10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Investigating the Link between Molecular Mass, Volatility, and Optical Properties of Light-absorbing Organic Aerosols
KHAIRALLAH ATWI, Zezhen Cheng, Rawad Saleh, University of Georgia
Abstract Number: 222 Working Group: Aerosol Chemistry
Abstract Organics produced from incomplete combustion span a wide range of light-absorption properties. We have directly investigated the relation between the light-absorption properties and molecular weight and volatility by conducting controlled combustion experiments using benzene as a model fuel. Combustion conditions were controlled by varying the flow rates of air and benzene and by controlling the temperature at which the combustion took place. At each combustion condition, the absorption coefficients (measured online using a photoacoustic spectrometer at 780, 532, and 420 nm) and the size distribution (measured using a scanning mobility particle sizer) were used to calculate the mass absorption cross section (MAC) and the absorption Ångström exponent (AAE). The particles were also collected on Quartz filters for offline analysis using laser desorption ionization time-of-flight mass-spectrometry (LDI-TOF-MS). The advantage of using LDI is that it can detect large molecules intact, allowing us to follow the formation of large organics from a several hundred to thousands of AMUs. Those measurements were also performed after heating the emissions in a thermodenuder operated at different temperatures to investigate the contribution of organics of different volatility to the total absorption cross section of the particles. The light-absorption properties of the organic combustion particles generated spanned a wide spectrum, with MAC at 532 nm ranging from 0.4 to 2.4 m2g-1 and AAE ranging from 9 to 3. The mass spectra produced using LDI-TOF-MS showed 24 AMU-spaced, regularly repeating peaks previously observed in the soot formation literature, with masses ranging between 200 and 2000 AMU. More importantly, there was a clear correlation of the molecular weight distribution of the organics with their light-absorption properties: the darker the samples (larger MAC and lower AAE), the more shifted were their mass spectra towards larger molecular sizes. Our experiments also show that as the particles were stripped of their more volatile components in the thermodenuder, their MAC increased and AAE decreased, indicating that the less volatile components are more absorptive. At a thermodenuder temperature of 400 C, the residual organics were extremely highly absorptive, with MAC and AAE of 3 m2g-1 and 1.5, respectively. Furthermore, the mass spectra of the thermodenuded samples had a largely diminished signal in the low molecular weight region, indicating that they consist of larger compounds. These findings provide physical evidence of the relation between the molecular weight of the compounds and their volatility and optical properties, with volatility decreasing and light-absorption across all wavelengths increasing with increasing molecular weight.