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

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.

Biomass-burning-derived particles from a wide variety of fuels: Part 2: Effects of photochemical aging on particle optical and chemical properties

“…Formation of nitro-aromatics is often linked to the enhanced absorptivity when NOx is present, and condensed phase reactions producing highly conjugated, potentially N-containing compounds also contribute to light absorption (Laskin et al, 2015). Garmash et al (2020) recently reported ROOR' dimer formation in the gas phase from OH oxidation of aromatics, which we speculate could also contribute to light absorption.…”

Molecular insights into new particle formation in Barcelona, Spain

“…Agriculture, waste management, and traffic are also all significant sources of low molecular weight alkylamines, such as DMA (Ábalos et al, 1999;Cadle and Mulawa, 1980;Hutchinson et al, 1982;Ge et al, 2011a), and are likely the source of amines found in this dataset. Activities such as composting and food industry are especially strong sources of trimethylamine (TMA) (Ge et al, 2011a). Although high emission fluxes of TMA are expected in this environment, they are not present in our spectra.…”

“…Ammonia and amines show enhancement for fullevent periods but not burst-event periods. Nucleation rates (at typical tropospheric sulfuric acid concentrations) are sensitive to amine concentrations in the range of a few pptv, with enhancements to amine mixing ratios past this point increasing the nucleation rate marginally (Almeida et al, 2013), while typical concentrations of DMA and other alkylamines vary from zero to a few pptv in the boundary layer (Ge et al, 2011a). Barcelona has been shown to contain ppbv levels of ammonia (Pandolfi et al, 2012) arising from both agriculture to the north (Van Damme et al, 2018) and anthropogenic activities such as waste management and traffic, with waste management being the primary ammonia source.…”

“…this will determine the number of hydrogen bond donating groups and therefore whether the NO − 3 CI-APi-ToF is sensitive to a molecule or not. Elevated insolation will result in enhanced photochemistry and thus more rapid RO q 2 formation rates, whereas elevated NO x will produce more HOMs with nitrate ester functionality (Garmash et al, 2020;, which tend towards higher volatilities and less efficient participation in particle formation (Ehn et al, 2014; and growth (Yan et al, 2020).…”

Measurement report: Effects of photochemical aging on the formation and evolution of summertime secondary aerosol in Beijing

“…The concentration of OA also persistently increased with ta, which was consistent with the fact that SOA (predominant species in OA) mainly originated from the photochemical process of VOCs (Fan et al, 2020;Fu et al, 2014;Hallquist et al, 2009). Higher production of RO2 will be beneficial to the formation of low volatility species and new particle formation (NPF) (Garmash et al, 2020;Tröstl et al, 2016), and then promote the formation of SOA through gas-particle partitioning (Odum et al, 1996). These results implied that the formation and evolution of PM pollution as well as OA was greatly dependent on the photochemical aging process and total oxidant present during the summertime, which need to be deeply explored to understand the growth of PM1.…”