The paper synthesizes ground-based aerosol measurement at a suburban site near Paris, France. Key measurements characterize the hygroscopicity under subsaturated conditions (RH = 90%) and the mass chemical composition – both at selected particle sizes and ranging between 100 to 250 nm. The authors obtain hygroscopic growth factors directly from HDTMA and estimated ones from AMS-based composition. Disagreement between the two is associated by the authors with heterogeneous mixing conditions, mingling low and high hygroscopicity particles. As an example, the authors display black carbon concentrations that lead to elevated mixing heterogeneity. Back-trajectory analysis informs on the air mass origins and their tendency for heterogeneous mixing. The paper is generally well written and the figures support the key findings. I recommend publication after resolving minor concerns.

Minor concerns

Sec. 2.3 The authors introduce particle size distributions, but neither show them in the paper nor in the supplemental material. If the authors did not find any distinct size distributions across air masses or mixing ratios, it is still worth expressing that.

Fig. 1 Please explain the upward pointing arrows. Assuming these are sources, are there any sinks worth highlighting?

l. 104 “The k will examine” seems an odd construct. Perhaps substitute with “k will inform”.

Fig. 5 Please indicate the uncertainty, for example, by showing a typical error bar per data set (e.g., on the far left or far right).

l. 251-254 (and throughout) Please be explicit about material that is being discussed (as opposed to results). The authors could use distinct phrases, such as “We speculate”, etc.

l. 262 Please define “inorganics” upon first use. Is it simply the sum of all species except “organics”?

l. 270 A correlation of 0.56 does not seems that high, leaving ~68% of the variance unexplained. Using “reasonable” seems confusing here.

ll. 274-275 It is unclear whether the authors motivate the next section here or whether they propose work that won’t be covered in this paper. Please clarify.

ll. 294-296 It appears as though the authors are suggesting that high hygroscopicity pollutants here. Please clarify and, if true, perhaps introduce in Section 1.

l. 396 Perhaps replace “another” with “other”.

Typos

l. 116 and also l. 209 Please remove parentheses within parentheses.

The paper uses long term in situ ground measurements within the Paris region to characterize κ hygroscopicity under subsaturated (RH = 90%). Measurements obtained chemical composition and growth factor (GF) for particles between 100 to 250 nm using HR-ToF-AMS and H-TDMA, respectively. Standard deviation (σ) of growth factor results were used to explain the degree of particle mixing state. The study found that external mixing had a great influence in hygroscopicity for the measured particles and that ZSR mixing rule is unreliable in predicting hygroscopicity for a wide distribution of these particles. Additionally, backward trajectories were used to analyze sources of the varied air masses and provide insight to the results of the study. The paper is a good application of measurements to identify important considerations (mixing state, chemical composition) in predicting hygroscopicity in subsaturated conditions. I recommend publication after addressing some minor comments.

Minor comments:

Page 5, Line 141: Does a variance of +/- 3% RH have a significant effect on GF measurements in this study? GF especially for lower soluble compounds may vary significantly within the +/- 3% RH range. If there is an effect, I would recommend mentioning this as a source of uncertainty

Section 2.3: Provide more information/description regarding the particle size distribution in the results as it is introduced in methodology

Page 6, Line 184: Is the 5-10% uncertainty in RH or in GF? Please clarify

Section 2.5: The composition of the particles indicates both hydrophilic and hydrophobic compounds present (varied solubility). There are many different variations of κ  prediction, such as Petters & Kreidenweis 2008 which considers solubility, and Nakao 2017 which uses O:C ratio to consider solubility and κ. These papers, along with other papers considering low solubility/hydrophobic compounds, found that accounting for the volume fraction dissolved as opposed to ZSR alone can improve κ predictions. Is this something that has been considered in κ_chem ?

Line 189: Is κ_i in the ZSR rule for kappa chem determined from measurement or derived from Köhler theory? Please clarify

Page 8, Line 245: Please clarify why a GF of 1.2 is used as the cut off for hydrophobic and hydrophilic compounds - theoretically a GF of 1.2 at 90% RH would exhibit a κ of ~0.08 (some water uptake) . Is there a specific paper that highlights why this is the cut off? If so please cite.

Page 9, Line 269-270 and Figure 4: Why is a correlation of 0.56 reasonable? This seems a bit low - please clarify

Page 12, Line 360: Reference that GF-PDF equations are in the supplemental information or include in text directly under methodology as it is a highlight of the results

Conclusion section: Previous laboratory and modeling studies (e.g., but not limited to Riemer et al., 2019, Razafindrambinina et al., 2022) have highlighted the effects of mixing state on hygroscopicity - how do the conclusions of this work tie into previous hygroscopicity/mixing state studies and add novelty? It would be helpful to strengthen the conclusions with these implications and highlight the uniqueness of this work when it comes to studying mixing state effects on hygroscopicity.