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Atmospheric Research 53 2000 29–43 www.elsevier.comrlocateratmos
Modeling of scavenging processes in clouds: some remaining questions about the partitioning of gases
among gas and liquid phases
N. Chaumerliac , M. Leriche, N. Audiffren
LAMPr OPGC, CNRS and UniÕersite Blaise Pascal, 24 AÕ. des Landais 63177 Aubiere Cedex France
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Abstract
Clouds can play an important role by affecting the chemical composition of the troposphere through modification of photolysis rates, and by redistributing compounds through the vertical
transport of species and their removal by wet deposition and finally by aqueous phase chemical reactions within cloud water or precipitation water. Several examples of the effects of clouds on
tropospheric chemistry are shown, using a box model or a mesoscale model illustrating the role of clouds on hydrogen peroxide: its partitioning between the gas and aqueous phases, including
deviations from Henry’s law. The main results are that deviations from Henry’s law exist even for small droplets, which are located on the edges of orographic clouds while equilibrium is attained
in the center of the cloud. The partitioning of gases is a function of the cloud development
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conditions such as the air mass in which the cloud has been formed continental vs. maritime , the Ž
. microphysical properties cloud water content, rainwater content . q 2000 Elsevier Science B.V.
All rights reserved.
Keywords: Aqueous phase chemistry; Henry’s law; Cloud chemistry modeling
1. Introduction
More than 50 of the Earth’s surface is covered by clouds and theoretical calcula- Ž
. tions of Ravishankara 1997 have shown that clouds can alter the chemical composition
of the atmosphere on a global scale. Clouds interact in many ways with chemicals on a wide range of scales, from micrometers up to thousands of kilometers.
Corresponding author. Tel.: q33-4-73407372; fax: q33-4-73271657. Ž
. E-mail address: chaumerlopgc.univ-bpclermont.fr N. Chaumerliac .
0169-8095r00r - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž
. PII: S 0 1 6 9 - 8 0 9 5 9 9 0 0 0 4 1 - 1
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On a large scale thousands of kilometers , clouds are organized in broad and complex systems that are responsible for the transport of species from the boundary
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layer to the free troposphere Renard et al., 1994; Edy et al., 1996 . Tracer redistribution can be greatly changed in case of precipitating clouds systems due to their efficient
scavenging. Photochemical processes can be modified through cloudrradiation interac- Ž
. tions Thompson, 1984 . Within these systems, each individual cloud is the host of
complex microphysical processes that influence the partitioning of species among the Ž
. air, the cloud and the precipitation Gregoire et al., 1994 . Finally, on microscale level,
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gas absorption and chemical reactions greatly depend on the microstructure of the cloud such as the droplet spectrum. Therefore, one has to consider complex interfacial transfer
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between gaseous and liquid phases Wurzler et al., 1995; Ricci et al., 1997 . Moreover, these small scale features cannot be ignored at larger scales because
removal processes and radiative properties of clouds that perturb photochemistry depend Ž
. on the microphysical characteristics of the clouds Madronich, 1987 .
In order to simulate such complex interactions on the whole range of scales at which they are important, it is necessary to use several types of models from box chemical
model, to mesoscale models. In this paper, scavenging processes that occur in clouds and their dependency on the
fine microphysical features, such as droplet size, liquid water content will be discussed for one particular chemical species, hydrogen peroxide, which is both a soluble and a
reactive compound in clouds. The way clouds interact with this particular species will be described in details. In particular, deviations from Henry’s law can occur in clouds for
this species at the sudden apparition of the aqueous phase or in a cloud, presenting
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different types of granulometry cloud droplets vs. raindrops .
2. Non equilibrium kinetics: mass-transfer of H O between gas and liquid phases
