Nussbaumer et al use observations from three campaigns across Europe (CYPHEX in Cyprus, HOPE in southern Germany, and HUMPAA in Finland) to calculate production of HCHO and O3.  They find that, across all locations, HCHO production can be closely approximated with only production from methane, acetaldehyde, isoprene, and methanol.  They also find that the ozone production regime varies by location.  This is an interesting paper that is suitable for publication in ACP once the minor issues below are addressed.

Line 37: You could also cite Fried et al, 2011 who found similar results about methane being the dominant contributor to HCHO production in the remote atmosphere.

Fried, A., et al. (2011). "Detailed comparisons of airborne formaldehyde measurements with box models during the 2006 INTEX-B and MILAGRO campaigns: potential evidence for significant impacts of unmeasured and multi-generation volatile organic carbon compounds." Atmospheric Chemistry and Physics 11(22): 11867-11894.

Line 108: I agree that, since the isoprene yield is bounded, this estimate doesn’t have a large impact on the overall message of the paper, but for the case of CYPHEX, this range still gives a factor of 2 difference in the potential values of HCHO production from isoprene.  I think it’s appropriate to either add more discussion to justify your assumption that [HO2] = [RO2] (box modeling studies from similar environments?) or use the data you have to try and estimate the ratio. An alternative method of calculating P(O₃) is from k_NO+HO2[NO][HO₂] + k_NO+RO2[NO][RO₂].  If you equate production from this method with that from the JNO2 method, you could potentially estimate RO2 that way.  For HUMPPA, at least, there also appears to be box modeling results by one of the co-authors that could be used to evaluate this assumption.

Figure 1: The light grey labels on the blue background are very difficult to read when you print this out.  I would recommend changing the font color.

Section 2.3.1: What time resolution are you using for your P(HCHO) and P(O3) calculations?  Do you average everything to the photolysis measurement frequency (10 mins?) or something else?

Line 202: There’s an extra ß in Hohenpeißenberg.

Line 211: You need more discussion about how you arrived at the boundary layer heights.  At the very least, summarize what was outlined in Fischer et al.

Line 234: Similar to the previous comment, how do you come up with the 20% uncertainty for the boundary layer height, if you don’t have observations?

Line 272: Here and elsewhere, when discussing diurnal profiles, it’s much more intuitive to use local time instead of UTC, especially since you are referencing multiple sites that have different UTC offsets.

Figure 5: Is this in local or UTC?  Please label.  Also, for panel a, there is a lot of overplotting.  Could you redo the figure where production rates from the individual species are stacked on top of each other to prevent the over plotting?

Figure 6: I assume this is for all data averaged together?  Does this include both daytime and nighttime values?  Please indicate.

Line 302: Where do you get the 20% value from to vary the yields?

Line 311: You attribute the nighttime increase in HCHO to local emissions from traffic.  Looking at the supplementary figure, it looks like on the night of July 27^th, the HCHO concentration nearly doubles.  Even with a low boundary layer, that seems too large to be just mobile emissions.  Have you looked at ΔHCHO/ΔCO or ΔHCHO/ΔNOx to see if this is in line with what you would expect from traffic emissions?  Couldn’t this also just be a new air mass moving in or advection from a different region?  Do you have meteorological observations you could use to look at this?

Line 315: I think you need a little more discussion about the deposition.  Instead of there being no deposition, couldn’t it just be that the deposition rate is lower than the rate of increase from whatever processes is leading to the increased nighttime HCHO, whether it’s advection or direct emission?

Line 402: Do you have thoughts as to why the contribution from acetaldehyde photolysis was so much greater at HOPE vs CYPHEX?  Is it from larger acetaldehyde concentrations or just a result of higher photolysis related to the higher altitude?

Line 457: Duncan et al use ratios of tropospheric column HCHO/NO₂ to estimate the ozone production regime.  While they do show that this is similar for PBL values, here you are just using surface observations.  Please give more justification on how the values you use to estimate the ozone production regime could be affected by this difference.  Do you have any evidence that the PBL is well-mixed enough to make this assumption?  Also, as you cite in your paper, Schroeder et al have pointed out that the values cited in Duncan vary by location.  Do your results change at all when taking this into consideration?

The manuscript by Nussbaumer et al. provides the first HCHO budget calculations across Europe using in situ measurements as opposed to evaluating the HCHO budget using model simulations. Additionally, the authors show that HCHO production is dominated primarily by the oxidation of methane, methanol, acetaldehyde, and isoprene during three campaigns (CYPHEX, HOPE, and HUMPPA) representing a coastal, mountain, and forested site, respectively. The HCHO yield from isoprene and fraction of methyl peroxy radicals (CH₃O₂) forming HCHO were also shown as an alternative method for determining whether a location is NO_x or VOC-limited (or in some transition regime).

General comments: While the oxidation of methane, methanol, acetaldehyde, and isoprene are the dominant VOC precursors to HCHO production, care should be taken throughout the manuscript to never imply that only these four chemical species make up the entirety of the HCHO budget. HCHO is commonly used as a VOC tracer and comes from more than simply those four species as the authors showed with the pie chart in Figure 6. As an example, Line 75 is misleading since it states that "HCHO production can be accounted for by the oxidation of methane, methanol, acetaldehyde, and isoprene" which implies 100%. Rather, it should read "...predominantly accounted for..." or some other conditional phrasing.

The manuscript fits well within the scope of ACP and provides a good, detailed analysis of uncertainty. I recommend publication after attention to the previous general comment and the following specific comments/technical corrections.

Specific Comments:

- For the reader, explicitly define somewhere in the text what is meant by "atmospheric variability" (i.e., what factors control this uncertainty)

- Whenever mentioning the detection limit of an instrument (for instance, in Section 2.3.1), the integration time necessary to achieve that detection limit should be mentioned.

- Figure 1: There is space above (or below) the HCHO precursors to write the actual chemical name for each of the chemical formulas (i.e., acetone, MHP, etc.)

- Line 197: The determination of the acetaldehyde and formaldehyde photolysis frequencies are from a parameterization using IUPAC quantum yield data and measurements of j(NO₂) and j(O¹D). This parameterization should be explicitly shown in the SI for the reader.

- Figure 4: At least for the example shown, it is odd that points were selected when the HCHO mixing ratio was still increasing and were included in the fit. In this particular case, the slope would be underestimated and the deposition velocity would be biased. What motivated the decision to always select points between 21:00 - 01:30 UTC as opposed to looking at the underlying nighttime HCHO mixing ratio data over several hours?

- Lines 395-396: Please create pie charts for HOPE and HUMPPA (as done for CYPHEX) since this readily shows the contributions of the four dominant precursors as well as the other reactions included in your chemical mechanism. Could either place resulting figure in main text or SI.

Line 475: Explicitly state that specialized instrumentation is still required (particularly for OH and HO₂) for these alternative methods of determining the chemical regime.

Technical Corrections:

Throughout text: Formatting for d[HCHO]/dt is inconsistent (for example, line 260 and 275)

Line 404: Misspelling of acetaldehyde

Figure S7A: Please clarify whether the data is from CYPHEX or HUMPPA

Figures: Font size on axes should be increased since the axes are hard to read when printed