Comprehensive Molecular Characterization of Atmospheric Brown Carbon by High Resolution Mass Spectrometry with Electrospray and Atmospheric Pressure Photoionization

Abstract

Light-absorbing components of atmospheric organic aerosols, which are collectively termed “brown carbon” (BrC), are ubiquitous in the atmosphere. They affect absorption of solar radiation by aerosols in the atmosphere and human health as some of them have been identified as potential toxins. Understanding the sources, formation, atmospheric evolution, and environmental effects of BrC requires molecular identification and characterization of light-absorption properties of BrC chromophores. Identification of BrC components is challenging due to the complexity of atmospheric aerosols. In this study, we employ two complementary ionization techniques, atmospheric pressure photo ionization (APPI) and electrospray ionization (ESI), to obtain broad coverage of both polar and nonpolar BrC components using high-resolution mass spectrometry (HRMS). These techniques are combined with chromatographic separation of BrC compounds with high performance liquid chromatography (HPLC), characterization of their light absorption with a photodiode array (PDA) detector, and chemical composition with HRMS. We demonstrate that this approach enables more comprehensive characterization of BrC in biomass burning organic aerosols (BBOAs) emitted from test burns of sage brush biofuel. In particular, we found that nonpolar BrC chromophores such as PAHs are only detected using positive mode APPI. Meanwhile, negative mode ESI results in detection of polar compounds such as nitroaromatics, aromatic acids, and phenols. For the BrC material examined in this study, over 40% of the solvent-extractable BrC light absorption is attributed to water insoluble, nonpolar to semipolar compounds such as PAHs and their derivatives, which require APPI for their identification. In contrast, the polar, water-soluble BrC compounds, which are detected in ESI, account for less than 30% of light absorption by BrC.

Impact of anthropogenic and biogenic sources on the seasonal variation in the molecular composition of urban organic aerosols: a field and laboratory study using ultra-high-resolution mass spectrometry

“…Such compounds are formed through the oxidation of aromatic VOCs in the presence of NO x (Forstner et al, 1997;Jang and Kamens, 2001;Hamilton et al, 2005;Sato et al, 2012;Irei et al, 2015). In earlier work, these compounds were observed at urban and rural locations and were mainly associated with biomass-burning activities (Kitanovski et al, 2012;Iinuma et al, 2010;Claeys et al, 2012;Kourtchev et al, 2013Kourtchev et al, , 2014bZhang et al, 2013;Mohr et al, 2013). The same peaks (C 5 H 6 O 4 N and C 7 H 7 O 4 N) were also abundant in measurements in the Pearl River Delta during the harvesting period (Lin et al, 2012).…”

Regional Differences of Chemical Composition and Optical Properties of Aerosols in the Tibetan Plateau

“…Fractions of water-insoluble organic carbon (WIOC) were calculated as the difference of OC and WSOC mass concentrations. The details of instruments and measurement procedures are published elsewhere (Feng et al, 2016;Lin et al, 2018;Xu et al, 2015).…”

“…The plot of DBE vs. n C can be used to infer the distribution of potential BrC active components (Lin et al, 2018). Since light absorption of BrC requires conjugated double bonds, the potential chromophores are suggested to have DBE/C ratio greater than that of linear polyenes (Lin et al, 2018), which is highlighted in brown in Figure 11. .…”

“…The light absorption by BrC is expected to increase with altitude because BrC can be transported to high altitudes by deep convection, and it can be also formed by in-cloud multiphase reactions . Molecular-level studies of BrC composition show that the light absorbing compounds are polycyclic aromatic hydrocarbons (PAH) and their oxygenated derivatives, nitroaromatics, phenols, and aromatic acids (Desyaterik et al, 2013;Lin et al, 2016;Lin et al, 2017;Lin et al, 2018;Xie et al, 2017). Generally, more absorbing BrC is typical for biomass burning organic aerosol (BBOA) and anthropogenically influenced secondary organic aerosol (anthropogenic SOA) (Washenfelder et al, 2015;Zhang et al, 2011).…”

“…Generally, more absorbing BrC is typical for biomass burning organic aerosol (BBOA) and anthropogenically influenced secondary organic aerosol (anthropogenic SOA) (Washenfelder et al, 2015;Zhang et al, 2011). Molecular-level studies of BrC composition show that the light absorbing compounds are polycyclic aromatic hydrocarbons (PAH) and their oxygenated derivatives, nitroaromatics, phenols, and aromatic acids (Desyaterik et al, 2013;Lin et al, 2016;Lin et al, 2017;Lin et al, 2018;Xie et al, 2017). Furthermore, light absorbing spectral properties of BrC are pH dependent, which is important for evaluating the ambient radiative forcing in modeling studies (Lin et al, 2017;Phillips et al, 2017).…”