General comments

This study presents a new version of the regional urban road network air quality modelling system IAQMS-street v2.0 to simulate urban background and road network pollution. The manuscript is generally well organised, at this stage the reviewer has a positive attitude towards its publication. However, there are still some points regarding the numerical conditions that should be further explained and modified. The detailed comments of the reviewer are as follows

Specific comments

Line 163 “The fixed time step for interfacing between NAQPMS and MUNICH was 20 min, i.e., the same as that of the regional model.” Is the time step 20 min also for MUNICH? As this study focuses on pollutant diffusion and chemical reaction at the street scale (100 m). The reviewer thinks that 20 min is too long. For example, the previous study on MUNICH with chemical reactions used a time step of 100 s. https://doi.org/10.5194/gmd-15-7371-2022

Please justify that the simulation results based on 20 min can achieve similar simulation accuracy with a smaller time step (e.g. 5 min). In addition, a sensitivity test should also be carried out on the time step for the interface between NAQPMS and MUNICH (e.g. 5 min).

Figure 1. How high is the bottom layer in this study? Please provide a reasonable explanation that the simulation results are not dependent on the height of the bottom layer.

Figure 7. Why is the number of NO observations less than NO2 and O3? Also, it seems that only the daily averaged data are shown. The hourly averaged data should be shown as the prediction accuracy of the peak concentration is important.

Technical Corrections

IAQMS-street v2.0 could use more CPU time than IAQMS-street v1.0. It is helpful for the potential user of IAQMS-street v2.0 to know the detailed CPU time in this study for different scenarios.

In this manuscript, the authors designed a two-way coupled regional-urban–street network air quality model system, and applied and evaluated it in a megacity, Beijing, China. The topic is of great interesting to recognize the complex interactions of air pollutants between larger different scales in spatial dimension inducing by emissions, mass transform andchemisty among the scales. The manuscripts is generally well organized and the analysis is mostly sound. But some details need modify and some ambiguous presentation need clarify. I recommend a minor revision and my comments listed below.

Specific comments:

1. The title should include air quality, indicating you designed a two-way coupled air quality model system.
2. In line 11 of the abstract, “gap” could be better replaced by “different” or other word implicating the large different between the concentration in reginal and street scales.
3. In line 27 and in the context, cannot say “O3 emissions”.
4. In line 121, dc-->dC;
5. In line 135, definition γ as mass transfer efficiency may better than mass flux.
6. In line 150, “ in Eq. (3),” could be in Eq. (4)?
7. In figure 3b, I found a truck, but the "truck up" and "truck down" were both zero. Were the statistics (in red) right?
8. In figure 7, we can find the simulated NO and NO2 in regional (NAQPMS) higher than that in network simulations. So，what’s the means of the presentation in abstract “the concentration of NOx at street scale is higher than that at the regional scale,”?