Contents lists available at ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv Analysis of reaction products formed in the gas phase reaction of E,E-2,4- hexadienal with atmospheric oxidants: Reaction mechanisms and atmospheric implications I. Colmenar a,b , P. Martin a,b,* , B. Cabañas a,b , S. Salgado a,b , E. Martinez a,b a Universidad de Castilla La Mancha, Departamento de Química Física, Facultad de Ciencias y Tecnologías Químicas, Avda. Camilo José Cela S/N, 13071 Ciudad Real, Spain b Universidad de Castilla La Mancha, Instituto de Combustión y Contaminación Atmosférica (ICCA), Camino Moledores S/N, 13071 Ciudad Real, Spain ARTICLE INFO Keywords: E,E-2,4-hexadienal Atmospheric oxidants Reaction products E/Z-butenedial Particulate matter Nitrate compounds ABSTRACT An analysis of reaction products for the reaction of E,E-2,4-hexadienal with chlorine atoms (Cl) and OH and NO 3 radicals has been carried out at the first time with the aim of obtaining a better understanding of the tropo- spheric reactivity of α,β-unsaturated carbonyl compounds. Fourier Transform Infrared (FTIR) spectroscopy and Gas Chromatography-Mass Spectrometry with a Time of Flight detector (GC-TOFMS) were used to carry out the qualitative and/or quantitative analyses. Reaction products in gas and particulate phase were observed from the reactions of E,E-2,4- hexadienal with all oxidants. E/Z-Butenedial and maleic anhydride were the main products identified in gas phase. E-butenedial calculated molar yield ranging from 4 to 10%. A significant amount of multifunctional compounds (chloro and hydroxy carbonyls) was identified. These compounds could be formed in particulate phase explaining the ∼90% of unaccounted carbon in gas phase. The reaction with Cl atoms in the presence of NOx with a long reaction time gave Peroxy Acetyl Nitrate (PAN) as an additional product, which is known for being an important specie in the generation of the photochemical smog. Nitrated compounds were the major organic products from the reaction with the NO 3 radical. Based on the identified products, the reaction mechanisms have been proposed. In these mechanisms a double bond addition of the atmospheric oxidant at C4/ C5 of E,E-2,4-hexadienal is the first step for tropospheric degradation. 1. Introduction E,E-2,4-Hexadienal is an unsaturated aldehyde with a medium chain length (H 3 CCH=CHCH=CHCHO). A review on the available data for the sources and fate of this compound was published in the IARC monograph Vol 101 (2013). E,E-2,4-Hexadienal is naturally present as an auto-oxidation product of polyunsaturated fatty acids of plant and animal origin (Dulio et al., 1999; Haahr et al., 2000; Pandey-Rai et al., 2006; De Freitas Ferreira et al., 2016). Furthermore, the production and use of 2,4-hexadienal as a food additive and chemical intermediate may result in its release into the atmosphere. E,E-2,4-Hexadienal could therefore be present in the atmosphere as an emission from vegetation due to leaf wounding (IARC, 2013) and it is also formed in situ by the atmospheric degradation of volatile organic compounds (VOCs) such as toluene (Dumdei et al., 1988). The presence of E,E-2,4-hexadienal in the atmosphere has been detected in several field studies (IARC, 2013). As an aldehyde and an unsaturated compound E,E-2,4-hexadienal may significantly influence the oxidizing capacity of the lower atmo- sphere. In an effort to establish the impact of unsaturated aldehydes on air quality and tropospheric chemistry in general, it is necessary to determine the rates and mechanisms for atmospheric degradation of these compounds. In order to achieve this goal, studies on the kinetics and reaction products must be carried out. As E,E-2,4-hexadienal is a high-weight carbonyl compound it probably forms aerosols as the main reaction products during its tropospheric degradation. Evidence for the formation of nanoparticles has been reported in different studies on the heterogeneous reactivity of this compound with H 2 SO 4 (Zhao et al., 2005; Wang et al., 2010, 2011). In the case of the homogeneous reactivity of E,E-2,4-hexadienal only two kinetic studies have been reported in the literature. These studies show that the main degradation routes are the reactions with OH radicals and sunlight (O’Connor et al., 2006; Colmenar et al., 2014). However, the gas phase reactivity with chlorine atoms should not be dismissed since it is a fast reaction (3.98 × 10 -10 molec -1 cm 3 s -1 , https://doi.org/10.1016/j.atmosenv.2017.12.027 Received 20 March 2017; Received in revised form 13 December 2017; Accepted 18 December 2017 * Corresponding author. Universidad de Castilla La Mancha, Departamento de Química Física, Facultad de Ciencias y Tecnologías Químicas, Avda. Camilo José Cela S/N, 13071 Ciudad Real, Spain. E-mail address: mariapilar.martin@uclm.es (P. Martin). Atmospheric Environment 176 (2018) 188–200 Available online 21 December 2017 1352-2310/ © 2017 Elsevier Ltd. All rights reserved. T