Insights into the O : C-dependent mechanisms controlling the evaporation of α-pinene secondary organic aerosol particles

Buchholz, Angela; Lambe, Andrew T.; Ylisirniö, Arttu; Li, Zijun; Tikkanen, Olli-Pekka; Faiola, Celia; Kari, Eetu; Hao, Liqing; Luoma, Olli; Huang, Wei; Mohr, Claudia; Worsnop, Douglas R.; Nizkorodov, Sergey A.; Yli-Juuti, Taina; Schobesberger, Siegfried; Virtanen, Annele

The volatility of oxidation products of volatile organic compounds (VOCs) in the atmosphere is a key factor to determine if they partition into the particle phase contributing to secondary organic aerosol (SOA) mass. Thus, linking volatility and measured particle composition will provide insights into SOA formation and its fate in the atmosphere. We produced inline-formulaα-pinene SOA with three different oxidation levels (characterized by average oxygen-to-carbon ratio; inline-formula M4inlinescrollmathml true chem normal O : normal C normal ‾ = normal 0.53 60pt13ptsvg-formulamathimgd6f09714e1b0bee06ef073a3b8555f32 acp-19-4061-2019-ie00001.svg60pt13ptacp-19-4061-2019-ie00001.png , 0.69, and 0.96) in an oxidation flow reactor. We investigated the particle volatility by isothermal evaporation in clean air as a function of relative humidity (RH inline-formula<2 %, 40 %, and 80 %) and used a filter-based thermal desorption method to gain volatility and chemical composition information.

We observed reduced particle evaporation for particles with increasing inline-formula M6inlinescrollmathml true chem normal O : normal C normal ‾ 27pt13ptsvg-formulamathimg309430b98a0463bbe12fadecf7fdef58 acp-19-4061-2019-ie00002.svg27pt13ptacp-19-4061-2019-ie00002.png ratio, indicating that particles become more resilient to evaporation with oxidative aging. Particle evaporation was increased in the presence of water vapour and presumably particulate water; at the same time the resistance of the residual particles to thermal desorption was increased as well. For SOA with inline-formula M7inlinescrollmathml true chem normal O : normal C normal ‾ = normal 0.96 60pt13ptsvg-formulamathimgf6f35b380852b89b49305fcf77c256a5 acp-19-4061-2019-ie00003.svg60pt13ptacp-19-4061-2019-ie00003.png , the unexpectedly large increase in mean thermal desorption temperature and changes in the thermogram shapes under wet conditions (80 % RH) were an indication of aqueous phase chemistry. For the lower inline-formula M8inlinescrollmathml true chem normal O : normal C normal ‾ 27pt13ptsvg-formulamathimga8df7c557e221716ba5a2725ac2445d9 acp-19-4061-2019-ie00004.svg27pt13ptacp-19-4061-2019-ie00004.png cases, some water-induced composition changes were observed. However, the enhanced evaporation under wet conditions could be explained by the reduction in particle viscosity from the semi-solid to liquid-like range, and the observed higher desorption temperature of the residual particles is a direct consequence of the increased removal of high-volatility and the continued presence of low-volatility compounds.

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Buchholz, Angela / Lambe, Andrew T. / Ylisirniö, Arttu / et al: Insights into the O : C-dependent mechanisms controlling the evaporation of α-pinene secondary organic aerosol particles. 2019. Copernicus Publications.

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