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Jost Heintzenberg and Robert J. Charlson

Abstract

The purpose of this paper is to document the key literature references and to describe the design philosophy. the principles of the instrument, the various possible designs, calibration, systematic errors, applications to scientific problems and inherent limitations. According to the design philosophy established in the original publication, instruments are devised to directly measure the relevant integral aerosol parameters, thus eliminating the need for assumptions about particle size distribution, particle shape and composition, complex Mie calculations, and the unknown uncertainties associated with them. The key parameter measured by the integrating nephelometer is the scattering component of extinction as a function of wavelength. This philosophy subsequently allows two approaches to the determination of several parameters—direct measurement with the aid of the integrating nephelemeter and calculation via the Mie formalism. Comparison of calculated and measured values for a parameter allows closure studies; that is, the difference between them is an objective measure of the uncertainty that is inherent in the combined set of measured and calculated parameter values.

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Bert Bollin, Georg Witt, and Robert J. Charlson
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Maria Cristina Facchini, Mihaela Mircea, Sandro Fuzzi, and Robert J. Charlson
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Ari Laaksonen, Pekka Korhonen, Markku Kulmala, and Robert J. Charlson

Abstract

A generalized reformulation of the Köhler theory to include the effect of soluble gases and slightly soluble aerosol substances is presented. A single equation is derived that takes into account 1) the Kelvin effect; 2) the Raoult effect caused by highly soluble aerosol material (salt); 3) increase in droplet radius due to an undissolved, insoluble, or slightly soluble core; 4) contribution of solute into the droplet by a slightly soluble substance; and 5) contribution of hygroscopic material into the droplet by a soluble trace gas allowed to deplete from the gas phase because of the condensational growth of the droplets (assuming a monodisperse size distribution). Model calculations are presented for a system in which the aerosol is composed of a slightly soluble CaSO4 core coated with ammonium sulfate, and the gas phase contains HNO3. It is shown that the resulting equilibrium curves allow the occurrence of stable, unactivated droplets with radii up to about 10 μm at realistic ambient conditions. The equilibrium curves show in some cases local minima and maxima, the reasons for and consequences of which are discussed. The results of this study suggest that a new definition for “activated droplet” is needed.

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Frida A.-M. Bender, Anders Engström, Robert Wood, and Robert J. Charlson

Abstract

The hemispheric symmetry of albedo and its contributing factors in satellite observations and global climate models is evaluated. The analysis is performed on the annual mean time scale, on which a bimodality in the joint distribution of albedo and cloud fraction is evident, resulting from tropical and subtropical clouds and midlatitude clouds, respectively. Hemispheric albedo symmetry is not found in individual ocean-only latitude bands; comparing the Northern and Southern Hemisphere (NH and SH), regional mean albedo is higher in the NH tropics and lower in the NH subtropics and midlatitudes than in the SH counterparts. This follows the hemispheric asymmetry of cloud fraction. In midlatitudes and tropics the hemispheric asymmetry in cloud albedo also contributes to the asymmetry in total albedo, whereas in the subtropics the cloud albedo is more hemispherically symmetric. According to the observations, cloud contributions to compensation for higher clear-sky albedo in the NH come primarily from cloud albedo in midlatitudes and cloud amount in the subtropics. Current-generation climate models diverge in their representation of these relationships, but common features of the model–data comparison include weaker-than-observed asymmetry in cloud fraction and cloud albedo in the tropics, weaker or reversed cloud fraction asymmetry in the subtropics, and agreement with observed cloud albedo asymmetry in the midlatitudes. Models on average reproduce the NH–SH asymmetry in total albedo over the 60°S–60°N ocean but show higher occurrence of brighter clouds in the SH compared to observations. The albedo bias in both hemispheres is reinforced by overestimated clear-sky albedo in the models.

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Cynthia H. Twohy, Philip A. Durkee, Barry J. Huebert, and Robert J. Charlson

Abstract

Aerosol particles can act as cloud condensation nuclei and thereby influence the number and size of droplets in clouds. Consequently, anthropogenic particles have the potential to influence global climate by increasing cloud albedo and decreasing precipitation efficiencies. Enhanced cloud reflectances associated with increases in panicle number have been observed, but our understanding of these interactions has been hindered by incomplete empirical studies and models of limited scope.

In this study, aerosol and droplet size distributions were measured on 13 research flights in stratiform clouds within 300 km west of the northern California coast. The chemical composition of the droplet solute was also assessed. Microphysical and chemical properties indicated that most of the clouds were influenced by pollution from the North American continent, but pristine marine clouds were sampled on one flight during westerly flow conditions. Data from this flight and another, representing a pristine and polluted environment, were compared with high-resolution satellite observations.

In the polluted case, particle and droplet number concentrations decreased, mean droplet size increased, and satellite-derived reflectance at 3.7 μm decreased with increasing distance from the northern California urban region. Relative to the unpolluted stratiform cloud, the polluted cloud had, on average, a sulfate concentration that was higher by an order of magnitude, droplet number concentrations higher by a factor of 6, droplet sizes smaller by a factor of 2, and 3.7-μm reflectance that was higher by a factor of 2. However, no significant difference in the visible reflectance was detected between the two cases, probably a result of differences in liquid water path.

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David S. Ensor, William M. Porch, Michael J. Pilat, and Robert J. Charlson

Abstract

The possible climatic effects of the secular increase of aerosols from man's activities have been coupled with the microphysics of the aerosol properties. The magnitude of the critical aerosol absorption coefficient to backscatter coefficient, (b abs/ b bs)critical, was estimated for a model atmosphere corresponding to cooling or heating of the earth with increasing aerosol concentration. The b abs/b bs ratio was calculated with Mie theory assuming a Junge particle size distribution and spherical particles as a function of the imaginary part of the particle refractive index (particle light absorption) and the size distribution slope. Comparing the b abs/b bs ratio calculated from Mie theory to the critical b abs/ b bs, cooling might ensue if the imaginary part is less than 10−3 while heating may result if it is greater than 0.1.

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Theodore L. Anderson, Robert J. Charlson, David M. Winker, John A. Ogren, and Kim Holmén

Abstract

Tropospheric aerosols are calculated to cause global-scale changes in the earth's heat balance, but these forcings are space/time integrals over highly variable quantities. Accurate quantification of these forcings will require an unprecedented synergy among satellite, airborne, and surface-based observations, as well as models. This study considers one aspect of achieving this synergy—the need to treat aerosol variability in a consistent and realistic way. This need creates a requirement to rationalize the differences in spatiotemporal resolution and coverage among the various observational and modeling approaches. It is shown, based on aerosol optical data from diverse regions, that mesoscale variability (specifically, for horizontal scales of 40–400 km and temporal scales of 2–48 h) is a common and perhaps universal feature of lower-tropospheric aerosol light extinction. Such variation is below the traditional synoptic or “airmass” scale (where the aerosol is often assumed to be essentially homogeneous except for plumes from point sources) and below the scales that are readily resolved by chemical transport models. The present study focuses on documenting this variability. Possible physical causes and practical implications for coordinated observational strategies are also discussed.

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Stephen E. Schwartz, Robert J. Charlson, Ralph A. Kahn, John A. Ogren, and Henning Rodhe

Abstract

The observed increase in global mean surface temperature (GMST) over the industrial era is less than 40% of that expected from observed increases in long-lived greenhouse gases together with the best-estimate equilibrium climate sensitivity given by the 2007 Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). Possible reasons for this warming discrepancy are systematically examined here. The warming discrepancy is found to be due mainly to some combination of two factors: the IPCC best estimate of climate sensitivity being too high and/or the greenhouse gas forcing being partially offset by forcing by increased concentrations of atmospheric aerosols; the increase in global heat content due to thermal disequilibrium accounts for less than 25% of the discrepancy, and cooling by natural temperature variation can account for only about 15%. Current uncertainty in climate sensitivity is shown to preclude determining the amount of future fossil fuel CO2 emissions that would be compatible with any chosen maximum allowable increase in GMST; even the sign of such allowable future emissions is unconstrained. Resolving this situation, by empirical determination of the earth’s climate sensitivity from the historical record over the industrial period or through use of climate models whose accuracy is evaluated by their performance over this period, is shown to require substantial reduction in the uncertainty of aerosol forcing over this period.

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