This manuscript (Manuscript ID: egusphere-2025-5655) presents an original and timely investigation into the widespread occurrence of large molecular methylsiloxanes in ambient aerosols. The topic is both novel and important, as it highlights a previously underrecognized group of synthetic organic compounds within atmospheric particles. The multidisciplinary approach, combining measurements from diverse environments with detailed chemical characterization, is a significant strength of the study. The results are convincing and suggest that these compounds may play a more substantial role in atmospheric chemistry than previously appreciated. Overall, the work represents a valuable contribution to atmospheric chemistry and aerosol science, and I recommend publication after minor revision.

Here are some comments in details:

The term “large molecular methylsiloxanes” is central to the study but is currently used somewhat broadly. Please provide a clearer operational definition in the Methods section, including typical molecular weight ranges and the key diagnostic ions used for mass spectrometric identification.

While the study benefits from data collected at multiple sampling locations, it would be helpful to comment briefly on how representative these sites are of broader regional or global conditions. A short discussion of potential sampling biases (e.g., proximity to emission sources, meteorological influences) and the expected variability of methylsiloxane mass fractions across seasons and regions would strengthen the manuscript.

Line 21: The manuscript states that methylsiloxanes constitute 2.0–4.3% of the “non-refractory organic aerosol mass”. Please clarify that this percentage refers specifically to the non-refractory organic component and acknowledge that refractory or less volatile compounds are not included. A brief explanation would improve the transparency of the interpretation.

Line 70: “the Hague” should be corrected to “The Hague”.

Line 74: “pre-backed” should be revised to “pre-baked”.

Line 139: “methylsilxanes” should be corrected to “methylsiloxanes”.

Line 155: Please add the preposition “at” after “occurring”.

Line 187: Replace “was” with “were”.

Line 192: The phrase “60–90% lower” appears inconsistent with the numbers presented and should be revised to “10–40% lower”.

Line 234: The statement “traffic currently recognized as their only known source” is overly absolute. Please rephrase to acknowledge the possibility of additional or emerging sources.

Line 345: The phrase “three to four orders of magnitude lower than the methylsiloxane intakes in urban and coastal regions” risks overgeneralizing all urban and coastal environments. Consider qualifying this comparison more carefully.

Line 411: Replace “may exceed that of other synthetic compounds” with “may be comparable to or exceed that of other synthetic compounds” for greater accuracy.

Review report of “Widespread occurrence of large molecular methylsiloxanes in ambient aerosols”

Methylsiloxanes are considered as an emerging class of pollutants. The authors have developed a novel analytical method called TD-PTR-TOF-MS, which enables to identify methylsiloxanes in the aerosol samples, and showing their widespread presence in ambient PM samples collected at diverse seasons and locations, accounting for 2-4% of organic aerosol mass fraction. The possible sources, correlation with long chain HC and atmospheric stability of methylsiloxanes are also discussed. In the end, the authors call for further attention to their potential health and climate impact.

I like this analytical idea based on Si isotope patten in combination with thermal desorption, which is highly selective and is actually a type of non-targeted strategy to identify Si-containing compounds in complicated samples, without using commercial standards. I strongly recommend it for publication and only have a few questions for the authors to address.

1. Can this analytical method be used to identify gas-phase methylsiloxanes? Also, it seems methylsiloxanes or cyclic volatile methylsiloxanes are sub-class of Si-containing compounds. Could other sub-class of Si-containing compounds (not methylsiloxanes or cyclic volatile methylsiloxanes) exist in the PM samples and be detected by TD-PTR-TOF-MS?

2. Can these methylsiloxanes compounds be resolved and analyzed by other analytical instrumentation, e.g. HPLC-Orbitrap MS? If possible, please discuss the current available method and analytical challenge of methylsiloxanes in the introduction section

3. It seems the quantification of individual methylsiloxanes was established based on PTR transmission curve. Could any commercial methylsiloxanes standards be used for establishing calibration curve? For instance, I wonder whether this quantification method could be tested or validated for methylsiloxanes standards, e.g. by spiking methylsiloxanes standards with known mass onto the filter? This could examine the reliability of this quantification method

4. For each PM sample, the chemical profiles of methylsiloxanes were collected at different temperatures. Those produced at lower temperature were assigned to be methylsiloxanes with small molecular weight, while higher temperature was associated with large molecular weight. It seems this desorbing and/or depolymerization process is highly dynamic. I wonder what is the temporal trend or evolution process from the PTR results by increasing the temperature during the experiment? Can the authors give an example specifically? This would help to understand how individual methylsiloxanes and total methylsiloxanes finally being quantified and converted to mass loading on the filter

5. Section 2.3 - For D5 (C10H30O5Si5), the first isotope peak (m/z 372) you mentioned actually merges both C13 and Si29 isotopes, and therefore in total accounts for 36% of main peak at m/z 371. Same for the second isotope peak at m/z 373. Am I right?

6. Section 4.1-4.3 could be merged as one section in Section 3.4 as “atmospheric implication”, and the discussion of health and climate relevant impact could be shortened and condensed a bit