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Dean A. Hegg

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Lawrence F. Radke, Peter V. Hobbs, and Dean A. Hegg

Abstract

Airborne measurements over periods of several hours were made in the effluents that collected in the boundary layer in the form of “ground clouds” when an Atlas/Centaur and Titan III rocket were launched at night-time from Cape Canaveral, Florida. The ground cloud produced by the ATLAS was dry, whereas that produced by the TITAN was initially wet, then dry, and finally wet again. Both clouds dispersed primarily in the horizontal plane. Their volumes at time t (min) were given by V = V 0 t n where V 0 = 1.3 × 106 m3 and n = 0.98 for the ATLAS and V 0 &equals 1.76 × 107 m3 and n = 0.94 for the TITAN.

The ATLAS ground cloud initially contained elevated concentrations of NO, N02, hydrocarbons and particulate mass. However, dispersion of the cloud quickly reduced these concentrations and the light-scattering coefficient of the cloud. Gas-to-particle conversion (postulated to be the result of the oxidation of NO to NO2 followed by the Formation of NH4NO3) produced smoke particles at a rate of - ∼1016 s−1 in the ATLAS ground cloud but these did not contribute significantly to the total mass of particles in the cloud.

Gas-to-particle conversion in the TITAN ground cloud during its dry phase (probably produced by the reaction of HCI, from the rocket exhausts, with NH3, from the ambient air, to produce NH4Cl) created mass at a sufficient rate (∼0.1 μg m−3 min−1) to provide the potential for a significant source of pollution for several days.

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Po-Fat Yuen, Dean A. Hegg, and Timothy V. Larson

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Model calculations am presented for continental scenarios that demonstrate that the heterogeneity in the chemistry of different size cloud drops can have a significant impact on the amount of sulfate produced in cloud, its size distribution, and the consequent integral light-scattering efficiency of sulfate. The results are contrasted with previous calculations for a marine scenario.

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Dean A. Hegg, Lawrence F. Radke, and Peter V. Hobbs

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Preliminary measurements of several physical and chemical parameters associated with clouds in two cases of onshore flow over western Washington suggest that the physical and chemical properties of maritime, cloudy air passing over this region change over relatively small spatial and temporal scales (∼100–200 km, and 5- 15 h, respectively). These scales are similar to those for changes in precipitation chemistry in this region. This tentative conclusion concerning the scales for air mass changes differs from the assumption usually made concerning air mass characteristics and transport distances in the eastern United States.

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Dean A. Hegg, Ronald J. Ferek, and Peter V. Hobbs

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Cloud condensation nucleus (CCN) spectral data are presented for the Arctic in spring, which considerably augment the existing meager CCN database for the Arctic. Concurrent measurements of sulfate mass suggest that men of the CCN were commonly not sulfate. Sulfate was more closely associated with particles below the CCN size range. Some measurements of the microphysical structure of Arctic status clouds are aim described.

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Dean A. Hegg, Peter V. Hobbs, Ronald J. Ferek, and Alan P. Waggoner

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Airborne measurements of aerosol light-scattering efficiencies are presented for a portion of the northeast Atlantic seaboard of the United State during July 1993. The measurements suggest a value for the sulfate light-scattering efficiency in the range 2.2–3.2 m2 g−1, which is lower than the value used in recent modeling assessments of the climate impact of aerosols. In general the sulfate light-scattering efficiency decreased with increasing altitude in a manner consistent with concurrent measurements of aerosol size distributions. Some limited measurements of cloud condensation nuclei and sea-salt particles are also presented.

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Po-Fat Yuen, Dean A. Hegg, Timothy V. Larson, and Mary C. Barth

Abstract

Comparison of in-cloud sulfate production by a bulk-parameterized cloud model, a modified bulk parameterized model, and an explicit microphysical model for a wide variety of scenarios has been used as the basis for deriving a parameterization of the effects of heterogeneous cloud chemistry on in-cloud sulfate production. The parameterization, essentially a transfer function relating bulk and explicit model predictions, can be easily employed in large-scale Eulerian cloud models and has been demonstrated to have significant impact on predictions of sulfate deposition.

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Brian I. Magi, Peter V. Hobbs, Thomas W. Kirchstetter, Tihomir Novakov, Dean A. Hegg, Song Gao, Jens Redemann, and Beat Schmid

Abstract

Airborne in situ measurements of vertical profiles of the aerosol light scattering coefficient, light absorption coefficient, and single scattering albedo (ω 0) are presented for locations off the East Coast of the United States in July–August 2001. The profiles were obtained in relatively clean air, dominated by airflows that had passed over Canada and the Atlantic Ocean. Comparisons of aerosol optical depths (AODs) at 550 nm derived from airborne in situ and sun-photometer measurements agree, on average, to within 0.034 ± 0.021. A frequency distribution of ω 0 measured in the atmospheric boundary layer off the coast yields an average value of ω 0 = 0.96 ± 0.03 at 550 nm. Values for the mass scattering efficiencies of sulfate and total carbon (organic and black carbon) derived from a multiple linear regression are 6.0 ± 1.0 m2 (g SO=4)−1 and 2.6 ± 0.9 m2 (g C)−1, respectively. Measurements of sulfate and total carbon mass concentrations are used to estimate the contributions of these two major components of the submicron aerosol to the AOD. Mean percentage contributions to the AOD from sulfate, total carbon, condensed water, and absorbing aerosols are 38% ± 8%, 26% ± 9%, 32% ± 9%, and 4% ± 2%, respectively. The sensitivity of the above results to the assumed values of the hygroscopic growth factors for the particles are examined and it is found that, although the AOD derived from the in situ measurements can vary by as much as 20%, the average value of ω 0 is not changed significantly. The results are compared with those obtained in the same region in 1996 under more polluted conditions.

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Kathleen K. Crahan, Dean A. Hegg, David S. Covert, Haflidi Jonsson, Jeffrey S. Reid, Djamal Khelif, and Barbara J. Brooks

Abstract

Although the importance of the aerosol contribution to the global radiative budget has been recognized, the forcings of aerosols in general, and specifically the role of the organic component in these forcings, still contain large uncertainties. In an attempt to better understand the relationship between the background forcings of aerosols and their chemical speciation, marine air samples were collected off the windward coast of Oahu, Hawaii, during the Rough Evaporation Duct project (RED) using filters mounted on both the Twin Otter aircraft and the Floating Instrument Platform (FLIP) research platform. Laboratory analysis revealed a total of 17 species, including 4 carboxylic acids and 2 carbohydrates that accounted for 74% ± 20% of the mass gain observed on the shipboard filters, suggesting a possible significant unresolved organic component. The results were correlated with in situ measurements of particle light scattering (σ sp) at 550 nm and with aerosol hygroscopicities. Principal component analysis revealed a small but ubiquitous pollution component affecting the σ sp and aerosol hygroscopicity of the remote marine air. The Princeton Organic-Electrolyte Model (POEM) was used to predict the growth factor of the aerosols based upon the chemical composition. This output, coupled with measured aerosol size distributions, was used to attempt to reproduce the observed σ sp. It was found that while the POEM model was able to reproduce the expected trends when the organic component of the aerosol was varied, due to large uncertainties especially in the aerosol sizing measurements, the σ sp predicted by the POEM model was consistently higher than observed.

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Ralph A. Kahn, Tim A. Berkoff, Charles Brock, Gao Chen, Richard A. Ferrare, Steven Ghan, Thomas F. Hansico, Dean A. Hegg, J. Vanderlei Martins, Cameron S. McNaughton, Daniel M. Murphy, John A. Ogren, Joyce E. Penner, Peter Pilewskie, John H. Seinfeld, and Douglas R. Worsnop

Abstract

A modest operational program of systematic aircraft measurements can resolve key satellite aerosol data record limitations. Satellite observations provide frequent global aerosol amount maps but offer only loose aerosol property constraints needed for climate and air quality applications. We define and illustrate the feasibility of flying an aircraft payload to measure key aerosol optical, microphysical, and chemical properties in situ. The flight program could characterize major aerosol airmass types statistically, at a level of detail unobtainable from space. It would 1) enhance satellite aerosol retrieval products with better climatology assumptions and 2) improve translation between satellite-retrieved optical properties and species-specific aerosol mass and size simulated in climate models to assess aerosol forcing, its anthropogenic components, and other environmental impacts. As such, Systematic Aircraft Measurements to Characterize Aerosol Air Masses (SAM-CAAM) could add value to data records representing several decades of aerosol observations from space; improve aerosol constraints on climate modeling; help interrelate remote sensing, in situ, and modeling aerosol-type definitions; and contribute to future satellite aerosol missions. Fifteen required variables are identified and four payload options of increasing ambition are defined to constrain these quantities. “Option C” could meet all the SAM-CAAM objectives with about 20 instruments, most of which have flown before, but never routinely several times per week, and never as a group. Aircraft integration and approaches to data handling, payload support, and logistical considerations for a long-term, operational mission are discussed. SAM-CAAM is feasible because, for most aerosol sources and specified seasons, particle properties tend to be repeatable, even if aerosol loading varies.

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