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<front>
<journal-meta>
<journal-id journal-id-type="publisher">ACP</journal-id>
<journal-title-group>
<journal-title>Atmospheric Chemistry and Physics</journal-title>
<abbrev-journal-title abbrev-type="publisher">ACP</abbrev-journal-title>
</journal-title-group>
<issn pub-type="epub">1680-7324</issn>
<publisher><publisher-name>Copernicus GmbH</publisher-name>
<publisher-loc>Göttingen, Germany</publisher-loc>
</publisher>
</journal-meta>
<article-meta>
<article-id pub-id-type="doi">10.5194/acp-10-6537-2010</article-id>
<title-group>
<article-title>Tropospheric ozone variations at the Nepal Climate Observatory-Pyramid (Himalayas, 5079 m a.s.l.) and influence of deep stratospheric intrusion events</article-title>
</title-group>
<contrib-group><contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>Cristofanelli</surname>
<given-names>P.</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
</contrib>
<contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>Bracci</surname>
<given-names>A.</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
<xref ref-type="aff" rid="aff2">
<sup>2</sup>
</xref>
</contrib>
<contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>Sprenger</surname>
<given-names>M.</given-names>
</name>
<xref ref-type="aff" rid="aff2">
<sup>2</sup>
</xref>
</contrib>
<contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>Marinoni</surname>
<given-names>A.</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
</contrib>
<contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>Bonafè</surname>
<given-names>U.</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
</contrib>
<contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>Calzolari</surname>
<given-names>F.</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
</contrib>
<contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>Duchi</surname>
<given-names>R.</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
</contrib>
<contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>Laj</surname>
<given-names>P.</given-names>
</name>
<xref ref-type="aff" rid="aff3">
<sup>3</sup>
</xref>
</contrib>
<contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>Pichon</surname>
<given-names>J.M.</given-names>
</name>
<xref ref-type="aff" rid="aff4">
<sup>4</sup>
</xref>
</contrib>
<contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>Roccato</surname>
<given-names>F.</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
</contrib>
<contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>Venzac</surname>
<given-names>H.</given-names>
</name>
<xref ref-type="aff" rid="aff4">
<sup>4</sup>
</xref>
</contrib>
<contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>Vuillermoz</surname>
<given-names>E.</given-names>
</name>
<xref ref-type="aff" rid="aff5">
<sup>5</sup>
</xref>
</contrib>
<contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>Bonasoni</surname>
<given-names>P.</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
<xref ref-type="aff" rid="aff5">
<sup>5</sup>
</xref>
</contrib>
</contrib-group><aff id="aff1">
<label>1</label>
<addr-line>Institute for Atmospheric Science and Climate, National Research Council, Bologna, Italy</addr-line>
</aff>
<aff id="aff2">
<label>2</label>
<addr-line>ETHZ, Zurich, Switzerland</addr-line>
</aff>
<aff id="aff3">
<label>3</label>
<addr-line>Laboratoire de Glaciologie et Géophysique de l&apos;Environnement, St Martin d&apos;Hères Cedex, France</addr-line>
</aff>
<aff id="aff4">
<label>4</label>
<addr-line>Laboratoire de Meteorologie Physique, CNRS, Université Blaise Pascal, Aubiere cedex, France</addr-line>
</aff>
<aff id="aff5">
<label>5</label>
<addr-line>EV-K2-CNR Committee, 24126 Bergamo, Italy</addr-line>
</aff>
<pub-date pub-type="epub">
<day>16</day>
<month>07</month>
<year>2010</year>
</pub-date>
<volume>10</volume>
<issue>14</issue>
<fpage>6537</fpage>
<lpage>6549</lpage>
<permissions>
<license xlink:type="simple">
<license-p>This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.</license-p>
</license>
</permissions>
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<self-uri xlink:href="http://www.atmos-chem-phys.net/10/6537/2010/acp-10-6537-2010.pdf">The full text article is available as a PDF file from http://www.atmos-chem-phys.net/10/6537/2010/acp-10-6537-2010.pdf</self-uri>
<abstract>
<p>The paper presents the first 2 years of continuous surface ozone (O&lt;sub&gt;3&lt;/sub&gt;)
observations and systematic assessment of the influence of stratospheric
intrusions (SI) at the Nepal Climate Observatory at Pyramid (NCO-P;
27°57&apos; N, 86°48&apos; E), located in the southern Himalayas at 5079 m a.s.l..
Continuous O&lt;sub&gt;3&lt;/sub&gt; monitoring has been carried out at this GAW-WMO station
in the framework of the Ev-K2-CNR SHARE and UNEP ABC projects since March
2006. Over the period March 2006–February 2008, an average O&lt;sub&gt;3&lt;/sub&gt; value
of 49&amp;plusmn;12 ppbv (&amp;plusmn;1&amp;\delta;) was recorded, with a large annual
cycle characterized by a maximum during the pre-monsoon (61&amp;plusmn;9 ppbv)
and a minimum during the monsoon (39&amp;plusmn;10 ppbv). In general, the
average O&lt;sub&gt;3&lt;/sub&gt; diurnal cycles had different shapes in the different
seasons, suggesting an important interaction between the synoptic-scale
circulation and the local mountain wind regime.
&lt;br&gt;&lt;br&gt;
Short-term O&lt;sub&gt;3&lt;/sub&gt; behaviour in the middle/lower troposphere (e.g. at the
altitude level of NCO-P) can be significantly affected by deep SI which,
representing one of the most important natural input for tropospheric
O&lt;sub&gt;3&lt;/sub&gt;, can also influence the regional atmosphere radiative forcing. To
identify days possibly influenced by SI at the NCO-P, a specially designed
statistical methodology was applied to the time series of observed and
modelled stratospheric tracers. On this basis, during the 2-year
investigation, 14.1% of analysed days were found to be affected by SI.
The SI frequency showed a clear seasonal cycle, with minimum during the
summer monsoon (1.2%) and higher values during the rest of the year
(21.5%). As suggested by back-trajectory analysis, the position of the
subtropical jet stream could play an important role in determining the
occurrence of deep SI transport on the southern Himalayas.
&lt;br&gt;&lt;br&gt;
We estimated the fraction of O&lt;sub&gt;3&lt;/sub&gt; due to SI at the NCO-P. This analysis
led to the conclusion that during SI O&lt;sub&gt;3&lt;/sub&gt; significantly increased by
27.1% (+13 ppbv) with respect to periods not affected by such events.
Moreover, the integral contribution of SI (O&lt;sub&gt;3S&lt;/sub&gt;) to O&lt;sub&gt;3&lt;/sub&gt; at the NCO-P
was also calculated, showing that up to 13.7% of O&lt;sub&gt;3&lt;/sub&gt; recorded at the
measurement site could be possibly attributed to SI. On a seasonal basis,
the lowest SI contributions were found during the summer monsoon (less than
0.1%), while the highest were found during the winter period (up to
24.2%). Even considering the rather large uncertainty associated with
these estimates, the obtained results indicated that, during non-monsoon
periods, high O&lt;sub&gt;3&lt;/sub&gt; levels could affect NCO-P during SI, thus influencing
the variability of tropospheric O&lt;sub&gt;3&lt;/sub&gt; over the southern Himalayas.</p>
</abstract>
<counts><page-count count="13"/></counts>
</article-meta>
</front>
<body/>
<back>
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