Ryan et al. provide an interesting and thorough report of the deployment of a MAX-DOAS instrument in central London and use its measurements to explore impact of the 2022 heatwaves on NO2 and HCHO concentrations, and subsequent effects on O3 production. The measurements are also compared with TROPOMI overpasses when available and highlight the complementary nature of the new measurements. The O3 production section is of wider interest in terms of urban air quality policy, as the authors note, with reference to the UK’s goals of reducing NO2 emissions.



The paper is well written and formatted, and subject to some minor comments, should be accepted for publication in ACP.



Minor comments:

• ca. figure 8 / lines 380 – the ratio between in-situ isoprene and HCHO appears to dramatically change during the 18^th July. It would be interesting to have some discussion as the why this is, especially as the increase in HCHO happens before the increase in isoprene. I suspect it has something to do with the location of Marylebone Road relative to the MAX-DOAS view.
• The lowest MAX-DOAS layer is described as ~55m, but the instrument is located at 60m above ground, and all elevation angles are listed as positive inclinations. Is this correct? Adding some information to section 2.1 for clarification would be useful.

Technical Comments:

• Figure 1 a – the labels collide with the location pins in some cases – adjust spacing to fix.
• The “DOFs/DOFS” initialism’s “s” character is inconsistently capitalised throughout the manuscript.  

Ryan et al. presented a valuable report about the ozone pollution episodes during 2022 heatwaves in Central London via the MAX-DOAS measurements. NO2 and HCHO VCDs of TROPOMI were firstly validated by ground-based MAX-DOAS. In addition, lowest layer retrieved NO2 and HCHO from MAX-DOAS were compared with in-situ NO2 and isoprene, respectively. Regarding the daytime ozone production, VOCs-limited regime is identified for non-heatwave days according to the MAX-DOAS HCHO-to-NO2 tropospheric vertical column ratios. Temperature favors the biogenic isoprene emissions and further the increase of ozone concentrations exceeding the regulatory standard. Influenced heavily by traffic emission, the compliance status may be changed under the conditions that stricter controls on NOx vehicle emissions and frequenter and severer heatwave. Overall, the paper is well organized and written, however, there still some comments need to be addressed before it can be considered to be accepted for ACP journal.

Main concerns:

Since the vertical profiles of NO2 and HCHO can be obtained by the RAPSODI algorithm, why only the column density and lowest layer results were used to discuss in the paper? I would like to suggest the authors present the characteristics of the vertical pattern of NO2 and HCHO during heatwave days and non-heatwave days at least.

Moreover, considering the air mass transport described in Line 48-49, the HCHO-to-NO2 ratio may also be analyzed in different heights.

Minors:

1. Line 45-47, It’s better to also provide background information about the VOCs emission in Central London, when discuss the ozone production there.

2. Line 115-120, in addition to the DLs of individual DSCDs, the authors should provide a more detailed table for the spectral analysis configurations. Besides, the performance of the spectral analysis should be evaluated, such as the range of RMS? DSCDs errors? An example plot of spectral fitting? And any filtering of the DSCDs before be introduced into the profile retrieval?

3. Fig. 4, why datasets of elevation 20° not be presented?

4. Fig. 6, MAX-DOAS HCHO:NO2 is VCD to VCD or lowest layer to lower layer? Same comments on Fig. 8.

5. Fig. 8 and related discussion, the dependency of isoprene-to-HCHO ratio to NO2 need to be investigated, also isoprene-to-NO2 ratio.