General Comments:

This manuscript is well written and the results are nicely structured and discussed in great details. The authors present a field investigation of the effect of fog conditions on the chemical and physical properties of soot-containing particles through secondary processing during winter in the San Joaquin Valley. Enrichment in oxygenated organic aerosols and ammonium nitrate on BC-coating were observed during the fog event. The fog-related oxidized organic aerosols, are produced mainly by relatively fast aqueous phase processing of BBOA within fog droplets. The use of the potassium to BC ratio to estimate the fraction of BB contributing to the secondary formed OOA_BC was a very interesting approach.

The comparison with the colocated AMS ensemble measurements provide valuable insights on the mixing state of the PM₁, with high-fog conditions favoring internally mixed BC with secondary OA and ammonium nitrate. Furthermore, they evidence a potential link between soot particle coated with secondary material from frog processing and changes in light absorption properties. The coating of BC by SOA and ammonium nitrate also lead to more hygroscopic soot particles and therefore more effective CCN.

I recommend this manuscript for publication after addressing minor comments. In particular, the authors caution about potential bias on the use of the CH₂O₂⁺ fragment as tracer for aqueous processing. The influence of intense NO₂⁺ signal, especially during the fog event when NO₃ concentrations drastically increased, which may bias high the intensity of CH₂O₂⁺ (see comments).

Minor Comments:

1. Page 4 line 129: “the laser vaporizer RIE is varies from this”, remove “is” and maybe add at least the RIE used in the SI. Is it 0.05 for rBC based on (Collier et al. 2018)?
2. Page 7 lines 219-221: “In contrast, K₃SO₄⁺ concentrations were elevated during the high-fog period compared to the low-fog period. The correlations between K₃SO₄⁺ and SO_4,BC concentrations showed variable slopes, gradually decreasing over the course of the high-fog period (Fig. 2b).” Are those different slopes linked to various source of SO_4,BC or K₃SO₄⁺? And why are K₃SO₄⁺ concentrations high at the beginning of the fog period (following NO_3,BC) and then decrease whereas BBOA concentrations are low and FOOA ones (Figure S4) remain stable?
3. Page 13 line 411-413 and 425-428: Although the CH₂O₂⁺ signal appears real based on Figure S13a, its intensity is much lower than the NO₂⁺ fragment, which could bias high the intensity of CH₂O₂⁺ signal. This is even more relevant during the high-fog period, as NO₃ concentrations increase and so does NO₂⁺ and CH₂O₂⁺. If that’s the case, their temporal variations would match (as shown in Figure S6b) and as a result, they will be grouped into a same factor by PMF. You may want to show the m/z 46 HR fitting peak averaged over the low and high-fog periods to support your point. I appreciate the caution about the peak fitting in the conclusion (page 16 line 500-502), but it might be better to address it earlier in the discussion.
4. What is the contribution of BC-containing particles to PM₁ fraction between low and high-fog periods? Could be added to Fig 1.

Supplementary information:

1. S1.1: What was the RH after the dryer, as it may affect the optical measurements.
2. S1.3 lines 85-90: In the laser mode vaporization of the SP-AMS, f_CO2 can result from non-refractory CO₂⁺ of the organic coating and refractory CO₂⁺ from BC thermal decomposition, do you think it could result in an overestimation of κ_𝑂𝑟𝑔?
3. Fig S8: if available, the diurnal variations of T, RH and wind speed/direction during low and high-fog period could be useful information to add in this figure.
4. Fig S11: “Top panels show the scaled residual between the measured and modeled size distributions.” Is there something missing in the figure?

This manuscript describes the chemical composition of BC aerosols, particle optical properties, sources of soot particles, and atmospheric processes affecting the properties of BC coatings. Studies have found that aqueous-phase reactions facilitated by fog droplets had a significant impact on the thickness and chemical composition of BC coatings, which represents interesting and meaningful academic research. I recommend the publication of the manuscript after the following points have been addressed.

I only have several minor questions about the manuscript.

Point 1: Lines 172-173: “These observations provide clear evidence that the presence of fog droplets promoted the formation of nitrate on BC particles”, is it also possible that the presence of nitrate can cause high fog environments?

Point 2: Lines 194-195: The proportion of organic compounds in the high-log period is smaller than that in the low-log period, but the concentration of Org_BC is higher in the high-log period. More details of the possible mechanisms should be discussed here.

Point 3: There is no apparent logical relationship between the subsections, the article logic and analysis sequence need to be reorganized for a more natural transition between the subsections.

Point 4: The mention of "However, the diurnal profiles of HOA_BC showed notable differences between the two periods" in Line 261 appears to have a hasty explanation. Evidence to prove specific differences in traffic patterns could make it more convincing, and the mentioning of the possible relationship between these differences and variations in boundary layer height seems somewhat abrupt.

Point 5: lines 423-424 “This finding is in contrast to measurements in China, where primary BB emissions accounted for 30% of the oxalate mass”, however, it did not mention the differences or the specific details of the data which has “no significant relationship between CH₂O₂⁺ and BBOA_BC (r² = 0.02)”.