Comment on acp-2020-1202

The manuscript presents evidence on how important strong pollution sources emittting SO2 can influence or initiate nucleation events downwind of the sources in remote areas. The study is mostly based on case studies. One case study is presented in detail in the main manuscript. Three other case studies are presented in the supplemental part whereof in one case measured acids and ion clusters are missing, in a second one measured ion clusters are missing and in a third one measured ion clusters are missing and other explanations are listed for the respective NPF event.

The study delivers interesting results on pollution induced new particle formation, but some more details especially on statistics with respect to the whole measurement period are strongly recommended to include before final publication. The manuscript misses a detailed overview to put the case studies in the context of the full measurement period which was a couple of month in the winter period. An overview of all nuclation events, with regard to levels of SO2 concentration, sulfuric acid concentration (measured and modelled), wind direction, available UV radiation, quality of event (number of clusters and further growth, etc.) would place the events in a context which is needed the evaluate the abundance of natural and anthropogenic nucleation events during the measurement period. Right now the manusctipt delivers case studies within a larger measurement period and it is difficult to follow the significancee of the findings. What kind of types of NPF was observed in March? Were there periods when conditions for NPF were favourable based on pollutant emissions, but NPF did not take place? In addition, the paragraph on growth of observed particles and their contribution to CCN remains a bit vague and needs to be elaborated on. Figure S1 shall be included in the main manuscript.

Introduction
Page 1, line 25-26: Comment: I would say that SO2 contributes to acidification of … by atmospheric aerosol and cloud formation … (check the sense of the sentence). Comment: 300 km is not so close in terms of distance between the observations and the emissions. I recommend to write the disctance explicitely here and also at other places in the manuscript (see abstract). 300 km gives some time for transport and corresponding processing! 2 Methods

2.1
Site and time of the study -

Instrumentation
Pag4, line 103: The part of the DMPS for ultrafine particle detection malfunctioned … Pag4, line 109-114: The paragraph is misunderstanding, write exactly when this instrument was used and since when with the switcher, etc.

Nucleation rate calculation
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Sulfuric acid proxy calculation
This whole section leaves some questions. Because of missing data, the authors make a number of assumptions on the sulfuric acid concentration. Sulfuric acid is calculated using SO2 oxidation by OH which is proxied by global radiation; Criegee Intermediates proxied by monoterpene and ozone concentrations, condensation on pre-existing aerosol, assumption on monoterpenes, and global radiation assumption was used. This needs to be verified and some general evaluation of this method using the campaign dataset should be discussed.

Trajectory analysis
-3 Results and Discussion 3.1 New particle formation during the measurement period General comment: As the study is mainly based on case studies, an overview table of these studies is needed stating the differences and similarities of the different events. The descrption here is otherwise confusing to evaluate which events follow certain patterns and which do not. What about situations where NPF would be expected to happen based on the general conditions, but it did not?
Page 6, line 170 -179: Comment: Please redo the figure, it is not possible to see specific days because of the overall scaling of the time axis. It might be an idea to add boxes where events have taken place and label these boxes with the respective event dates.

Meteorological situation and trace gas concentrations
Page 7, line 204-205: Comment: It must be possible with Hysplit to calculate boundary layer height aalong the trajectory. This is very useful to see that air masses even at low altitudes were not above the boundary layer height and air was not mixed in from above.

Introduction
Page 2