Reply on RC2

This manuscript provides a comprehensive, methodological analysis of the individual and combined effects of COVID-related emission changes and meteorological variability on air quality over Central Europe during the core period of the COVID-19 lockdown in 2020. The study design is thoughtful and sound. The results provide a valuable contribution to the rapidly-growing set of studies investigating this topic, especially by highlighting the complex interactions between meteorology and emissions for key pollutants and cautioning against attributing observed concentration changes directly to changes in emissions without performing an in-depth analysis of potential confounding factors.

The introduction section could potentially be shortened by either eliminating or reducing the summary of results from previous studies.

It was shortened
My only major comment is to consider adding analysis for modelled PM2.5 species to provide additional context on how changes in total PM2.5 are driven by how different processes (emission changes vs. meteorology) affect individual PM2.5 components (e.g. primary vs. secondary,inorganic vs. organic).
We added results for ammonium, nitrate, and sulphate in the appendix and we briefly discuss this in the context of interactions between these inorganic aerosol components when precursor emissions (here NOx emissions) are strongly reduced. We refrained from discussing results for EC and OC in order to not further extend the paper that is already quite long.

Specific comments:
Page 1, line 22: remove comma after "both" done Page 2, line 46: suggest moving "also" after "weather conditions" done Page 2, line 48: To my knowledge, Goldberg et al. (2020) is a notable exception to this statement and might be cited here: Goldberg, D. L., Anenberg, S. C., Griffin, D., McLinden, C. A., Lu, Z., & Streets, D. G. (2020). Disentangling the impact of the COVID-19 lockdowns on urban NO2 from natural variability. Geophysical Research Letters, 47, e2020GL089269. https://doi.org/10.1029/2020GL089269 The statement does not imply that there are no studies about the interaction of the meteorological situation and the lockdown emission reduction, but that they care rare. We included the publication by Goldberg et al. later in the introduction when we describe previous studies.
Page 2, lines 49 -54: This section seems to summarize results obtained later in the paper without explicitly saying so, but without providing any separate reference, either. I suggest either providing a reference or removing it from this portion of the manuscript.

We shortened this paragraph and included a statement about meteorological influences on photochemistry in the remaining text.
Page 4, line 126: were the COVID-19 lockdown effects considered in the IFS-CAMS fields used as boundary conditions? If not, does this introduce an additional level of uncertainty into the analysis, especially as it relates to the role of meteorology and longer-range air mass transport? IFS-CAMS fields do not consider lockdown effects. We added a sentence about this on page 4. Consequently, effects of lockdown measures outside Europe, e.g. in North America and Africa, on intercontinental transport are not considered in our simulations. However, the simulations consider emissions changes in entire Europe, while the evaluation is performed for Central Europe only. This setup already consider medium range transport inside Europe and reduces effects of intercontinental pollutant transport. In addition, intercontinental transport will not play a major role during the major lockdown period because the Großwetterlage with a blocking high pressure system in Central Europe did not favour this. In conclusion, we believe that the uncertainties caused by neglecting lockdown measures outside Europe are much lower compared to the inherent model uncertainties, which are now given in section 4.3 (former section 6.1).
Page 5, lines 145-146: suggest moving "best" from the end of the sentence to before "reproduces" done Page 5, line 169: can you please provide a reference for the NMVOC split profiles used inthis analysis?
The data was provided by Jeroen Kuenen from TNO in a personal communication. There is currently no reference for the data available, which is why we cited it as "personal communication" and added an acknowledgement to Jeroen Kuenen.
Page 6, line 187: add comma after "time series data" done Page 6, What was the rationale for not assuming any changes in shipping emissions between 2016 and 2020? This is based on data for the Baltic Sea published by HELCOM and the Finnish Transport and Communications Agency ( see https://portal.helcom.fi/meetings/MARITIME%2020-2020-787/Documents/Pre sentation%204_Ship%20emissions%20in%20the%20Baltic%20Sea%20area% 202006%20-%202019.pdf) that shows a stable or even decreasing shipping emissions in the Baltic Sea when only IMO registered ships (i.e. bigger ships) are considered. We conclude from this that also in the North Sea shipping emissions will most likely not show significant changes between 2016 and 2020.
In order to keep the description of the basic emission construction for 2020 concise, we do not explain this further in subsection 3.1 Page 7, lines 224 -228: You may want to state upfront that this approach cannot distinguish between passenger cars and trucks which likely had very different activity changes resulting from the lockdown. This limitation is discussed in Section 6.2 but in my opinion should be mentioned here.
We added that vehicle types cannot be distinguished.
Page 7, line 237: most readers likely aren't familiar with the term RoRo for certain types of ferries, please define or spell out.