General comments

The article investigates the effect of the UHI circulation and the related temperature and humidity fields on the air pollution in the city of Hangzhou (China) using the modeling chain WRF-CMAQ. This topic is within the aim of ACP, however in my opinion the article does not present significant novel contributions.  Concerning the general structure of the paper, the title is appropriate and coherent with the content. The overall presentation is well structured and clear, the introduction provides a comprehensive description of the scientific background of the work and the number and quality of the reference are appropriate. I think that authors should state more clearly the aim and the novelty of their work. Concerning the methodology, I think that a brief description of the study area is missing and could be appropriate. The numerical methods applied are well established, even if not exhaustively described (see “Specific comments”).  Furthermore, in my opinion authors should: 1) delve into a more exhaustive analysis and discussion of the results, 2) extend the analysis of the results to a longer period (than a single day run), 3) consequently enhance the conclusions. For example, the role of the wind speed intensity is just mentioned only at the end of the Results section.

Specific comments

Abstract:

Lines 25-26: “This is mostly attributed to the UHI circulation (~90%) rather than changes in the air temperature (~5%) and humidity (~4%).” In my opinion this sentence does not help the reader to understand the results in this context.

Methodology:

Line 88: Why a dated version of the WRF model (i.e., 3.9.1) has been used? Can the authors motivate this choice?

Line 94: I have some doubts about the vertical resolution. As is known, BEP needs a high resolution near the surface to be used effectively. The sentence "the lowest 20 of which are below 2 km to better resolve the processes within the BL" does not provide enough information on the resolution near the soil.  Furthermore, in my opinion a more detailed description of the physics options in the modeling chain (WRF-CMAQ) could be provided to the reader, maybe in the supplement. For example, which PBL option (in WRF) has been applied?

Lines 94-95: “A 10-day simulation (from 95 00:00 UTC 10 September to 00:00 UTC 20 September 2017) was conducted”. A focused investigation on the UHI circulation should be performed during a period not affected by other local phenomena, such as the sea-land breezes or a heat wave. Why this period has been simulated? Can the authors explain if the absence of other local circulations has been verified? And if so, how? Furthermore, Fig. 2 shows an increasing trend in the PM2.5 concentration compared to the previous days starting precisely from 17-18 September. Can authors comment on this?

Model evaluation: Can the authors explain why the evaluation is performed only in terms of PM2.5? Temperature and humidity are key parameters in this analysis; therefore I would expect the validation of the model also in terms of these variables, together with the wind speed.

Results and discussion:

Lines 184-185: “In the 10-day experiment, the strongest UHI effect occurred on September 18, 2017, which will be discussed in the following sections”.  In my opinion a single day run does not provide sufficient data for a high quality analysis. Also, how is UHI quantified? How was the UHI on September 18th found to be the strongest over the 10-day period? Please clarify these aspects.

Lines 193-195: “Kang et al. (2014) suggested that the maximum surface UHI intensity occurs in the evening, while the strongest UHI circulation appears in the afternoon.” I think that some comments on the temporal characterization of the UHI circulation in the present results might also be interesting.

Lines 337-338: “In the 10-day simulation period, 7 UHI cases occurred in Hangzhou. The average (12:00‒17:00 LT each day) UHI intensity of these cases varied from 1.4 °C to 1.9 °C.”. I have some concerns about the position of this sentence at the end of the Results section. Furthermore, could the authors comment the occurrence of 7 UHI cases in the 10-day period considered? It could be interesting to clarify what happened in the remaining three days and how the UHI intensity was computed.

Lines 338-339: “the UHI circulation, which is characterized by vertical wind speed, is strengthened by 0 cm s−1 to 10 cm s−1 in the BL and 0 cm s−1 to 5 cm s−1 in the LFT”. This sentence is not clear. Please rephrase. Moreover, please clarify the type of speed values. Are they maximum or average values over the 10-day period? Or what time do they refer to? Can authors please provide more details on this?

Lines 340-341: “These UHI effects decreased the PM2.5 concentrations in the BL by 1% to 26%, and increased the PM2.5 concentrations in the LFT by 5% to 21%.” As for the point above, it might be useful to have more details on the listed PM2.5 values.

Lines 342-343: “The result of the simulations show that the impact of the UHI on aerosol is highly dependent on the intensity of the UHI effect.” The resultS show… Furthermore, this sentence is a bit to general. Could authors rephrase it?

Aerosol pollution is of great concern in many megacities all over the world. It is meaningful to study how urbanization affects local meteorology and air pollution. By using WRF-CMAQ model, this manuscript assessed the effects of UHI on the distribution and formation of aerosols in urban atmosphere. The authors separated the impacts of UHI circulation, temperature and humidity, and quantified the contributions of them on inorganic aerosols in the lower free troposphere. Generally, this manuscript is well organized. The findings are very interesting, the discussion is of scientific meaning, and the results can improve the understanding of the formation of inorganic aerosols in urban areas. This manuscript could be considered for potential publication after the following minor revisions.

1) In 2.2, it is better to show the results of the comprehensive model validation for meteorological parameters, such as temperature, relative humidity and wind speed.

2) Line 145: “The NMB, NME, MFB, and MFE…” should be “The NMB, NME, IOA, MFB, and MFE…”.

3) In 3.2, the process analysis technique was used, but it was not introduced in methodology. It is better to briefly describe it in section 2.

4) Lines 337-343: The authors should demonstrate that the conclusions (about the impact of UHI on inorganic aerosol in the lower free troposphere) are universal rather than just in a specific case. Although other 7 UHI cases in the 10-day simulation period were mentioned in the last paragraph, the discussion was not clear. More details about the 7 UHI cases should be provided in section 3.

5) Please check the English and avoid the typo errors.