Estimating cloud concentration nuclei number concentrations using aerosol optical properties: role of particle number size distribution and parameterization
<p>The concentration of cloud condensation nuclei (CCN) is an essential parameter affecting aerosol–cloud interactions within warm clouds. Long-term CCN number concentration (<span class="inline-formula"><i>N</i><sub>CCN</sub></span>) data are scar...
Main Authors: | , , , , , , , , , , , , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2019-12-01
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Series: | Atmospheric Chemistry and Physics |
Online Access: | https://www.atmos-chem-phys.net/19/15483/2019/acp-19-15483-2019.pdf |
Summary: | <p>The concentration of cloud condensation nuclei (CCN) is an essential
parameter affecting aerosol–cloud interactions within warm clouds. Long-term
CCN number concentration (<span class="inline-formula"><i>N</i><sub>CCN</sub></span>) data are scarce; there are a lot more
data on aerosol optical properties (AOPs). It is therefore valuable to
derive parameterizations for estimating <span class="inline-formula"><i>N</i><sub>CCN</sub></span> from AOP measurements. Such
parameterizations have already been made, and in the present work a new parameterization
is presented. The relationships between <span class="inline-formula"><i>N</i><sub>CCN</sub></span>, AOPs, and size
distributions were investigated based on in situ measurement data from six
stations in very different environments around the world. The relationships
were used for deriving a parameterization that depends on the scattering
Ångström exponent (SAE), backscatter fraction (BSF), and total
scattering coefficient (<span class="inline-formula"><i>σ</i><sub>sp</sub></span>) of PM<span class="inline-formula"><sub>10</sub></span> particles. The analysis
first showed that the dependence of <span class="inline-formula"><i>N</i><sub>CCN</sub></span> on supersaturation (SS) can be
described by a logarithmic fit in the range SS <span class="inline-formula"><1.1</span> %, without
any theoretical reasoning. The relationship between <span class="inline-formula"><i>N</i><sub>CCN</sub></span> and AOPs was
parameterized as <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi>N</mi><mi mathvariant="normal">CCN</mi></msub><mo>≈</mo><mo>(</mo><mo>(</mo><mn mathvariant="normal">286</mn><mo>±</mo><mn mathvariant="normal">46</mn><mo>)</mo></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="93pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="1db18230eee7b12e7ed258c4199f0c7d"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-19-15483-2019-ie00001.svg" width="93pt" height="13pt" src="acp-19-15483-2019-ie00001.png"/></svg:svg></span></span>SAE ln(SS/(<span class="inline-formula">0.093±0.006</span>))(BSF <span class="inline-formula">−</span> BSF<span class="inline-formula"><sub>min</sub></span>) <span class="inline-formula">+</span> (<span class="inline-formula">5.2±3.3</span>))<span class="inline-formula"><i>σ</i><sub>sp</sub></span>, where BSF<span class="inline-formula"><sub>min</sub></span> is the minimum BSF, in practice
the 1st percentile of BSF data at a site to be analyzed. At the lowest
supersaturations of each site (SS <span class="inline-formula">≈0.1</span> %), the average bias,
defined as the ratio of the AOP-derived and measured <span class="inline-formula"><i>N</i><sub>CCN</sub></span>, varied from
<span class="inline-formula">∼0.7</span> to <span class="inline-formula">∼1.9</span> at most sites except at a
Himalayan site where the bias was <span class="inline-formula">>4</span>. At SS <span class="inline-formula">>0.4</span> % the average bias ranged from <span class="inline-formula">∼0.7</span> to <span class="inline-formula">∼1.3</span> at most sites. For the marine-aerosol-dominated site Ascension Island the bias was
higher, <span class="inline-formula">∼1.4</span>–1.9. In other words, at SS <span class="inline-formula">>0.4</span> % <span class="inline-formula"><i>N</i><sub>CCN</sub></span> was estimated with an average uncertainty of approximately
30 % by using nephelometer data. The biases were mainly due to the biases
in the parameterization related to the scattering Ångström exponent
(SAE). The squared correlation coefficients between the AOP-derived and
measured <span class="inline-formula"><i>N</i><sub>CCN</sub></span> varied from <span class="inline-formula">∼0.5</span> to <span class="inline-formula">∼0.8</span>. To
study the physical explanation of the relationships between <span class="inline-formula"><i>N</i><sub>CCN</sub></span> and
AOPs, lognormal unimodal particle size distributions were generated and
<span class="inline-formula"><i>N</i><sub>CCN</sub></span> and AOPs were calculated. The simulation showed that the
relationships of <span class="inline-formula"><i>N</i><sub>CCN</sub></span> and AOPs are affected by the geometric mean
diameter and width of the size distribution and the activation diameter. The
relationships of <span class="inline-formula"><i>N</i><sub>CCN</sub></span> and AOPs were similar to those of the observed
ones.</p> |
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ISSN: | 1680-7316 1680-7324 |