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.
Highlights
Lignin is a biopolymer of woody plant tissue mainly composed of three aromatic alcohols: p-coumaryl, coniferyl, and sinapyl alcohols (Simoneit, 2002)
The research presented was mainly focused on the isolation and structure elucidation of the main guaiacol products obtained by photonitration in the aqueous phase
Methods were optimized for studying the composition of the formed guaiacol secondary organic aerosol (SOA) and purifying the main guaiacol nitroproducts, respectively
Summary
Lignin is a biopolymer of woody plant tissue mainly composed of three aromatic alcohols: p-coumaryl, coniferyl, and sinapyl alcohols (Simoneit, 2002). Angiosperm (hardwood) lignins are derived from products of sinapyl alcohol, while gymnosperm (softwood) lignins mainly contain products of coniferyl alcohol and to a lesser extent sinapyl alcohol products. Grass (Gramineae) lignins mainly contain products of p-coumaryl alcohol (Simoneit, 2002). Hardwood burning produces mainly syringol (2,6-dimethoxyphenol) derivatives and guaiacol (2-methoxyphenol) derivatives to a lesser extent, whereas softwood burning exclusively results in guaiacol derivatives (Simoneit, 2002). The enrichment of methoxyphenols in fog water was usually 3 to 4 times higher than that calculated from Henry’s law.
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