2011 - Cooperative Institute for Research in Environmental Sciences ...

Margaret Tolbert
Laboratory Studies of Clouds and Aerosols
FUNDING: NASA, NATIONAL SCIENCE FOUNDATION
Cirrus clouds,
composed of water ice,
cover up to 30 percent
of the Earth’s surface at
any time, and subvisible
cirrus are almost always
present in parts of the
tropics. Cirrus and subvisible
cirrus clouds play
an important role in the
climate system as well as
in controlling the amount
of water getting into the
stratosphere. The clouds
are usually optically thin
in visible wavelengths,
allowing most, but not
all, sunlight to reach the
Earth’s surface. In contrast,
the outgoing infrared
radiation is efficiently absorbed by cirrus ice particles.
While the net effect of cirrus clouds on climate is usually
a warming at the surface, the microphysical properties
of the clouds dictate the overall climatic impact. The microphysical
properties, in turn, depend on the nucleation
mechanism of ice in the atmosphere. In laboratory studies,
our research group is examining ice nucleation on a wide
range of possible atmospheric aerosols including organics,
minerals, sulfates and combinations of these species.
To study ice nucleation, we are using a combination
of optical and Raman microscopy. In an environmental
cell, we expose aerosols to increasing relative humidity
at low temperature and detect ice nucleation using
optical microscopy. We then evaporate the ice and use
Raman spectroscopy to identify the chemical nature of the
particles that nucleated ice. In this way, we can identify
the species most likely to nucleate ice, and also determine
the atmospheric conditions necessary for ice nucleation.
In addition to laboratory studies on well-defined particles
of known composition, studies are also probing the icenucleating
ability of particles collected in the field.
Other work in our laboratory is examining the direct
effect of aerosols on climate. Here cavity ring down aerosol
spectroscopy is used to determine the real and imaginary
refractive indices of particles likely to be present in
the atmosphere. Studies are also performed to determine
how the particle optical properties change upon exposure
to increased relative humidity. In this work, studies are
performed on both well-characterized laboratory samples
and on model secondary organic aerosol particles formed
in chambers. Ongoing work is examining how the optical
properties of the particles change as they are aged through
simulated oxidation in the atmosphere.
In parallel to studies of clouds and aerosols in the
Earth’s atmosphere, additional studies are probing clouds
and aerosols in other planetary environments. In one
project, we are studying the chemical composition and
optical properties of the organic haze that completely
52 CIRES Annual Report 2011
Figure 1. Raman map of a particle collected at Storm Peak Laboratory
during Storm Peak Aerosol and Cloud Characterization Study (SPACCS)
in 2010. Many of the observed particles were composed of mixtures of
sulfates and organics. Raman mapping allows the mixing state of the
particles to be determined. This particle shows a sulfate core with an
organic coating. Ice nucleation studies are performed on the samples
collected during the Storm Peak study.
Figure 2. Particles scatter and absorb sunlight and, thus, directly impact
climate and visibility. Large (wet) particles scatter more light than small
particles. We are using cavity ring down (CRD) aerosol extinction spectroscopy
to probe the direct effect of particles on climate.
shrouds Titan, a moon of Saturn. Another project focuses
on understanding heterogeneous (gas-surface) chemistry
in the Martian environment. Finally, studies that compare
these faraway places today with our own Earth, billions
of years ago, are underway to examine Earth’s earliest
atmosphere.