Comments to the manuscript titled “Origin, transport and processing of organic aerosols at different altitudes in coastal Mediterranean urban areas” by Clara Jaén et al.

The authors measured 68 organic molecular markers in diurnal and nocturnal PM₁₀ in an urban background site and in a nearby elevated sub-urban background site, in cold and warm periods in Barcelona. By applying MCR-ALS method, six different sources were resolved based on the profile of organic molecular markers. This manuscript has a good discussion of the influence of sources, altitude, seasons and diurnal differences. However, there are some concerns about the source apportionment methods and stability of the results. Firstly, the manuscript lacks the basic information of the MCR-ALS method and the rational of choice among the other source apportionment methods, e.g., Positive Matrix Factorization (PMF). Secondly, two weeks of diurnal and nocturnal sampling in cold and warm periods will have 28 samples, which is far behind the substance number (i.e., 67) input in the model. Therefore, the stability of results and the choice among different runs will be a big concern for the solidity of results of discussion. For example, for receptor models, depending on the number of degrees of freedom per variable, the suggested minimum number of samples (N) needs is N>30+(V+3)/2, where V represents the number of species and should be 65 here in the study (Henry et al., 1984). PMF has several sensitivity analysis, i.e., factor choosing, bootstrapping (BS) mapping and displacement (DISP) diagnostics. Please indicate the similar analysis for MCR-ALS to make sure the source apportionment result is solid and strong enough to support the subsequent discussions. Thirdly, given the limited samples number involved in the analysis, some interpreted source is not trusty, e.g., diurnal and nocturnal traffic and fresh secondary. Specially, for fresh secondary, the sampling matric, i.e., PM₁₀ is far from the new particle generation (e.g., PM₁ or smaller). Fourthly, the interpretation is based on source indicators like PAHs, hopanes, retene and others but in the real environment most of them are not single-source specific, fossil fuel combustion, vehicle emission and biomass burning will be integrated (Bi et al., 2008; Shen et al., 2012). Therefore I suggest the authors to reduce the number of sources and make the results more solid. Other multivariate analysis that don’t have strict limit on degrees of freedom, i.e., unsupervised hierarchical clustering analysis, could be supplemented to support the source apportionment results. Also, the source apportionment of other studies conducted in Barcelona should be compared with and discussed in the manuscript in detail. Given the above main concern, for now the manuscript is not qualified for ACP but it could be considered after major revision.

Specific comments:

1. Lines 49: The source of hopanes is not single.
2. Lines 116-117: Please indicate the sunset and sunrise time of sampling period and explain why the daytime and nighttime sampling period are chosen and different in cold and warm seasons.
3. Lines 121-123: Please indicate which type of quantification method was used and explain why full scan methods was employed other than MRM methods.
4. Lines 133-134: Please explain why different length of column was used to analysis different compounds.
5. Lines 141-144: Although there is only high-abundance nitro-PAHs are analyzed in this study, using two different instruments with two different source, EI and CI is weird. Please provide more evidence that they could provide equivalent results.
6. Lines 147-149: Please indicate the difference between method limit of detection (MLOD) and instrument limit of detection (ILOD) and provided detailed data in the Supporting Information given there are several instruments and methods involved in this study.
7. Lines 157-160: There might be some bias because the site difference of different data.
8. Lines 180: It is hard to understand the temporal evolution of potential temperature (θ).
9. Lines 189: It is confusing about the nocturnal decoupled behaviour for ozone.
10. Line 223-226: Retene dosen’t always indicate a single source, especially in different regions.
11. Lines 231-232: It is not clear to attribute the concentration difference to different primary and secondary. Please explain in more detail.
12. Lines 298: Please provide more information and method details about the MCR-ALS. Also, the difference of typical source apportionment tools like PMF should be explained in the discussion.
13. Lines 235-238: The evidence is not strong enough to support the nocturnal traffic source.
14. Lines 488-489: The evidence is not strong enough to support the new particle formation, especially due to the sampling matrix of PM₁₀.

Technical corrections:

1. Some font of citations is larger than others, e.g., Lines 39-40, Lines 225-226, Lines 255-256, 277-278, 347-348.
2. Lines 114: There should be one blank between number and unit.
3. Lines 121-123: The number should be subscript. Please check in the whole manuscript.
4. The language in the manscript needs to be modified by tools such as AI, e.g., Lines 186-188.
5. Lines 235: The x of NO_x should be subscript. Please scrutinize in the manuscript.
6. Lines 405-406: Confusing symbol before the number here.

References:

Bi, X., Simoneit, B.R.T., Sheng, G., Fu, J., 2008. Characterization of molecular markers in smoke from residential coal combustion in China. Fuel 87, 112–119. https://doi.org/10.1016/j.fuel.2007.03.047

Henry, R. C., Lewis, C. W., Hopke, P. K., & Williamson, H. J., 1984. Review of receptor model fundamentals. Atmospheric Environment (1967), 18(8), 1507-1515. https://doi.org/10.1016/0004-6981(84)90375-5

Shen, G., Tao, S., Wei, S., Zhang, Y., Wang, R., Wang, B., Li, W., Shen, H., Huang, Y., Yang, Y., Wang, W., Wang, X., Simonich, S.L.M., 2012. Retene emission from residential solid fuels in China and evaluation of retene as a unique marker for soft wood combustion. Environ. Sci. Technol. 46, 4666–4672. https://doi.org/10.1021/es300144m

The authors conducted PM10 sampling (both day and night) at two different sites in Barcelona and performed chemical analysis to report molecular markers during two distinct seasons, which is commendable. I have a few concerns listed below:

• Only 28 samples to run the source-apportionment analysis?
• description and logic behind using this apportionment method
• L105:… PM10 was collected at two background sampling stations were installed at two altitudes .. needs rephrasing
• L116: Any justification for different times chosen for sampling?
• L157: ‘This information’ means ‘Meteorological parameters’? Moreover, it can be understood that the met station was not the same as the air quality monitoring station. Please mention this limitation in the text.
• L218: missing space after unit.
• L294: provide the reason for such results.
• Section 3.3: Some information must also be provided on ‘already well-established’ apportionment methods such as PMF.
• L325-6: ‘methyl phenanthrenes’ or ‘methylphenanthrenes’?
• L430: ‘Vertical’ should be rephrased.