Seasonality of the particle number concentration and size distribution: a global analysis retrieved from the network of Global Atmosphere Watch (GAW) near-surface observatories
<p>Aerosol particles are a complex component of the atmospheric system which influence 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 tr...
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| Autores principales: | , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , |
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| Formato: | article |
| Lenguaje: | EN |
| Publicado: |
Copernicus Publications
2021
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| Materias: | |
| Acceso en línea: | https://doaj.org/article/93e4ecf90c694fdbb5b7d15408614ff0 |
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| Sumario: | <p>Aerosol particles are a complex component of the atmospheric system which
influence 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 (including wet and dry deposition), 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 (<span class="inline-formula"><i>N</i><sub>tot</sub></span>) and number size distribution (PNSD, from 39
sites). A sensitivity study was first performed to assess the impact of data
availability on <span class="inline-formula"><i>N</i><sub>tot</sub></span>'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.</p>
<p><span id="page17187"/>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 (<span class="inline-formula">∼</span> 10<span class="inline-formula"><sup>2</sup></span> cm<span class="inline-formula"><sup>−3</sup></span>) and show
a clear seasonality, which is also visible in the shape of the PNSD, while
diel cycles are in general less evident, 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 (<span class="inline-formula">∼</span> 10<span class="inline-formula"><sup>3</sup></span>–10<span class="inline-formula"><sup>4</sup></span> cm<span class="inline-formula"><sup>−3</sup></span>) 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 <span class="inline-formula"><i>N</i><sub>tot</sub></span> (<span class="inline-formula">∼</span> 10<span class="inline-formula"><sup>2</sup></span>–10<span class="inline-formula"><sup>3</sup></span> cm<span class="inline-formula"><sup>−3</sup></span>). 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 <span class="inline-formula"><i>N</i><sub>tot</sub></span> observed at these stations.
Based on available PNSD measurements, CCN-sized particles (considered here
as either <span class="inline-formula"><i>></i>50</span> nm or <span class="inline-formula"><i>></i>100</span> nm) can represent from a
few percent to almost all of <span class="inline-formula"><i>N</i><sub>tot</sub></span>, corresponding to seasonal medians on
the order of <span class="inline-formula">∼</span> 10 to 1000 cm<span class="inline-formula"><sup>−3</sup></span>, with seasonal patterns
and a hierarchy of the site types broadly similar to those observed for
<span class="inline-formula"><i>N</i><sub>tot</sub></span>.</p>
<p>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.</p> |
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