Earth Observation Data Group

Abstract:

During recent years ozone depletion has been detected not only over polar regions but also over mid-latitude areas. This study analyzed daily total ozone (TO) data from three south-western European locations in order to detect long-time TO trends by means of a filtering technique. Correlation analysis with atmospheric circulation patterns was carried out to explain the decreasing trends observed. Results appear to show a strong correlation between TO decrease and the North Atlantic Oscillation and Arctic Oscillation Indices throughout recent decades. On the other hand, the trends also indicate that, at least during the last ten years, TO variations cannot be explained solely by natural atmospheric cycles over the studied area.

Abstract:

The primary effect of a volcanic eruption is to alter the composition of the stratosphere by the direct injection of ash and gases. On average, there is a stratospherically significant volcanic eruption about every 5.5 years. The principal effect of such an eruption is the enhancement of stratospheric sulphuric acid aerosol through the oxidation and condensation of the oxidation product H2SO4. Following the formation of the enhanced aerosol layer, observations have shown a reduction in the amount of direct radiation reaching the ground and a concomitant increase in diffuse radiation. This is associated with an increase in stratospheric temperature and a decrease in global mean surface temperature (although the spatial pattern of temperature changes is complex). In addition, the enhanced aerosol layer increases heterogeneous processing, and this reduces the levels of active nitrogen in the lower stratosphere. This in turn gives rise to either a decrease or an increase in stratospheric ozone levels, depending on the level of chlorine loading.

Abstract:

A new approach has been developed to calculate wavelength-dependent complex refractive indices from spectral data of laboratory generated mimic polar stratospheric cloud (PSC) particles. Previous determinations of refractive index for mimic PSC aerosols and thin-films have used Kramers-Kronig based methods. To determine the refractive index this approach requires two aerosol spectra of the same composition, but with different mean radii; i.e. one spectrum of small particles with a negligible scattering component, and another with larger particles and stronger scattering component. Our analysis takes a more fundamental approach by first determining the complex dielectric constant, which can then be related to the complex refractive index at a given wavelength. The dielectric constant is calculated with a damped harmonic oscillator model using band parameters for each oscillator. This results in a much smaller number of unknowns for the retrieval analysis, enabling determination of refractive index from a single infrared extinction spectrum and negating the need for two spectra of the same composition but different mean radii, which can be difficult to obtain experimentally. Preliminary results are presented from laboratory measurements of supercooled ternary (H2SO4 / HNO3 / H2O) solution (STS) aerosols.

Abstract:

The extinction spectra of aqueous sulfuric acid aerosols fully covering the mid-IR to visible regions from 750 to 23,000 cm(-1) (13.9-0.4 mu m) have been measured in the laboratory with a Fourier-transform spectrometer. Both large and small aerosol particles with compositions of approximately 60-70-wt.% H2SO4 were generated and their spectra recorded at 230 and 294 K. The spectra were fitted to a model incorporating roam-temperature refractive-index data [Appl. Opt. 14, 208 (1975)] and Mie theory calculations to characterize the composition and size distributions of the aerosol samples. (C) 1999 Optical Society of America.