Investigating the dependence of light-absorption properties of combustion carbonaceous aerosols on combustion conditions

Abstract

We performed controlled combustion experiments to investigate the dependence of the mass absorption cross-section (MAC) and absorption Ångström exponent (AAE) of combustion carbonaceous aerosol emissions on combustion conditions. Using benzene and toluene as fuels, we obtained a wide range of combustion conditions by varying the combustion temperature and equivalence ratio. We also used nitrogen as a passive diluent to tune the combustion conditions. We calculated MAC and AAE from multi-wavelength light-absorption measurements using a photoacoustic spectrophotometer and aerosol mass loadings estimated from thermal-optical analysis. Starting with relatively low-temperature and fuel-rich combustion conditions and progressively increasing the temperature and/or decreasing the equivalence ratio, we produced emissions with progressive change from weakly absorbing brown carbon (BrC) (MAC at 532 nm (MAC532) = 0.24 m2/g and AAE = 8.6) to strongly absorbing BrC (MAC532 = 2.1 m2/g and AAE = 3.1) to mixtures of black carbon (BC) and strongly absorbing BrC (MAC532 = 7.7 m2/g and AAE = 1.5). These findings indicate that combustion conditions are important in dictating the light-absorption properties of the emitted aerosols. Furthermore, regardless of fuel type and combustion conditions, the emitted aerosols exhibit a unified continuum of light-absorption properties that can be characterized by MAC532 and AAE pairs. The MAC532 and AAE pairs are well-correlated with the elemental carbon-to-organic carbon ratio (EC/OC), which is a proxy of combustion conditions, confirming previous findings that EC/OC is a practical basis for parameterizing the light-absorption properties of combustion carbonaceous aerosols.Copyright © 2019 American Association for Aerosol Research