Abstract
Abstract. The aim of this study was to investigate oxidation of SOA formed from ozonolysis of α-pinene and limonene by hydroxyl radicals. This paper focuses on changes of particle volatility, using a Volatility Tandem DMA (VTDMA) set-up, in order to explain and elucidate the mechanism behind atmospheric ageing of the organic aerosol. The experiments were conducted at the AIDA chamber facility of Karlsruhe Institute of Technology (KIT) in Karlsruhe and at the SAPHIR chamber of Forchungzentrum Jülich (FZJ) in Jülich. A fresh SOA was produced from ozonolysis of α-pinene or limonene and then aged by enhanced OH exposure. As an OH radical source in the AIDA-chamber the ozonolysis of tetramethylethylene (TME) was used while in the SAPHIR-chamber the OH was produced by natural light photochemistry. A general feature is that SOA produced from ozonolysis of α-pinene and limonene initially was rather volatile and becomes less volatile with time in the ozonolysis part of the experiment. Inducing OH chemistry or adding a new portion of precursors made the SOA more volatile due to addition of new semi-volatile material to the aged aerosol. The effect of OH chemistry was less pronounced in high concentration and low temperature experiments when lower relative amounts of semi-volatile material were available in the gas phase. Conclusions drawn from the changes in volatility were confirmed by comparison with the measured and modelled chemical composition of the aerosol phase. Three quantified products from the α-pinene oxidation; pinonic acid, pinic acid and methylbutanetricarboxylic acid (MBTCA) were used to probe the processes influencing aerosol volatility. A major conclusion from the work is that the OH induced ageing can be attributed to gas phase oxidation of products produced in the primary SOA formation process and that there was no indication on significant bulk or surface reactions. The presented results, thus, strongly emphasise the importance of gas phase oxidation of semi- or intermediate-volatile organic compounds (SVOC and IVOC) for atmospheric aerosol ageing.
Highlights
Atmospheric aerosol particles are of importance both for human health effects and the effect on the climate by direct and indirect influence on the radiation budget
The comparison of different evaporative systems should be taken with some care since the evaporation of secondary organic aerosols (SOA), obtained Volume Fraction Remaining (VFR), usually is measured in a non-equilibrium mode and will depend on the residence time and design of the heating unit (Riipinen et al, 2010)
In a series of experiments within the MUCHACHAS camthe time-dependent trace for the semi-volatile model product paign, volatility was used as a signature for changes in comin Fig. 4 is consistent with APCI-MS observations of semi- position of SOA
Summary
Atmospheric aerosol particles are of importance both for human health effects and the effect on the climate by direct and indirect influence on the radiation budget. There are still areas of large uncertainty, especially regarding the oxidation steps of the initial precursor molecules and the actual identification and properties of products formed in these processes (Kroll et al, 2008). These reactions and the properties of the resulting aerosols are of concern for the atmospheric ageing processes of SOA and are linked to both condensed and gas phase processes (Jimenez et al, 2009; Donahue et al, 2011; Kroll et al, 2011). The term SOA ageing is a change of SOA properties with time and corresponding ageing processes may take place both in gas and condensed phase
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