We calculated the wavelength-dependent MAC of the emissions from real-time absorption coefficient measurements using a 3-wavelength (780, 532, and 420 nm) photoacoustic spectrophotometer and an Aethalometer and integrated size distribution measurements using a scanning mobility particle sizer.
We used benzene and toluene as model fuels. Starting with relatively low-temperature and fuel-rich combustion experiments and progressively increasing the temperature and/or the air/fuel ratio, we produced carbonaceous aerosol samples with progressive change from weakly absorptive BrC (MAC at 532 nm ~ 0.22 m2/g and AAE ~ 8) to highly absorptive BrC (MAC at 532 nm ~ 2.0 m2/g and AAE ~ 3) to mixtures of BC and highly absorptive BrC (MAC at 532 nm ~ 3.0 m2/g and AAE ~ 1.5). These results cover the wide range of values reported in the literature and indicate that the carbonaceous aerosol light-absorption properties depend strongly on combustion conditions.
We also attempted to obtain correlations between the light-absorption properties and two indicators of combustion conditions: the modified combustion efficiency (MCE = ∆CO2 / (∆CO2 + ∆CO)) and the organic to total carbon ratio (OC/TC). CO and CO2 concentration were measured using FTIR and OC/TC was measured offline using an OCEC Analyzer. We found stronger correlations between MAC at 532 and AAE with OC/TC than with MCE, indicating that OC/TC is a better predictor of light-absorption properties than MCE.