Clara M. Nussbaumer et al. presented the NOx fluxes from airborne measurements in Los Angeles during the aircraft campaign RECAP-CA. They showed both NOx concentrations and fluxes were higher in the weekdays and lower in the weekend. They also showed the difference between their calculated NOx fluxed and NOx emissions from the CARB inventory. The observations are valuable and very useful to the emission community. The paper is generally well rewritten. However, I still have some minor concerns before it can be published. 

line 94: please add what is NOy, what species are included in the NOy ?

line 160-162: it was mentioned that the values of the NOx flux are dominated by the atmospheric variability. Can you explain a little more about it?

line 174 you are using the boundary layer height, where did you get the boundary layer height? Is it measured or modeled boundary layer height? What is the uncertainty of the boundary layer height?

Line 177 ‘the fit’ is the the linear fit of Fz and z/zi. Please mention it here.

Line 187. Please make it more clear what is the sensitivity study. Can you also provide a figure of the vertical divergence versus the dimensionless which excluding data points within the upper 20 % of the boundary layer? I get very confused by looking at Figure s4 and Figure s5. It would be nice if you also use different colors to indicate the density of the points.

Line 195-199: the footprint calculation is dependent on many variables and using the KL04 model. What are the meteo input for the model? Do you use measured data or data from meteo models?

Figure 4. What do the black lines indicate in figure 4? Are they the flight paths? Please mention it in the caption.

Line 300, what are ‘area emissions’?

Line 310-313, I don’t understand this part. The airport emissions were not captured by the measurements. How about comparing your NOx fluxes with the CARB emissions excluding aircraft emissions?

Line 325-330: In Figure 6 and Figure s10, NOx fluxes are quite different. Which one shows better results? Is it necessary to include the correction of vertical divergence in the flux calculation? Why are the emissions enhanced over Downtown Los Angeles and the inland highways in San Bernardino, but lower in the coastal region and Santa Ana? Please Add more discussion about the influence of vertical divergence.

Line 332 change ‘in-situ’ to ‘airborne’

Section 4: The conclusion section is only a short summary of the results. Please also indicate the implication of the study. What can we learn from the difference between the estimated NOx fluxes and the CARB inventory? What is your conclusion after investigating the influence of vertical divergence.  Also discuss the limitation of the study and recommendations for future study.