The manuscript by Bigi et al. attempts to bridge between in situ measurements (obtained through with multi-wavelength absorption photometry) and AERONET sunphotometer retrievals, in the air pollution hotspot of the Po Valley. Established modelling approaches are followed for the disaggregation of BC-BrC components and also for the apportionment of BC fossil fuel and biomass burning sources at urban traffic and background locations. The insights gained by the paper are important in terms of how much in-situ absorption monitoring translates into actual absorption aloft and whether it can be used for radiative forcing estimates. The characterization of the intra-urban variation of absorbing aerosol especially between different site types can lead also to useful inferences in the ambient exposure domain, thus, the authors are encouraged to expand towards the analysis of intersite associations/contrasts for in-situ source-specific BC and BrC. There should be also some attempt to discuss the transport of absorbing pollutants within or from outside the Po Valley. The paper is generally clearly written, with pertinent references and adequate discussion. It also recognizes uncertainties and limitations of the monitoring/modelling approaches used. It can be considered for publication after exploring the above mentioned directions and addressing the following specific comments.

Specific Comments

Introduction, 3rd paragraph: The health effects from exposure to BrC should also be mentioned, given that a substantial part of light-absorbing OC are of polycyclic aromatic nature and therefore linked to oxidative stress induction and carcinogenic effects. 

l48-49: Given the existing ambiguity in the definition of eBC, this compilation should be recognized as a rather challenging task.

l59-65: Why go into such details about these methods when they are not used in the present study?

l101: Sources such as? Discuss.

l132: Give some more details about the fuel types that are used for domestic heating and emit PM in the area. The impact of residential biomass burning should be introduced because BB aerosols are integral to this study.

l134-136: This statement might be a bit ambitious, and taken out of context depending on how the reader understands the extent of the Po Valley. Moreover, the limited impact of industry that the authors claim for the Modena area, might not be the case in other more industrialized cities. I would suggest to tone this part down.

l142-143: Indicate the distance from the nearby road for the UB site, and the traffic intensity in the adjacent road for the UT site. It could be also useful to mention some inter-site differences observed for the regulatory pollutants, to illustrate how clear the UT-UB distinction is.

l155: The aggregation procedure here is unclear. What is meant by “custom”?

l157: A flow rate over 100cc is necessary to use the DualSpot compensation, but what factored in using different seasonal flow rates? Is there some recommendation by the authors?

l210-211: Describe in brief the screening process.

Section 2.3: A separate uncertainty section is not necessary here. Mover this information to the respective sections.

Section 2.4.1: I understand that the model has been presented already in literature, however, it is necessary to include a description of the procedure here and maybe some of the key equations. Otherwise, the reader will not understand how the different AAE values that are preselected are put into use.

l245: Do you consider that there are uncertainties around this assumption? For example, at a traffic impacted location, there could be some traffic-related BrC expected. This would also probably mean that AAE-BC and AAE-FF are not identical.  Please discuss and recognize the limitations.

l250: The selection of a fixed AAE-BrC value is critical for the calculations and it has to be justified better here. Provide more information on how the 3.9 value was derived (method, location, season, dominant sources etc.).

l292-301: I don’t think that a whole paragraph introducing the Figures is necessary. You can guide the reader through the presentation of the results.

l302-303: Mean values are mentioned here, while in the Figure-Table, the medians are displayed. Maybe consider a homogenization of the presentation.

Figure 2: A couple of things stand out here and should be discussed. First, in the holidays the nighttime peak of BCff absorption at UB is comparable to the workdays, and also bigger than that at UT. Second, at the UT site, nighttime BCbb and BrC absorptions become larger in holidays than in workdays.  

l302-304: Is there an increasing interannual trend for absorption in cities of the Po valley? Discuss.

l321: It should be “BrC estimated”.

Table 2: The vehicle count parameter should be expressed in vehicles-per-day units.

l350-355: Not much new in this paragraph and not in the core of the study. I suggest omitting it or condensing it to a sentence.

l380-387: The enhancement of BrC for winds of the southern direction should be explained, since in this study BrC is considered as a source-specific variable (BB-related). It can be observed that the winds related to the increase are only moderate. So it should be examined if there is indeed a BB source area or it is a low-wind stagnation effect during nighttime when the highest BrC levels are expected (it should be also noted that it is observed only in holidays).  

l396-398: Did you consider compensating for the wavelength discrepancy by adjusting in situ absorptions by the calculated absorption AAE?

Conclusions: The section repeats numerical results from the previous parts of the manuscript. Some more implications of the findings, regarding atmospheric absorption research and urban BC exposure should be added.

Technical corrections

l31: Delete “over the Earth”

l33: Delete “depending…specifics”

l43” That should be “increased eBC concentrations”. Same at next line

l314: “good agreement”

l400: “overestimation”

l400: “by some concurrent conditions: (a)”

l402: “at the rural…”

l405: “contribution”

l450: “mean absolute deviation”

l479: “with Ispra and Modena”