The Role of the Stratosphere in Global Change

The paper presents an introduction to scientific problems related to ozone in the stratosphere. Key uncertainties in our understanding of the chemistry and dynamics of this region are pointed out. Model simulations of the ozone evolution during the 1980s are discussed to illustrate the importance of heterogeneous chemical reactions in the lower stratosphere.

The importance of tropospherically forced waves in determining both the climatological mean state and the disturbance structure of the middle atmosphere is discussed. Some of the essentials of gravity waves, planetary waves, and equatorial circulations are presented. This is done by appealing to basic physical principles, using as little mathematics as possible.

The role of homogeneous (gas-phase) chemical reactions in controlling the stratospheric ozone concentration is briefly reviewed. An outline of definitions, conventions, and data sources is followed by a specific discussion of the most important ozone destroying reactions. The catalytic HO_x, NO_x, and CℓO_x cycles are described.

The chemistry of the stratosphere is strongly influenced by the presence of small particles composed of sulfuric acid, nitric acid and other materials. The ubiquitous background stratospheric aerosol layer is composed of sulfuric acid droplets, while the clouds observed in the polar winter stratosphere (the polar stratospheric clouds, or PSCs) are composed of nitric acid ices. Chemical reactions can occur efficiently on the particle surfaces, and in solution in the case of liquid droplets. Such reactions affect the concentrations of chlorine and nitrogen species in the lower stratosphere, and play a critical role in ozone depletion. Indeed, the “ozone hole” has been shown to be initiated by “heterogeneous” reactions occurring on PSC particles. The origins and properties of sulfate aerosols, PSCs and other observed stratospheric particles are surveyed. Anthropogenic influences on these aerosols are discussed. The heterogeneous chemistry of polar stratospheric clouds, and the chemical processing of air in contact with such clouds, are illustrated using detailed model simulations. The injection of sulfur and chlorine into the stratosphere by volcanic eruptions is also investigated. HCl scavenging in volcanic eruption plumes is quantified based on an analysis of the dynamics, physical chemistry and microphysics of eruption columns. It is shown that very little chlorine is likely to enter the stratosphere in volcanic plumes because of efficient HCl absorption in supercooled water that condenses on sulfuric acid aerosols. The possible role of sulfate aerosols — both of volcanic and background origin — as a medium for heterogeneous chemical reactions is assessed. It is argued that the sulfate aerosols can produce significant chemical pertubations in regions of the atmosphere where temperatures drop below about 200 K. The potential contribution of sulfate aerosols to ozone depletion at high latitudes is discussed. Outstanding scientific issues concerning stratospheric aerosols and their chemical effects are summmarized.

The general problem of trace gas mixing in the stratosphere is considered. Mixing is divided into two parts: mixing which occurs as part of the tracer realignment with the flow streamlines, and mixing which occurs as part of the evolution of the fluid flow. The mixing process is fundamentally driven by fluid strain or wind shear. The straining produces trace gas filaments which thin until the molecular viscosity scale is reached. At this point the trace gas comes into a dynamical (and chemical) balance with the ambient flow. The further evolution of the trace gas is highly correlated with dynamical tracers, such as potential vorticity, or with other long lived trace gases. Subsequent mixing of the trace gas follows the mixing of the dynamical tracers. Estimates the time required for the trace gas to reach dynamical equilibrium show that linear shear or even shear variation associated with the observed stratospheric energy spectrum cannot bring the trace gas to the dynamical balance as rapidly as is observed. Nonlinear phenomena producing random strain probably accelerate the collapse of trace gas filaments to viscous scales. This suggests that random strain models may be appropriate at meso and synoptic scales under appropriate conditions.

Main components of the methane atmospheric cycle are outlined and contemporary understanding of their input in the global change is reviewed. Special attention is paid to methane atmospheric content increase influence on atmospheric composition and on the expected climate changes. The connections of the latter with methane source and sink intensities is also discussed.

The use of 3-D assimilation model-derived dynamics in transport-chemistry models is a relatively new research methodology that has been used to interpret aircraft, ground-based remote sensing, balloon, and satellite data for the stratosphere. The unique aspect of these studies is that since the output of the assimilation procedure is a statistically optimal representation of dynamics, the time-varying output from this type of transport-chemistry model may be compared with sequences of actual observations. Some applications of this technique are presented relating to LIMS observations of nitric acid; the relation of satellite observed fields to model results; studies of the stratospheric ozone budget; and study of polar processing in relation to UARS CIO data.

A simple discussion of the processes which maintain the mean state of the stratosphere is given in order to demonstrate how the stratosphere might influence the tropospheric circulation. A series of GCM studies which indicate that the strength of the polar night jet is important in determining the planetary wave structures in the troposphere are then summarized. These experiments are used to determine the tropospheric response to stratospheric changes during northern winter, when the largest influence is expected. The climatological response and the time scale on which the response develops has been determined using separate experiments. Finally, problems with the physical parameterizations which are typically encountered when extending tropospheric GCMs to include the full stratosphere and lower mesosphere are summarized.

The energy balance of the stratosphere is discussed and the role of radiative heating in determining the climatology of the stratosphere is highlighted. Requirements for radiation transfer calculations in the middle atmosphere are discussed. Simple models are used to illustrate the effect of uncertainties in radiative heating calculations on the modelled stratospheric climatology.

Influences of the stratospheric circulation on the troposphere during the northern winter were mainly studied in the past in the context of linear response of planetary waves to changes in the propagation characteristics in a modified stratosphere. However, results of numerical experiments with a general circulation model (GCM) suggest that more complicated nonlinear processes are also involved. In the present study, the effects of changes in the stratospheric circulation on the troposphere are investigated by comparing observed results with those obtained from the GCM.

The chemical composition and dynamics of the tropical stratosphere are largely determined by forcing from the underlying troposphere,transmitted through the tropical tropopause. The properties of the tropopause region of the atmosphere are reviewed, with emphasis on the roles of Pacific sea-surface temperature variability in the El Niño mode and the stratospheric QBO as the principal factors that determine interannual variations.

The longest continuous set of daily analyses of stratospheric constant pressure levels covers 34 years, but the levels are all below 25 km. These analyses are for the Northern Hemisphere and have no equivalent on the Southern Hemisphere. Data from single stations go back another five to eight years. The attempts here to link qualitatively some of the interannual variability in the stratosphere to forcings from outside the stratosphere therefore deal with samples that are not necessarily representative of long periods.

In addition to the random interannual variability which is inherent in the atmosphere-ocean system, some of the interannual changes in the stratospheric circulation are associated with the following:

The stratosphere exerts an influence on the troposphere, and tropospheric climate change, in a number of ways. This paper will concentrate on the radiative interaction. The stratosphere (and overlying atmosphere) intercepts some of the solar radiation incident at the top of the atmosphere and reduces the energy reaching the troposphere. The stratosphere also emits thermal infrared (i.r.) energy, some of which reaches, and warms, the troposphere. Similarly, the radiative balance of the stratosphere is influenced by thermal i.r. emission by the troposphere.

The middle atmosphere is the place where the largest changes of anthropogenic origin have been observed in the last decades. The development of the Antarctic Ozone hole is the best known of such changes, but it is shown in this review that large changes are observed in the temperature, partly under the influence of the ozone depletion, and partly as a consequence of the increase in the concentration of greenhouse gases, mainly CO₂, CH₄ and H₂O. The temperature changes are compared with what is expected from the measured changes in atmospheric composition using different models. In both the stratosphere and the mesosphere, the temperature has decreased faster than predicted by any model. The fact that whereas the temperature is increasing in the troposphere, a cooling is already observed just above the tropopause, and this could be a major source of perturbation in the wave propagation between the two layers, and have an impact on the general circulation and therefore on the climate.

Satellites have allowed us to measure the global distribution of the atmosphere’s ozone content on a daily basis. This lecture describes how this data can be used to determine ozone trends, emphasizing some of the difficulties which must be overcome to determine an accurate trend from satellite data. The experience gained from nearly 14 years of TOMS data and 11 years of SBUV data have shown that accurate trends can be obtained if special attention is given to the problem of calibration drift.

Infrared remote sensing observations from the ground, using the sun as source of radiation, constitute a powerful tool for monitoring the state of our environment. The observational effort carried out at the International Scientific Station of the Jungfraujoch, Switzerland, is used in this paper as a typical example to stress out the performances achievable by that technique. When based on high spectral resolution and high signal-to-noise observations, it can overcome the bulk of the absorption produced by gases with high concentration in the troposphere and contribute efficiently to the quantification and monitoring of trace gases predominantly concentrated in the stratosphere.

The European Arctic Stratospheric Experiment (EASOE), funded by national sources and the Commission of the European Communities, began in mid November 1991 and completed its main measurements phase at the end of of March 1992. The research was prompted by the decline in northern hemisphere ozone values during the last decade and need to understand the role of halocarbons in the decline. A variety of measurements have covered a wide area from Central Europe, across the North Atlantic and the European region including Russia. The measurements of stratospheric composition (including ozone and other species involved in ozone chemistry such as nitrogen and chlorine compounds) comprise regular ground-based and ozone sonde measurements from more than 20 sites, observations within the ozone layer by experiments on 41 large balloons and nearly 100 flights of research aircrafts. A large amount of data has been collected and analysis will continue for many months. Campaign planning, instrumentation, as well as preliminary results and findings are presented below.

The SAGE II (Stratospheric Aerosol and Gas Experiment II) sensor was launched into a 57° inclination orbit aboard the Earth Radiation Budget Satellite (ERBS) in October 1984. During each sunrise and sunset encountered by the orbiting spacecraft, the instrument (Mauldin et al., 1985) uses the solar occupation technique to measure attenuated solar radiation through the Earth’s limb in seven channels centered at wavelengths ranging from 0.385 to 1.02 μm. The exo-atmospheric solar irradiance is also measured in each channel during each event for use as a reference in determining limb transmittances. The transmittance measurements are inverted using the “onion-peeling” approach (Chu et al., 1989) to yield 1-km vertical resolution profiles of aerosol extinction (at 0.385, 0.453, 0.525, and 1.02 μm), ozone, nitrogen dioxide, and water vapor. The focus of the measurements is on the lower and middle stratosphere, although retrieved aerosol, water vapor, and ozone profiles often extend well into the troposphere under non-volcanic and cloud-free conditions. SAGE II was preceded into orbit by sister instruments SAM II (Stratospheric Aerosol Measurement II), which has been measuring 1.0-μm aerosol extinction in the polar regions since 1978, and SAGE I, which provided near global measurements of aerosol extinction (at 0.45 and 1.0 μm), ozone, and nitrogen dioxide from 1979–1981 (McCormick et al., 1979).

The Atmospheric Trace Molecule Spectroscopy (ATMOS) experiment employs a Fourier transform spectrometer to record infrared solar spectra at orbital sunrises and sunsets from on board the Space Shuttle. The data returned from 19 occultations covered by the instrument’s first flight as part of the Spacelab 3 payload in April, 1985, have been analyzed for the profiles of some thirty atmospheric constituents. These results included a number of species not previously detected or measured, an investigation of the No_y and Cl_x budgets, and, as a whole, have been used as the input for critically evaluating stratospheric photochemical models. The instrument was flown again on the ATLAS-1 shuttle mission in March, 1992, where it obtained data through nearly 100 solar occultation events located between latitudes of 30°N and 55°S. Results from the 1985 mission as well as preliminary results from the more recent 1992 flight are summarized here.

The atmospheric parameters controlling the UV-B irradiance at the Earth’s surface are shortly described. The long-term changes in the stratospheric and tropospheric composition affecting the UV radiative transfer are reviewed in order to define the appropriate strategy for global UV-B monitoring. The broad-band radiometer and spectral irradiance measurements currently performed are presented and discussed.

The ozone layer provides an effective shield against harmful ultraviolet (UV) radiation from the sun. Most of the ozone in the atmosphere is located in the stratosphere between 20 and 30 km above the Earth’s surface. In addition to ozone, aerosols and clouds alter the amount of ultraviolet radiation reaching the biosphere. In this paper we discuss how to compute the ultraviolet radiation at the ground and at various levels in the ocean. This will allow us to assess how changes in ozone abundance, aerosol loading and cloud cover will influence UV exposure. As will be discussed the modeling of UV penetration through the atmosphere is at a relatively mature stage compared to the modeling of UV penetration through the combined atmosphere/ocean system.

The photochemistry of the troposphere is highly non-linear, and may be changing due to emissions of gases related to human activities. Increases in tropospheric ultraviolet (UV) radiation, due to stratospheric ozone depletion, may also perturb the troposphere.

Enhancements in UV radiation (due to stratospheric ozone depletion) may affect a number of biological processes. Most of these processes are strong functions of wavelength, making them more or less sensitive to ozone depletion. For some of the most sensitive biological systems, significant UV dose enhancements may have already occurred.

There is strong evidence that marine organisms in the upper layers of the sea are influenced by increased ultraviolet radiation resulting from declines in the thickness of stratospheric ozone. Early evidence supporting this hypothesis included the fact that wavelengths of potentially damaging ultraviolet radiation can penetrate to ecologically significant depths and laboratory findings that many marine organisms are extremely sensitive to this radiation. Laboratory results within the past few years have provided significantly improved estimates of the biological weighting function for damage to phytoplankton by UV radiation, and investigated possible protective and repair mechanisms in these organisms. Recent field work, making use of the Antarctic ozone hole to provide variable UVB flux on a natural phytoplankton community, has provided the first conclusive evidence for a direct ozone-related effect on an aquatic system giving further evidence of the sensitivity of marine organisms to UVB. Much recent work has been motivated by the large springtime depletion of stratospheric O ₃ over the Antarctic which has also led to increased accuracy in atmospheric models necessary for the quantitative computation of UV fluence at high latitudes.

Potential damage to photosynthesis and other plant processes by increased UV-B has been demonstrated at the physiological level and for a few species at the stand level in the field. Other responses of vegetation including shifts in competitive balance of plant species may be of equal importance. Interactions with other environmental factors and difficulties in making realistic assessments are discussed.

Solar radiation causes malignant melanoma of the skin, non- melanocytic skin cancer, and possibly cancer of the lip and melanoma of the eye. UVB is most likely the wavelength range mainly responsible for these effects.

Ultraviolet radiation-B (280–320nm) is linked directly to skin cancer formation and is also associated with significant immunological alterations leading to local and systemic immunosuppression in mammals. The mechanism as well as the short or long-term effects of such immunological perturbations is largely unknown. This mechanism may, however, play a critical role in human skin cancer development and other UVB-related disorders.