Characterization of ultrafine particles and the occurrence of new particle formation events in an urban and coastal site of the Mediterranean area

Dinoi, Adelaide; Gullì, Daniel; Weinhold, Kay; Ammoscato, Ivano; Calidonna, Claudia Roberta; Wiedensohler, Alfred; Contini, Daniele

doi:https://doi.org/10.5194/acp-2022-512

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https://doi.org/10.5194/acp-2022-512

Preprints

18 Aug 2022

Status: this preprint is currently under review for the journal ACP.

Characterization of ultrafine particles and the occurrence of new particle formation events in an urban and coastal site of the Mediterranean area

Adelaide Dinoi¹, Daniel Gullì², Kay Weinhold³, Ivano Ammoscato², Claudia Roberta Calidonna², Alfred Wiedensohler³, and Daniele Contini¹ Adelaide Dinoi et al.

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¹Institute of Atmospheric Sciences and Climate, ISAC-CNR, S. P. Lecce-Monteroni km 1.2, 73100 Lecce, Italy
²Institute of Atmospheric Sciences and Climate, ISAC-CNR, Zona Industriale, I-88046 Lamezia Terme (CZ), Italy
³Leibniz Institute for Tropospheric Research, 04318, Leipzig, Germany

Received: 29 Jul 2022 – Discussion started: 18 Aug 2022

Abstract. In this work, new particle formation events (NPF) occurred at two locations in Southern Italy, the urban background site of Lecce (ECO station) and the coastal site of Lamezia Terme (LMT station), are identified and analysed. The study aims to compare the properties of NPF events at the two sites located 225 km away from each other and characterized by marked differences in terms of emission sources and local weather dynamics. Continuous measurements of particle number size distributions, in the size range from 10 nm to 800 nm, were performed at both sites by a Mobility Particle Size Spectrometer (MPSS). The occurrence of NPF events, observed throughout the study period lasted five years, produced different results in terms of frequency of occurrence, 25 % of the days at ECO and 9 % at LMT. NPF events showed seasonal patterns, higher frequency during spring and summer at the urban background site, while at the coastal site during the autumn-winter period. Some of these events happened simultaneously at both sites, indicating the occurrence of the nucleation process on a large spatial scale (regional event). Cluster analysis of 72 h back-trajectories showed that during the NPF events the two stations were influenced by similar air masses, most of which originating from the North-Western directions. Local meteorological conditions characterized by high pressure, with a prevalence of clear skies, low level of relative humidity (RH < 52 %), and moderate winds (3–4 m s^-1) dominated the NPF events at both sites. Notable differences were observed in SO₂ and PM_2.5 concentrations, resulting in ~65 % and ~80 % lower at LMT compared to ECO, respectively. It is likely that the lower level of SO₂, recognized as one of the main gas precursors involved in the nucleation process, can be responsible for the smaller NPF frequency of occurrence (~60 % less than ECO) observed in LMT.

Adelaide Dinoi et al.

Status: open (until 29 Sep 2022)

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RC1:
'Comment on acp-2022-512', Anonymous Referee #1, 27 Sep 2022 reply
The MS deals with the properties of NPF events and particle number concentrations in various size fractions at an urban background site in Lecce and at a costal location in central Mediterranean, southern Italy. It presents valuable results and conclusions and contributes to the growing knowledge on the atmospheric nucleation and consecutive particle growth phenomenon in this larger region. However, the MS could be and should be improved substantially in several ways before deciding whether it is acceptable or not for publication in the ACP. The corrections can hopefully be accomplished by a very careful and thorough revision of the present version.

Major concerns

The MS is too much of the descriptive character. Lots of simple statistical results are just supplied without interpreting them or putting them into appropriate frameworks or formulating clear conclusions or messages from them. Examples could be large parts of Sect. 3.2, lines 221–228 or Table 3. Further possibilities for improved interpretations could involve e.g. explaining and better comparing the seasonality of NPF events and diurnal concentration patterns of various particle number size fractions. Further important references on urban NPF could also be added to this purpose.

The SO₂ is often used in the existing interpretations (e.g. in Sect. 3.3). Despite the fact that 1) its photochemical oxidation to the key nucleating vapour of H₂SO₄ was shown to be slow, complex and of less direct influence on the NPF, and 2) the authors possess all necessary properties and variables for deriving the proximity value of H₂SO₄ (which is more directly connected to the process) either by the classical method of Petäjä et al. (ACP, 2009) without the scaling factor or by its improved estimation proposed in Dada et al. (ACP, 2020). The authors may want to amend this part, which could contribute to the improved overall quality of the final MS.

It was shown in several publications that the size range below 10 nm is crucial for identifying and characterising NPF events in particular in cities (e.g. Nieminen et al., ACP, 2018). The authors are asked to discuss how they avoided the limitations imposed by their relatively large measurable diameter of 10 nm in Lecce. For instance, how did this fact influence the share of the undefined days.

The frequency of missing days were relatively large around 22% at both sites. It is wondered how these days were distributed over the years or the measurement campaign since the NPF occurrence frequency showed a remarkable seasonal dependency, which could possibly impact on the representativity of the remaining days.

The reader can have the feeling at several places (e.g. in lines 154–160) that the nucleation or NPF processes and the particle growth process are not clearly distinguished. For instance, it cloud be clarified what the authors meant by “the temporal evolution of the events”.

It is not described properly how some important properties were obtained. An example could be the lines 222–224 where the CS is mentioned only very briefly. By the way, this (in a more detailed extent) should be shifted from the section Results and discussion to e.g. Sect. 2.3 since this is not their result. In this respect, it is also mentioned that the original NSF in Salma et al., ACP, 2017 was further developed, and its NSF_GEN and NSF_NUCL are more informative than the original form and should be used or at least mentioned. It is not clear (lines 225–228) how the start and end times of the growth events and more importantly, the geometric diameters Dp1 and Dp2 were derived and whether the latter were modal median diameters or something else.

Figure 4 (and possibly some others as well) contains too many lines and it is difficult to follow. In addition, it should be discussed whether a local time involving the daylight-saving time (clock change in the EU) or UTC+1 or else were used as the time scale. This could be related to the shift in the positions of the diurnal peaks in different seasons.

Line 239 and further: the phenomenon or process is somewhat more sophisticated. The authors perhaps want to include and discuss the NPF occurrence with respect to the ratio of sources and sinks of low-volatility vapours and not just the amount of CS alone.

Minor comments

Line 33: use either primary or emission (source).

The references should be ordered chronologically and not alphabetically, e.g. lines 36–37.

Some abbreviations are not explained, e.g. line 101: MPSS, or Table 3 W, Sp, S and A.

Line 105: is TSI Inc. really based in Rome, Italy?

Line 239: replace "discouraged" by not favoured or something similar.

What is the advantage of using a CNR4 net radiometer which measures the energy balance between incoming short-wave and long-wave far infrared radiation versus surface-reï¬ected short-wave and outgoing long-wave radiation to measuring global or direct solar radiations.

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Citation: https://doi.org/10.5194/acp-2022-512-RC1

Adelaide Dinoi et al.

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Short summary

In this study, particle number size distribution analysis was performed with the purpose of characterizing new particle formation (NPF) events occurred in two areas of southern Italy during 5 years of measurements. The identification of NPF events produced different results in terms of frequency and seasonality. Some of the main variables involved in the process, the local atmospheric conditions in which the events occurred and the role of the air masses were discussed and compared.


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