402
L. Deguillaume et al.
But recently, the metabolic activity of microorganisms (bacteria, fungi, yeasts) has
been shown to influence cloud chemistry [3]. In clouds, microorganism abilities to
survive tend to activate their metabolism. They can resist and survive even they are
exposed to harsh conditions (cold shock, presence of oxidants, osmotic shock, UV
radiation,…) encountered in clouds. Cells in clouds can uptake molecular compounds
used as nutrients, thus contributing to cloud chemistry. More specifically, microorganisms can degrade organic compounds (such as organic acids, formaldehyde or
methanol) due to their carbon metabolism. They also modify the chemical budget of
oxidants. For example, they can defend from the presence of H 2 O 2 thanks to their
oxidative stress metabolisms; this impacts the formation of HO
• radicals that are produced by H 2 O 2 photolysis [6]. Rates of biotic and abiotic transformations determined
in microcosms mimicking cloud environments and directly in cloud waters clearly
highlighted that biodegradation are competitive with radical chemistry. However,
those experiments have been done under bulk laboratory conditions. But the cloud
system is a multiphase medium where chemical and biological transformations are
impacted by microphysical processes and mass transfers between the various phases.
The present study aims at integrating biological reactions studied in the lab into a
cloud chemistry model reproducing microphysical and mass transfer processes.
64.2 Model Results
Four chemical compounds (formic and acetic acids, formaldehyde and H 2 O 2 ) were
selected and their biodegradation rates were experimentally evaluated. Three bacteria
strains were chosen for their capacity to efficiently degrade these compounds; those
strains have been isolated from cloud waters collected at the puy de Dôme station in
France. These biodegradation transformations and the associated rates complete the
CLEPS aqueous mechanism. The cloud chemistry model used in this study considers
explicit chemistry in both gas and aqueous phases together with the dynamical mass
transfer between these two phases. The aqueous mechanism is based on a new protocol that provides a representation of most probable oxidation pathways of organic
compounds [5]. Radical versus microbiology efficiency were confronted modifying
“key” environmental parameters (temperature and actinic flux).
The chemical scenario is representative of low NO x conditions. In order to get
a chemical photostationary state, a first simulation is performed using only the gas
phase version of the model for a period of 31 days. Aqueous phase appears at noon
the 31th day and lasts 12 h. An initial concentration of 1 µM of iron in the aqueous
phase is considered, which is representative of values measured in continental cloud
water [2]. This allows recycling oxidants resulting from the iron redox cycles. The
size of the cloud droplets is fixed at 10 µM and the liquid water content is equal to
3 10
−7 vol/vol. The initial acidity is set to pH = 4 and evolves afterwards.
Simulations are realized to represent two seasons (summer vs. winter). For the
summer case, the actinic flux is at noon 1.5 higher than for a winter case. For the
summer case, the temperature is prescribed to 17 °C and for wintertime conditions,
L. Deguillaume et al.
But recently, the metabolic activity of microorganisms (bacteria, fungi, yeasts) has
been shown to influence cloud chemistry [3]. In clouds, microorganism abilities to
survive tend to activate their metabolism. They can resist and survive even they are
exposed to harsh conditions (cold shock, presence of oxidants, osmotic shock, UV
radiation,…) encountered in clouds. Cells in clouds can uptake molecular compounds
used as nutrients, thus contributing to cloud chemistry. More specifically, microorganisms can degrade organic compounds (such as organic acids, formaldehyde or
methanol) due to their carbon metabolism. They also modify the chemical budget of
oxidants. For example, they can defend from the presence of H 2 O 2 thanks to their
oxidative stress metabolisms; this impacts the formation of HO
• radicals that are produced by H 2 O 2 photolysis [6]. Rates of biotic and abiotic transformations determined
in microcosms mimicking cloud environments and directly in cloud waters clearly
highlighted that biodegradation are competitive with radical chemistry. However,
those experiments have been done under bulk laboratory conditions. But the cloud
system is a multiphase medium where chemical and biological transformations are
impacted by microphysical processes and mass transfers between the various phases.
The present study aims at integrating biological reactions studied in the lab into a
cloud chemistry model reproducing microphysical and mass transfer processes.
64.2 Model Results
Four chemical compounds (formic and acetic acids, formaldehyde and H 2 O 2 ) were
selected and their biodegradation rates were experimentally evaluated. Three bacteria
strains were chosen for their capacity to efficiently degrade these compounds; those
strains have been isolated from cloud waters collected at the puy de Dôme station in
France. These biodegradation transformations and the associated rates complete the
CLEPS aqueous mechanism. The cloud chemistry model used in this study considers
explicit chemistry in both gas and aqueous phases together with the dynamical mass
transfer between these two phases. The aqueous mechanism is based on a new protocol that provides a representation of most probable oxidation pathways of organic
compounds [5]. Radical versus microbiology efficiency were confronted modifying
“key” environmental parameters (temperature and actinic flux).
The chemical scenario is representative of low NO x conditions. In order to get
a chemical photostationary state, a first simulation is performed using only the gas
phase version of the model for a period of 31 days. Aqueous phase appears at noon
the 31th day and lasts 12 h. An initial concentration of 1 µM of iron in the aqueous
phase is considered, which is representative of values measured in continental cloud
water [2]. This allows recycling oxidants resulting from the iron redox cycles. The
size of the cloud droplets is fixed at 10 µM and the liquid water content is equal to
3 10
−7 vol/vol. The initial acidity is set to pH = 4 and evolves afterwards.
Simulations are realized to represent two seasons (summer vs. winter). For the
summer case, the actinic flux is at noon 1.5 higher than for a winter case. For the
summer case, the temperature is prescribed to 17 °C and for wintertime conditions,