Research Focus and Motivation
The focus of this
research is to investigate physical and chemical influences on free
tropospheric populations of aerosols involved in the formation of ice clouds, including
heterogeneous ice nuclei (IN) and soluble aerosols which can freeze
homogeneously in upper tropospheric conditions. This research extends
specialized studies of the spectroscopy of ice forming nuclei to discern
factors contributing to their sources, abundance and effectiveness. The primary
component of this work is a field study conducted
during Spring 2004 at a mountaintop laboratory in Colorado, at 3.2 km MSL, with
exposure to free tropospheric air and minimal nearby urban influences.
The motivation for
a springtime measurement campaign is the mounting evidence that mineral dust
aerosols are major components of heterogeneous IN populations and the observed
high concentrations of dust particles in the spring over elevated sites in the
The project exploits the linkage, demonstrated already
in a pilot project, of two novel measurement systems: (1) a continuous-flow ice
nucleus instrument capable of measuring the number concentrations of ice
nucleus aerosol particles active by particular ice formation mechanisms over
the full tropospheric range of supercooled temperatures; and (2) instruments
capable of characterizing the size and composition of the residuals of
nucleated ice particles in real time (single and bulk aerosol mass
spectrometers) or after collection (electron microscopy). The physical and
chemical characteristics of particles active as ice nuclei are compared with
those of the ambient atmospheric aerosol to determine which characteristics are
associated with the ability to form ice crystals at typical cirrus temperatures
and supersaturations. These characteristics can also be used to infer sources
of active ice nuclei, as well as factors that diminish effectiveness of
freezing.
Research Objectives
The specific objectives of
the proposed work are as follows:
Objective 1:
Determine the concentration of aerosols that nucleate ice, as a function of
relative humidity and temperature, during the peak in annual mineral dust
loading in the Western US.
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The left panel is from VanCuran and Cahill (2002,
J. Geophys. Res., 107, d.o.i: .1029/2002JD002204)
indicating the peak of monthly percent frequency of fine aerosol due to Asian
dust transport to the continental U.S. as interpreted from IMPROVE network
data. This indicates the spring peak in such dust influences and the stronger
influences at higher altitude sites. The right panel indicates the fine soil
contribution to aerosol mass loading at the nearby Mt. Zirkel Wilderness
IMPROVE site, contrasting the historic April-May period planned for current study
to the November period of the previous INSPECT study.
Objective 2:
Describe the chemical composition of those particles which nucleate ice.
Objective 3:
Determine the role of natural vs. anthropogenic components in ice nuclei.
Objective 4:
Elucidate the role that various aerosol components, specifically organics, play
in the enhancement or suppression of ice formation.
Objective 5:
Develop a consistent method for the ongoing study of IN.
Objective
6: Investigate the relation between physical and chemical properties of
background aerosol particles (sizes, water uptake, CCN activity) and ice nuclei
Objective
7: Composite data for use in numerical modeling applications