the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 15 Oct 2024
Molecular characterization of organic aerosols in urban and forested areas of Paris using high resolution mass spectrometry
Abstract. In order to study aerosols in environment influenced by anthropogenic and biogenic emissions to variable extents, PM1 samples were collected during the summer 2022 in the greater Paris area (ACROSS campaign, Atmospheric Chemistry Of the Suburban Forest, June 14 to July 25) at two locations that represent the urban Paris and the suburban forested area. They were analyzed using high resolution mass spectrometry (HRMS) together with total carbon (TC) by a thermo-optical method. Both sites are compared here to explore differences in aerosol composition from urban and forested environments. The TC analysis shows similar organic carbon (OC) concentrations at both sites (3.2 ± 1.8 µg m-3 for Paris and 2.9 ± 1.5 µg m-3 for Rambouillet), and higher elemental carbon (EC) values in the urban area. Both OC and EC concentrations did not show significant variations for daytime and nighttime conditions. This work highlights the influence of anthropogenic inputs into the chemical composition of urban and forested areas, derived into the presence of CHO and CHON compounds but also the detection of two sulfur-containing compounds (C5H12SO7 and C10H17NSO7), which could be tentatively assigned as organosulfates. A smaller number of aromatic compounds were observed for clean periods, that better represent the local biogenic and anthropogenic contributions in Rambouillet and Paris, respectively.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2024-3015', Anonymous Referee #1, 01 Nov 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-3015/egusphere-2024-3015-RC1-supplement.pdf
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RC2: 'Comment on egusphere-2024-3015', Anonymous Referee #2, 12 Nov 2024
Pereira et al. present analysis of organic aerosols collected during a period in summer in Paris and a forested site outside of Paris. The aerosol was collected on filters during "daytime" and "nighttime" during the course of the sampling period and analyzed offline with a Sunset OC/EC analyzer and high resolution mass spectrometry. Comparison between the two sites in EC and OC mass concentration as well as a select analysis of elemental composition for different days was conducted. Some similarities and differences between the sites was observed.
Though the paper may be of interest for the ACP community, there are many aspects the authors need to address prior to publication. Along with the concerns brought up by the other reviewer, which cover many of my concerns as well, the authors need to provide further analysis and description for the following aspects of the paper:
1) Methods--
1a) It is not clear how the aerosol was sampled onto the filters. Was there an impactor to ensure only PM1 was sampled? Was the high volume aerosol sampler located outside or inside? Was there any sample lines? If there were sample lines, what material was used?
1b) It is stated that the aerosol was sampled at ~20 m above ground in Paris and at ground level at the forested site (line 111 - 112). If the sampler was this low, how much is soil emissions and canopy/below canopy emissions and chemistry impacting the aerosol, compared to the urban site? Can it really be assumed that similar organic aerosol is being observed between the two sites if the sampling is being conducted so far into the forest?
1c) Why were 6:00 - 22:00 and 22:00 - 06:00 time selected for filter sampling? How does this correspond to rush hour or other traffic related emissions as well as other anthropogenic activities, such as cooking? If there was transport from Paris to the forested site, how long is the transport, and how would that impact the aerosol sampled during these times (e.g., how much would the nighttime filter from the forested site be more correlated to the daytime filter from Paris)?
1d) It is not clear in the discussion about extraction how the extractions were being conducted (line 149 - 151). Was methanol used to extract the material, or was it in the slurry ice?
1e) What solvent was used for the ESI source (line 155 - 159)? The choice of solvent will impact background and sensitivity/selectivity.
1f) It is not clear why CH families were not discuss or report (line 169+). Was this due to type of solvent and/or ionization mode? Similar to how much CHN and CHS may be a large fraction (e.g., reviewer #1 comment), how much is being missed by not reporting CH? E.g., CH would potentially be associated with primary organic aerosol, as discussed in the introduction; however, if the sampling method is biased to not observed primary organic aerosol, how are the results in comparing Paris to the forested site impacted by neglecting this source?
1g) It is not clear nor supported why using O/C and H/C ratios to describe aliphatic and aromatic domains. How justified is this using standards or comparing compounds that may have similar O/C, H/C, and/or elemental ratios but are different between aliphatic and aromaticity? Why isn't the double bound equivalency and/or aromaticity equivalency used instead?
1h) What is condensed aromatics? How is this different from aromatics?2) Figure 2 is difficult to interpret. For the OC and EC plot, it appears the legend may not match the traces. Also, it is not clear which site is which for wind direction/wind speed. Something that may be useful for Fig. 2 is highlighting time periods where Paris is potentially directly impacted the forested site.
3) Line 222: It needs to be stated that you are comparing the entire sampling period in Paris vs the short period in the forested site. Also, what is the average for Paris during the time period the forested site was sampling for better comparison?
4) Line 228 - 234: The comparisons here are hard to follow. There are lines where two different concentrations are reported for OC, and it is not clear if it should be OC and EC.
5) Table 1 and Sect. 3.1.1 -- It is surprising that there is minimal day / night differences between EC and OC for the sites. How are emissions, chemistry, and boundary layer dynamics playing into impacting these mass concentrations? Why does it appear there is no impact from emissions on EC between day and night?
6) Sect. 3.2 and Fig. 3 -- Trying to conduct comparisons between the different samples and follow the listing of different suggested molecular identification is difficult here. One potential way to improve comparisons is to either plot the day / night comparison as day with positive values and night as negative values or a scatter plot of day vs night for one site and day vs day, night vs night, and/or day vs night between urban and forested site. This may provide a better way to evaluate similarities and differences between the sites and differences in chemistry.
7) Table 2 / Sect. 3.2 -- This table is very uninterpretable. It is difficult to try to compare not only for one site the differences in compound class, but also between urban and forested site. This table may be better for SI and instead some visual way to compare these to better show similarities/differences. Further, though there may be similar CHO (or other classes) across urban and forested, does that really mean that the aerosol is similar? E.g., CHO is dominated by ketones, aldehydes, and/or alcohols in urban area but acids or overall more oxidized material in forested region, but the percent remains approximately the same, it would appear that the organic aerosol is similar when it really is not. Finally, it is surprising how dominant the aromatic/condensed aromatic is in the forested site. Is this all due to transport or something else, as biogenic VOCs/SOA are generally not considered to be aromatic.
8. Fig. 4 is really nice to better represent and emphasize similarities and differences between the sites. Highly recommend diving a little bit more in the discussion/analysis here.
9. Sect. 3.3 -- This section is very confusing and potential misleading (e.g., correlation does not mean causation). As example, line 389 discusses how relative humidity is negatively correlated with NOx and EC, and this negative correlation is due to RH impacting production of acidic gases, which is not accurate. NOx may produce nitric acid, but it depends on the VOC reactivity, OH concentration, etc. (highly non-linear) and may/may not be correlated with relative humidity. As discussed by Reviewer #1, better description of the chemistry that would lead to the formulas/families provided here and other sections and why there may be correlation is needed.
Citation: https://doi.org/10.5194/egusphere-2024-3015-RC2
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