J. C. Wilson

Particle Formation in the Upper Tropical Troposphere: A Source of Nuclei for the Stratospheric Aerosol

Atmospheric measurements and numerical calculations described here indicate that binary homogeneous nucleation of H2SO4-H2O particles occurs in the upper tropical troposphere. Particle concentrations decrease with increasing altitude above the tropical tropopause as a result of coagulation during the upward air transport produced by stratospheric circulations. During the extended periods of time that volcanic eruptions do not strongly influence stratospheric particle number concentrations, particles formed in the upper tropical troposphere provide nuclei upon which oxidized sulfur gases condense in the stratosphere. This particle source, coupled with aerosol microphysical properties and atmospheric transport, governs the number concentration of particles in the lower tropical and mid-latitude stratosphere.

Observed OH and HO2 in the upper troposphere suggest a major source from convective injection of peroxides

ER-2 aircraft observations of OH and HO_2 concentrations in the upper troposphere during the NASA/STRAT campaign are interpreted using a photochemical model constrained by local observations of O_3, H_2O, NO, CO, hydrocarbons, albedo and overhead ozone column. We find that the reaction Q(^(1)D) + H_2O is minor compared to acetone photolysis as a primary source of HO_x (= OH + peroxy radicals) in the upper troposphere. Calculations using a diel steady state model agree with observed HO_x concentrations in the lower stratosphere and, for some flights, in the upper troposphere. However, for other flights in the upper troposphere, the steady state model underestimates observations by a factor of 2 or more. These model underestimates are found to be related to a recent (< 1 week) convective origin of the air. By conducting time-dependent model calculations along air trajectories determined for the STRAT flights, we show that convective injection of CH_3OOH and H_2O_2 from the boundary layer to the upper troposphere could resolve the discrepancy. These injections of HO_x reservoirs cause large HO_x increases in the tropical upper troposphere for over a week downwind of the convective activity. We propose that this mechanism provides a major source of HO_x in the upper troposphere. Simultaneous measurements of peroxides, formaldehyde and acetone along with OH and HO_2 are needed to test our hypothesis.

Emission Measurements of the Concorde Supersonic Aircraft in the Lower Stratosphere

Emission indices of reactive gases and particles were determined from measurements in the exhaust plume of a Concorde aircraft cruising at supersonic speeds in the stratosphere. Values for NOx (sum of NO and NO2) agree well with ground-based estimates. Measurements of NOx and HOx indicate a limited role for nitric acid in the plume. The large number of submicrometer particles measured implies efficient conversion of fuel sulfur to sulfuric acid in the engine or at emission. A new fleet of supersonic aircraft with similar particle emissions would significantly increase stratospheric aerosol surface areas and may increase ozone loss above that expected for NOx emissions alone.

Measurements of high number densities of ice crystals in the tops of tropical cumulonimbus

Imaging and light scattering instruments were used during the January/February 1987 STEP Tropical Experiment at Darwin, Australia, to measure ice crystal size distributions in the tops of tropical cumulonimbus anvils associated with tropical cyclones and related cloud systems. Two light scattering instruments covered particles from 0.1-μm to 78-μm diameter. Particles larger than 50-μm diameter were imaged with a two-dimensional Grey optical array imaging probe. The measurements were made at altitudes ranging from 13 to 18 km at temperatures ranging from −60° to −90°C. Additional measurements made in continental cumulonimbus anvils in the western United States offer a comparative data set. The tropical anvil penetrations revealed surprisingly high concentrations of ice crystals. Number densities were typically greater than 10 cm−3 with up to 100 cm−3 if one includes all particles larger than 0.1 μm and can approach condensation nuclei in total concentration. In order to explain the high number densities, ice crystal nucleation at altitude is proposed with the freezing of fairly concentrated solution droplets in equilibrium at low relative humidities. Any dilute liquid phase is hypothesized to be transitory with a vanishingly short lifetime and limited to cloud levels nearer −40°C. Homogeneous nucleation of ice involving H2SO4 nuclei is attractive in explaining the high number densities of small ice crystals observed near cloud top at temperatures below −60°C. The tropical size distributions were converted to mass using a spherical equivalent size, while the continental anvil data were treated as crystalline plates. Comparisons of the ice water contents integrated from the mass distributions with total water contents measured with NOAA Lyman-alpha instruments require bulk densities equivalent to solid ice for best agreement. Correlation between the two data sets for a number of flight passes was quite good and was further improved by subtraction of water vapor density values ranging between ice and water saturation. Ice water contents up to 0.07 g m−3 were observed in the tropical anvils with over 0.1 g m−3 in continental anvils. The size distributions in tropical anvils generally reveal mass modes at sizes of 20–40 μm. With rare exceptions, particles larger than 100 μm were not observed near the cloud tops. In continental cumulonimbus anvils, much larger plate crystals approaching 1 mm in size account for the majority of the ice water. Most of the ice crystal mass lofted to anvil altitudes falls to lower levels prior to evaporating. The anvils can experience strong radiational heating as well as cooling depending upon lower cloud cover, particle size distribution, and time of day.

Chemical loss of ozone in the arctic polar vortex in the winter of 1991-1992.

In situ measurements of chlorine monoxide, bromine monoxide, and ozone are extrapolated globally, with the use of meteorological tracers, to infer the loss rates for ozone in the Arctic lower stratosphere during the Airborne Arctic Stratospheric Expedition II (AASE II) in the winter of 1991-1992. The analysis indicates removal of 15 to 20 percent of ambient ozone because of elevated concentrations of chlorine monoxide and bromine monoxide. Observations during AASE II define rates of removal of chlorine monoxide attributable to reaction with nitrogen dioxide (produced by photolysis of nitric acid) and to production of hydrochloric acid. Ozone loss ceased in March as concentrations of chlorine monoxide declined. Ozone losses could approach 50 percent if regeneration of nitrogen dioxide were inhibited by irreversible removal of nitrogen oxides (denitrification), as presently observed in the Antarctic, or without denitrification if inorganic chlorine concentrations were to double.

In‐situ observations of mid‐latitude forest fire plumes deep in the stratosphere

We observed a plume of air highly enriched in carbon monoxide and particles in the stratosphere at altitudes up to 15.8 km. It can be unambiguously attributed to North American forest fires. This plume demonstrates an extratropical direct transport path from the planetary boundary layer several kilometers deep into the stratosphere, which is not fully captured by large-scale atmospheric transport models. This process indicates that the stratospheric ozone layer could be sensitive to changes in forest burning associated with climatic warming.

Institutions, ecosystems, and sustainability

Introduction A Framework for Exploring the Linkages Between Ecosystems and Human Systems - Cleveland et al Dynamic Systems Modeling - Costanza and Ruth Models Human-Ecosystem Interactions: A Simple Dynamic Integrated Model - Low et al Scale Misperceptions and the Spatial Dynamics of Socio-Ecological Systems with Examples in Fisheries - Wilson et al Investing in Irrigation Infrastructure - Ostrom et al Integrated Ecological Economic Modeling of the Patuxent River Watershed in Maryland - Costanza et al Models for Scoping and Consensus Building - Costanza and Ruth Modeling Summary/Conclusions - Low et al Empirical Studies Empirical Studies of Fisheries - Wilson et al CIPEC Forest Comparisons - Ostrom et al Irrigation Institutions in the Diverse Ecosystems of Nepal - Ostrom et al Empirical Studies at the National Level - Turner Empirical Summary/Conclusions Conclusions Conclusions and Remaining Questions Glossary

A comparison of large-scale atmospheric sulphate aerosol models (COSAM): overview and highlights

The comparison of large-scale sulphate aerosol models study (COSAM) compared the performance of atmospheric models with each other and observations. It involved: (i) design of a standard model experiment for the world wide web, (ii) 10 model simulations of the cycles of sulphur and 222Rn/210Pb conforming to the experimental design, (iii) assemblage of the best available observations of atmospheric SO=4, SO2 and MSA and (iv) a workshop in Halifax, Canada to analyze model performance and future model development needs. The analysis presented in this paper and two companion papers by Roelofs, and Lohmann and co-workers examines the variance between models and observations, discusses the sources of that variance and suggests ways to improve models. Variations between models in the export of SOx from Europe or North America are not sufficient to explain an order of magnitude variation in spatial distributions of SOx downwind in the northern hemisphere. On average, models predicted surface level seasonal mean SO=4 aerosol mixing ratios better (most within 20%) than SO2 mixing ratios (over-prediction by factors of 2 or more). Results suggest that vertical mixing from the planetary boundary layer into the free troposphere in source regions is a major source of uncertainty in predicting the global distribution of SO=4 aerosols in climate models today. For improvement, it is essential that globally coordinated research efforts continue to address emissions of all atmospheric species that affect the distribution and optical properties of ambient aerosols in models and that a global network of observations be established that will ultimately produce a world aerosol chemistry climatology.

Aerosol particles in the upper troposphere and lower stratosphere: Elemental composition and morphology of individual particles in northern midlatitudes

Atmospheric particles were collected in the midlatitude upper troposphere (UT) and lower stratosphere (LS) by inertial impaction for subsequent electron microscopy and individual particle elemental analysis. More than 97% of particles analyzed on impactor substrates exposed in the LS contained only O and S in detectable quantities; these particles are believed to be acidic sulfate. Nonsulfate materials seen in the remaining particles included soot, other c-rich substances and crustal materials. Although not predominantly sulfate, usually carried a sulfur-rich coating in the LS. Samples collected very near and just below the tropopause were also dominated by sulfates. The fraction of sulfate particles analyzed on impactor substrates exposed in the UT was 91-94% of the total particle concentration. Nonsulfate substances observed in the UT samples included crustal-type materials, hydrated marine salts, carbon-rich materials of several types, and metal-containing substances of uncertain origin. Most of these UT particles were not coated with detectable quantities of sulfate. 15 refs., 3 figs., 1 tab.

Aerodynamic particle size measurement by laser-doppler velocimetry☆

Abstract A method of measuring the aerodynamic diameter of aerosol particles is presented. Particles are accelerated in a converging nozzle and their velocity is measured near the exit with a laser-Doppler velocimeter. Nozzle parameters and flow conditions are chosen so that the particle velocity depends primarily on aerodynamic diameter. Experimental studies utilizing a test nozzle are reported. Particles of a known diameter in the range from 0.5 to 11.3 μm were accelerated under various flow conditions. Particle velocities were measured near the nozzle exit. The measurements demonstrated the feasibility of determining particle size from particle velocity. Theoretical studies of the test nozzle showed that it was possible to accurately calculate the particle velocities. Additional theoretical investigations showed that aerodynamic diameter can be determined from particle velocity if the particle motion is not too ultra-Stokesian. A nozzle configuration and flow rate are proposed for the measurement of the aerodynamic diameter of atmospheric aerosol in the range from 0.5 to 10 μm.