Chemical characterization of the main products formed through aqueous-phase photonitration of guaiacol

Abstract

Abstract. Guaiacol (2-methoxyphenol) and its derivatives can be emitted into the atmosphere by thermal degradation (i.e., burning) of wood lignins. Due to its volatility, guaiacol is predominantly distributed atmospherically in the gaseous phase. Recent studies have shown the importance of aqueous-phase reactions in addition to the dominant gas-phase and heterogeneous reactions of guaiacol, in the formation of secondary organic aerosol (SOA) in the atmosphere. The main objectives of the present study were to chemically characterize the main products of the aqueous-phase photonitration of guaiacol and examine their possible presence in urban atmospheric aerosols. The aqueous-phase reactions were carried out under simulated sunlight and in the presence of hydrogen peroxide and nitrite. The formed guaiacol reaction products were concentrated by solid-phase extraction and then purified with semi-preparative high-performance liquid chromatography (HPLC). The fractionated individual compounds were isolated as pure solids and further analyzed with liquid-state proton, carbon-13 and two-dimensional nuclear magnetic resonance (NMR) spectroscopy, and direct infusion negative ion electrospray ionization tandem mass spectrometry ((−)ESI-MS/MS). The NMR and product ion (MS2) spectra were used for unambiguous product structure elucidation. The main products of guaiacol photonitration are 4-nitroguaiacol (4NG), 6-nitroguaiacol (6NG), and 4,6-dinitroguaiacol (4,6DNG). Using the isolated compounds as standards, 4NG and 4,6DNG were unambiguously identified in winter PM10 aerosols from the city of Ljubljana (Slovenia) by means of HPLC/(−)ESI-MS/MS. Owing to the strong absorption of ultraviolet and visible light, 4,6DNG could be an important constituent of atmospheric "brown" carbon, especially in regions affected by biomass burning.

Direct observation of aqueous secondary organic aerosol from biomass-burning emissions

“…When the fraction of the biomass-burning aqSOA becomes larger, the AAE increases, with values up to 5. This result agrees with measurements of optical properties of aqSOA produced in laboratory experiments from photooxidation and nitration of phenolic compounds (13,17,39). The large AAE indicates that this OA component absorbs radiation efficiently in the near-UV and visible region, possibly contributing to a net positive radiative forcing.…”

“…Monosaccharides, dicarboxylic acids, and phenols emitted during wood combustion can act as aqSOA precursors (12)(13)(14)(15). A few studies investigated formation and properties of aqSOA from biomass combustion through laboratory photochemical oxidation of single compounds, such as levoglucosan and phenolic species (12,13,(16)(17)(18). However, ambient observations of aqSOA formation in environments with high wood-burning emissions are very limited.…”

Evolution of the chemical fingerprint of biomass burning organic aerosol during aging

“…Nitro-aromatic compounds have been observed at sites impacted by biomass burning before (Iinuma et al, 2010;Kitanovski et al, 2012Kitanovski et al, , 2014Mohr et al, 2013;Kahnt et al, 2013;Frka et al, 2016;Gaston et al, 2016) at concentration levels typically two orders of magnitude lower than that of levoglucosan (4-NC / LG ≈ 0.01-0.1, MNC / LG ≈ 0.01-0.08). Here, the 4-NC / LG and MNC / LG ratios vary from 0.25 to 0.5 and 0.06 to 0.15, respectively.…”

Evolution of the chemical fingerprint of biomass burning organic aerosol during aging

“…Nitro-aromatic compounds have been observed at sites impacted by biomass burning before (Iinuma et al, 2010, Claeys et al, 2012, Kitanovski et al, 2012, Mohr et al, 2013, Kahnt et al, 2013, Kitanovski et al, 2014, Frka et al, 2016, Gaston et al, 2016 at concentration levels typically two orders of magnitude lower than that of levoglucosan (4-NC/LG ≈ Atmos. Chem.…”