PROGRAM AREA OVERVIEW --

3. ATMOSPHERIC MEASUREMENT TECHNOLOGY

World-wide energy production is modifying the chemical composition of the atmosphere and is linked with environmental degradation and human health problems. The radiative transfer properties of the atmosphere may be changing as well. Various technological developments are needed for high accuracy and/or long term monitoring of these changes to support a strategy of sustainable and pollution-free energy development for the future.

Grant applications must propose Phase I bench tests of critical technologies. (“Critical technologies” refer to components, materials, equipment, or processes that overcome significant limitations to current capabilities, with respect to the subtopics that follow.) For example, grant applications proposing only computer modeling without physical testing will be considered non-responsive. Grant applications also should describe the purpose and benefits of any proposed teaming arrangements with government laboratories or universities in the technical approach or work plan. Applications submitted to any of the subtopics should support claims of commercial potential for proposed technologies, (e.g., endorsements from relevant industrial sectors, market analysis, or identification of potential spin-offs). Grant applications are sought only in the following subtopics:

a. Measurements of the Chemical Composition of Carbonaceous Aerosols—There is a need to develop improved measurement methods to characterize the bulk and the size-resolved chemical composition of ambient aerosols in real time, particularly carbonaceous aerosols. Improved measurements would facilitate the identification of the origin of aerosols, i.e. primary versus secondary and fossil fuel versus biogenic. Also, they would help to elucidate how aerosol particles are processed in the atmosphere by chemical reactions and by clouds, and how their hygroscopic properties change as they age. This information is important because relatively little is known about organic and absorbing particles, which are abundant in many locations in the atmosphere. In particular, there is a need for instruments suitable for real-time measurements of the composition of particles at the molecular level. Although recent advances have led to the development of new instruments such as particle mass spectrometers and single particle analyzers, these instruments still have important limitations in their ability to quantify black carbon vs. organic carbon, provide speciation of refractory and volatile organic compounds, and calibrate both organic and inorganic components. Therefore, grant applications are sought to improve these instruments, or develop new technology, in order to provide one or more of the following attributes related to the measurement of the chemical composition of carbonaceous aerosols: (1) quantifiable results over a wide range of compounds – a problem for laser ablation aerosol mass spectrometer methods; (2) measurements over a range of volatility so that dust, carbon, and salt are detectable – a problem for thermal decomposition aerosol mass spectrometers; (3) speciation of individual organics, including those containing oxygen, nitrogen, and sulfur; (4) identification of elemental carbon and other carbonaceous material, so that the makeup of the absorbing fraction is known; (5) measurements with high time resolution, an inherent problem with filter techniques; (6) identification of source markers, such as isotopic abundances in aerosols; and (7) the ability to probe the chemical composition of aerosol surfaces.

b. Instrumentation for Characterizing Atmospheric Aerosols—Improved instrumentation and techniques are required to understand other characteristics of atmospheric aerosols. This subtopic deals with three of them:

(1) Aerosol precursors. Improvements in gas phase chemistry are needed to further understand the evolution of aerosols in clouds. For example, gas phase measurements of H₂SO₄, a major aerosol precursor, have revealed a wealth of new information in the last decade. To make further progress, grant applications are sought to develop instruments that can make fast measurements of NH₃, ion clusters, and gas phase organics, substances that might either condense or dissolve into preexisting aerosols or cloud droplets.

(2) Aerosol absorption. The aerosol absorption coefficient, together with the aerosol scattering coefficient, determines the single-scattering albedo. This key aerosol property and the factors that contribute to it are critical for determining heating rates and climate forcing by aerosols. Therefore, grant applications are sought to develop reliable instruments for the in situ measurement of the single-scattering albedo for particles containing black and organic carbon, dust, and minerals. The measurements must cover the solar wavelengths (UV, visible, and near infrared), must not alter aerosol properties, and must address the influence of relative humidity.

(3) Aerosol size distributions. Knowledge of the particle size distribution is essential for describing both direct and indirect radiative forcing by aerosols. However, current techniques for determining these distributions are often ambiguous because of the assumption that the particles are spherical. In particular, the optical techniques most often used in the 0.5-10 µm size range have inherent problems. Therefore, grant applications are sought for techniques to determine the size distribution of ambient aerosols, in the 0.5-10 µm size range, that are not based on optical properties. The techniques must address the influence of relative humidity and must be integrated with the simultaneous measurements of such properties as mass, area (extinction) and number.

c. Oxygen A-band Spectrometer for Photon Pathlength Measurements—The high-resolution spectrometry of strongly absorbing atmospheric bands is capable of providing information about the distribution of scatterers and absorbers in the atmosphere. This information cannot be obtained by other passive optical remote sensing techniques. The Oxygen A-band (around 764 nm wavelength, in the near infra-red) is a particularly useful domain for remote sensing, with useful applications both in stand-alone instruments and in combination with millimeter-band cloud-sensing radar. Low-resolution A-band sensing has been used in downward looking platforms (satellite or high-altitude aircraft) to sense surface pressure and cloud-top heights. High-resolution A-band instruments were considered as part of the Picasso-CENA (now Calypso) and Cloudsat instrument packages and are being considered for future missions such as the Orbiting Carbon Observatory (OBO).

The highest performance achieved to date with an A-band instrument provides spectral coverage of at least the entire P-branch of the A-band, spectral resolution of 0.5 wavenumber or better, wavelength stability better than 1/20 of full-width at half-maximum (FWHM), and a far out-of-band (OOB) rejection ratio better than 10^-4. However, for practical purposes, this level of performance could be relaxed – a lower resolution of 5-10 wavenumbers would be sufficient to obtain two useful parameters, the mean and variance of the photon path length. Grant applications are sought to develop innovative optical detector methods to lower the cost of such moderate performance A-band instruments, in order to permit them to be more widely deployed. For this class of instruments, the OOB rejection ratio could be modestly lower, the wavelength stability must be comparable, and the stability of responsivity must be better, compared to the performance levels described above.

d. Measurement of the Size Distribution of Water Drops in Clouds—Existing in situ optical instruments, commonly used on research aircraft and at the surface, for measuring the drop size distribution of water clouds suffer inherent limitations. Because the measurement technology of these instruments (e.g., forward scattering and phase Doppler probes) is based on the light scattering properties of individual drops, the sample volume must be very small to prevent the coincidence of two or more drops from influencing the measurement. However, in clouds with low drop concentrations, these very small sample volumes make it difficult to obtain statistically significant drop samples when the drops are greater than approximately 50 µm in diameter. Yet, clouds with low drop concentrations and with drops greater than 50 µm are scientifically very important, because this is the regime in which drizzle drops (i.e., drops with diameters from 50 to 200 µm) are formed. The formation of drizzle can lead to a rapid modification of the cloud drop size distribution, which in turn has a strong influence on a cloud’s radiative properties, especially for marine and arctic stratus clouds. (For example, the formation of drizzle often leads to a rapid increase in effective drop radius and a corresponding decrease in number concentration, thereby decreasing the reflectance of a cloud.) Because stratus clouds cover a large portion of the earth, this process has a strong impact on the global radiative budget. Grant applications are sought to develop new instrument technology that is capable of providing statistically significant measurements of the size distribution of water drops with diameters from 3 to 200 µm in clouds that have a total drop concentration on the order of 10 to 100 per cubic centimeter. The new instrumentation should be capable of operation on research aircraft, tethered balloons, and on the ground without degradation in performance. The Phase I project should demonstrate the feasibility of the technology in the laboratory. In Phase II, an operational sensor should be built and tested on a research aircraft, on a tethered balloon, and on the ground. Potential commercial applications of the new sensor could extend to measurements of industrial and agricultural sprays.

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