ACP Atmospheric Chemistry and Physics ACP Atmos. Chem. Phys. 1680-7324 Copernicus Publications Göttingen, Germany 10.5194/acp-14-2871-2014 Aqueous-phase photochemical oxidation and direct photolysis of vanillin – a model compound of methoxy phenols from biomass burning Li Y. J. 1 Huang D. D. 2 Cheung H. Y. 1 Lee A. K. Y. 3 Chan C. K. https://orcid.org/0000-0001-9687-8771 1 2 Division of Environment, Hong Kong University of Science and Technology, Hong Kong, China Department of Chemical and Biomolecular Engineering, Hong Kong University of Science and Technology, Hong Kong, China Department of Chemistry, University of Toronto, Toronto, Canada 19 03 2014 14 6 2871 2885 Copyright: © 2014 Y. J. Li et al. 2014 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://acp.copernicus.org/articles/14/2871/2014/acp-14-2871-2014.html The full text article is available as a PDF file from https://acp.copernicus.org/articles/14/2871/2014/acp-14-2871-2014.pdf

We present here experimental results on aqueous-phase (A) photochemical oxidation (with UV and OH radicals generated from H<sub>2</sub>O<sub>2</sub> photolysis) and (B) direct photolysis (with only UV irradiation) of a methoxy phenol, vanillin (VL), as a model compound from biomass burning. Both on-line aerosol mass spectrometric (AMS) characterization and off-line chemical analyses were performed. AMS analyses of dried atomized droplets of the bulk reacting mixtures showed that VL almost entirely evaporates during the drying process. Large amounts of organic mass remained in the particle phase after reactions under both conditions. Under condition (A), AMS measured organic mass first increased rapidly and then decreased, attributable to the formation of non-volatile products and subsequent formation of smaller and volatile products, respectively. The oxygen-to-carbon (O : C) ratio of the products reached 1.5 after about 80 min, but dropped substantially thereafter. In contrast, organic mass increased slowly under condition (B). The O : C ratio reached 1.0 after 180 min. In off-line analyses, small oxygenates were detected under condition (A), while hydroxylated products and dimers of VL were detected under condition (B). Particle hygroscopic growth factor (GF) and cloud condensation nuclei (CCN) activity of the reacting mixtures were found to depend on both organic volume fraction and the degree of oxygenation of organics. Results show that (1) aqueous-phase processes can lead to the retention of a large portion of the organic mass in the particle phase; (2) once retained, this portion of organic mass significantly changes the hygroscopicity and CCN activity of the aerosol particles; (3) intensive photochemical oxidation gave rise to an O : C ratio as high as 1.5 but the ratio decreased as further oxidation led to smaller and more volatile products; and (4) polymerization occurred with direct photolysis, resulting in high-molecular-weight products of a yellowish color. This study demonstrates that aqueous-phase reactions of a methoxy phenol can lead to substantial amount of secondary organic aerosol (SOA) formation. Given the vast amount of biomass burning input globally, model representation of either the SOA budget or their subsequent effects would not be adequate if the contribution of SOA formation from aqueous-phase reactions of methoxy phenols is not considered.

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