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<article language="en">
	<journal>
		<journal_title>Atmospheric Chemistry and Physics Discussions</journal_title>
		<journal_url>www.atmos-chem-phys-discuss.net</journal_url>
		<issn>1680-7367</issn>
		<eissn>1680-7375</eissn>
		<volume_number>9</volume_number>
		<issue_number>2</issue_number>
		<publication_year>2009</publication_year>
	</journal>
	<doi>10.5194/acpd-9-8635-2009</doi>
	<article_url>http://www.atmos-chem-phys-discuss.net/9/8635/2009/</article_url>
	<abstract_html>http://www.atmos-chem-phys-discuss.net/9/8635/2009/acpd-9-8635-2009.html</abstract_html>
	<fulltext_pdf>http://www.atmos-chem-phys-discuss.net/9/8635/2009/acpd-9-8635-2009.pdf</fulltext_pdf>
	<start_page>8635</start_page>
	<end_page>8665</end_page>
	<publication_date>2009-04-01</publication_date>
	<article_title content_type="html">Aerosol- and updraft-limited regimes of cloud droplet formation: influence of particle number, size and hygroscopicity on the activation of cloud condensation nuclei (CCN)</article_title>
	<authors>
		<author numeration="1" affiliations="1,2">
			<name>P. Reutter</name>
			<email>preutter@uni-mainz.de</email>
		</author>
		<author numeration="2" affiliations="2,4">
			<name>J. Trentmann</name>
		</author>
		<author numeration="3" affiliations="1">
			<name>H. Su</name>
		</author>
		<author numeration="4" affiliations="3">
			<name>M. Simmel</name>
		</author>
		<author numeration="5" affiliations="1">
			<name>D. Rose</name>
		</author>
		<author numeration="6" affiliations="2">
			<name>H. Wernli</name>
		</author>
		<author numeration="7" affiliations="1">
			<name>M. O. Andreae</name>
		</author>
		<author numeration="8" affiliations="1">
			<name>U. Pöschl</name>
		</author>
	</authors>
	<affiliations>
		<affiliation numeration="1" content_type="html">Max Planck Institute for Chemistry, Biogeochemistry Department, Mainz, Germany</affiliation>
		<affiliation numeration="2" content_type="html">Institute for Atmospheric Physics, Johannes Gutenberg University Mainz, Mainz, Germany</affiliation>
		<affiliation numeration="3" content_type="html">Leibniz Institute for Tropospheric Research, Leipzig, Germany</affiliation>
		<affiliation numeration="4" content_type="html">German Weather Service, DWD Offenbach, Germany</affiliation>
	</affiliations>
	<abstract content_type="html">We have investigated the formation of cloud droplets under (pyro-)convective
conditions using a cloud parcel model with detailed spectral microphysics
and with the κ-Köhler model approach for efficient and realistic
description of the cloud condensation nucleus (CCN) activity of aerosol
particles. Assuming a typical biomass burning aerosol size distribution
(accumulation mode centred at 120 nm), we have calculated initial cloud
droplet number concentrations (&lt;i&gt;N&lt;sub&gt;CD&lt;/sub&gt;&lt;/i&gt;) for a wide range of updraft
velocities (&lt;i&gt;w&lt;/i&gt;=0.5–20 m s&lt;sup&gt;&amp;minus;1&lt;/sup&gt;) and aerosol particle number
concentrations (&lt;i&gt;N&lt;sub&gt;CN&lt;/sub&gt;&lt;/i&gt;=10&lt;sup&gt;3&lt;/sup&gt;–10&lt;sup&gt;5&lt;/sup&gt; cm&lt;sup&gt;&amp;minus;3&lt;/sup&gt;) at the cloud base.
Depending on the ratio between updraft velocity and particle number
concentration (&lt;i&gt;w&lt;/i&gt;/&lt;i&gt;N&lt;sub&gt;CN&lt;/sub&gt;&lt;/i&gt;), we found three distinctly different regimes of
CCN activation and cloud droplet formation:
&lt;br&gt;&lt;br&gt;
1. An aerosol-limited regime that is characterized by high &lt;i&gt;w&lt;/i&gt;/&lt;i&gt;N&lt;sub&gt;CN&lt;/sub&gt;&lt;/i&gt;
ratios (&gt;&amp;#x2248;10&lt;sup&gt;&amp;minus;3&lt;/sup&gt; m s&lt;sup&gt;&amp;minus;1&lt;/sup&gt; cm&lt;sup&gt;3&lt;/sup&gt;), high maximum values
of water vapour supersaturation (&lt;i&gt;S&lt;/i&gt;&lt;sub&gt;max&lt;/sub&gt;&amp;gt;&amp;#x2248;0.5%), and high
activated fractions of aerosol particles (&lt;i&gt;N&lt;sub&gt;CD&lt;/sub&gt;&lt;/i&gt;/&lt;i&gt;N&lt;sub&gt;CN&lt;/sub&gt;&lt;/i&gt;&amp;gt;&amp;#x2248;90%).
In this regime &lt;i&gt;N&lt;sub&gt;CD&lt;/sub&gt;&lt;/i&gt; is directly proportional to &lt;i&gt;N&lt;sub&gt;CN&lt;/sub&gt;&lt;/i&gt; and
practically independent of &lt;i&gt;w&lt;/i&gt;.
&lt;br&gt;&lt;br&gt;
2. An updraft-limited regime that is characterized by low &lt;i&gt;w&lt;/i&gt;/&lt;i&gt;N&lt;sub&gt;CN&lt;/sub&gt;&lt;/i&gt; ratios
(&lt;&amp;#x2248;10&lt;sup&gt;&amp;minus;4&lt;/sup&gt; m s&lt;sup&gt;&amp;minus;1&lt;/sup&gt; cm&lt;sup&gt;3&lt;/sup&gt;), low maximum values of water
vapour supersaturation (&lt;i&gt;S&lt;/i&gt;&lt;sub&gt;max&lt;/sub&gt;&lt;&amp;#x2248;0.2%), and low activated
fractions of aerosol particles (&lt;i&gt;N&lt;sub&gt;CD&lt;/sub&gt;&lt;/i&gt;/&lt;i&gt;N&lt;sub&gt;CN&lt;/sub&gt;&lt;/i&gt;&lt;&amp;#x2248;20%). In
this regime &lt;i&gt;N&lt;sub&gt;CD&lt;/sub&gt;&lt;/i&gt; is directly proportional to w and practically
independent of &lt;i&gt;N&lt;sub&gt;CN&lt;/sub&gt;&lt;/i&gt;.
&lt;br&gt;&lt;br&gt;
3. An aerosol- and updraft-sensitive regime, which is characterized by
parameter values in between the two other regimes and covers most of the
conditions relevant for pyro-convection. In this regime &lt;i&gt;N&lt;sub&gt;CD&lt;/sub&gt;&lt;/i&gt; depends
non-linearly on both &lt;i&gt;N&lt;sub&gt;CN&lt;/sub&gt;&lt;/i&gt; and &lt;i&gt;w&lt;/i&gt;.
&lt;br&gt;&lt;br&gt;
In sensitivity studies we have tested the influence of aerosol particle
hygroscopicity on &lt;i&gt;N&lt;sub&gt;CD&lt;/sub&gt;&lt;/i&gt;. Within the range of effective hygroscopicity
parameters that is characteristic for continental atmospheric aerosols
(κ&amp;#x2248;0.05–0.6), we found that &lt;i&gt;N&lt;sub&gt;CD&lt;/sub&gt;&lt;/i&gt; depends rather
weakly on the actual value of κ. Only for aerosols with very low
hygroscopicity (κ&lt;0.05) and in the updraft-limited regime also
for aerosols with higher than average hygroscopicity (κ&gt;0.3) did
the relative differential quotients (Δ&lt;i&gt;N&lt;sub&gt;CD&lt;/sub&gt;&lt;/i&gt;/&lt;i&gt;N&lt;sub&gt;CD&lt;/sub&gt;&lt;/i&gt;)/(&amp;Delta;κ/κ)
exceed values of ~0.2, indicating that a 50%
difference in κ would change &lt;i&gt;N&lt;sub&gt;CD&lt;/sub&gt;&lt;/i&gt; by more than 10%. Realistic
changes in the aerosol particle size distribution had practically no effect
on the aerosol-limited regime and limited influence on the aerosol- and
updraft sensitive regime (Δ&lt;i&gt;N&lt;sub&gt;CD&lt;/sub&gt;&lt;/i&gt;/&lt;i&gt;N&lt;sub&gt;CD&lt;/sub&gt;&lt;/i&gt;&lt;30%) but can
have strong effects at low supersaturation in the updraft-limited regime
(Δ&lt;i&gt;N&lt;sub&gt;CD&lt;/sub&gt;&lt;/i&gt;/&lt;i&gt;N&lt;sub&gt;CD&lt;/sub&gt;&lt;/i&gt;&amp;gt;30% at &lt;i&gt;S&lt;/i&gt;&lt;sub&gt;max&lt;/sub&gt;&lt;0.1%). Overall,
the results of this and related studies suggest that the variability of
initial cloud droplet number concentration in (pyro-)convective clouds is
mostly dominated by the variability of updraft velocity and aerosol particle
number concentration in the accumulation mode. Coarse mode particles and the
variability of particle composition and hygroscopicity appear to be play
major roles only at low supersaturation in the updraft-limited regime of CCN
activation (&lt;i&gt;S&lt;/i&gt;&lt;sub&gt;max&lt;/sub&gt;&lt;0.2%).</abstract>
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</article>

