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J. E. Frederick
and
R. D. Hudson

Abstract

Knowledge of the agronomic production of odd hydrogen in the dissociation of water vapor is limited by uncertainties in the penetration of solar irradiance in the Schumann-Rung bands of O2 and by incomplete information concerning the products of photolysis at Lyman alpha. Consideration of an error sources involved in computing the H2o dissociation rate in the wavelength region 175–200 nm leads to an estimated uncertainty of ±35% at an altitude of 90 km for an overhead sun. The uncertainty increases with decreasing altitude such that the true dissociation rate at 60 km for an overhead sun lies between 0.45 and 1.55 times the result computed using the best input parameters currently available. Calculations of the H2o dissociation rate by Lyman alpha should include the variation in O2 opacity across the solar line width. Neglect of this can lead to errors as large as 50% at altitudes where the process is the major source of odd hydrogen.

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J. E. Frederick
and
R. D. Hudson

Abstract

Oscillator strengths and predissociation linewidths deduced in recent studies predict a dissociation rate for O2 in the Schumann-Runge bands which is significantly larger in the upper stratosphere and lower mesosphere than previously believed. Error bars on molecular parameters required in the cross-section calculation translate into uncertainties in the dissociation rate which are less than ±10% at all altitudes where the Schumann-Runge bands are agronomically significant.

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J. E. Frederick
and
R. D. Hudson

Abstract

Absorption of solar photons by nitric oxide in the wavelength ranges 181.3–183.5 and 189.4–191.6 nm leads to predissociation of the molecule in the mesosphere and upper stratosphere. Molecular oxygen controls the penetration of the required solar irradiance via absorption in the Schumann-Runge bands, while attenuation due to ozone becomes significant in the upper stratosphere. The calculation of the nitric oxide dissociation rate is complicated by the need to include all rotational fine structure in both the NO and O2 cross sections. The dissociation rate computed here for the upper mesosphere is a factor of 3.6 less than that reported in past work when currently accepted values of the oscillator strengths and solar irradiance are used. In addition, improved molecular parameters describing the O2 cross section predict less attenuation of the dissociation rate with decreasing altitude than results previously available.

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James G. Hudson
,
Timothy J. Garrett
,
Peter V. Hobbs
,
Scott R. Strader
,
Yonghong Xie
, and
Seong Soo Yum

Abstract

Enhancements of droplet concentrations in clouds affected by four ships were fairly accurately predicted from ship emission factors and plume and background cloud condensation nucleus (CCN) spectra. Ship exhausts thus accounted for the increased droplet concentrations in these “ship tracks.” Derived supersaturations were typical of marine stratus clouds, although there was evidence of some lowering of supersaturations in some ship tracks closer to the ships where CCN and droplet concentrations were very high.

Systematic differences were measured in the emission rates of CCN for different engines and fuels. Diesel engines burning low-grade marine fuel oil produced order of magnitude higher CCN emissions than turbine engines burning higher-grade fuel. Consequently, diesel ships burning low-grade fuel were responsible for nearly all of the observed ship track clouds. There is some evidence that fuel type is a better predictor of ship track potential than engine type.

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Kerri A. Pratt
,
Andrew J. Heymsfield
,
Cynthia H. Twohy
,
Shane M. Murphy
,
Paul J. DeMott
,
James G. Hudson
,
R. Subramanian
,
Zhien Wang
,
John H. Seinfeld
, and
Kimberly A. Prather

Abstract

During the Ice in Clouds Experiment–Layer Clouds (ICE-L), aged biomass-burning particles were identified within two orographic wave cloud regions over Wyoming using single-particle mass spectrometry and electron microscopy. Using a suite of instrumentation, particle chemistry was characterized in tandem with cloud microphysics. The aged biomass-burning particles comprised ∼30%–40% by number of the 0.1–1.0-μm clear-air particles and were composed of potassium, organic carbon, elemental carbon, and sulfate. Aerosol mass spectrometry measurements suggested these cloud-processed particles were predominantly sulfate by mass. The first cloud region sampled was characterized by primarily homogeneously nucleated ice particles formed at temperatures near −40°C. The second cloud period was characterized by high cloud droplet concentrations (∼150–300 cm−3) and lower heterogeneously nucleated ice concentrations (7–18 L−1) at cloud temperatures of −24° to −25°C. As expected for the observed particle chemistry and dynamics of the observed wave clouds, few significant differences were observed between the clear-air particles and cloud residues. However, suggestive of a possible heterogeneous nucleation mechanism within the first cloud region, ice residues showed enrichments in the number fractions of soot and mass fractions of black carbon, measured by a single-particle mass spectrometer and a single-particle soot photometer, respectively. In addition, enrichment of biomass-burning particles internally mixed with oxalic acid in both the homogeneously nucleated ice and cloud droplets compared to clear air suggests either preferential activation as cloud condensation nuclei or aqueous phase cloud processing.

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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
, and
Ian Brooks

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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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
, and
Robert A. Pockalny

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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P. A. Durkee
,
K. J. Noone
,
R. J. Ferek
,
D. W. Johnson
,
J. P. Taylor
,
T. J. Garrett
,
P. V. Hobbs
,
J. G. Hudson
,
C. S. Bretherton
,
G. Innis
,
G. M. Frick
,
W. A. Hoppel
,
C. D. O’Dowd
,
L. M. Russell
,
R. Gasparovic
,
K. E. Nielsen
,
S. A. Tessmer
,
E. Öström
,
S. R. Osborne
,
R. C. Flagan
,
J. H. Seinfeld
, and
H. Rand

Abstract

Anomalously high reflectivity tracks in stratus and stratocumulus sheets associated with ships (known as ship tracks) are commonly seen in visible and near-infrared satellite imagery. Until now there have been only a limited number of in situ measurements made in ship tracks. The Monterey Area Ship Track (MAST) experiment, which was conducted off the coast of California in June 1994, provided a substantial dataset on ship emissions and their effects on boundary layer clouds. Several platforms, including the University of Washington C-131A aircraft, the Meteorological Research Flight C-130 aircraft, the National Aeronautics and Space Administration ER-2 aircraft, the Naval Research Laboratory airship, the Research Vessel Glorita, and dedicated U.S. Navy ships, participated in MAST in order to study processes governing the formation and maintenance of ship tracks.

This paper tests the hypotheses that the cloud microphysical changes that produce ship tracks are due to (a) particulate emission from the ship’s stack and/or (b) sea-salt particles from the ship’s wake. It was found that ships powered by diesel propulsion units that emitted high concentrations of aerosols in the accumulation mode produced ship tracks. Ships that produced few particles (such as nuclear ships), or ships that produced high concentrations of particles but at sizes too small to be activated as cloud drops in typical stratocumulus (such as gas turbine and some steam-powered ships), did not produce ship tracks. Statistics and case studies, combined with model simulations, show that provided a cloud layer is susceptible to an aerosol perturbation, and the atmospheric stability enables aerosol to be mixed throughout the boundary layer, the direct emissions of cloud condensation nuclei from the stack of a diesel-powered ship is the most likely, if not the only, cause of the formation of ship tracks. There was no evidence that salt particles from ship wakes cause ship tracks.

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