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- Author or Editor: Barry J. Huebert x
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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.
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.
Although continental-scale plumes of Asian dust and pollution reduce the amount of solar radiation reaching the earth's surface and perturb the chemistry of the atmosphere, our ability to quantify these effects has been limited by a lack of critical observations, particularly of layers above the surface. Comprehensive surface, airborne, shipboard, and satellite measurements of Asian aerosol chemical composition, size, optical properties, and radiative impacts were performed during the Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia) study. Measurements within a massive Chinese dust storm at numerous widely spaced sampling locations revealed the highly complex structure of the atmosphere, in which layers of dust, urban pollution, and biomass- burning smoke may be transported long distances as distinct entities or mixed together. The data allow a first-time assessment of the regional climatic and atmospheric chemical effects of a continental-scale mixture of dust and pollution. Our results show that radiative flux reductions during such episodes are sufficient to cause regional climate change.
Although continental-scale plumes of Asian dust and pollution reduce the amount of solar radiation reaching the earth's surface and perturb the chemistry of the atmosphere, our ability to quantify these effects has been limited by a lack of critical observations, particularly of layers above the surface. Comprehensive surface, airborne, shipboard, and satellite measurements of Asian aerosol chemical composition, size, optical properties, and radiative impacts were performed during the Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia) study. Measurements within a massive Chinese dust storm at numerous widely spaced sampling locations revealed the highly complex structure of the atmosphere, in which layers of dust, urban pollution, and biomass- burning smoke may be transported long distances as distinct entities or mixed together. The data allow a first-time assessment of the regional climatic and atmospheric chemical effects of a continental-scale mixture of dust and pollution. Our results show that radiative flux reductions during such episodes are sufficient to cause regional climate change.
As part of the U.K. contribution to the international Surface Ocean-Lower Atmosphere Study, a series of three related projects—DOGEE, SEASAW, and HiWASE—undertook experimental studies of the processes controlling the physical exchange of gases and sea spray aerosol at the sea surface. The studies share a common goal: to reduce the high degree of uncertainty in current parameterization schemes. The wide variety of measurements made during the studies, which incorporated tracer and surfactant release experiments, included direct eddy correlation fluxes, detailed wave spectra, wind history, photographic retrievals of whitecap fraction, aerosolsize spectra and composition, surfactant concentration, and bubble populations in the ocean mixed layer. Measurements were made during three cruises in the northeast Atlantic on the RRS Discovery during 2006 and 2007; a fourth campaign has been making continuous measurements on the Norwegian weather ship Polarfront since September 2006. This paper provides an overview of the three projects and some of the highlights of the measurement campaigns.
As part of the U.K. contribution to the international Surface Ocean-Lower Atmosphere Study, a series of three related projects—DOGEE, SEASAW, and HiWASE—undertook experimental studies of the processes controlling the physical exchange of gases and sea spray aerosol at the sea surface. The studies share a common goal: to reduce the high degree of uncertainty in current parameterization schemes. The wide variety of measurements made during the studies, which incorporated tracer and surfactant release experiments, included direct eddy correlation fluxes, detailed wave spectra, wind history, photographic retrievals of whitecap fraction, aerosolsize spectra and composition, surfactant concentration, and bubble populations in the ocean mixed layer. Measurements were made during three cruises in the northeast Atlantic on the RRS Discovery during 2006 and 2007; a fourth campaign has been making continuous measurements on the Norwegian weather ship Polarfront since September 2006. This paper provides an overview of the three projects and some of the highlights of the measurement campaigns.
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