METO 637 Lesson 8

Perturbations of the stratosphere Testing our knowledge of the stratosphere comes from a comparison of the measured and predicted concentrations. But an equal source of information is obtained by studying the response of the atmosphere to a sudden perturbation, e.g. a volcanic eruption. Longer term perturbations can also be used to test our knowledge, e.g. solar cycle variations.

Solar Proton Events Charged particles from the Sun are guided by the Earth’s magnetic field to the atmosphere at high latitudes. This provides a source of odd hydrogen and nitrogen. Every few years these events are greatly enhanced – solar proton event (SPE). These protons ionize molecular nitrogen, which in turn produces about 1.26 N atoms for every ion pair. Each ion pair is thought to produce two HOx species as well as the NOx. These produce more that 20% depletion of ozone in the upper stratosphere in the polar regions.

Solar ultraviolet irradiance The solar ultraviolet irradiance determines the overall level of ozone in the stratosphere, principally through the rate of atomic oxygen production. Small variations in the solar irradiance arise from changes in the Earth-Sun distance, and solar rotation. Largest change due to solar activity – linked to the Sunspot Cycle (~11 years). The change in the solar constant (mainly visible wavelengths) is less than 1% over a cycle. In the wavelength range of atomic oxygen production (~200 nm), however, the change over a solar cycle is about 10%, and in the region of ozone photolysis (~300 nm) thye change is about 2%. Hence ozone concentrations at 40 km, should show a change of about 4%

Solar cycle ratio Nov 89/Sep 86 Lean,J., Geophys. Res. Lett., 27, , 2000.

Solar cycle change in the O 2 dissociation rate vs Altitude

Tropics – QBO/El Nino Most of the source gases from the troposphere come are brought into the stratosphere within the strong convective uplift. Any modulation of this convective flow will produce changes in the stratosphere ozone level. Quasi-Biennial Oscillation – a switch on the lower stratosphere from Easterly to Westerly winds. Easterly winds inhibit upward transport ( about 2.3 years) El Nino – changes in the sea-surface temperature patterns. Sea surface temperature influences the strength of the convection.

VOLCANIC ACTIVITY Most volcanoes eject dust etc. into the troposphere where it is quickly rained out. However large volcanoes can eject large quantities of material into the stratosphere. Water and hydrochloric acid add to the amounts In the stratosphere. Sulfur compounds (SO 2, OCS, CS 2 ) transform into aerosols, which remain in the stratosphere for one to two years. These aerosols will tend to cool the troposphere, and scatter solar radiation back to space. Eruption of Mount Tambora in Indonesia led to 'year without a summer'

Mount Pinatubo Produced the largest natural perturbation of the stratosphere in the 20 th century. Estimated to have injected 20 M tonne of SO 2 into the stratosphere. SO 2 cloud encircled the earth in 22 days. Within two months all of the SO 2 had been converted into liquid H 2 SO 4 aerosol. Surface area about 100 times that before the eruption. This led to dynamical, radiative and chemical perturbations.

NO 2 and aerosol observations after Pinatubo

Fraction of the odd-oxygen loss rate

Mount Pinatubo Reductions of up to 60% in NO 2 were observed. Similar results were also obtained for NO. ‘Denoxification’. Nitric acid showed an increase, probably due to the heterogeneous reaction N 2 O 5 + H 2 O → 2HNO In the lower stratosphere, where the aerosols exist, the nitrogen family has an effect on the overall odd oxygen reduction, but they also act to inhibit the efficiencies of the catalytic cycles of the other families. Remove the nitrogen oxides, and the other cycles have a larger effect, than that of the nitrogen oxides.