Comment on acp-2021-282

The manuscript describes airborne data from the Zeppelin aerosol measurements during the Pegosos Campaigns in San Pietro Capofiume in 2012 and in Hyytiälä in 2013. The results are highly interesting and could be a useful data set to trace back the appearance and origin of nucleation mode particles in the atmosphere or, to characterize air masses that contain precursor material, as proposed in the abstract. In both cases, in SPC and in HTL the data on the scale of the spatial distribution suggest a significant contribution of horizontal transport on top of the diurnal cycle of vertical convection. An interesting new result is the signature of sulphuric acid found above the MBL before convection mixed a larger MBL volume. Getting into more detail a chemistry transport model would be useful, not included in the current study.

The manuscript describes airborne data from the Zeppelin aerosol measurements during the Pegosos Campaigns in San Pietro Capofiume in 2012 and in Hyytiälä in 2013. The results are highly interesting and could be a useful data set to trace back the appearance and origin of nucleation mode particles in the atmosphere or, to characterize air masses that contain precursor material, as proposed in the abstract. In both cases, in SPC and in HTL the data on the scale of the spatial distribution suggest a significant contribution of horizontal transport on top of the diurnal cycle of vertical convection. An interesting new result is the signature of sulphuric acid found above the MBL before convection mixed a larger MBL volume. Getting into more detail a chemistry transport model would be useful, not included in the current study.
A further well notified result is, that despite transport over major agricultural areas in the center of the eastern Po-Valley, west of Ferrara enhanced nucleation aerosol was not observed. This is a bit surprising after to the results of Kontkanen et al, 2016, who found NPF events each day with only one exception, a paper suggesting that NPF is more a general feature in the Po-Valley.
However, focusing on the current manuscript, the title promises new results on the 'onset of new particle formation in the planetary boundary layer usig an airborne platform (Zeppelin)' For such an investigation a precise time and location of the data points would be necessary. Unfortunately these data are not presented and on top there are several obvious timing problems clearly visible in the figures: Fig. 2, Trajectories were calculated for 12:00 UTC (see HYSLIT info in the plots, not for the morning as claimed in the figure caption). Already 2 hours difference might be critical for the wind direction and trajectory. Also, for a process occurring in a diurnal cycle with short lived compounds a trajectory for 72 h does not make a lot of sense.  Fig. 9 do not support the conclusions in the text. Low wind situation with a good chance to get into self-contamination problems in circle flights. The time spent for half a circle (20 m/sec and ~4 km diameter) is close to air mass transport time across circle. Timing requirements for such a local case study are even more stringent than just using the correct time zone. Even the internal timing of the NAIS or SMPS scanning loop becomes important . Is the observation of 3-10 nm particles in agreement with the growth rate and diurnal pattern of > 10 nm particles at HTL in Fig. 3?

Summary:
A three dimensional airborne study as presented here on a time critical process, 'the onset', requires a clear temporal and geographical identification of data points and a timely correct trajectory analysis. This is unfortunately missing in this manuscript, or, where it is at least partially available, often and obviously false due to incorrect time settings.
Finally the abstract claims: 'In Po Valley we observed NPF that was limited to a specific air mass', however, the air mass is neither specified nor characterized in the text. Air mass origin and composition in the text is speculative.
The reference list is incomplete. Citations are missing for: Dada et al, 2017, Dunne et al., 2016Gordon et al., 2017;Yu and Luo, 2009, Mohr et al, 2019, Pierce and Adams, 2009Stratmann et al, 2003, Junkermann andHacker, 2018;Sullivan et al, 2016;Vogel and Elbern, 2021 However, there are a few more issues that came up reading the manuscript not addressed but likely worth an investigation with a high resolution chemistry transport model: Interestingly, NPF is linked to convection rather than to air chemistry, how can this be explained? Convection starts with global radiation, air chemistry with UV radiation, respectively photolysis, about 1.5 h later and subsequently it should also take some time for the fresh clusters to grow into measurable particles (Kulmala et al, 2013).
What is the origin of the sulfuric acid (SA) at a time of the day (early morning) when there is not yet sufficient UV radiation to produce OH radicals for sulphur dioxide conversion and why is SA observed first in the 'clean' residual layer? What is the corresponding chemistry? The maximum solar radiation flux for June 30 is only ~ 10% different for SPC/Bologna (896) and Monte Cimone (976). Under conditions with more SO2 pollution, in the boundary layer the formation of sulphuric acid (SA) thus should be more intense.
Why are nanoparticles not observed in the north-western loop, north of the fiume, despite an advection of an air mass passing over the areas with intense agricultural activities and more intense ammonia emissions (see the Italian emission inventory, Taurino et al, 2020). Contrary, nanoparticles were observed the same day June 30 in air masses travelling over forested mountains southwest of Bologna. A contribution of the city of Bologna is unlikely based on HYSLIT trajectories for the day as well as a contribution of the Venice area as speculated in the text.