Observing the Stratospheric Submillimeter Spectrum for Detecting Threats to the Ozone Layer

Abstract

Stratospheric ozone protects life on Earth from solar ultraviolet radiation, but the ozone layer is fragile. The Antarctic ozone hole has shown that humankind's release of certain chemicals into the atmosphere can deplete ozone essentially completely in a region where the destruction process is operative. Early detection of a future threat, especially one that might operate on a global scale as severely as that now operating in a layer over Antarctica each October, is crucial. Globally observing the stratosphere submillimeter-wavelength spectrum can give early (potentially earliest-possible) detection of threats to the ozone layer. Hundreds of chemical species – including radicals that can reveal new destruction processes before they cause noticeable depletion of ozone – have submillimeter spectral lines that are detectable and measurable at abundances that can threaten ozone. Spectral lines are resolved at all stratospheric heights, providing definitive identification. Chemical species in all global regions and at all stratospheric heights can be measured each 24-hour period, both day and night, including in the presence of dense volcanic aerosol and ice clouds. New solid-state technology is available for the stratosphere submillimeter spectrum to be observed from satellite at wavelengths down to 0.1 mm by both passive and active limb sounding. Using this technology, we present a Submillimeter Observatory for the Stratosphere (SOS) concept. SOS economically combines the most valuable features of passive and active measurements: vertical profile measurements of passive and ultra-high sensitivity of active. Active and passive measurements are time-shared, the passive system is the receiver for the active, eliminating the need for a separate receiver satellite. Active measurement vertical resolution is obtained from the measured spectral line shape, eliminating the need for a constellation of satellites. Instruments operate at ambient temperature, eliminating the need for detector cooling. Projected SOS detectability is given for 455 chemical species. Active measurement daily 10° latitude zonal mean precisions with 2 m antenna are projected capable of detecting 440 species down to ∼10 ⁻¹² relative abundances, and 220 species down to ∼10 ⁻¹⁵ . Passive individual vertical profile measurements, made every 1.5° along the suborbital path with ∼2 km vertical resolution, have projected precision better than ∼10 ⁻⁹ relative abundance for 390 species. Passive daily 10° latitude zonal means with 5 km vertical resolution have projected precision capable of detecting 200 species down to ∼10 ⁻¹² . The fundamental limit on detectability is the stratosphere's spectral clutter floor. The practical limit is likely to be set by the ability to calibrate out instrumental spectral artifacts.