David C. Woods

In Situ Observations of Aerosol and Chlorine Monoxide After the 1991 Eruption of Mount Pinatubo: Effect of Reactions on Sulfate Aerosol

Highly resolved aerosol size distributions measured from high-altitude aircraft can be used to describe the effect of the 1991 eruption of Mount Pinatubo on the stratospheric aerosol. In some air masses, aerosol mass mixing ratios increased by factors exceeding 100 and aerosol surface area concentrations increased by factors of 30 or more. Increases in aerosol surface area concentration were accompanied by increases in chlorine monoxide at mid-latitudes when confounding factors were controlled. This observation supports the assertion that reactions occurring on the aerosol can increase the fraction of stratospheric chlorine that occurs in ozone-destroying forms.

Halite Particles Injected into the Stratosphere by the 1982 El Chichón Eruption

Halite particles about 2 micrometers in size were collected by a quartz crystal microbalance cascade impactor from the El Chich�n eruption cloud in the lower stratosphere during April and May 1982. These particles are probably derived from the erupted chloride-rich, alkalic magma. Enrichments of hydrogen chloride and increases in optical depolarization in the eruption cloud observed by lidar measurements may reflect the influence of the halite particles. There is evidence that the halite particles reacted with sulfuric acid after about 1 month, releasing gaseous hydrogen chloride, which can influence the catalytic destruction of ozone in the stratosphere.

Evolution of the Pinatubo volcanic cloud over Hampton, Virginia

A ground-based lidar system at NASA Langley Research Center in Hampton, Virginia, has monitored the stratospheric aerosol vertical distribution and loading since 1974. The eruption of Mt. Pinatubo in June 1991 produced the largest enhancement of stratospheric aerosol loading ever observed by lidar over this mid-latitude location. Low altitude layers (<20 km) were the first to arrive over Hampton in early August, the result of transport associated with a tropospheric anticyclonic cell over North America. The maximum peak scattering ratio, 34 at 22.4 km, and the maximum stratospheric integrated backscatter of 0.0053 sr−1, both at 694 nm, observed since the eruption were measured on February 20, 1992. After decreasing during the spring and summer of 1992, the aerosol burden increased significantly during the winter of 1992–3, evidence of poleward winter transport from the equatorial reservoir. Over the period from February 1992 to February 1994, the stratospheric aerosol loading decreased with an average 1/e decay time of 10.1 months. The vertical distribution, intensity, and transport of Pinatubo aerosols over Hampton, Virginia, are described in detail and compared with similar measurements after El Chichon.

Relationships between optical extinction, backscatter and aerosol surface and volume in the stratosphere following the eruption of Mt. Pinatubo

The eruption of the Mt. Pinatubo volcano in the Philippines in June 1991 has resulted in increases in the surface and mass concentrations of aerosol particles in the lower stratosphere. Airborne measurements made at midlatitudes between 15 and 21 km from August 1991 to March 1992 show that, prior to December 1991, the Pinatubo aerosol cloud varied widely in microphysical properties such as size distribution, number, surface and volume concentrations and was also spatially variable. Aerosol surface area concentration was found to be highly correlated to extinction at visible and near-infrared wavelengths throughout the measurement period. Similarly, backscatter at common lidar wavelengths was a good predictor of aerosol volume concentrations. These results support the use of satellite extinction measurements to estimate aerosol surface and of lidar measurements to estimate aerosol volume or mass if temporal changes in the relationships between the variables are considered.

Fine Particles in the Soufriere Eruption Plume

The size distributions of fine particles measured at tropospheric altitudes in the periphery of the eruption plume formed during the 17 April 1979 eruption of Soufriere Volcano and in the low-level effluents on 15 May 1979 were found to be bimodal, having peak concentrations at geometric mean diameters of 1.1 and 0.23 micrometers. Scanning electron microscopy and energy-dispersive x-ray analysis of the samples revealed an abundance of aluminum and silicon and traces of sodium, magnesium, chlorine, potassium, calcium, and iron in the large-particle mode. The submicrometer-sized particles were covered with liquid containing sulfur, assumed to be in the form of liquid sulfuric acid.

Hydrogen chloride and aerosol ground cloud characteristics resulting from space shuttle launches

Abstract Airborne measurements of gaseous HCl, gaseous and aerosol HCl, particulates, relative humidity and temperature were obtained in ground clouds produced during three Space Shuttle launches. Partitioning of HCl between HCl aerosol and gaseous HCl was investigated as the solid rocket exhaust cloud diluted with ambient air to evaluate the conditions under which aerosol formation occurs in the troposphere in the presence of hygroscopic HCl vapor. Equilibrium predictions for aqueous HCl aerosol formation generally agree with the measured HCl partitioning over HCl concentrations from 0.5 to 36 ppm. HCl concentration dispersion within four cloud segments at time t (min) was evaluated using the expression C = C 0 t α where C 0 varied from 145 to 2250 ppm and α varied from −1.14 to −1.73. Aerosol fallout from the exhaust clouds was measured with time by monitoring HCl concentrations and aerosol distributions 100 m below the cloud as it drifted away from the launch site. Significant amounts of HCl were found to be removed by fallout of particles in the 80–220 μm diameter range up to 30 min after launch.

Preferential concentration of certain elements in smaller aerosols emitted from aircraft engines

Aerosols from aircraft engines were collected with an Andersen eight‐stage cascade sampler for a period of 24 h. The aerosol samples from each stage were analyzed for their elemental composition using the proton‐induced x‐ray emission (PIXE) technique. Seventeen elements (Si, P, S, Cl, K, Ca, Ti, V, Fe, Ni, Cu, Zn, Br, Sr, Nb, Sn, and Pb) were positively identified and quantitated at each stage. Six elements (S, Ca, Fe, Zn, Sn, and Pb) showed fractional concentration increase with the decreasing aerosol size. Similar, but less well‐defined, trends were also observed for V and Ni. Silicon and chlorine, on the other hand, showed an opposite trend. Neutron activation analysis of bulk aerosol samples collected every 2 h over the same period showed correlation of concentration of Si, Ca, V, Ti, Zn, Br, and Sn with the density of air traffic at the airport. Analysis of the aviation fuel samples by PIXE indicates that major fractions of Pb, Sn, Br, Zn, Ni, Fe, V, Ca, and S observed in these aerosol studies come from the aircraft engine exhaust.Aerosols from aircraft engines were collected with an Andersen eight‐stage cascade sampler for a period of 24 h. The aerosol samples from each stage were analyzed for their elemental composition using the proton‐induced x‐ray emission (PIXE) technique. Seventeen elements (Si, P, S, Cl, K, Ca, Ti, V, Fe, Ni, Cu, Zn, Br, Sr, Nb, Sn, and Pb) were positively identified and quantitated at each stage. Six elements (S, Ca, Fe, Zn, Sn, and Pb) showed fractional concentration increase with the decreasing aerosol size. Similar, but less well‐defined, trends were also observed for V and Ni. Silicon and chlorine, on the other hand, showed an opposite trend. Neutron activation analysis of bulk aerosol samples collected every 2 h over the same period showed correlation of concentration of Si, Ca, V, Ti, Zn, Br, and Sn with the density of air traffic at the airport. Analysis of the aviation fuel samples by PIXE indicates that major fractions of Pb, Sn, Br, Zn, Ni, Fe, V, Ca, and S observed in these aerosol studies come ...

Twenty-six years of lidar monitoring of northern midlatitude stratospheric aerosols

Aerosols in the upper troposphere and low stratosphere have been monitored continuously during the past 26 years by a ground-based lidar system at the NASA Langley Research Center in Hampton, Virginia. The measurements were started in 1974 to support NASAs ongoing atmospheric research programs, and have produced one of the worlds longest continuous lidar records on northern mid- latitude aerosols. The 26-year record spans periods during which the stratospheric aerosol loading was greatly enhanced by highly explosive volcanic eruptions including, Fuego in 1974, El Chichon in 1982, and Mt. Pinatubo in 1991, each of which injected enormous quantities of aerosols and gases into the stratosphere. These lidar observations of volcanic aerosol plumes in the stratosphere over long time periods have provided insight into their potential impact on global climate and other atmospheric processes.

Examples of Realistic Aerosol Particles Collected in a Cascade Impactor

In this paper some scanning electron microscope photomicrographs showing examples of a variety of particulate aerosol shapes are presented. Compositions of particles also are presented. These particles represent samples from a number of different sources and locations including: solid propellant rocket motor plumes, active volcano plumes, the lower stratosphere over Sondrestrom, Greenland, and the upper troposphere over northern Texas. The particles were collected from aboard an aircraft with a cascade impactor which classified them according to aerodynamic size into 10 size intervals ranging from submicron to greater than 25 micrometers in diameter. The cascade impactor also served to measure the mass concentration as a function of particle size. The variety of shapes and compositions found among these particles suggest difficulties in obtaining reliable size distribution data from light scattering measurements.

Recent modifications, enhancements, and measurements with an airborne lidar system

The NASA Langley Research Centers 14-inch airborne aerosol lidar system, which is routinely flown on several NASA aircraft including the DC-8 and the P-3, has been upgraded with several modifications to enhance its measurement capabilities. A new 900 mJ, 10 pps Nd:YAG laser was added with the capability of producing 5 watts of power at 1064 nm, 2.5 watts at 532 nm and 1.5 watts at 355 nm. The existing detector package has been modified to accommodate the three wavelengths and to permit cross-polarization measurements at 532 nm. New software was developed for on- line data visualization and analysis, and computer- controlled laser alignment is being incorporated. The system is now capable of producing real-time color modulated backscatter plots. Other additions include a Pentium/90 processor, GPS (Global Positioning System) and ARINC (Aeronautical Radio Inc.) receivers for acquiring accurate aircraft position data. In 1992 and 1993 this system was flown on several airborne missions to map and characterize the stratospheric aerosol cloud produced by the 1991 eruption of the Mount Pinatubo volcano. Efforts to map the global distribution of Pinatubo were made on both daytime as well as nighttime flights from Moffett Field in California to the South Pacific, to Central and South America, to Australia and to Alaska. In September 1994, the system (aboard NASAs P-3) made correlative measurements along shuttle orbit ground tracks in support of the Lidar In-space Technology Experiment flown on the Space Shuttle. In this paper the system upgrades will be discussed and selected data obtained during these recent airborne campaigns will be presented.