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AASE2 Mission Statement

Part one Preamble (AASE II)

This statement has been prepared by scientists of the second Airborne Arctic Stratospheric Expedition (AASE-II). The mission was staged over a period of eight months from Moffett Field, California; Fairbanks, Alaska; Anchorage, Alaska; Stavanger, Norway; and Bangor, Maine. The mission used two aircraft to study the lower stratosphere: a high altitude ER-2 aircraft for in situ observations and a long range DC-8 for remote sensing observations. This summary represents the preliminary conclusions of the scientists at the end of the flight series.

The issue of ozone depletion is of widespread public concern. Hence, policy makers and the public should be kept abreast of the advances in the scientific understanding. It is in this spirit that we report our provisional interpretation of new data concerning stratospheric ozone in the Northern Hemisphere. A comprehensive interpretation of our findings will be forthcoming after a series of scientific meetings and the publication of peer-reviewed scientific papers.

Background to the Mission

In 1985, a large and unanticipated decrease in the abundance of ozone over Antarctica was reported by the British Antarctic Survey. Public concern was heightened in 1988 and again in 1991 by ground and satellite observations that showed ozone at northern mid latitudes in the winter had decreased 6-8% between 1979 and 1990. It is critical to recognize that while ozone exhibits considerable natural variability, decreases in ozone overhead without other offsetting atmospheric changes result in increased ultraviolet radiation reaching the Earth's surface. Biological and medical studies suggest that the accumulated exposure to these increases produce deleterious effects on mankind and other living organisms.

Predicting ozone loss in the Earth's stratosphere over the next decade require detailed knowledge of both chemical and transport processes. Two types of scientific investigation provide guidance for policy decisions: (1) mechanistic studies linking cause and effect, which serve as the foundation of our ability to look forward in time, and (2) global-scale studies in trends of atmospheric change. Improved understanding of mechanism and process was the focus of the second Airborne Arctic Stratosphere Expedition (AASE II).

The research phase described here, which began in August, 1991, is a continuation of episodic aircraft flights into the antarctic, arctic and mid-latitude stratosphere. More than one hundred flights have been made during the last five years by the ER-2 and DC-8, encompassing all latitudes from the South Pole to the North Pole. Results from the first of these aircraft campaigns, the Airborne Antarctic Ozone Experiment (AAOE) over Antarctica in 1987, demonstrated that chlorofluorocarbons (CFCs) released into the atmosphere caused dramatic springtime ozone erosion over the Antarctic. Those studies pinpointed chlorine monoxide and bromine monoxide as the species responsible for controlling the rate of ozone destruction and further indicated the importance of polar stratospheric clouds (PSCs) in producing chemical transformations that facilitate such destruction. In 1989, the first Airborne Arctic Stratospheric Expedition (AASE-I) was staged from Stavanger, Norway. During that mission, ER-2 flights into the arctic stratosphere revealed that chlorine monoxide and bromine monoxide were present at concentrations comparable to those observed over Antarctica in 1987. However, since the degree of ozone loss depends both on the ClO/BrO concentrations and on the duration of the elevated levels, the shorter period of cold temperatures in the Antarctic diminishes the impact on ozone.

AASE-II is comprised of four major program elements: a high altitude ER-2 aircraft, a long-range DC-8 aircraft, extensive meteorological predictions and analyses, which include an array of computational programs to correlate and interpret the aircraft observations, and finally, the Total Ozone Mapping Spectrometer (TOMS) on the Nimbus-7 satellite, which monitored the global distribution of total ozone. The instrument packages on the two aircraft measured an array of chemical species and other atmospheric parameters that are associated with the mechanisms that determine the distribution of ozone.

Meteorological analyses from the NOAA National Meteorological Center (NMC) provided the historical context, analysis and predictive capability for temperature, pressure and wind fields for the northern hemisphere during the field deployment.

Three question define the principal mission objectives for AASE-II:

    1. Will significant erosion of stratospheric ozone occur over the Arctic as stratospheric chlorine levels increase during the next decade?
    2. What are the causes of mid-latitude stratospheric ozone decreases in late fall through the early summer, revealed over the past decade by ground and satellite observations?
    Finally, given the eruption of Mt. Pintaubo in June of 1991, we address another issue:
  1. What effect do volcanoes have on the chemical processes that govern stratospheric ozone? In particular, could volcano aerosols modify depletion of stratospheric ozone associated with industrial halocarbons?

Part two: AASE-II Summary Statement

    • The chemical composition of the stratosphere was highly perturbed at the northern latitudes this winter. Most of the chlorine released from the CFCs in the stratosphere, where it resided as chemically stable inorganic chlorine (HCl and ClONO2), was converted to the reactive form (ClO). This transformation, observed in a sequence of flights, began in mid-December and was complete by mid-January. Concentrations of ClO were observed by the ER-2 to increase during January, exceeding 1500 pptv on January 20th. Appearance of high ClO concentrations correlated with the disappearance of HCl and with the onset of temperatures cold enough to form polar stratospheric clouds (PSCs). The onset of cold temperatures, high ClO, and the loss of HCl had been predicted based on laboratory measurements and was observed simultaneously in specific air masses for the first time in AASE-II.
    • Meteorological conditions over the Arctic this winter were characterized by a period from mid-December to the third week of January when minimum temperatures dropped below the threshold for PSC formation at ER-2 altitudes. This period was shorter than average. An analysis of HCl, ClONO2, NO, NO2, HONO2 and ClO concentrations from this mission, in the context of more than one hundred aircraft flights over the last five years, implies that ClO levels in excess of 1000 pptv should emerge at high latitudes in January during typical or colder year for the next two decades.
    • Temperatures warmed abruptly within the vortex the third week of January. The decline of ClO commenced as expected when temperatures warmed above the PSC threshold. ClO dropped slowly: by mid-February concentrations from 700 to 1000 pptv were typical in the vortex. The decline of ClO continued into late March.
    • Calculations based on the observed concentrations of HCl, ClONO2, NO, NO2, HONO2, ClO and BrO in the arctic vortex in January and February indicate approximately 20% ozone removal between 15 and 20km. Using data for ozone and tracers from the aircraft, evidence for comparable ozone removal was observed over a more limited altitude range. The percentage total column ozone loss is estimated to be about one half the percentage observed between 15 and 20 km. In the calculations, ozone removal is due in about equal measure to reaction involving chlorine and bromine.
    • The amount of ozone destroyed in a given year is controlled by two factors: (1) total chlorine and bromine concentrations in the stratosphere, both of which are increasing annually; (2) the timing and vertical extent of temperatures below the threshold for PSCs. If temperatures had remained cold into the third week of February, which has occurred several times in the last decade, greater amount of ozone would have been destroyed. The loss of ozone in winter 1991/1992, while significant, should not be described as an "ozone hole," a term coined to denote the sharp transition to dramatically suppressed O3 levels over Antarctica. In this hemisphere, it is essential to focus on the more broadly distributed erosion of ozone at both mid and high latitudes.
    • During 1992, the Total Ozone Mapping Spectrometer (TOMS) satellite measurements show that the hemispheric ozone average during January, February, and most of Mach was lower than any previous year in the TOMS record. TOMS measurements also showed that total ozone values in the mid-latitude maximum during February were 10-15% lower than any previous year in the TOMS record. The processes responsible for this ozone decrease are under investigation.
    • The mission revealed strong evidence for the influence of sulfate aerosols on stratospheric chemistry, particularly outside the polar vortex. Natural sulfate particles appear to suppress concentrations of NO and NO2, leading to enhance concentrations of ClO and BrO. The highest concentrations of ClO and BrO outside the vortex were observed in winter at high latitudes. The results are consistent with the view, expressed in the recent UNEP report, that ClO and BrO are likely to be implicated in recent reductions of column ozone amounts observed over midlatitudes. The largest change in midlatitude ozone levels are observed in late winter and occur in the same altitude regions where ClO and BrO are elevated throughout the winter.
    • The eruption of Mt. Pinatubo increased abundances of natural sulfate aerosol particles, potentially amplifying the effects of reactions which take place on the surfaces of particles. No significant direct injection of chlorine by the volcano was observed. There was no evidence for significant influence of Pinatubo aerosols on the chemistry of the polar vortex. In the vortex, the removal of NO, NO2, and HCl, and the large enhancements of ClO appear to be triggered by formation of PSCs.
  • Enhanced sulfate loading from Mt. Pinatubo may affect regions of the atmosphere where the fractional conversion resulting from pre-Pinatubo aerosol loading is incomplete, which is increasingly true at lower latitudes and as altitudes above 20 km. Column amounts of NO2 were observed by the DC-8 to be notably depressed at mid-latitudes this year, as compared to past years. Ozone levels were reduced within the Pinatubo aerosol layers in the tropics.