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ABSTRACT: In situ data from the 2nd Aerosol Characterization Experiment (ACE-2) were used to describe the aerosol optical properties in a marine environment perturbed by continental sources, such as the outflow of European aerosol and dust outbreaks from North Africa. The data consist of airborne measurements made with an integrating nephelometer and absorption photometer.The cases investigated in the ACE-2 experiment included vertical profiles flown in dusty, polluted and clean airmasses. While it was possible to describe the extensive optical properties (scattering and absorption coefficients) for the clean cases in a more general way, the polluted and dusty cases showed much variability from case to case. The intensive properties (single-scattering albedo and Ångström exponent) showed little variability with height, in agreement with other studies. A key result in this paper is the strong relationship between single-scattering albedo and transport time for the polluted cases.Calculations of aerosol optical depth (AOD) made using in situ measurements showed that in the marine-dominated airmasses the contribution from the free troposphere (FT) to the total columnar AOD is not negligible. In the polluted cases, the marine boundary layer (MBL) dominated the contribution to the total AOD, when this was topped by a clean FT. In the cases when pollution outbreak in the MBL was accompanied by a dusty FT, most of the columnar AOD was due to the dust layer.Assessment of the horizontal variability of the aerosol field, achieved by employing the dry scattering coefficient as proxy data, showed that there was variability in the aerosol concentration at scales below the airmass scale in which the aerosol was embedded. This shows that the concept that the aerosol concentration varies horizontally at the same scale as the airmass can lead to significant errors in the aerosol modelling schemes, in particular on the estimates of the impact of aerosols on the Earth's radiative balance.
Tellus B 06/2005; 57(3):247 - 260. · 4.38 Impact Factor
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ABSTRACT: In situ measurements of dry aerosol optical properties were made during the North Atlantic Aerosol Characterization Experiment in the vicinity of the Canary Islands. Profiles of aerosol scattering in 3 wavelengths and absorption coefficients are presented for 2 cases where North African mineral dust layers were found above a marine boundary layer (MBL) influenced by varying pollution level. The scattering and absorption coefficients showed large differences in magnitude and variability; not only between the 2 cases of mineral dust (e.g., the mean scattering in the green wavelength was 9.8×10−6±1.6×10−6 m−1 and 2.1×10−5±7.3×10−6 m−1), but also within a given layer (e.g., from less than 1×10−5 to 3.1×10−5 m−1 during the second flight). This was also true for the polluted MBL where it is likely due to origin, age and history of the air mass as well as on the mixing state of the MBL. The scattering, backscattering and absorption coefficients have been used to calculate aerosol optical properties such as hemispheric backscatter fraction, single scattering albedo, aerosol optical depth and the Ångström exponent, which are also presented. The hemispheric backscatter fraction was found to be smaller in the mineral dust layer than in the MBL, as was the wavelength dependence on scattering. The single scattering albedo of dry aerosols was found to vary between 0.73±0.12 and 0.91±0.12 in the dust layer and showed large variability within and between the dust cases. This might be due to the mixing state of the dust layer, but could also be explained by the dust having different optical properties depending on local origin within the same major source region.
Tellus B 02/2003; 52(2):526 - 545. · 4.38 Impact Factor
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Kevin J. Noone,
Doug W. Johnson,
Jonathan P. Taylor,
Ronald J. Ferek,
Tim Garrett,
Peter V. Hobbs,
Philip A. Durkee,
Kurt Nielsen, Elisabeth Öström,
Colin O'Dowd,
Michael H. Smith,
Lynn M. Russell,
Richard C. Flagan,
John H. Seinfeld,
Lieve De Bock,
René E. Van Grieken,
James G. Hudson,
Ian Brooks,
Richard F. Gasparovic,
Robert A. Pockalny
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ABSTRACT: A case study of the effects of ship emissions on the microphysical, radiative, and chemical properties of polluted marine boundary layer clouds is presented. Two ship tracks are discussed in detail. In situ measurements of cloud drop size distributions, liquid water content, and cloud radiative properties, as well as aerosol size distributions (outside-cloud, interstitial, and cloud droplet residual particles) and aerosol chemistry, are presented. These are related to remotely sensed measurements of cloud radiative properties.
The authors examine the processes behind ship track formation in a polluted marine boundary layer as an example of the effects of anthropogenic particulate pollution on the albedo of marine stratiform clouds.
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Kevin J. Noone, Elisabeth Öström,
Ronald J. Ferek,
Tim Garrett,
Peter V. Hobbs,
Doug W. Johnson,
Jonathan P. Taylor,
Lynn M. Russell,
Richard C. Flagan,
John H. Seinfeld,
Colin D. O’Dowd,
Michael H. Smith,
Philip A. Durkee,
Kurt Nielsen,
James G. Hudson,
Robert A. Pockalny,
Lieve De Bock,
René E. Van Grieken,
Richard F. Gasparovic,
Ian Brooks
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ABSTRACT: The effects of anthropogenic particulate emissions from ships on the radiative, microphysical, and chemical properties of moderately polluted marine stratiform clouds are examined. A case study of two ships in the same air mass is presented where one of the vessels caused a discernible ship track while the other did not. In situ measurements of cloud droplet size distributions, liquid water content, and cloud radiative properties, as well as aerosol size distributions (outside cloud, interstitial, and cloud droplet residual particles) and aerosol chemistry, are presented. These are related to measurements of cloud radiative properties. The differences between the aerosol in the two ship plumes are discussed;these indicate that combustion-derived particles in the size range of about 0.03–0.3-μm radius were those that caused the microphysical changes in the clouds that were responsible for the ship track.
The authors examine the processes behind ship track formation in a moderately polluted marine boundary layer as an example of the effects that anthropogenic particulate pollution can have in the albedo of marine stratiform clouds.
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Beat Schmid,
John M. Livingston,
Philip B. Russell,
Philip A. Durkee,
Haflidi H. Jonsson,
Donald R. Collins,
Richard C. Flagan,
John H. Seinfeld,
Santiago Gassó,
Dean A. Hegg, Elisabeth Öström,
Kevin J. Noone,
Ellsworth J. Welton,
Kenneth J. Voss,
Howard R. Gordon,
Paola Formenti,
Meinrat O. Andreae
Tellus, v.52B, 568-593 (2000).
