The Airborne Southern Hemisphere Ozone Experiment (ASHOE) is designed to examine the causes of ozone loss in the Southern Hemisphere lower stratosphere and to investigate how the loss is related to polar, mid-latitude, and tropical processes. ASHOE will be conducted in concert with the campaign, Measurements for Assessing the Effects of Stratospheric Aircraft (MAESA), whose focus is to provide information about stratospheric photochemistry and transport for assessing the potential environmental effects of stratospheric aircraft. These combined objectives will be met by a series of flights of the National Aeronautics and Space Administration (NASA) ER-2 high-altitude research aircraft from Christchurch, New Zealand, and on transit flights from Moffett Field, California, to Christchurch via Hawaii and Fiji. Flights of the remotely piloted vehicle Perseus, or possibly balloons, to higher altitudes in the tropics and mid-latitudes will also be made. There are several compelling reasons to study processes relating to ozone depletion in the Southern Hemisphere and tropical lower stratosphere:
The possibility of the ozone hole spreading to mid-latitudes, either by fluid mechanical peel-off of filaments of vortex air or by actual equatorward expansion of the vortex, is a controversial issue.
o Recent work has shown that ozone loss in the lower stratosphere is a major factor in calculations of the greenhouse effect. Unless the processes that cause ozone loss at various latitudes are understood, it will be difficult to predict the net effect of human- influenced emissions on surface warming.
o The role of aerosols in perturbing lower stratospheric chemistry, and hence depleting ozone, and enhancement of aerosols by volcanic eruptions remains poorly characterized.
o The transport of material injected into the lower stratosphere at tropical and mid-latitudes is a central issue in assessing the effects on stratospheric ozone by exhaust emissions from possible future high-speed civil transport aircraft (HSCTs).
o The rates of production and destruction of ozone and reactive trace gases (such as the nitrogen oxides) in the polar, mid-latitude, and tropical regions have never been directly tested by measurements.
ASHOE/MAESA will take place in four phases through the Antarctic winter of 1994: late March to early April, late May to early June, late July to early August, and October. The ER-2 component will be conducted in a manner similar to the highly successful polar ozone experiments from Punta Arenas, Chile, in August/September 1987; Stavanger, Norway, in January/February 1989; Fairbanks, Alaska, in October, 1991; and Bangor, Maine, from November 1991 through March 1992. Christchurch has been chosen as a site because of its comparatively benign surface wind conditions and its location, which should permit ER-2 observations both inside and outside the Antarctic vortex. The Perseus and/or balloon measurements will extend the altitude range of analysis in the tropical and northern mid-latitudes.
ASHOE/MAESA is an international effort with participants from nine countries. It involves scientists from a wide range of organizations, including: several NASA Centers, the National Oceanic and Atmospheric Administration, the National Center for Atmospheric Research (sponsored by the National Science Foundation), many universities, private research companies, the New Zealand National Institute of Water and Atmospheric Research, the United Kingdom Meteorological Office, the European Center for Medium-Range Weather Forecasting, and the Cooperative Research Center for Southern Hemisphere Meteorology (Australia).
Historically, when the ozone content of the stratosphere was calculated in the 1960's using the known solar flux for a hypothetical motionless atmosphere of pure oxygen diluted 1:4 by chemically inert molecular nitrogen, two major discrepancies became apparent: [1] There was too much ozone by a factor of 2 to 3. [2] The highest ozone column abundances overhead were computed to be in the tropics, contrary to observation. Discrepancy [1] signals the inadequacy of the pure oxygen reactions: #001# Discrepancy [2] points to the importance of the global wind systems in redistributing the ozone-rich air produced in the tropical stratosphere. The source of the material for extra photochemical reactions to destroy more ozone is in the air entering the stratosphere at the tropical tropopause, probably mostly through thunderstorms over the western tropical Pacific during northern winter. This air contains water, nitrous oxide, methane and chlorofluorocarbons. In the 1970's it was realized that these molecules could be a source of reactive species which could destroy ozone, so resolving the historical discrepancy. In and above the ozone layer, these source molecules are converted to reactive species by solar ultraviolet radiation and the subsequent photochemistry: #001# where the main source of O(1D) is photodissociation of ozone at wavelengths shorter than 320 nm. The reactive species then undergo chain reactions which destroy ozone: #001# where X can be H, OH, NO, Cl or Br. The chain carried by Cl and ClO, for example, can have a length of ~ 104; one chlorine atom can destroy 10,000 ozone molecules. During autumn and winter the upper stratospheric air containing ClO, NO2, OH etc. moves poleward, cools and sinks. Hydrogen, nitrogen, chlorine and bromine are predominantly not in the reactive forms of OH, HO2, NO, ClO and BrO, however. The chain carriers tend to recombine at the higher pressures and in the low levels of illumination characteristic of the polar night into the more harmless forms on the right of the following equations; a balance is formed, with less of the reactive species present than at mid-latitudes: #001# The polar vortex initially contains, therefore, most of the reactive, ozone destroying material in harmless forms, nitric and hydrochloric acid vapors and chlorine nitrate. However, the Antarctic vortex is sufficiently cold that every winter it can form polar stratospheric clouds: #001# These mixed nitric acid-water crystals can form at temperatures less than -77#161#C for typical abundances of water (5 ppmv) and nitric acid (10 ppbv) in the lower stratosphere. By early June, temperatures approach the frost point, about -85#161#C, and ice clouds can form. Most of the nitric acid and over half the water molecules are irreversibly lost by gravitational sedimentation of the crystals. The crystals in polar stratospheric clouds can intervene in the gaseous photochemistry, converting HCl and ClONO2 to reactive forms. For example: #001# In the absence of NO2 (most of the reactive nitrogen has been lost by gravitation sedimentation of crystals, and the remainder is in the form of HNO3, which is slow to photodissociate at polar latitudes), a different chain reaction can occur: #001# Note that the rate determining step (r.d.s.) is proportional to the square of the reactive chlorine abundance and that at long slant paths of the incident solar beam in the polar region, there is little ozone production to balance the loss. In the denitrified Antarctic vortex the ozone hole results; less spectacular loss occurs in the warmer Arctic vortex. Heterogeneous chemistry is also important outside the vortex, via the effect of sulfuric acid aerosol on the partitioning of reactive nitrogen: #001# This reaction effectively pumps the reactive nitrogen species NO, NO2, NO3 and N2O5, which are all closely coupled by fast photochemical reactions, into nitric acid vapor, which is relatively slow to photodissociate. The low NO2 levels permit ClO to rise compared to an aerosol-free atmosphere, causing extra ozone loss. The Mount Pinatubo eruption in June 1991 increased the surface area of sulfuric acid aerosol in the lower stratosphere by a factor of 20 to 30, by which point the availability of aerosol surface is no longer the limiting factor. By the time of ASHOE/MAESA, much of this Pinatubo aerosol will have left the stratosphere, leaving a sharp contrast between the Antarctic vortex and its mid-latitude environs during 1994. The ER-2 is equipped with a payload designed to examine the photochemistry, the aerosol and PSC characteristics and also with tracer measurements to provide information in the meteorological motions and how they transport air among tropical, mid-latitude and polar regions. In this way ASHOE can address broad questions about the relative roles of Antarctic ozone loss spreading to mid-latitudes and in situ loss induced by sulfuric acid aerosol, in a quantitative manner. These broad questions have many detailed questions connected with them, which also must be addressed. The mid-latitude ozone loss in the southern hemisphere has been observed to be approaching 10% per decade in the annual total column average from 1979-1991, with the greatest losses occurring in the lower stratosphere during winter and spring. ASHOE is primarily targeted at understanding this loss. MAESA concentrates on the atmospheric effects of the exhaust of a proposed fleet of high speed (supersonic) civil transport (HSCT). Most of this exhaust will be released in the northern middle latitudes near 20 kilometers, and about twenty percent will be released in the tropics. The concern is the effect of this exhaust on stratospheric composition, primarily ozone, and thus upon the radiation balance of the lower atmosphere. Observations from the ER-2 during the second Airborne Arctic Stratospheric Expedition (AASE II) and the Stratospheric Photochemistry, Aerosols and Dynamics Expedition (SPADE) show that heterogeneous chemistry on sulfate aerosols, particularly the conversion of N2O5 to HNO3 by reaction with water, alters the catalytic destruction of ozone by the chemical families. As a result, a large fraction of the emitted NOx is converted to HNO3, and only small amounts of ozone are predicted to be destroyed by the high speed civil transport. This simple picture has complications, however. If the emitted NOx and water vapor increase the probability of formation of mixed nitric acid and water clouds (PSC's), then more ozone loss will occur by the polar ozone chemistry that was described above. Second, if a significant fraction of aircraft exhaust is transported to higher altitudes and lower latitudes where heterogeneous chemistry is less effective, then direct catalytic destruction of ozone by emitted NOx will occur. These complications imply that both polar and tropical processes must be examined in order to better assess the effects of high speed civil transport.
Project Office
Mail Stop 245-5
National Aeronautics and Space Administration
Ames Research Center
Moffett Field, CA 94035-1000
