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).

Technical:

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.

This technical note presents the effect of the storage in different conditions on the molecular composition of aerosol produced in chamber and collected from ambient air. I absolutely agree with the authors on the crucial need to understand the effect of storage on the organic matter composition and I particularly appreciate the dual approach, taking into account the trends of specific m/z and on the global composition of the samples. The objectives of the study are clear and the methodology is well written and detailed. This work deserves to be published.

Nevertheless, few points can be improved:

• The authors investigate 3 storage conditions for filters and extracts: room temperature, -20°C and -80°C. I would have added also 4°C. This could be extremely interesting for the extracts, because it is the temperature of the autosampler for many instruments (LC-MS, IC-MS…). Do the authors have an idea of the effect of storage in refrigerated conditions?
• Regarding the storage conditions, at lines 56-61, the authors reported that storage conditions were evaluated for inorganic ions, EC and OC analysis. No references are reported. What are the current guidelines from research infrastructures? Are they comparable with those presented in this work?
• SOA samples seems more perishable than ambient aerosol samples: few hours of sampling are enough to change the composition of the organic matter. Sampling in chamber should be performed in refrigerated conditions?
• The extraction procedure reported at line 98-99 is reported in previous studies or was designed for this study in particular?
• The PCA enables a global view of the variation of the composition of the samples with storage. However, I miss some details of the statistic study, like the methodology for the normalization of the intensity of the peaks. This is essential when the analysis is performed on a large database like mass spectrometry results. I also feel that the PCA can be better exploited to understand the effect of the storage: is there a trend in the clusters? For example, looking at the room temperature extracts in Fig 1, I would expect that short storage times would be closer to the initial sample.
• I found very interesting the behaviour of cis-pinonic acid. May it be considered as a proxy to study the desorption or the sublimation of semivolatile organic compounds?
• HVAS = ? high volume aerosol sample, I imagine
• I think that the objective of this study is to provide some guidelines on the storage of filters and extracts for aerosol samples. I expected to find these guidelines in the conclusions but I didn’t. In my opinion, the authors should put the basis for a standardization of the methodologies in aerosol sampling for molecular characterisation and clearly state the procedure to adopt in order to improve the comparison of samples.

The manuscript by Resch et al. presents an interesting novel study on the influence of storage conditions on chemical composition of collected aerosol filter samples. 

In general the manuscript is well written and presents the results in a straightforward way. But it still needs a careful language check. I have pointed to some suggestions for this, but an additional check is needed. 

My main concern is the lack of detailed description of laboratory sample generation and conditions during collection from ambient air, as well as considerations of the difference in chemical composition between laboratory generated SOA and ambient aerosols as described in details below.

L. 12: I suggest to include here which oxidants were used to generate the SOA.

L20: It is not clear what you mean by “illustrate”. Please use a more scientific description. Please add information about the temperatures.

L21: I suggest to add more information about your results here to describe what you mean by “significant”.

L53-61: This section contains speculations and broad criticism of previous studies. I suggest to change the wording to avoid these broad accusations of unspecified previous studies. 

L70: Please state purities if known.

Section 2.2: The conditions for laboratory generation of SOA samples should be described in much more detail including the OCU in the main manuscript. How much ageing does this compare to (oxidant concentrations, days)?

Furthermore the dates of ambient sampling should be listed, as well as supporting measurements such as PM levels.

L90: Please provide this information, maybe in the SI.

L101-102: What was the precision of these volumes? Please give the appropriate number of digits.

L113-115: These lines need revision regarding: liquid chromatographic measurements, split sampler, for separations. Please rewrite to clarify and improve the (scientific) language.

Section 3.3. It is important to consider the influence of the time of year on the results. 

It is very interesting that the conclusion in section 3.3 is that ambient samples are less affected by the storage conditions. Could this difference be due to a high content of freshly formed reactive species (HOMs, peroxides etc.) in the laboratory-generated SOA?

Conclusion L359: This sentence seems quite speculative.

Fig S4: Which type of SOA is shown? 

Fig. S2 and S5: Numbers in dark blue markers are not visible.

Minor comments:

Line 10 artefactual – please check that this is a correct word

L18: Seems that a word is missing after chromatography – with? coupled to?

L50: exhibit -> show?

L62: define – I assume you mean “identify” or “characterize”.

L127: AGC target of 3E6 – please clarify

L164: I suggest to mention the oxidants again.

L297: generates -> generated