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 dopant-assisted 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 (MACbulk) measured at λ350 nm for the BrC fraction of aerosol emitted from the hottest undiluted flame (T max = 1946 K) was 0.49 m2 g–1, while 0.004 m2 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.

Analysis of particulate matter (PM) is critical for the assessment of human exposures to potentially harmful agents, notably combustion-generated PM; specifically polycyclic aromatic hydrocarbons (PAHs) found in them and associated with carcinogenic and mutagenic effects. In this study, we quantify the presence and concentrations of PAHs with low molecular weight (LMW) and higher molecular weight (HMW) in combustion-generated PM using excitation-emission matrix (EEM) fluorescence spectroscopy. PM samples were generated in a laminar diffusion inverted gravity flame reactor (IGFR) operated on Ethylene and Ethane. Fuel dilution by Ar in 0% to 90% range controls the flame temperature, the maximum flame temperature decreases with fuel dilution. The colder flames result in lower PM yields; however, the PM PAH content increases significantly. Temperature thresholds for PM transition from low to high organic carbon content were characterized based on the maximum flame temperature (1814K-1864K) and highest soot luminosity region temperature (1600K-1650K). Principal component regression (PCR) analysis of the EEM spectra correlates to GCMS data, R 2 values of 0.98 for LMW and 0.99 for HMW PAHs. The agreement demonstrates that EEM analysis can be used to determine relative concentrations of organic carbon and PAH fractions in combustion PM, and can be related to PM health effects and used in the environmental studies.File list (2) download file view on ChemRxiv Inverted Flame PAH-PCR_SI.pdf (2.01 MiB) download file view on ChemRxiv Inverted Flame PAH-PCR_manuscript.pdf (836.36 KiB)

<p></p><p>Analysis of particulate matter (PM) is critical for the assessment of human exposures to potentially harmful agents, notably combustion-generated PM; specifically polycyclic aromatic hydrocarbons (PAHs) found in them and associated with carcinogenic and mutagenic effects. In this study, we quantify the presence and concentrations of PAHs with low molecular weight (LMW) and higher molecular weight (HMW) in combustion-generated PM using excitation-emission matrix (EEM) fluorescence spectroscopy. PM samples were generated in a laminar diffusion inverted gravity flame reactor (IGFR) operated on Ethylene and Ethane. Fuel dilution by Ar in 0% to 90% range controls the flame temperature, the maximum flame temperature decreases with fuel dilution. The colder flames result in lower PM yields; however, the PM PAH content increases significantly. Temperature thresholds for PM transition from low to high organic carbon content were characterized based on the maximum flame temperature (1814K-1864K) and highest soot luminosity region temperature (1600K-1650K). Principal component regression (PCR) analysis of the EEM spectra correlates to GCMS data, R²values of 0.98 for LMW and 0.99 for HMW PAHs. The agreement demonstrates that EEM analysis can be used to determine relative concentrations of organic carbon and PAH fractions in combustion PM, and can be related to PM health effects and used in the environmental studies. </p><p></p>

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