The subject manuscript reports on trends in atmospheric composition during the COVID-19 lockdown in Paris, as compared with meteorologically similar days in previous years. The authors develop and apply a 3-layer analog approach for identifying meteorologically similar days in different years; the layers include synoptic (sea-level pressure), regional (air mass trajectory), and local (temperature and relative humidity). Unlike some of the early studies of COVID-19 on air quality, the study includes analysis of secondary pollutants, including secondary organic aerosol, and composition and origin of particulate matter. The focus on the role of meteorology in interpreting air quality measurements is a strength of the paper, and the A3Q methodology can be widely applicable provided data are available. For the manuscript to be of broad interest and of high impact, some revisions are needed. The results presented in section 4 are clear, but this may be of limited interest to broader ACP readership, and more appropriate for a Measurement Report. However, with better characterization of the limitations of existing approaches to account for differences in meteorology (see scientific merit below) and editing to remove unclear language (see presentation quality below), the manuscript may be suitable for publication in ACP.

Scientific Merit:

Significant changes in meteorology would be expected in the transition from winter to spring (i.e., pre-lockdown period and lockdown period) and even from early to late spring (i.e., beginning of lockdown period to end of lockdown period). Thus, the  pre-lockdown and lockdown comparisons in pollutant concentrations (Fig. 1) and air mass trajectories (Fig. 2) are important for illustrating the point, but should not be a main focus for justifying the A3Q approach. For example, in lines 171-173 it is stated that the continental sector is under-represented and the oceanic sector over-represented when comparing the lockdown period with other reference periods. This is clear for the pre-lockdown period; however, within the measurement uncertainty there appears to be no statistically significant difference for the continental sector across the other reference years (over the same date range) and statistically different but (potentially) small differences for the oceanic sector in three of the four periods. Similar comparisons for temperature and precipitation for the LP and references periods would be valuable. Minor comment-the date range listed in 161 does not match any of those in Fig. 1.

The authors need to better demonstrate that the A3Q approach provides a significantly better solution than comparing with a range or ranges of previous years over the same days (i.e., see Parker et al. GRL, 2020). It would be useful to see some of the pollutant concentrations presented in Fig. 1 with the results using the different analogs (Fig. 7).

presentation quality:

Clarity of the manuscript could be improved with organizational changes and additional editing. The language in many places is quite challenging, and the main points are obscured or unclear.

The introduction, as written, includes one paragraph about COVID-19 restrictions, perturbations in human activity, and associated opportunities to better understand atmospheric composition and chemistry. Indeed, there have been a large number of studies published on air quality during COVID-19, some of which are cited by the authors. It is suggested that the introduction be reorganized such that these two pieces (motivation and current published studies) be combined as a single paragraph. The introduction could then be expanded to describe the current state-of-the-science regarding air quality in France/Europe and the importance of considering meteorology in air quality studies, which are both relevant to the conclusions and only cursorily described in the second paragraph of the introduction. A more concise discussion of secondary chemistry and PM composition would also strengthen the introduction and the manuscript as a whole.

There are several statements that are unclear as written. A few examples follow, but this is not an exhaustive list. It is recommended that the manuscript be reviewed for these unclear or ambiguous statements.

Lines 49-51: “Without climatologically representative values, comparisons of concentrations observed during and outside the lockdown periods shall thus free themselves from differences in weather.”  The authors are making the valid point that care must be taken when comparing air quality measurements over different time periods because much of the variability can be driven by meteorology. However, this specific sentence seems to contradict that point, though it is very difficult to interpret.

Lines 55-58: The authors state- “Air quality shall not be restrained to NOx, O3, and PMx only, and limited number of studies so far has treated air quality as a whole, notably taking PM chemistry into account.” The sentences before and after suggest that the authors are referencing the need to consider speciation of PM. Depending on the objectives of a study, this may be a critical aspect, but isn’t necessarily a requirement for air quality studies.

Lines 176-177: “…it may also explain why these methodologies are associated to an increase of eg NO3, SO4 and OOA, due to an underestimation of business-as-usual concentrations for LP2020 meteorological conditions.” It seems the authors here are suggesting that if the air masses for LP2020 were correctly represented/compared, and if there was no lockdown, then the NO3, SO4, and OAA concentrations in the reference periods may not have been higher than business-as-usual LP2020. However, there was no business-as-usual LP2020. Maybe the authors mean to suggest that the differences in concentrations, without accounting for meteorology/air mass origin, are not only due to changes in human activity and are likely exaggerated due to fewer air masses of continental origin in prior years.

minor comments:

lines 79-83: The latter part of this sentence is unclear-it is clear that the measured fractions were corrected for collection and ionization efficiency, but it is not clear what it meant by “successfully participated in” intercomparison. Is this for the measurements being reported or is this something that has been done previously?

line 140: Please provide some additional details regarding ZeFir.

line 186: How is “sunny” quantified?

line 227: Why would wetter days lead to enhanced condensation of semi-volatile compounds?

lines 230-231: Annotation (brackets) is unclear (also in Table 2).

section 3.2.4-The presentation of the sensitivity tests is confusing as written, with both “S” used to indicate a scenario and also “scenario”.

line 295: “specie” should be “species”

The manuscript by Petit et al. firstly reports the responses of aerosol and gaseous composition to COVID-19 lockdown by an innovative methodology “Application for Air Quality”. Their results provide a comprehensive understanding on the impacts of traffic-related reduction on primary and secondary species. I suggest that major revision is needed for the manuscript prior to be published in this journal.

Major comments:

1. The impacts of lockdown on aerosol chemistry were evaluated using Analog Application for Air Quality (A³Q) approach. Please give a detail discussion on the reliability and uncertainty (or limitation) of the method. I suppose that the method of comparing with reference period could be moved to the supplementary material because the focus should be on A³
2. Please elaborate how to calculate the absolute and relative changes of aerosol and gaseous species due to lockdown.
3. The Aerosol Chemical Speciation Monitor (ACSM) were used to measure aerosol composition. Since there are two versions of ACSMs nowadays, i.e., quadruple and Time-of-Flight ACSM, the author should clarify this in the Method. A composition dependent CE or a constant CE is used in this study?
4. For Sec. 2.2, the POA-constrained PMF (or ME-2) was performed on OA matrix to resolve three OA factors (i.e., HOA, BBOA, OOA) during January-May 2020. The source apportionment results during June 2011- March 2018 were obtained from previous studies. It is unclear whether similar PMF method and OA components were used between the two periods. Please specify it.
5. The elemental ratio and oxidation degree of SOA were calculated using I-A method. Which fragments or m/z were used for O:C and H/C calculation? Considering that ACSM detect species with unit mass resolution, I am afraid that the absolute values of element ratios were questionable to some extent.
6. Figure 3, the scatter plots of simulated versus observed species contain two markers, i.e., small dots and six solid dots. Please explain it in the figure caption. Compared to the secondary inorganic aerosol, both POA factors and OOA showed much lower R during the evaluation period (Table 4). Did this mean that the organics might not be well reproduced by this model? If this, the quantification of OA changes during 2020 lockdown might also be affected.
7. The time periods used for calculation and discussion were a bit confused. In line 205-210, the author said that “the study period covers 92 days from March 1^st –May 31^st 2020”. However, January-February 2020 was chosen for model performance evaluation. Why only January-February 2020 was used? Which period was referred to as “lockdown” through the study? Please declare it in the Method.
8. In line 390-400, is there other evidence that NO₃ formed from long-range transportation? The total changes of NO₃ can be quantitatively apportioned into regional decrease and advected contribution? Please elaborate the estimation method.

Minor comments:

1. Please define BC_ff and BC_wb when first mentioned.
2. The table caption should be placed on the top to table instead of the bottom.
3. Although this is not a final publication version of ACP, the author should carefully check the output styles of the references as ACP recommended. For example, complete and abbreviated journal names
4. The abstract could be more simplified, particularly for the descriptions before A³Q method.