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<article language="en">
	<journal>
		<journal_title>Atmospheric Chemistry and Physics</journal_title>
		<journal_url>www.atmos-chem-phys.net</journal_url>
		<issn>1680-7316</issn>
		<eissn>1680-7324</eissn>
		<volume_number>9</volume_number>
		<issue_number>18</issue_number>
		<publication_year>2009</publication_year>
	</journal>
	<doi>10.5194/acp-9-7067-2009</doi>
	<article_url>http://www.atmos-chem-phys.net/9/7067/2009/</article_url>
	<abstract_html>http://www.atmos-chem-phys.net/9/7067/2009/acp-9-7067-2009.html</abstract_html>
	<fulltext_pdf>http://www.atmos-chem-phys.net/9/7067/2009/acp-9-7067-2009.pdf</fulltext_pdf>
	<start_page>7067</start_page>
	<end_page>7080</end_page>
	<publication_date>2009-09-24</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>
		</author>
		<author numeration="2" affiliations="1">
			<name>H. Su</name>
			<email>hsu@mpch-mainz.mpg.de</email>
		</author>
		<author numeration="3" affiliations="2,4">
			<name>J. Trentmann</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="1">
			<name>S. S. Gunthe</name>
		</author>
		<author numeration="7" affiliations="2">
			<name>H. Wernli</name>
		</author>
		<author numeration="8" affiliations="1">
			<name>M. O. Andreae</name>
		</author>
		<author numeration="9" affiliations="1">
			<name>U. PÃ¶schl</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">now at: 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 Îº-KÃ¶hler 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;=0.25â€“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;=200â€“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;â‰ˆ10&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;&gt;â‰ˆ0.5%), and high
activated fractions of aerosol particles (&lt;i&gt;N&lt;sub&gt;CN&lt;/sub&gt;&lt;/i&gt;/&lt;i&gt;N&lt;sub&gt;CN&lt;/sub&gt;&lt;/i&gt;&gt;â‰ˆ90%).
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;â‰ˆ10&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;â‰ˆ0.2%), 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;â‰ˆ20%). In
this regime &lt;i&gt;N&lt;sub&gt;CD&lt;/sub&gt;&lt;/i&gt; is directly proportional to &lt;i&gt;w&lt;/i&gt; 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 (transitional 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 size
distribution and 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 (Îºâ‰ˆ0.05â€“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 Îº. A compensation of changes in
Îº and &lt;i&gt;S&lt;/i&gt;&lt;sub&gt;max&lt;/sub&gt; leads to an effective buffering of &lt;i&gt;N&lt;sub&gt;CD&lt;/sub&gt;&lt;/i&gt;. Only
for aerosols with very low hygroscopicity (Îº&lt;0.05) and also in
the updraft-limited regime for aerosols with higher than average
hygroscopicity (Îº&gt;0.3) did the relative sensitivities âˆ‚ln&lt;i&gt;N&lt;sub&gt;CD&lt;/sub&gt;&lt;/i&gt;/âˆ‚lnÎºâ‰ˆ
(Î”&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;Îº/Îº) exceed values of ~0.2,
indicating that a 50% difference in Îº would change &lt;i&gt;N&lt;sub&gt;CD&lt;/sub&gt;&lt;/i&gt;
by more than 10%.
&lt;br&gt;&lt;br&gt;
The influence of changing size distribution parameters was stronger than
that of particle hygroscopicity. Nevertheless, similar regimes of CCN
activation were observed in simulations with varying types of size
distributions (polluted and pristine continental and marine aerosols with
different proportions of nucleation, Aitken, accumulation, and coarse mode
particles). In general, the different regimes can be discriminated with
regard to the relative sensitivities of &lt;i&gt;N&lt;sub&gt;CD&lt;/sub&gt;&lt;/i&gt; against &lt;i&gt;w&lt;/i&gt; and &lt;i&gt;N&lt;sub&gt;CN&lt;/sub&gt;&lt;/i&gt;
(âˆ‚ln&lt;i&gt;N&lt;sub&gt;CD&lt;/sub&gt;&lt;/i&gt;/âˆ‚ln&lt;i&gt;w&lt;/i&gt; and âˆ‚ln&lt;i&gt;N&lt;sub&gt;CD&lt;/sub&gt;&lt;/i&gt;/âˆ‚ln&lt;i&gt;N&lt;sub&gt;CN&lt;/sub&gt;&lt;/i&gt;).
We propose to separate the different regimes by relative
sensitivity ratios, (âˆ‚ln&lt;i&gt;N&lt;sub&gt;CD&lt;/sub&gt;&lt;/i&gt;/âˆ‚ln&lt;i&gt;w&lt;/i&gt;)/(âˆ‚ln&lt;i&gt;N&lt;sub&gt;CD&lt;/sub&gt;&lt;/i&gt;/âˆ‚ln&lt;i&gt;N&lt;sub&gt;CN&lt;/sub&gt;&lt;/i&gt;)
of 4:1 and 1:4, respectively.
&lt;br&gt;&lt;br&gt;
The results of this and related studies suggest that the variability of
initial cloud droplet number concentration in convective clouds is mostly
dominated by the variability of updraft velocity and aerosol particle number
concentration in the accumulation and Aitken mode. Coarse mode particles and
the variability of particle composition and hygroscopicity appear to 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>

