AASE II Mission Overview

The sudden onset of ozone depletion in the Antarctic vortex set a precedent for both the time scale and the severity of global change. The AIRBORNE ANTARCTIC OZONE EXPERIMENT (AAOE) staged from Punta Arenas, Chile, in 1987, established that CFCs and Halons are the source of chlorine and bromine radicals that, in turn, control the rate of ozone destruction, that the vortex is depleted in nitrogen oxides, reactive nitrogen, and water vapor and that diabatic cooling during the antarctic winter leads to subsidence within the vortex core importing air from higher altitudes and lower latitudes. This last conclusion is based on observed dramatic distortion in the tracer fields, most notably N2O.

In 1989 the AIRBORNE ARCTIC STRATOSPHERIC EXPEDITION (AASE) staged from Stavanger, Norway, using the same aircraft employed for AAOE, the NASA ER-2 and the NASA DC-8, discovered that while NOx and to some degree NOy were perturbed within the arctic vortex, there was little evidence for desiccation. Under these (in contrast, to the antarctic) marginally perturbed conditions, however, ClO was found to be dramatically perturbed such that a large fraction of the available (inorganic) chlorine resided in free radical and free radical dimer form.

This leaves two abiding issues for the northern hemisphere:

1. WILL SIGNIFICANT OZONE EROSION OCCUR WITHIN THE ARCTIC VORTEX IN THE NEXT TEN YEARS AS CHLORINE LOADING IN THE STRATOSPHERE APPROACHES 5 ppbv? THAT IS, WILL AN "OZONE HOLE" APPEAR IN THE NORTHERN HEMISPHERE BY DECADE'S END?

2. WHICH MECHANISMS ARE RESPONSIBLE FOR THE OBSERVED OZONE EROSION POLEWARD OF 30 DEGREES N IN THE WINTER/SPRING NORTHERN HEMISPHERE REPORTED IN SATELLITE OBSERVATIONS?

II. Scientific Questions

While there are literally dozens of specific questions that must be dealt with quantitatively in the course of answering the above questions, for the sake of coherence they can be categorized as follows:

Question 1: How do the meteorological (pressure, temperature, wind) fields, radiation (UV, VIS, IR) fields, tracer (N20. CH4, CFC-11) fields, ozone, and water vapor fields evolve during set up, maintenance, and break up of the vortex?

The thrust of this question is to define for the first time the structure of the stratosphere poleward of 40 latitude to as high an altitude as possible.

• When does diabatic cooling begin to distort the "normal" summertime tracer fields? What is the temporal evolution of potential temperature and potential vorticity fields as the polar jet develops?
• To what degree is there stratospheric- tropospheric exchange along isentropic surfaces?
• To what degree is there exchange between inner and outer vortex air during the lifetime of the vortex?
• What is the origin of air that occupies the core of the vortex?
• Do rates of subsidence reflected in distortions to the tracer fields match subsidence rates inferred for diabatic cooling rates determined from observations of radiant divergence?
• What is the latitude and altitude (referenced, for example, to surfaces of constant N20) dependence of thermal tendency, eddy heat transport, and mean heat transport from 400 to the pole in both hemispheres?

Question 2. What are the threshold conditions for significant repartitioning of reactive species within the nitrogen, chlorine, bromine, and hydrogen families?

The thrust of this question is to understand how the reactive compounds, most notably the catalytically active free radicals, respond to the dropping temperatures that initiate polar stratospheric cloud formation both inside and within the environs of the vortex.

• What is the mechanism for the "NOxon cliff'-the removal of NOx from the gas phase? Does the phenomenon require the presence of PSCs?
• What conditions (temperature, pressure, HONO2 and H2O, vapor pressure) trigger the formation of PSCs? At what threshold temperature do Type I and Type II PSCs form? Is temperature alone a valid indication of these phase transitions?
• Do NO, NO2, ClO HONO2, and HCl concentration fields evolve in the temperature and PSC fields in a manner consistent with heterogeneous conversion of HCl and ClONO2 to gas phase C12 and solid phase nitric acid trihydrate? How important is the N2Os/H2O reaction in the denitrification process, to the concentration of NOx and HOX radicals in the lower stratosphere?
• Do PSC's appear outside the vortex, for example, in gravity wave events? How important are these localized adiabatic processes to the overall effectiveness of halogen and nitrogen repartitioning?
• Which processes govern the transition from "normal" conditions to deNOxification (removal of NOx from the gas phase), to denitrification (removal of NOy from the gas phase), to dehydration (irreversible removal of H20 in all phases)?
• What factors control the kinetics of HCl/ClONO2 conversion to C1O?
• What factors control growth rates and subsidence rates of PSCs?

Question 3. How do ozone loss rates resulting from free radical catalytic processes evolve with time as a function of altitude, latitude, and solar zenith angle? How much ozone is destroyed by chemical processes within the vortex?

This question raises both (a) intravortex loss, which has been crudely quantified in a limited region within the antarctic vortex, as well as (b) more broadly distributed ozone erosion in the northern hemisphere well outside the arctic vortex. How will this situation evolve as chlorine loading approaches 5 ppbv?

• How do the concentration fields of the rate limiting radicals (CIO, BrO, N02, OH, H02) evolve within time as a function of altitude, latitude, and solar zenith angle poleward of 40 degrees?
• Where do each of the rate limiting steps in the catalytic cycles peak as a function of altitude, latitude, region, and season?
• What is the evolution of the correlation of ozone versus the tracer fields ~N20, CH4, CFC11), potential temperature, and potential vorticity between altitudes of 10 and 30 km poleward of 40 degrees?
• Does the degree of ozone loss regressed against the tracer fields (N20, CH4, CFC-11) correspond to the ozone loss determined from dO3/dt surfaces using observed concentrations of the rate limiting radicals?
• How much air is "processed" within the vortex during its lifetime?
• At what rate and by what process does CIO return to "normal" levels of CION02 and HCl? What are the chemical and dynamical signatures of air as it spins out from the vortex following vortex break up?
• Why is enhanced CIO so smoothly distributed over large regions outside the vortex? Why is it "capped" at ~ 100 pptv?
• How will the picture change as the stratosphere approaches 5 ppbv of total chlorine?

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