Scientific Theme: Advanced Modeling and Observing Systems

Scientific Theme: Regional Processes
ongoing. Laboratory experiments are planned to examine the composition of secondary organic aerosol and its
relation to aerosol hygroscopicity.
MILESTONE CSD09.2:
Plan for and participate in the Gulf of Mexico Atmospheric Composition and Climate Study in
August/September 2006, specifically investigating the role of urban aerosol in cloud microphysics
and cloud evolution in Houston.
ACCOMPLISHMENTS FOR CSD09.2:
During the summer of 2006, an airborne study of aerosol-cloud interactions was undertaken through a partnership
between NOAA, the California Institute of Technology, the Center for Interdisciplinary Remotely-Piloted Aircraft
Studies (CIRPAS), and various university collaborators. The primary objectives of this study were to:
1. Study the regional-scale aerosol chemical, optical, and radiative properties in Texas/Gulf of Mexico;
2. Acquire an extensive data set on aerosol, cloud and radiation interactions to evaluate models that predict
cloud drop number concentration, cloud evolution, and cloud radiative properties;
3. Provide sub-orbital support and validation for satellite remote sensing of aerosol and aerosol-cloud
interactions.
The CIRPAS Twin Otter, the primary platform for this study, was instrumented with a suite of aerosol samplers,
cloud probes, and radiation measurements. It performed 22 flights between August 21 and September 15. Fourteen
of these focused on aerosol-cloud-radiation interactions. All 22 flights yielded valuable measurements of aerosol
composition and optical properties from a range of Houston-area sources (urban aerosol, petrochemical plants,
power plants, etc.) as well as aerosol transported to the region (Saharan dust and forest fires).
To date (with much of the analysis still in progress) a number of interesting results are emerging:
1. CIRES/CSD boundary layer model appears to capture successfully the main statistical properties of the
cloud macroscale (cloud fraction and cloud size) and microphysical fields (liquid water and drop
concentration).
2. Observed aerosol effects on cloud microphysics show that in the polluted air in Houston, a relatively small
fraction of the aerosol accumulation mode is activated. This is in contrast to cleaner air where activated
fractions are higher. The model simulations capture this low activated fraction reasonably well.
3. Clouds are significant sources of organic aerosol (oxalate).
MILESTONE CSD09.3:
Study the relative humidity dependence of aerosol light extinction for surrogate atmospheric
aerosol using cavity ring-down aerosol extinction spectroscopy.
ACCOMPLISHMENTS FOR CSD09.3:
This laboratory research helps determine improved parameterization for the relative humidity dependence of aerosol
light extinction for atmospheric aerosol and accuracy of radiative forcing calculations. The relative humidity
dependence of aerosol light extinction (fσ(ep)(80%RH, Dry)) at 532 nm for non-absorbing surrogate atmospheric
aerosols was measured to determine the influence of particle size, composition (inorganic vs. organic), and mixing
state (internal vs. external) on aerosol light scattering. Results for mixtures of NaCl and (NH4)2SO4 with a few
dicarboxylic acids are included. For atmospheric conditions, the variability in the RH dependence of aerosol light
scattering (fσ(sp)(RH, Dry)) is most sensitive to aerosol composition and size. The influence of the mixing state on
fσ(sp)(RH, Dry) is small. These laboratory results imply that fσ(sp)(RH, Dry) can be reasonably estimated from the
aerosol size distribution and composition (inorganic/organic) using the mass-weighted average of fσ(sp)(RH, Dry) for
the individual components.
This study also used laboratory-generated particles to examine the connection between aerosol light extinction,
chemical composition, and hygroscopicity for particles composed of internal mixtures of ammonium sulfate and
water-soluble organic compounds. The extinction coefficient (σep) at 532 nm was measured for size-selected
particles at