Molecular Composition and the Optical Properties of Brown Carbon Generated by the Ethane Flame

Atmospheric "Brown Carbon" (BrC) is a complex mixture of organic compounds with diverse composition and variability of its light-absorbing properties. BrC formed by incomplete combustion of fossil fuels and biomass has been shown to be an important contributor to the light absorption by atmospheric aerosols. Previous reports provided substantial molecular information of BrC related to biomass burning emissions; however, very few studies describe BrC generated from hydrocarbon fuel combustion. The work presented here is the first study that identifies and characterizes BrC formed in the controlled flame combustion of ethane, one of the most basic hydrocarbon fuels. To understand the molecular composition and optical properties of BrC, we used an analytical platform that includes high-performance liquid chromatography (HPLC) coupled to photodiode array (PDA) detection, followed by dopantassisted atmospheric pressure photoionization (APPI) and high-resolution mass spectrometry (HRMS). For this study, six soot samples were generated in a custom-built inverted gravity flame reactor (IGFR) at different combustion settings. The temperature of the diffusion flame was controlled by fuel dilution with argon (up to 80% v/v) and was measured to be in the range of 1750-1950 K Basic characterization of the samples (i.e., mass loading, OC/soot ratio) was employed, followed by molecular speciation of BrC chromophores. A vast majority of BrC chromophores identified in these samples are oxygenated polycyclic aromatic hydrocarbons (O-PAHs) and unsubstituted PAHs. Nearly 90% of the total BrC absorbance was attributed to approximately equal contributions from the groups of: O-PAHs, low- and high-molecular-weight PAHs referred as PAH < BaP and PAH > BaP (i.e., smaller and larger than Benzo[a]pyrene (BaP), respectively). The mass absorption coefficient (MAC(bulk)) measured at lambda(35)(0 mm) for the BrC fraction of aerosol emitted from the hottest undiluted flame (T-max = 1946 K) was 0.49 m(2) g(-1), while 0.004 m(2) g(-1) was measured for aerosol emitted from the colder flame (T-max = 1863 K, 67% dilution). The optical properties of BrC generated in the hottest flame are comparable to previous measurements of BrC generated from gasoline combustion of motor vehicles.