The first direct experiments on the ER-2 aircraft recast our understanding of ozone loss in the lower stratosphere. Commercial aircraft sales represent an international market measured in tens of billions of dollars annually. Development of the HSCT is a main component in the international battle for leadership in this field. A key issue for this development is that the nitrogen oxide/particulate/water vapor effluent from the proposed aircraft could trigger both enhanced ozone loss in the stratosphere and radiative changes linked, through water vapor changes and cloud formation, to climate changes. Senate hearings in the early 1970s hinged significantly on the prospect of damage to the ozone layer by large fleets of supersonic transports resulting from NOx emissions.6 Equally important, however, is recognition that, if an aircraft is detrimental to global ozone and/or climate, business decisions to build such an aircraft are compromised.

NASA is carrying out a research effort with airborne missions7 to test fundamental ideas about processes that control tropospheric and stratospheric ozone and, in particular, how the proposed HSCT and subsonic aircraft may alter those processes. The past three years have witnessed two important developments in our understanding of processes that control the catalytic destruction of ozone in the lower stratosphere. The first development emerged out of simultaneous NOx/ NOy observations8 during NASA's research and analysis airborne mission to the Arctic. The mission found that aerosols (minute liquid droplets) have a dramatic impact on the fraction of reactive nitrogen tied up in free radical form (NO and NO2). These ER-2 in situ observations clearly demonstrated that NOx was converted to NOy, thereby providing a natural “sink” for any reactive nitrogen compound added to the lower stratosphere and, in particular, for the combustion effluent from the proposed Mach 2.4 HSCT. This result constitutes the first serious challenge to the two-decades-old premise that catalytic destruction of ozone in the lower stratosphere is dominated by nitrogen radicals (NOx). It was this fundamental tenet—that ozone removal in the lower stratosphere is rate limited by NO 2 —combined with the realization that a significant fleet of supersonic transports would add appreciably to the nitrogen oxide budget of the lower stratosphere, that impugned supersonic transports in the early 1970s.9

The second key development emerged from NASA's Stratospheric Photochemistry, Aerosol, and Dynamics Expedition of May 1993 and has subsequently been confirmed in more recent airborne missions. This ER-2 mission was the first to include a new generation of solid-state laser experiments capable of detecting OH and HO2, thereby completing an ensemble of instruments capable of simultaneous in situ detection of each of the rate-limiting radicals in the dominant catalytic cycles (NO2, ClO, BrO, and HO2) and of the key coupling radicals NO and OH. These ER-2 observations 10 demonstrated the predominance of odd hydrogen HOx) and halogen free radical (ClOx and BrOx) catalysis in determining the rate of removal of ozone in the lower stratosphere. A single catalytic cycle, rate limited by HO2 + O3 → HO + O2 + O2, was found to account for nearly half the total O3 removal in the midlatitude northern hemisphere lower stratosphere. Halogen radical chemistry was found to be responsible for 30 percent of the



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