Reduced effective radiative forcing from cloud–aerosol interactions (ERF<sub>aci</sub>) with improved treatment of early aerosol growth in an Earth system model
<p>Historically, aerosols of anthropogenic origin have offset some of the warming from increased atmospheric greenhouse gas concentrations. The strength of this negative aerosol forcing, however, is highly uncertain – especially the part originating from cloud–aerosol interactions. An importan...
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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/67704069f4a248cfaa16b3ea4b43f121 |
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| Sumario: | <p>Historically, aerosols of anthropogenic origin have offset some of the warming from increased atmospheric greenhouse gas concentrations. The
strength of this negative aerosol forcing, however, is highly uncertain – especially the part originating from cloud–aerosol interactions. An
important part of this uncertainty originates from our lack of knowledge about pre-industrial aerosols and how many of these would have acted as
cloud condensation nuclei (CCN). In order to simulate CCN concentrations in models, we must adequately model secondary aerosols, including new
particle formation (NPF) and early growth, which contributes a large part of atmospheric CCN. In this study, we investigate the effective radiative
forcing (ERF) from cloud–aerosol interactions (<span class="inline-formula">ERF<sub>aci</sub></span>) with an improved treatment of early particle growth, as presented in
<span class="cit" id="xref_text.1"><a href="#bib1.bibx9">Blichner et al.</a> (<a href="#bib1.bibx9">2021</a>)</span>. We compare the improved scheme to the default scheme, OsloAero, which are both embedded in the atmospheric component of the
Norwegian Earth System Model v2 (NorESM2). The improved scheme, OsloAeroSec, includes a sectional scheme that treats the growth of particles
from 5–39.6 <span class="inline-formula">nm</span> in diameter, which thereafter inputs the particles to the smallest mode in the pre-existing modal aerosol scheme. The
default scheme parameterizes the growth of particles from nucleation up to the smallest mode, a process that can take several hours. The
explicit treatment of early growth in OsloAeroSec, on the other hand, captures the changes in atmospheric conditions during this growth time
in terms of air mass mixing, transport, and condensation and coagulation.</p>
<p>We find that the <span class="inline-formula">ERF<sub>aci</sub></span> with the sectional scheme is <span class="inline-formula">−</span>1.16 <span class="inline-formula">W m<sup>−2</sup></span>, which is 0.13 <span class="inline-formula">W m<sup>−2</sup></span> weaker compared to the
default scheme. This reduction originates from OsloAeroSec producing more particles than the default scheme in pristine, low-aerosol-concentration
areas and fewer NPF particles in high-aerosol areas. We find, perhaps surprisingly, that NPF inhibits cloud droplet activation in
polluted and/or high-aerosol-concentration regions because the NPF particles increase the condensation sink and reduce the growth of the larger particles
which may otherwise activate. This means that in these high-aerosol regions, the model with the lowest NPF – OsloAeroSec – will have the highest cloud
droplet activation and thus more reflective clouds. In pristine and/or low-aerosol regions, however, NPF enhances cloud droplet activation because the NPF
particles themselves tend to activate.</p>
<p>Lastly, we find that sulfate emissions in the present-day simulations increase the hygroscopicity of secondary aerosols compared to
pre-industrial simulations. This makes NPF particles more relevant for cloud droplet activation in the present day than the pre-industrial
atmosphere because increased hygroscopicity means they can activate at smaller sizes.</p> |
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