Seasonality of the particle number concentration and size
distribution: a global analysis retrieved from the network of Global
Atmosphere Watch (GAW) near-surface observatories
Authors: Rose Clémence , Collaud Coen Martine , Andrews Elisabeth , Lin Yong , Bossert Isaline , Lund Myhre Cathrine , Tuch Thomas , Wiedensohler Alfred , Fiebig Markus , Aalto Pasi , Alastuey Andrés , Alonso-Blanco Elisabeth , Andrade Marcos , Artíñano Begoña , Arsov Todor , Baltensperger Urs , Bastian Susanne , Bath Olaf , Beukes Johan Paul , Brem Benjamin T. , Bukowiecki Nicolas , Casquero-Vera Juan Andrés , Conil Sébastien , Eleftheriadis Konstantinos , Favez Olivier , Flentje Harald , Gini Maria I. , Gómez-Moreno Francisco Javier , Gysel-Beer Martin , Hallar A. Gannet , Kalapov Ivo , Kalivitis Nikos , Kasper-Giebl Anne , Keywood Melita , Kim Jeong Eun , Kim Sang-Woo , Kristensson Adam , Kulmala Markku , Lihavainen Heikki , Lin Neng-Huei , Lyamani Hassan , Marinoni Angela , Martins Dos Santos Sebastiao , Mayol-Bracero Olga L. , Meinhardt Frank , Merkel Maik , Metzger Jean-Marc , Mihalopoulos Nikolaos , Ondracek Jakub , Pandolfi Marco , Pérez Noemi , Petäjä Tuukka , Petit Jean-Eudes , Picard David , Pichon Jean-Marc , Pont Veronique , Putaud Jean-Philippe , Reisen Fabienne , Sellegri Karine , Sharma Sangeeta , Schauer Gerhard , Sheridan Patrick , Sherman James Patrick , Schwerin Andreas , Sohmer Ralf , Sorribas Mar , Sun Junying , Tulet Pierre , Vakkari Ville , van Zyl Pieter Gideon , Velarde Fernando , Villani Paolo , Vratolis Stergios , Wagner Zdenek , Wang Sheng-Hsiang , Weinhold Kay , Weller Rolf , Yela Margarita , Zdimal Vladimir , Laj Paolo 
Published: January 2021

Abstract. Aerosol particles are a complex component of the atmospheric system that influences climate directly by interacting with solar radiation, and indirectly by contributing to cloud formation. The variety of their sources, as well as the multiple transformations they may undergo during their transport, result in significant spatial and temporal variability of their properties. Documenting this variability is essential to provide a proper representation of aerosols and cloud condensation nuclei (CCN) in climate models. Using measurements conducted in 2016 or 2017 at 62 ground based stations around the world, this study provides the most up-to-date picture of the spatial distribution of particle number concentration (Ntot) and number size distribution (PNSD, from 39 sites). A sensitivity study was first performed to assess the impact of data availability on Ntot's annual and seasonal statistics, as well as on the analysis of its diel cycle. Thresholds of 50 % and 60 % were set at the seasonal and annual scale, respectively, for the study of the corresponding statistics, and a slightly higher coverage (75 %) was required to document the diel cycle. Although some observations are common to a majority of sites, the variety of environments characterizing these stations made it possible to highlight contrasting findings, which, among other factors, seem to be significantly related to the level of anthropogenic influence. The concentrations measured at polar sites are the lowest (~102 cm−3) and show a clear seasonality, which is also visible in the shape of the PNSD, while diel cycles are in general barely marked, due notably to the absence of a regular day-night cycle in some seasons. In contrast, the concentrations characteristic of urban environments are the highest (~103–104 cm−3) and do not show pronounced seasonal variations, whereas diel cycles tend to be very regular over the year at these stations. The remaining sites, including mountain and non-urban continental and coastal stations, do not exhibit as obvious common behaviour as polar and urban sites and display, on average, intermediate Ntot (~102–103 cm−3). Particle concentrations measured at mountain sites, however, are generally lower compared to nearby lowland sites, and tend to exhibit somewhat more pronounced seasonal variations as a likely result of the strong impact of the atmospheric boundary layer (ABL) influence in connection with the topography of the sites. ABL dynamics also likely contribute to the diel cycle of Ntot observed at these stations. Based on available PNSD measurements, CCN-sized particles (i.e. > 50–100 nm) can represent from a few percent to almost all of Ntot, corresponding to seasonal medians in the order of ~10 to 1000 cm−3, with seasonal patterns and a hierarchy of the site types broadly similar to those observed for Ntot. Overall, this work illustrates the importance of in-situ measurements, in particular for the study of aerosol physical properties, and thus strongly supports the development of a broad global network of near surface observatories to increase and homogenize the spatial coverage of the measurements, and guarantee as well data availability and quality. The results of this study also provide a valuable, freely available and easy to use support for model comparison and validation, with the ultimate goal of contributing to improvement of the representation of aerosol-cloud interactions in models, and, therefore, of the evaluation of the impact of aerosol particles on climate.