Molecular characterization of brown carbon (BrC) chromophores in secondary organic aerosol generated from photo-oxidation of toluene

Abstract

Atmospheric brown carbon (BrC) is a significant contributor to light absorption and climate forcing. However, little is known about a fundamental relationship between the chemical composition of BrC and its optical properties. In this work, light-absorbing secondary organic aerosol (SOA) was generated in the PNNL chamber from toluene photo-oxidation in the presence of NOx (Tol-SOA). Molecular structures of BrC components were examined using nanospray desorption electrospray ionization (nano-DESI) and liquid chromatography (LC) combined with UV/Vis spectroscopy and electrospray ionization (ESI) high-resolution mass spectrometry (HRMS). The chemical composition of BrC chromophores and the light absorption properties of toluene SOA (Tol-SOA) depend strongly on the initial NOx concentration. Specifically, Tol-SOA generated under high-NOx conditions (defined here as initial NOx/toluene of 5/1) appears yellow and mass absorption coefficient of the bulk sample (MACbulk@365 nm = 0.78 m(2) g(-1)) is nearly 80 fold higher than that measured for the Tol-SOA sample generated under low-NOx conditions (NOx/toluene < 1/300). Fifteen compounds, most of which are nitrophenols, are identified as major BrC chromophores responsible for the enhanced light absorption of Tol-SOA material produced in the presence of NOx. The integrated absorbance of these fifteen chromophores accounts for 40-60% of the total light absorbance by Tol-SOA at wavelengths between 300 nm and 500 nm. The combination of tandem LC-UV/Vis-ESI/HRMS measurements provides an analytical platform for predictive understanding of light absorption properties by BrC and their relationship to the structure of individual chromophores. General trends in the UV/Vis absorption by plausible isomers of the BrC chromophores were evaluated using theoretical chemistry calculations. The molecular-level understanding of BrC chemistry is helpful for better understanding the evolution and behavior of light absorbing aerosols in the atmosphere.

Effect of relative humidity on the composition of secondary organic aerosol from the oxidation of toluene

“…The low-NO x mass spectrum shown in Fig. 1 is qualitatively similar to the low-NO x mass spectrum of toluene SOA discussed in a previous study in which it was prepared in a different smog chamber but analyzed by the same nano-DESI instrument (Lin et al, 2015).…”

Fogs and Air Quality on the Southern California Coast

“…This work also demonstrates the need for additional research to determine the extent of in-fog formation of CHO, CHOS, CHNO, and CHNOS species. The formation of these species can result in increased organic aerosol mass formation (McNeill et al, 2012) and absorption of ultraviolet and visible light by aerosol (Lin et al, 2015), both of which are important for climate and air quality. The regional O&NG processing VOC emissions observed in this study are likely to be precursors for the CHO, CHOS and CHNOS species.…”

Relative importance of black carbon, brown carbon, and absorption enhancement from clear coatings in biomass burning emissions

“…Potential sources of BrC include emissions from biomass burning (Kirchstetter et al, 2004;Moosmüller et al, 2009;Chen and Bond, 2010;Lack et al, 2012a;Saleh et al, 2014;Washenfelder et al, 2015); incomplete combustion of fossil fuels, especially coal (Bond, 2001;Yang et al, 2009;Olson et al, 2015); and secondary organic aerosols (Saleh et al, 2013;Zhang et al, 2013;Lin et al, 2015). There exists significant uncertainty concerning the relative contribution of each of these source types to total BrC concentrations, but several studies have identified biomass burning as a potentially significant source (Washenfelder et al, 2015;McMeeking et al, 2014;Lack et al, 2012a).…”