Multi-generation OH oxidation as a source for highly oxygenated organic molecules from aromatics

Olga Garmash,Carl Percival,Oskari Kausiala,Astrid Kiendler-Scharr,Gordon Mcfiggans,Thomas F Mentel,Mattias Hallquist,Jürgen Wildt,Einhard Kleist,Thomas J Bannan,Defeng Zhao,Michael Priestley,Ralf Tillmann,Mikael Ehn,Torsten Berndt,Matti P Rissanen,Iida Pullinen,Sebastian Schmitt,Åsa M Hallquist

doi:10.5194/acp-20-515-2020

Abstract

Abstract. Recent studies have recognised highly oxygenated organic molecules (HOMs) in the atmosphere as important in the formation of secondary organic aerosol (SOA). A large number of studies have focused on HOM formation from oxidation of biogenically emitted monoterpenes. However, HOM formation from anthropogenic vapours has so far received much less attention. Previous studies have identified the importance of aromatic volatile organic compounds (VOCs) for SOA formation. In this study, we investigated several aromatic compounds, benzene (C6H6), toluene (C7H8), and naphthalene (C10H8), for their potential to form HOMs upon reaction with hydroxyl radicals (OH). We performed flow tube experiments with all three VOCs and focused in detail on benzene HOM formation in the Jülich Plant Atmosphere Chamber (JPAC). In JPAC, we also investigated the response of HOMs to NOx and seed aerosol. Using a nitrate-based chemical ionisation mass spectrometer (CI-APi-TOF), we observed the formation of HOMs in the flow reactor oxidation of benzene from the first OH attack. However, in the oxidation of toluene and naphthalene, which were injected at lower concentrations, multi-generation OH oxidation seemed to impact the HOM composition. We tested this in more detail for the benzene system in the JPAC, which allowed for studying longer residence times. The results showed that the apparent molar benzene HOM yield under our experimental conditions varied from 4.1 % to 14.0 %, with a strong dependence on the OH concentration, indicating that the majority of observed HOMs formed through multiple OH-oxidation steps. The composition of the identified HOMs in the mass spectrum also supported this hypothesis. By injecting only phenol into the chamber, we found that phenol oxidation cannot be solely responsible for the observed HOMs in benzene experiments. When NOx was added to the chamber, HOM composition changed and many oxygenated nitrogen-containing products were observed in CI-APi-TOF. Upon seed aerosol injection, the HOM loss rate was higher than predicted by irreversible condensation, suggesting that some undetected oxygenated intermediates also condensed onto seed aerosol, which is in line with the hypothesis that some of the HOMs were formed in multi-generation OH oxidation. Based on our results, we conclude that HOM yield and composition in aromatic systems strongly depend on OH and VOC concentration and more studies are needed to fully understand this effect on the formation of HOMs and, consequently, SOA. We also suggest that the dependence of HOM yield on chamber conditions may explain part of the variability in SOA yields reported in the literature and strongly advise monitoring HOMs in future SOA studies.

Highlights

Oxygenated organic molecules (HOMs) have been identified as large contributors to atmospheric secondary organic aerosol (SOA) in forested environments (Ehn et al, 2014; Öström et al, 2017; Bianchi et al, 2019)
We confirmed the production of highly oxygenated organic molecules (HOMs) in the OH-induced oxidation of aromatic compounds
In benzene oxidation experiments in the flow reactor, we most likely observed first-generation HOMs formed after a single OH attack

Summary

Introduction

Oxygenated organic molecules (HOMs) have been identified as large contributors to atmospheric secondary organic aerosol (SOA) in forested environments (Ehn et al, 2014; Öström et al, 2017; Bianchi et al, 2019). HOMs form through a process called autoxidation, where intramolecular hydrogen shifts in organic peroxy radicals are followed by addition of molecular oxygen (Crounse et al, 2013), causing a rapid increase in the oxygen content of the molecules. A second study investigated HOM formation from the OH oxidation of seven different aromatics, finding HOM yields mainly within 0.1 %–1 % for single-ring aromatics, and a few percent for two polycyclic aromatics, naphthalene and biphenyl (Molteni et al, 2018). These yields are comparable in magnitude with those reported from ozone and OH oxidation of monoterpenes As aromatics are thought to be the most efficient precursors of SOA in urban areas (Kroll and Seinfeld, 2008), further studies of HOM formation, as well as their contribution to SOA, are necessary

Methods

Results

Conclusion