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Direct determinations of polar stratospheric cloud composition

Objectif

There are three primary objectives with this instrument development project:
- to improve the design of an existing Lyman-` hygrometer to give the instrument increased sensitivity and a lower detection limit for water vapor such that it can be used in conjunction with the CVI to measure the condensed water content of PSCs and upper-tropospheric cirrus clouds. - to adapt a laser-diode spectrometer system used to measure HNO3 for use in stratospheric conditions and incorporate it into the CVI sampling system. This instrument would then be used to measure the nitric acid content of polar stratospheric cloud elements.
- to prepare an instrument payload which can be used to directly and in real time measure the composition of the condensed phase in PSCs with quantitative determination of H2O and HNO3 contents. Together with simultaneous measurements of the cloud element number concentration (COPAS project) the goal of the experiment is a comprehensive in situ analysis of the condensed phase in polar stratospheric clouds.

The project involves developing two new instruments for measuring H2O and HNO3, incorporating the
instruments into a common payload for deployment on the M-55 Geophysica aircraft, and interfacing the
combined instrument payload with the Counterflow Virtual Impactor (CVI) probe developed as part of the
COPAS project.
a) H2O Instrument
The new H2O instrument design will be based on current Lymanhygrometers presently in use on various
aircraft payloads. A new instrument based upon these hygrometers which will be optimised for use in the
stratosphere will be constructed. The largest challenge will be to increase the sensitivity of the new hygrometer
by a factor of 100 relative to the instruments now in use. There are several ways to achieve this decrease in
detection limit: a) increase the path length between the Lyman- light source and the detector; b) increase the
strength of the light source; c) add fluorescence detectors in parallel with the absorption detectors; d) increase
the enrichment factor of the CVI probe. Part of the initial design effort will be to determine the optimal
combination of these approaches in terms of achieving a small, lightweight and robust hygrometer.
b) HNO3 instrument
The measurement of the HNO3 concentration with high sensitivity can be performed by means of absorption
measurements of laser diode radiation at 7.5mm over a long path, as it can be realised in a multipass gas cell.
The project will develop and test all the optical systems for detecting the absorption lines of HNO3 and be
responsible for certification of the optical systems. For this purpose, an improved Herriot cell (after a
preliminary test with a White cell) will be employed. It is planned to develop a folded-type Herriot cell
exhibiting relative insensitivity to small misalignment and displacements of the mirrors which can result from
mechanical vibrations.
c) Chemistry Payload/CVI Consolidation
Once the two instruments are constructed and initial tests performed, they will be physically consolidated into
a single payload and tested. Due to weight and space constraints, all of the CVI instrumentation will have to
be co-located on the M-55. The flow requirements of the instruments will to a large extent determine the exact
configuration of the CVI probe.

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Coordinateur

STOCKHOLM UNIVERSITET
Contribution de l’UE
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Adresse
12,Svante Arrheniusväg 12, Arrhenius Laboratory
106 91 STOCKHOLM
Suède

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