Paul J. DeMott talks with
ScienceWatch.com and answers a few questions about
this month's Emerging Research Front Paper in the field of
Geosciences.
Article: African dust aerosols as atmospheric ice
nuclei
Authors: DeMott,
PJ;Sassen, K;Poellot, MR;Baumgardner, D;Rogers,
DC;Brooks, SD;Prenni, AJ;Kreidenweis, SM
Journal: GEOPHYS RES LETT, 30 (14): art. no.-1732 JUL 17
2003
Addresses: Colorado State Univ, Dept Atmospher Sci, Ft
Collins, CO 80526 USA.
Colorado State Univ, Dept Atmospher Sci, Ft Collins, CO
80526 USA.
Univ Alaska Fairbanks, Inst Geophys, Fairbanks, AK 99775
USA.
Univ N Dakota, Dept Atmospher Sci, Grand Forks, ND 58201
USA.
Univ Nacl Autonoma Mexico, Mexico City 04510, DF,
Mexico.
Natl Ctr Atmospher Res, Boulder, CO 80307 USA.
Why do you think your paper is highly
cited?
I think the paper highlighted a growing recognition that mineral dusts may
play complex roles in affecting climate, due not only to their impacts on
radiative transfer but also due to their action as favored nuclei for
forming warm and cold clouds. Our paper confirmed, through actual airborne
measurements, the particular action of mineral dusts as nuclei for the
formation of ice in clouds, even after transport for thousands of
kilometers.
The paper is one of the first that demonstrates this via field measurements
and not through only theoretical or modeling arguments. This behavior of
dusts is not yet explicitly accounted for in climate change assessments and
requires consideration also in the context of net impacts (thermodynamic
and microphysical) of dust plume ingestion by hurricanes in their formative
stages. Yet, mineral dusts travel widely in the atmosphere at all times of
the year and so it is vital to understand their microphysical interactions
with all types of clouds.
Does it describe a new discovery, methodology, or
synthesis of knowledge?
Previous knowledge on the ice nucleation behavior of mineral dusts in the
atmosphere came from measurements of composition of the central nuclei of
snow crystals and from laboratory studies of specific minerals or mixes of
these. It stood to reason that if one encountered a concentrated dust
plume, such as that we encountered within the Saharan aerosol layer near
Florida, the number concentrations of ice nuclei would be greatly elevated,
but until recently these ice nuclei number concentrations could not be
measured in real-time from aircraft.
"The influence of man’s
activities on climate is something that has
long been of concern to scientists, and is
now increasingly of concern to many people
around the world."
Our measurements confirmed that ice nuclei concentrations increased when
the aircraft penetrated the dust layers. Together with other measurements
of cloud properties, we showed that the dust particles help ice particles
to form at warmer temperatures and in greater numbers in dust-affected
clouds. The newness involved was the technological capability to reliably
measure the relatively small number concentrations of ice nuclei (sometimes
only 1 in 10,000 of cloud condensation nuclei) in real-time, to collect the
nuclei to confirm their nature, and to have enough supporting measurements
(lidar, other aerosol measurements) along to clearly associate the ice
nuclei with the transported dust layer. We were the only group able to make
these types of measurements at that time.
Would you summarize the significance of your paper in
layman's terms?
This research made it crystal clear that the emission and transport through
the atmosphere of mineral dusts is a major global source for particles that
form ice in clouds, a process that is a necessary first step in
precipitation formation for many clouds on earth. By implication, such
particles represent a large potential lever on the hydrologic cycle, so new
measurements are needed and models ultimately need to carefully consider
their sources and sinks in the context of human impacts and climate change.
How did you become involved in this research and were
any particular problems encountered along the way?
My involvement in this research came about through my interests in ice
nucleation and cloud microphysics, which led us naturally from studies in
the laboratory to ones on aircraft in the atmosphere. Our goals in the
experiment discussed in the paper were to understand the mechanisms by
which aerosols in the lower atmosphere impact clouds through the depth of
the troposphere. We approached this through detailed measurements of the
aerosols entering clouds and through similar analyses of particles that had
activated to form ice; those collected directly as ice crystals (and
evaporated to release the nuclei) from within the colder regions of clouds.
The measurements themselves involved 10+ years of team development of an
instrument to measure ice nuclei with fast response by exposing aerosol to
similar conditions as might be encountered in a cloud. As any atmospheric
experimentalist knows, instrument development efforts are oftentimes works
in progress. Each campaign brings its stress, defeats, and successes, and
one can only hope that over time the trend is firmly toward the latter. The
dust layer measurements we made were conducted within the last two days of
a month-long project that began with an inoperable instrument. We left
exhausted, but delighted that on return to our flight base on these last
two days, we were able to obtain a unique data set that has stimulated
long-term interest.
Where do you see your research leading in the
future?
The period of time following the paper release has seen an explosion of
research into the impacts of aerosols on clouds and precipitation. Much new
effort has been expended in laboratories throughout the world to
investigate the behaviors of collected mineral dusts and their specific
components as cloud nuclei. Simultaneously, many more investigators have
now fashioned new instrumentation for obtaining measurements of ice
nucleating aerosols in the laboratory and in the atmosphere. I see a period
in which these measurements will be made widely and tied closely to
documenting real effects on clouds. This information will be assimilated
toward developing and improving theoretical and parametric descriptions
that can be put in models that account for source emission through
atmospheric and climate impacts.
This also largely describes where my own research is leading. At the same
time, mineral dusts are not the only players. We need to understand direct
and indirect human contributions to the populations of cloud-active
aerosols via our land and energy usages, and we need to understand the
dynamic contribution that may come also from biological particles, certain
of which represent some of the best ice nucleators known to man. Therefore,
I am pursuing and anticipate more cross-disciplinary research in this area.
Do you foresee any social or political implications for
your research?
The influence of man's activities on climate is something that has long
been of concern to scientists, and is now increasingly of concern to many
people around the world. As an example of implications of our work,
activities that result in desertification of larger regions of the earth
could lead to increased emissions of dusts active as ice nuclei in clouds,
with unknown consequences for the global hydrological cycle. Changes in
precipitation pattern and amounts might serve to moisten dry areas, or
might lead to additional drying of arid regions, or might have no
discernable impact.
Particles in the air know no boundaries, so changes in natural emissions or
actions in other regions that alter human sources of aerosols can influence
clouds at long distances. As we increase our understanding of these complex
processes and improve models by incorporating details on aerosol impacts at
the cloud or storm scale, we will better have the ability to enter
discussions across national and political boundaries about strategies to
minimize or mitigate climate impacts.
Dr. Paul J. DeMott
Senior Research Scientist/Scholar
Department of Atmospheric Science
Colorado State University
Fort Collins, CO, USA
Keywords: UNITED-STATES; MINERAL DUST; DESERT DUST; TRANSPORT;
ISRAEL.