General Comments

This study examines an interesting dust event that originated from the deserts in central Asia, most probably from the dried playas of the desiccated Aral Sea (Aralkum desert) and after following an unusual westwards movement affected the northern part of Italy 3-4 days later. This is an interesting dust episode affecting Europe, mostly because of the trajectory of the dust plume, since dust storms originated from Central Asia usually affect the eastern part of Iran, Afghanistan and/or they are transporting eastwards affecting Pamirs, Tian-Shan mountains and western China (Li et al., 2019, AtmosRes; https://doi.org/10.1016/j.atmosres.2019.06.013), Zhang et al. (2020; https://doi.org/10.1016/j.atmosenv.2020.117734) and references therein. Therefore, authors should give more emphasis on the uniqueness of this event. However, the dust source area for this event is not shown in the current figures and authors should provide at least satellite imagery for this dust event over Central Asia, at least for its genesis and transport. Another important issue is the comprehensive examination of the meteorological patterns that triggered and transported towards the west this dust event. The synoptic meteorology maps should definitely cover the central Asia and authors should explore the role of the Caspian Sea High (and the Siberian anticyclone) on this dust event. Many previous works have shown that the Caspian Sea high plays an important role in dust activity and transport of dust plumes over central and southwest Asia (i.e. Kaskaoutis et al., 2016, Glob. Plan.Change; https://doi.org/10.1016/j.gloplacha.2015.12.011), Labban et al. (2021; https://doi.org/10.1002/joc.6611), Hamidianpour et al. (2021; https://doi.org/10.1016/j.atmosres.2021.105711) and references therein. Furthermore, the Caspian Sea High in combination with the low over the North Sea, create the North Sea –Caspian Pattern (NCP) that significantly modulates climatic conditions over the Middle East and the Mediterranean, which may also affect the dust activity (you may see Brunetti and Kutiel, 2011; Nat. Hazards), Kutiel and Benaroch, (2002; Theor. Appl. Climat.). So, more emphasis should be given in satellite monitoring of the dust plume and transport and on the meteorological dynamics that carried the dust plume toward Balkans and Italy. On the other hand, the structure of the paper should be increased at several parts and some texts need revision. In the analytic comments below, I emphasize with specific comments to parts/lines of the paper that need revision and/or more discussion and clarification.

Specific comments

Lines 33-34: This cannot be a likely consequence from this dust storm, since the PM and pollution levels are not high. You may insist more on modulation of dust aerosol properties as a consequence of the mixing processes and not to health effects.

Lines 51-52: Here, you totally ignored the deserts in central Asia, China and Mongolia.

Line 65: Does it mean that these meridian pressure gradients are due to African Highs and south European lows? I do not think that this is right. For example, Sharav cyclones in north Africa are low-pressure systems moving across north Sahara. This sentence should be revised and better clarified if authors believe that discussing these dust mechanisms is required. 

Lines 67-68: This is mostly right, but apart from the regional scale mechanisms large-scale dynamics, like NAO variability, also affects the dust activity across the Mediterranean. You may see a recent paper by Kaskaoutis et al. (2019, Atmos.Res.) and several references therein about these phenomena, as well as about Sharav cyclones (e.g. Bou Karam et al., 2010, J.G.R.).

Line 84-86: For this statement, you may see the recent review study by Li et al. (2021; Geoscience Frontiers, 101180, https://doi.org/10.1016/j.gsf.2021.101180).

Section 2.1. The measured particle size range from the various OPC instruments should be mentioned in Section 2.1.

Line 131: According to satellite imagery (worldview website), the dust event was initially started over Aralkum on 24 March 2020. So, the beginning day cannot be considered as that when the dust plume affected Italy i.e. 27 March.

I would recommend to move the section 2.2 after the measurements description at the end of section 2.

The wind regime is the most important parameter in order to analyze the synoptic meteorology prevailed during the unusual dust-plume pathway. Therefore, vector winds should be superimposed in the already existed meteorological figures or should be plotted separately.

Section 2.4. What's the instrument for PM10 measurements?

Before the synoptic analysis (Section 3.1), authors should provide some satellite imagery for the dust emissions over Aralkum and the transport of a dust plume towards the Black Sea and the Balkans on the next days. By analyzing satellite imagery, I saw that the dust plume originated from Aralkum (mostly) on 24 March. These figures are absolutely necessary for somebody to follow the whole material. Unfortunately, there was extensive cloudiness over central-north Italy and western Balkans that obscured any dust plume towards Italy. This can be analysed via CAMS reanalysis (as authors did in Fig. 1).

Line 243: Why likely? Are you not sure about the source region of the examined dust event? Furthermore, the area that you described here is a vast arid-desert area with several active sources of dust like Karakum, Aralkum, Kyzylkum, Caspian depression, etc. You may see Shi et al. (2019, Atmos.Environ; doi: 10.1016/j.atmosenv.2019.117176) for more details.

Line 248: “with winter and spring as the driest seasons”. I do not think that this statement is right. previous works over the region show that winter and spring are the wettest seasons. You may see Li et al. (2019, Atmos.Res.; https://doi.org/10.1016/j.atmosres.2019.06.013) and Li et al. (2021; https://doi.org/10.1016/j.gsf.2021.101180).

Lines 248-249: You do not provide specific reasons for the analysis of the meteorological patterns here. The only reason is that you just selected the dusty period, and this is absolutely logic.

Lines 250-253: The synoptic maps should be redrawn, since they do not cover the dust source region, which is central Asia. There is coverage of the north Atlantic, but not of the source region. So, the reader cannot see the specific meteorological conditions that dominated over the area. Second, the PM10 maps correspond to CAMS reanalysis and are not satellite observations as mentioned here. 

In general, The synoptic analysis should be rewritten in view of the new synoptic maps and emphasis should be given also over central Asia. One of the most important meteorological factors that determines the climate, dust activity and atmospheric circulation over the region is the Siberian High and its expansion over the Caspian Sea in late spring and summer. You may see previous recommended works by Li et al. (2019, 2021; Labban et al., 2021, Intern.J. Climatology; Hamidianpour et al., 2021, Atmos.Res.). So, the position and intensity of the Siberian/Caspian Sea High is very important to be discussed here. Authors may also see the anomaly from the mean climatology of the MSLP.

For the AERONET analysis some few more discussions and references to previous works about modification of columnar aerosol properties during dust events in Italy should be added.

I believe that the last section 3.3.3 should be omitted and more analysis should be given in the previous ones. There are no obvious differences in the chemical composition of aerosols due to arrival or the dust plume. Especially, carbonaceous aerosols, sulfate, nitrate and other trace elements that are mostly related to anthropogenic pollution do not exhibit notable peaks that can be directly attributed to the dust plume. I would recommend to skip this section.

Line 481: There was not a change in dust optical properties. The aerosol optical properties were changed due to arrival of the dust plume.

Line 482: That's the first time that Aralkum is referred as the dust source region. This analysis should be performed before.

Line 482-487: This is a very long sentence which should be split into two or three.

Conclusions may be reconstructed giving more emphasis on the source region and specific meteorological conditions that carried dust from central Asia to north Italy.

Minor comments

Lines 18-21: You may also include the dates in the abstract.

Line 23: “plume” instead of “cloud”

Line 50: Revise as "dust sources" or "dust-source regions"

Line 84: replace “reminded” with "mentioned".

Line 100: Revise this sentence “during transport” does not make good sense.

Line 111: Revise as "was analyzed or investigated or studied.."

Lines 113-114: This sentence makes no sense. Something is missing. It should be rephrased.

Line 134: Reconstructed is not the appropriate term here. The sentence should be revised.

Line 139: Delete double use of "we have"

Line 156: 2.3 Heading. Revise as "particle"

Lines 162-165: This sentence needs revision.

Line 177: You should revise this sentence. The equation 1was not previously indicated.

Line 253: I think this is 25 March.

Line 342: “evolution” or “variation” instead of trend. You did not examine the trends here.

Lines 348-353: This sentence is very long and it should be revised and split into two. You may use Italian references for COVID-19 lockdown in Italy.

Line 389. "settling" instead of "selection".

Lines 391-392: Define the time frame in order to make it clear.

Line 420: Add "and" between the concentrations.

Figure 6 caption. In this figure there are two measurement sites in Bologna, not three.

Line 467, 476: You should complete these sentences.

This study presents an interesting dust event that originated from Central Asia and was evident in parts of Southern Europe, with a specific focus in Italy. The event is discussed mainly by means of particle number densities and particle size distributions, derived from optical particle counters located at three different sites, namely Bologna, Trieste and Mt. Cimone. While the paper is overall well written, some parts of it need revisions and the syntax could be improved.

Specific comments:

• Section 2.1: Since the instrumentation is not uniform for all three sampling sites, I would recommend summing up all the relevant information in a table.
• Section 2.2 would fit better in the end of Section 2.
• The authors should explain their choice of using the total, fine and coarse mode AOD at the 500 nm wavelength.
• Section 3.1 needs to be rewritten given more focus to the synoptic conditions above Central Asia. In addition, Fig. 1 needs to be revised and include the dust source region.
• Both sections 3.3.2 and 3.3.3 would benefit from a more in depth analysis.
• A.2(a): The backtrajectories ending at 1.7 km and 2.7 km altitude originate from the Sahara region. In line 330 the authors state that the air mass “it did not give origin to visible peaks in the OPC time-series”. Please elaborate more on that statement. Perhaps a peak would is visible for other fractions.

Minor comments:

• Please make sure that all acronyms have been defined properly in the manuscript (e.g., EUSAAR in line 129).
• Lines 345-346: A reference for the statement would be useful.
•  Figures 5 and 9 are difficult to read.

This paper provides a detailed investigation of a strong dust event over Italy during March 2020. The most interesting fact is that the origin of this dust event is located in central Asia, which is a rare source of dust for Europe. The synoptic conditions before and during the dust event are also discussed. The evolution of the event is monitored by in-situ measurements from optical particle counters (OPC) at three stations located in northern Italy. These measurements are complemented by back-trajectories analysis, air quality products from the Copernicus Atmosphere Monitoring Service (CAMS), AERONET AOD, and vertical profiles from a Lidar Ceilometer. Some information is also provided regarding the particles’ chemical composition. The paper is generally well written, however there are some points that must be clarified or better explained.

Line 25: "supported by AOD (Aerosol Optical Depth) maps"  You are not showing AOD maps in the manuscript. However, I think that the paper would greatly benefit by including a map of satellite derived AOD (such as from MODIS ) or reanalysis AOD data from MERRA-2.

Line 106 “aerosol vertical behavior” please rephrase to “vertical distribution”

Sect. 2.4 Please provide more information about the two air quality stations of the ARPAE Regional Environmental Protection Agency in Bologna

AERONET: Please mention if you used “all-points” or “daily averages” data.

Line 244: what do you mean by "ecologically active"?

Lines: 251-252: "derived from satellite observations" You used data from CAMS product, which merges model and observation data. Please rephrase to "derived from CAMS".

Lines 253-267: You discuss the synoptic conditions during 20-24 March without providing the respective maps. I suggest to include the Z500 maps for the aforementioned period in a supplement/appendix. Please include in the supplement/appendix the maps for Z850 during the episode days. Also, consider including in Figure 1 the Z500 and PM10 concentrations maps for the 29 March.

Line 270: "with a double curvature in correspondence of the Balkans" ? What do you mean by "double curvature"? Please rephrase.

Lines 350-351: "besides the seasonal evolution pattern naturally leading to a decrease going towards the warmer season at this latitude" Please provide a reference.

Fig. 4: What about the polar plot and the wind speed & direction time-series for CMN?

You mention (line 375) that "the dust event was preceded and followed by weak winds, lower than 2 m/s" . However, the time series in Fig. 4 does not support this statement. For example, for Trieste, between 25-27 March we observe a persistent NE flow (ws>10m/s). During 28-29 March the wind speed decrease, while it increases again in 30 March. Over Bologna the wind speed is much lower compared to Trieste and does not exhibit clear patterns. Moreover in March 28 (day with high dust concentrations) the mean wind speed seems to be lower than before the event. Please elaborate more on the results presented in Figure 4, taking also into account the previous discussion of the synoptic conditions.

Line 406: please provide a short description of the algorithm and its possible shortcomings.

Line 408: You state that there is "a negative bias" with respect to the experimental values recorded by the Regional Environmental Protection Agency in Bologna. However, according to Fig 6a the solid lines representing the concentration values recorded in Bologna ARPAE stations are slightly above the blue-dashed lines (OPC data in Bologna). Also, the red dashed line (Trieste) seems to fits better with the ARPAE line. Ιs there a chance that something went wrong in Fig6? Are you sure that colors represent the appropriate time-series? Please check.

Line 416-417: "while two relative maxima were detected on 27 March (36.4 μg m-3) and 22 March (35.0 μg m-3)" Please check the reported dates, they do not seem to be in agreement with the figure.

Line 421-423: According to Fig 6, the Trieste time-series are shorter (20/3 - 6/3) than those of Bologna, (1/3 - 18/4). From witch dates the mean PM10 values (before and after the dust event) in Trieste are calculated? Averaging data from different dates can lead to non-comparable results between Trieste and Bologna regarding the pre- and post-event mean PM10 concentrations.

Fig. 6, caption: "Bologna at three air quality stations from the ARPAE network"  possible typo: three-> two

Sect. 3.3.1, AERONET: Please elaborate more on the spectral variation of AOD. For example, on 28 March the decrease of AOD with wavelength is smaller. This probably is caused by a larger coarse particles fraction.

Line 484: “The change in dust optical properties”. There is no change in dust optical properties. The aerosol optical properties changed due to the dust intrusion.

Sect. 3.3.3, general comment: Please elaborate more on the findings regarding the chemical composition, pointing also to the differences between the analyzed dust event and typical Saharan dust events. 

conclusions: please write 2-3 sentences mentioning the synoptic conditions resulting in the dust event.