The authors present a valuable dataset, with two years of MAX-DOAS measurements in Southern California. They compare these measurements with TROPOMI data and examine the diurnal variation of lower troposphere vertical column densities (VCD) and surface mole fraction for NO₂ and HCHO. They further combined the dataset with ozone measurements from Pico Rivera to understand the ozone sensitivity in this region. They find that NO₂ and HCHO have different vertical distributions throughout the day, which significantly impact the derived HCHO to NO2 (FNR) ratio, whether derived from surface values or total columns. This ratio has been used for identifying the threshold values that indicate a shift between NOx-limited and VOC-limited ozone production regimes. The finding has important implications on how to interpret satellite-derived FNR in the context of ozone sensitivity. The dataset is interesting, but I find that the analysis can be further improved. Here a few comments:

1. There is very little discussion about the details of comparison between MAX-DOAS and TROPOMI. Was there any quality control on TROPOMI level 2 data? Did you filter out any data that is considered low quality?

Would it be possible that the difference between MAX-DOAS and TROPOMI NO2/HCHO VCD (Figures 2 and 3) can be due to the difference in vertical sensitivity and a priori profiles ? This has been discussed in several studies (Dimitropoulou et al., 2020; Verhoelst et al., 2021)? Could the authors examine these discrepancies on both NO₂ and HCHO, possibly by recalculating TROPOMI values using mean MAX-DOAS vertical profiles? While I do not suggest extensive additional work, this step would improve our understanding of the observed differences between MAX-DOAS and TROPOMI results.

2. Are there any other ground-based in situ or remote-sensing measurements of HCHO and NO2 available in this region? If so, do they support the MAX-DOAS data presented here? It would strengthen the study if the MAX-DOAS results were compared with other data sources in the area.

3. This work has been comparing with Jin et al. (2020), which is a study based on satellite data and noontime ozone. Note that ozone production often peaks around noontime, which coincides with satellite overpass time. It is unclear if the authors are focusing on noontime observations for their ozone sensitivity analysis. I recommend focusing on the surface and column FNR comparison specifically at noontime, as done in Jin et al. (2020).

4. In Figure 5, the authors show that surface or LT-VCD derived threshold FNR are lower than the satellite derived values, due to a large fraction of HCHO aloft. This is likely due to secondary production of HCHO that takes place aloft while NO2 is from primary emissions, leading to different vertical distribution. It would be useful to have some discussion on the possible reasons for different vertical distributions between HCHO and NO, and potential impact on ozone production.

5. Figure 4. Can you add ozone diurnal profile here to Figure 4?

Technical comments:

6. Line 136 Kim 2016 wrong citation.

7. Please define FNR in its first appearance.

References

Dimitropoulou, E., Hendrick, F., Pinardi, G., Friedrich, M. M., Merlaud, A., Tack, F., De Longueville, H., Fayt, C., Hermans, C., Laffineur, Q., Fierens, F., and Van Roozendael, M.: Validation of TROPOMI tropospheric NO₂ columns using dual-scan multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements in Uccle, Brussels, Atmospheric Meas. Tech., 13, 5165–5191, https://doi.org/10.5194/amt-13-5165-2020, 2020.

Verhoelst, T., Compernolle, S., Pinardi, G., Lambert, J.-C., Eskes, H. J., Eichmann, K.-U., Fjæraa, A. M., Granville, J., Niemeijer, S., Cede, A., Tiefengraber, M., Hendrick, F., Pazmiño, A., Bais, A., Bazureau, A., Boersma, K. F., Bognar, K., Dehn, A., Donner, S., Elokhov, A., Gebetsberger, M., Goutail, F., Grutter de la Mora, M., Gruzdev, A., Gratsea, M., Hansen, G. H., Irie, H., Jepsen, N., Kanaya, Y., Karagkiozidis, D., Kivi, R., Kreher, K., Levelt, P. F., Liu, C., Müller, M., Navarro Comas, M., Piters, A. J. M., Pommereau, J.-P., Portafaix, T., Prados-Roman, C., Puentedura, O., Querel, R., Remmers, J., Richter, A., Rimmer, J., Rivera Cárdenas, C., Saavedra de Miguel, L., Sinyakov, V. P., Stremme, W., Strong, K., Van Roozendael, M., Veefkind, J. P., Wagner, T., Wittrock, F., Yela González, M., and Zehner, C.: Ground-based validation of the Copernicus Sentinel-5P TROPOMI NO₂ measurements with the NDACC ZSL-DOAS, MAX-DOAS and Pandonia global networks, Atmospheric Meas. Tech., 14, 481–510, https://doi.org/10.5194/amt-14-481-2021, 2021.

This measurement report provides an overview of MAX-DOAS measurements performed from Whittier College over the course of ~2 years. Measurements of NO₂ and HCHO from MAX-DOAS were analyzed and compared to TROPOMI retrievals. Diurnal variations of vertical column densities were also analyzed and demonstrated similar patterns of variability observed in previous studies. Uniquely, using a combination of surface and MAX-DOAS column measurements, the ratios of HCHO to NO2 (FNR) were analyzed to better understand the ozone production regimes in the eastern LA area. FNR values were found to be lower over the Whittier College site compared to downtown Los Angeles. The findings highlight the need to carefully distinguish near surface chemistry from above surface chemistry when using FNRs to better disentangle ozone chemistry.

Line 85: The authors reference “high spatial resolution grid” when talking about retrieving vertical profiles. I think the authors meant to say “high vertical resolution grid”. Please clarify.

Line 104: Check for switches between present and past tense throughout the manuscript.

 Line 120-125: It would be useful to briefly mention the possible reasons why TROPOMI would be higher during the study period and besides the possible long range transport of wildfire plumes. Have previous studies, whether in agreement or contrast, highlighted any biases in the TROPOMI HCHO apriori profile in urban areas?