AEROSOL

To gain improved understanding and model-based representation of aerosol radiative influences an IOP (Intensive Observational Period) was conducted at the Department of Energy's Atmospheric Radiation Measurement (ARM) Southern Great Plains Site in north central Oklahoma, in May 2003. This experiment used ground and airborne measurements of aerosol absorption, scattering, and extinction over the ARM SGP site to characterize the routine ARM aerosol measurements, and help resolve differences between measurements and models of diffuse irradiance at the surface. The assessments of aerosol optical thickness and aerosol absorption was carried out in conjunction with measurements of downwelling direct and diffuse irradiance as a function of wavelength and altitude. The IOP carried out a variety of closure experiments on aerosol optical properties and their radiative influence. Measurements of the aerosol chemical composition and size distribution allowed testing of the ability to reconstruct optical properties from these measurements. Additional effort was directed toward measurement of cloud condensation nucleus concentration as a function of supersaturation and relating CCN concentration to aerosol composition and size distribution. This relation was central to description of the aerosol indirect effect.

Activity Summary

The Aerosol IOP was conducted between May 5-31, 2003 over the ARM SGP site. There were a total of 16 science flights, for a total of 60.6 flight hours, conducted by the CIRPAS Twin Otter aircraft on 15 days during this period. Most of the Twin Otter flights were conducted under clear or partly cloudy skies to assess aerosol impacts on solar radiation. Additional flights were used to target mostly cloudy skies to assess aerosol/cloud interactions, test our understanding and model representation of aerosol activation, and to test how well surface remote sensing of the indirect effect works.

The field deployment phase of the Aerosol IOP was successful in a number of ways. First, the instruments deployed on the aircraft and on the surface generally worked very well and acquired the data required to address the IOP objectives. These instruments included both well established and newly developed airborne instruments to measure aerosol optical properties (scattering, absorption, and extinction), aerosol size distribution, and cloud condensation nuclei (CCN), as well as surface based instruments to measure aerosol composition, aerosol optical properties, and cloud condensation nuclei. A newly developed stabilized platform for zenith viewing radiometric instruments was also deployed on the CIRPAS Twin Otter aircraft and worked well for most of the mission, including the flights designed to assess direct and diffuse radiation. Additional airborne and surface instruments acquired the desired measurements of solar direct and diffuse measurements. Additional surface-based instruments deployed at the SGP Central Facility, including several instruments from investigators funded by the DOE Atmospheric Chemistry Program (ACP), measured aerosol size distributions, aerosol mass, and composition.

During the IOP, several different datasets that were used to address the IOP objectives were successfully acquired. One objective of the IOP was to measure aerosol optical properties (scattering, absorption, and extinction) using a number of different instruments simultaneously with measurements of direct and diffuse solar radiation in order to better understand and model the impact of aerosols on direct and diffuse radiation. Particular emphasis was placed on the role of aerosol absorption. Data to examine this issue and to accurately characterize aerosol absorption were acquired during a number of flights. Another objective was to examine the ability of routine Raman lidar and In Situ Aerosol Profiling (IAP) measurements to characterize ambient aerosol extinction. Four coordinated flights were conducted so that the IAP and Raman lidar measurements could be evaluated using the more extensive instrumentation on the CIRPAS Twin Otter. This IOP also was the first time the ARM program acquired airborne CCN measurements over the SGP site. These data were used to assess how aerosols impact cloud properties and help develop algorithms to use routine ARM surface and remote sensing data to model cloud properties and their impact on radiation.

Several new and/or upgraded instruments were deployed over the ARM SGP for the first time during this mission. For example, two new instruments, a cavity ring-down cell (NASA Ames) that measured aerosol extinction and a photoacoustic sampler (Desert Research Institute) that measured aerosol absorption, were deployed. A newly developed stabilized platform for zenith viewing radiometric instruments was also deployed and worked well. New surface and airborne instruments (Cal Tech and DRI) to measure Cloud Condensation Nuclei were also deployed.

Several times during the IOP, elevated aerosol layers were observed over the ARM SGP site. These layers, which were present 2-5 km above the surface, are often the result of the transport of smoke, dust, or pollution from long distances away. Observations of these layers during the IOP indicated that these layers may be more common than originally thought, and can have a substantial impact on the atmospheric radiation budget. CIRPAS Twin Otter flights were also conducted in coordination with overpasses of NASA Terra and Aqua satellites. These aircraft and satellite measurements will be used to evaluate how the satellite, airborne, and surface network measurements can be used to study the horizontal variability of aerosols and clouds.

The success of this IOP was due to the hard work and dedicated efforts from a large team of ARM and ACP scientists and investigators, CIRPAS Twin Otter and Cessna pilots, crew, and support personnel, SGP site personnel, ARM infrastructure support, weather forecaster, and support from Greenwood Aviation at the Ponca City airport. We thank ARM for their support of this IOP.

Timeline

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