Stratospheric Sulfate Aerosol Research Articles

Two‐dimensional assessment of the impact of aircraft sulphur emissions on the stratospheric sulphate aerosol layer

We have developed a two‐dimensional sulphate aerosol model, which includes most of the important microphysical processes governing the formation and evolution of stratospheric sulphate particles. Simulations of the background aerosol layer shows that important aerosol properties (surface area density, optically active particle concentration, mass mixing ratio) are strongly latitude dependent. This model is used to investigate if the sulphur aircraft emissions may have caused the 60% increase in large aerosol particles observed over the 1979–1990 time period. Results suggest that although aircraft may represent a substantial source of sulphate below 20 km, the rise in air traffic is insufficient to account for the apparent trend in stratospheric aerosol particles.

Stratospheric sulfate aerosol in and near the northern hemisphere polar vortex: The morphology of the sulfate layer, multimodal size distributions, and the effect of denitrification

Measurements were made of stratospheric sulfate aerosols using a passive cavity aerosol spectrometer and a condensation nucleus counter on a NASA ER–2 aircraft in the Airborne Arctic Stratospheric Experiment of 1989. The problems of representative and accurate sampling and particle evaporation were explicitly addressed in the design of the inlets and reduction of the data. The measurements suggest that the sulfate aerosol is bimodal in the polar vortex above the mass mixing ratio maximum in the sulfate layer. It appears that a nuclei mode of small, newly formed particles exists in this region. A stronger case is made for a nuclei mode in the upper few kilometers of the troposphere and in the lower few kilometers of the stratosphere. This mode is probably a global phenomenon occurring in all seasons. Comparison of denitrified and nondenitrified air suggests that denitrification removes some of the larger sulfate particles.

Heterogeneous conversion of N2O5 TO HNO3 on background stratospheric aerosols: Comparisons of model results with data

A two dimensional photochemical model is used to examine the effects of heterogeneous processing by a parameterized lower stratospheric sulfate aerosol layer on model calculations. The single reaction considered is N2O5 + H2O → 2HNO3, at a rate consistent with recent laboratory measurements. The calculations are compared with data, to see if the heterogeneous chemistry improves the agreement between model calculations and data. The data used are Limb Infrared Monitor of the Stratosphere (LIMS) measurements of HNO3 and NO2, and in situ measurements of NO, NOy, and ClO taken by the NASA ER‐2 aircraft. A contradictory picture results. With heterogeneous processing, model HNO3 agrees better with LIMS HNO3; however, LIMS NO2, and NO2/HNO3 agree more closely with the results of a standard gas‐phase calculation. High‐latitude NO/NOy from in situ measurements in winter also favor standard chemistry, while the seasonal variation of NO and NO2 are better represented when the heterogeneous reaction is included. ClO is also better simulated when the heterogeneous reaction is included. These comparisons suggest that the assumptions made to parameterize the sulfate aerosol chemistry result in a rate of heterogeneous processing that is too vigorous. However, the lack of consistency in the different comparisons leaves the situation ambiguous, and a definite conclusion is unwarranted.

Evidence for denitrification in the 1990 Antarctic spring stratosphere: II. Lidar and aerosol measurements

Balloonborne aerosol soundings from McMurdo Station, Antarctica in 1990, documented the occurrence of polar stratospheric clouds (PSCs) during both fast and slow cooling events as well as the background stratospheric sulfate aerosol. PSCs formed under slow cooling predominated and in this case the size distributions were found to be bimodal with mode radii of 0.08 and 2–3 µm, similar to previous measurements in Antarctica. The aerosol soundings were also compared to lidar measurements at McMurdo in three cases. In the one PSC layer formed from fast cooling, the best agreement between measured and calculated scattering ratio was found using an index of refraction of 1.37, suggesting an amorphous nitric acid/water composition rather than crystalline nitric acid trihydrate. For the background aerosol, calculations using an index of 1.5 were generally in best agreement with the measured values.

Role of heterogeneous conversion of N2O5 on sulphate aerosols in global ozone losses

THERE is a serious discrepancy between estimates of the downward trend in the column abundance of ozone at middle and high latitudes derived from ground-based and satellite data, and those obtained by models that calculate the effect of increases in atmospheric chlorine concentrations, but include only gas-phase chemistry1,2. Recent measurements3,4 of the reaction rate of N2O5 on sulphate aerosols yield very fast rates for a wide range of water content, indicating that this reaction could take place in the stratospheric sulphate aerosol layer, which is present around the globe and year-round between ~14–25 km altitude. Here we include this reaction in a two-dimensional model and find that the calculated decadal ozone trends at both high and middle latitudes agree much more closely with the trends deduced from observations. Inclusion of the reaction also significantly increases the predicted concentrations of key species such as OH, ClO and HNO3, and decreases those of NO and NO2. Measurements of these species in and near the sulphate layer are needed to confirm the importance of this reaction in the observed decrease of atmospheric ozone.

Middle Atmosphere Aerosols and Their Effects

The middle atmosphere holds a rich variety of particulate matter, ranging from meteoritic debris to sulfate aerosols to polar stratospheric ice clouds. Volcanic eruptions strongly perturb the stratospheric sulfate (JUNGE) layer. High-altitude “noctilucent” ice clouds condense at the cold menopause. The properties of these particles—including composition, size and geographical distribution—are discussed, and their global effects—including chemical, radiative and climatic roles—are surveyed. Polar stratospheric clouds (PSCs) catalyze reactions of chlorine compounds that “activate” otherwise inert chlorine reservoirs, leading to severe ozone depletions in the southern polar stratosphere during austral spring. PSCs also modify the composition of the polar stratosphere through complex physicochemical processes, including dehydration and denitrification. The sulfate aerosol layer can reflect solar radiation and increase the planetary albedo, thereby cooling the surface and altering the climate. Major volcanic eruptions, which enhance the stratospheric sulfate aerosol burden by a factor of 100 or more, may cause significant global climatic anomalies. Sulfate aerosols also appear capable of activating stratospheric chlorine reservoirs on a global scale. Accordingly, if atmospheric concentrations of chlorine (associated with anthropogenic use of chlorocarbons) increase by a factor of two or more in future decades, significant worldwide ozone depletions could occur.

The El Chichon volcanic cloud in the stratosphere: lidar observation at Fukuoka and numerical simulation

The stratospheric volcanic cloud from the eruption.of El Chichon, Mexico, on 4 April 1982 was observed routinely by a Nd: YAG lidar system from 18 April 1982 at Kyushu University, Fukuoka, Japan. The observed layers of the cloud above 20 km were in the easterly wind region and those below 20 km were in the westerly region. The main part of the cloud mass was in the upper layer. This upper layer broadened slowly until September 1982, then broadened rapidly and merged with the lower layer as the easterly wind changed to the westerly wind. The vertical eddy diffusion coefficient estimated from the broadening of the upper layer was much smaller than the value usually used in the one-dimensional model calculation of chemical components until September and subsequently remained at about the same value. The increase of the integrated backscattering coefficient (IBC) was about two orders of magnitude larger than the largest increase after volcanic injections for the last 10 years. The IBC reached a maximum value on 3 May and gradually decreased until August 1982, then re-increased until December 1982. The IBC between December 1982 and February 1983 was about the same value as in May 1982. Using the one-dimensional stratospheric sulfate aerosol model simulations it was concluded that to explain the broadening of the upper layer an eddy diffusion coefficient of about 10 2cm 2s -1 would be needed in the easterly wind region in summer. It was also concluded that the IBC re-increase was caused after advective horizontal transport from lower to higher latitudes by chemical reactions within the upper layer without meridional diffusion during summer and that the transport was controlled by nucleation, which gives rise to small particles, a decreasing settling velocity of the volcanic cloud and then the cloud being less affected by horizontal transport.

The 1980 eruptions of Mount St. Helens: Physical and chemical processes in the stratospheric clouds

We investigate the physical and chemical properties of the stratospheric clouds produced by the recent eruptions of the Mount St. Helens volcano. The large and diverse set of observational data collected in the high‐altitude plumes of the May 18, May 25, and June 13, 1980, eruptions is organized and analyzed for evidence of the processes at work. The data are used to guide and constrain detailed model simulations of the volcanic clouds. For this purpose a comprehensive one‐dimensional model of stratospheric sulfate aerosols, sulfur precursor gases, and volcanic ash and dust is utilized. The model accounts for gas‐phase and condensed‐phase (heterogeneous) chemistry in the clouds, aerosol nucleation and growth, and cloud expansion. Computational results are given for the time histories of the gaseous species concentrations, aerosol size distributions, and ash burdens of the eruption clouds. The long‐term buildup of stratospheric aerosols in the Northern Hemisphere, and the persistent effects of injected chlorine and water vapor on stratospheric ozone, are also investigated. We conclude that SO2, water vapor, and ash were probably the most important substances injected into the stratosphere by the Mount St. Helens volcano, both with respect to their widespread effects on composition and their impact on climate. Despite the seemingly large magnitude of the Mt. St. Helens eruption, we also find that the volcano had little influence on the climate (≲0.05 K surface cooling in the Northern Hemisphere) or on stratospheric ozone (<0.2% maximum hemispherical depletion). The remaining problems in the field of volcanic cloud physics and chemistry, and the requirements for future experimental work, are also discussed.

Absolute rate constant for the reaction of O(3P) with CH3SSCH3 from 270 to 329 K

Rate constants for the oxidation reactions of organic sulfur compounds have become increasingly important due to the potential role of these compounds in the formation of tropospheric and stratospheric sulfate aerosols. The present study deals with the reaction of the ground state oxygen atom with dimethyl disulfide (DMDS). The discharge fast flow–resonance fluorescence technique was used to monitor the decay of O(3P) concentration in the presence of excess CH3SSCH3 under pseudo-first-order conditions. Over a temperature range from 270 to 329 K, and a pressure range from 0.52 to 2.60 Torr, no variation of the rate constant with temperature and pressure was observed. The results are best represented by the temperature and pressure independent value of (2.12±0.22)×10−10 cm3 molecule−1 s−1, which is about one-half the theoretical collision frequency. The zero activation energy for this reaction is discussed in terms of the temperature dependence of other sulfur compound–oxygen atom reactions reported in the literature.

The stratospheric sulfate aerosol layer: Processes,models, observations, and simulations

After briefly reviewing the observational data on the stratospheric sulfate aerosol layer, the chemical and physical processes that are likely to fix the properties of the layer are discussed. We present appropriate continuity equations for aerosol particles, and show how to solve the equations on a digital computer. Simulations of the unperturbed aerosol layer by various published models are discussed and the sensitivity of layer characteristics to variations in several aerosol model parameters is studied. We discuss model applications to anthropogenic pollution problems and demonstrate that moderate levels of aerospace activity (supersonic transport and space shuttle operations) will probably have only a negligible effect on global climate. Finally, we evaluate the possible climatic effect of a ten-fold increase in the atmospheric abundance of carbonyl sulfide.

Absolute rate constant for the reaction of O(3P) with CH3SCH3 from 272 to 472 K

Rate constants for the oxidation reactions of organic sulfur compounds have become increasingly important due to their potential role in the formation of tropospheric and stratospheric sulfate aerosols. The present study deals with the reaction of the ground state oxygen atom with dimethyl sulfide. A discharge fast flow–resonance fluorescence technique was used to follow the decay of O(3P) atoms in the presence of excess CH3SCH3 under pseudo-first-order conditions. Kinetic data, obtained over the temperature range 272 to 472 K, are well represented by the Arrhenius expression, kbi= (1.28±0.12) ×10−11 exp[(803±60)/RT] cm3 molecule−1 sec−1. The results are compared to those of previous studies which employed different experimental techniques.

OCS, stratospheric aerosols and climate

Carbonyl sulphide (OCS) is found to be the predominant sulphur-bearing compound in our atmosphere1–3. It contributes to the formation of stratospheric sulphate aerosol particles4, which affect the Earth's radiation balance and climate5–7. Using recently obtained data, we estimate that OCS has a global source of ∼5 tg per year (tg = 1012 g) and a lifetime of roughly 1 yr. We calculate that increasing anthropogenic emissions of OCS could cause measurable climate alterations within the next century. Numerous sources of OCS have been identified (see Fig. 1). Crutzen et al.8 estimate that natural and agricultural fires contribute 0.2–0.3 tg of OCS to the atmosphere each year. Adams et al.9 measured average OCS emission rates for a variety of common soils of about 0.004 g m−2 yr−1, which may be extrapolated to a global OCS source of nearly 0.5 tg yr−1. Adams et al.9 also noted OCS emissions several thousand times greater than average above saline marshes. Carbonyl sulphide has been detected near cattle feedlots in concentrations as high as 6,000 p.p.b.v.10. Volcanoes and fumaroles seem to represent a minor source of OCS (refs 11,12). We estimate that the direct contributions of biospheric processes to the OCS budget may be ∼1 tg yr−1.

A One-Dimensional Model Describing Aerosol Formation and Evolution in the Stratosphere: I. Physical Processes and Mathematical Analogs

Abstract We have developed a time-dependent one-dimensional model of the stratospheric sulfate aerosol layer. In constructing the model, we have incorporated a wide range of basic physical and chemical processes in order to avoid predetermining or biasing the model predictions. The simulation, which extends from the surface to an altitude of 58 km, includes the troposphere as a source of gases and condensation nuclei and as a sink for aerosol droplets; however, tropospheric aerosol physics and chemistry are not fully analyzed in the present model. The size distribution of aerosol particles is resolved into 25 discrete size categories covering a range of particle radii from 0.01–2.56 µm with particle volume doubling between categories. In the model, sulfur gases reaching the stratosphere are oxidized by a series of photochemical reactions into sulfuric acid vapor. At certain heights this results in a supersaturated H2SO4–H2O gas mixture with the consequent deposition of aqueous sulfuric acid solution on th...

Stratospheric Aerosol Measurements IV: Global Time Variations of the Aerosol Burden and Source Considerations

Abstract Balloon-borne measurements of the stratospheric sulfate aerosol from late 1971 to mid-1974, a quiescent period in terms of large volcanic eruptions at stations ranging from 85°N to 90°S, are utilized in a study of the global spatial and temporal variations and for sulfur budget and aerosol source considerations. Similarities in the aerosol loading in the two hemispheres, both spatial and temporal, are evident. An apparent long-term decay in total aerosol appears to have occurred globally during the period suggesting a transient source. SO2 budgetary considerations and model calculations favor a larger injection of SO2 than would be expected from a quasi-static natural exchange of tropospheric air.