Technical note: Effects of storage conditions on molecular-level composition of organic aerosol particles
Abstract. A significant fraction of atmospheric aerosol particles, which affect both the Earth’s climate and human health, can be attributed to organic compounds and especially secondary organic aerosol (SOA). To better understand the sources and processes generating organic aerosol particles, detailed chemical characterization is necessary, and particles are often collected onto filters and subsequently analyzed by liquid chromatography mass spectrometry (LC-MS). A downside of such offline analysis techniques is the uncertainty regarding artefactual changes in composition occurring during sample collection, storage, extraction and analysis. The goal of this work was to characterize how storage conditions and storage time may affect the chemical composition of SOA generated from β-pinene and naphthalene, as well as from urban atmospheric aerosol samples. SOA samples were produced in the laboratory using an aerosol flow tube and collected on PTFE filters, whereas ambient samples were collected onto quartz filters with a high-volume air sampler. To characterize temporal changes of SOA composition, all samples were extracted and analyzed immediately after collection, but were also stored as aqueous extracts or as filters for 24 hours and up to 4 weeks at three different temperatures of +20 °C, -20 °C or -80 °C, to assess whether a lower storage temperature would be favorable. Analysis was conducted using ultra high-performance liquid chromatography high resolution mass spectrometry (UHPLC-HRMS). Both principal component analysis (PCA) and time series of selected compounds were analyzed to identify the compositional changes over time. We illustrate that the chemical composition of organic aerosols remained stable during low temperature storage conditions, while storage at room temperature led to significant changes over time, even at short storage times of only one day. This indicates that it is necessary to freeze samples immediately after collection, and this requirement is especially important when automated ambient sampling devices are used where filters might be stored in the device for several days before being transferred to a laboratory.
Julian Resch et al.
Status: open (until 09 Jun 2023)
- RC1: 'Comment on egusphere-2023-840', Anonymous Referee #1, 03 May 2023 reply
Julian Resch et al.
Julian Resch et al.
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This study of filter stability before the analysis has been long overdue, and I am really happy the authors found the time to do it, and do it well! The conclusion is something people suspected but had no proof for: filters and extracts stored at room temperature change in composition significantly on a times scale of a day or so. This is important, and I expected many offline studies of aerosols to cite this study in the future. I recommend publishing after minor revisions.
I do not think this has to be be a technical note, it can also be a regular paper, especially if the authors can offer a hypothesis or a possible explanation for why aging processes in solution are different from those on a filter (Figures 1 and 3).
If it is practical I would make the X- and Y-scales on all PCA figures (Figures 1, 3, 5, S2, S5, S6, S7, S8) the same
Legend labels in Figures 5, S6, S7, S8 should say -20C and -80C instead of 20 and 80 (as they do in the rest of the PCA figures)
Line 166 and line 236: Intensity of 7E5 is not too meaningful – it is better to specify it in relationship to the peak signal (such as X% of the largest observed peak). Similar comment for line 292.
Figure 3: this figure can be much improved – the traces can be split into separate stacked panels, the text labels can be more readable, and the four most intense peaks mentioned in line 192 of the paper can be explicitly identified.