While the authors have acknowledged R2’s comments, the new manuscript has not addressed them.

Comment 1 (background NOx averaging): R2 has requested more information to assess the analysis of background NOx. R2 asked that evidence be given to whether the time averaging adequately captures background NOx. The response that no other studies exist with which to compare long-term background NOx studies is not sufficient. The referenced short-term studies are not useful as these report tens of ppb NOx, so they must be referring to something other than background NOx. Figure 2 shows evidence that local effects are important in PC2, but could this indicate an issue with the MDA8 NOx metric generally? Have the authors looked at Ox (= NO2 + O3), this would remove the variability due to O3 and NOx changes due to storage of O3 as NO2. 

Comment 2 (background NOx measurements): This comment has not been addressed. If NOx is averaged over the same time period as MDA8 then it is most susceptible to interferences by higher nitrogen oxides, as the MDA8 typically centers over the hours of active photochemistry. R2 is asking whether the trend in NOx is in part driven by chemiluminescence instruments having different sensitivity to organic nitrates than to HNO3. Under high-NOx chemistry, HNO3 is the dominant NOx termination product, but at intermediate and low NOx, organic nitrate production is more important. If the chemiluminescence instruments are more sensitive to one type of nitrate over another, the trend in observed NOx may be impacted. The authors should present evidence to address the concern raised by R2. The relevance of the discussion of the HNO3 lifetime is not clear to me.

Comment 3 (background NOx is large): More should be done here. While the aircraft studies are carried out over a short time period, they are still informative. Can the results be compared over the same time window? Second, due to concerns over interferences by higher oxides, high background NOx deserves special attention. Ozone is produced throughout the boundary layer, if the background NOx estimate is influenced by very-local sources, then it is not representative of background NOx, including vertically.

Comment 4 (MDA8 O3): I agree with the referee that MDA8 use needs context. The fraction of O3 on all days attributed to the background is not important, what is important is the contribution to MDA8 on exceedance days. I do not understand why the authors have chosen the MDA8 metric but are not concerned with compliance/exceedances. As acknowledged in the NOx discussion, the MDA8 may not be an ideal averaging window for assessment of background NOx concentrations. If the analysis aims to assess trends in background O3 and NOx outside of the regulatory context, then perhaps there is a better metric.