Stratosphere

The terrestrial stratosphere lies above the troposphere and has an upward-warming region associated with ultraviolet heating by the ozone layer.

Transport of air and trace species across the tropopause. Downward ozone transport can supply the troposphere even when local tropospheric photochemical production is unchanged. Regional pathways, mixing and residence times determine the resulting burden.

The wave-driven mean stratospheric circulation with tropical ascent and extratropical descent. Changes in wave propagation and momentum deposition can alter its strength and transport times. Its response redistributes ozone and trace gases; local chemistry and transport must be considered together.

A wintertime region of strong circumpolar flow that can isolate polar stratospheric air and sustain cold conditions. Isolation, cloud formation, sunlight return and subsequent breakup jointly influence seasonal ozone chemistry. Cooling alone does not determine the full ozone evolution.

An increase of atmospheric carbon dioxide can cool the stratosphere through enhanced infrared cooling relative to non-infrared heating and through changes in the spectrum of upwelling radiation. A local emissivity-only explanation is incomplete because absorption and emission both change. The response depends on ozone solar heating, spectral opacity, altitude and adjustments of the layers below.^[ref]

Cold-region stratospheric particles that can support heterogeneous reactions activating reactive halogen chemistry. Their occurrence and effect depend on temperature, available condensable species, halogen reservoirs and sunlight history.

Removal of reactive nitrogen from an air mass through formation and sedimentation of nitric-acid-containing particles. This can delay reformation of chlorine nitrate and prolong active chlorine chemistry. It is a physical/chemical atmospheric removal process, distinct from biological denitrification.

Conversion of relatively unreactive chlorine reservoirs into forms that produce active radicals in sunlight. Heterogeneous reactions on appropriate particle surfaces can turn hydrogen chloride and chlorine nitrate into photolabile species. Available surface composition, temperature, reservoir supply and subsequent illumination all matter; adding arbitrary particles does not guarantee activation.