which has the ability to characterize and calibrate radiometric instruments with spectrally pure sources of differing flux levels over a wide range of wavelengths (ultraviolet to infrared), with detectors tied to cryogenic cavity radiometers (absolute accuracy 0.001 percent for power measurement) and apertures measured at NIST.


Key uncertainties in the composition and properties of aerosol particles and their role in clouds require measurements and models under controlled conditions. Laboratory and process studies are needed to resolve these questions by four types of measurements. First, measurement of the composition and thermodynamic properties of organic and inorganic components, together with development of intelligent parameterizations of these properties, is necessary to describe the properties of individual particles. Second, measurements of spectrally resolved imaginary refractive indices are needed to determine absorbing properties. The third type of measurement characterizes the morphology and reactivity of particle surfaces. Finally, surface tension and wettability of organic particles must be measured in order to predict cloud droplet activation properties.

The thermodynamic properties of organic and some mineral components are not well understood. Most particle types show a strong hysteresis effect between the relative humidity for deliquescence (conversion from solid to liquid) and the relative humidity for efflorescence (conversion from liquid to solid). This hysteresis effect is not well characterized in the laboratory or in the field, yet it plays a critical role in particle optical properties. The partitioning of semivolatile organic compounds between the gas and the aerosol phase also needs to be better determined as a function of temperature and aerosol phase composition.

The optical properties of organic compounds present in aerosols are poorly known. Much of the existing information is limited to the UV and driven by the needs of the polymer industry. Very little information exists for spectrally resolved imaginary refractive indices in the visible spectrum. Because these measurements are relatively routine but very time-consuming, there has been little interest in the research community in collecting the required database of optical properties.

The third type of measurement is the reactivity of ambient particles, given their shapes and structures. Such reactivity may include probabilities of inorganic and organic reactions that affect particle lifetime, distribution, and optical behavior. Porosity and surface area strongly determine the rates and yields of heterogeneous chemical reactions, yet very little is known about these characteristics for ambient particles.

A fourth property of chemical mixtures that is not well understood for

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