Professor Roy Grainger

Abstract:

We present a new method to determine mesospheric electron densities from partially reflected medium frequency radar pulses. The technique uses an optimal estimation inverse method and retrieves both an electron density profile and a gradient electron density profile. As well as accounting for the absorption of the two magnetoionic modes formed by ionospheric birefringence of each radar pulse, the forward model of the retrieval parameterises possible Fresnel scatter of each mode by fine electronic structure, phase changes of each mode due to Faraday rotation and the dependence of the amplitudes of the backscattered modes upon pulse width. validation results indicate that known profiles can be retrieved and that chi(2) tests upon retrieval parameters satisfy validity criteria. Application to measurements shows that retrieved electron density profiles are consistent with accepted ideas about seasonal variability of electron densities and their dependence upon nitric oxide production and transport. (C) 2004 Elsevier Ltd. All rights reserved.

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.