Earth Observation Data Group

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.

Abstract:

Observations of Mount Pinatubo aerosol as measured by the ISAMS instrument on UARS are presented for December 1991. These observations show a tropical reservoir of high aerosol with steep gradients across the subtropics. On the 450 K isentrope there is a ring of high aerosol in midlatitudes which is separated from the tropical reservoir by an approximately zonal band of lower aerosol values. The ISAMS aerosol data are compared with output from the off-line three-dimensional transport model SLIMCAT. Integrations are performed with isentropic transport only, diabatic transport only, isentropic and diabatic transport, and isentropic, diabatic and sedimentary transport of aerosol. Integrations using winds and temperatures from UKMO and ECMWF are compared. It is shown that even in the lower stratosphere, cross-isentropic transport over the timescale of a month can have a significant effect on the distribution of aerosol. The SLIMCAT integrations with isentropic transport only do not form a ring of high aerosol in midlatitudes on the 450 K isentrope. Inclusion of diabatic transport in the SLIMCAT integration shows that this ring forms from diabatic descent of air containing higher aerosol concentrations from aloft. Isentropic transport of high-aerosol air from the tropics into midlatitudes on the 500 K isentrope, followed by diabatic descent, further increases the values in the midlatitude ring on the 450 K isentrope. Inclusion of the sedimentation of aerosol in SLIMCAT integrations significantly offsets the diabatic ascent in the tropics. The SLIMCAT integrations, particularly those run using UKMO analyses, produce too much transport of midlatitude air into the tropics. The implications for accurate modeling of the tropical reservoir are discussed. Copyright 1999 by the American Geophysical Union.

Abstract:

Observations of Mount Pinatubo aerosol as measured by the ISAMS instrument on UARS are presented for December 1991. These observations show a tropical reservoir of high aerosol with steep gradients across the subtropics. On the 450 K isentrope there is a ring of high aerosol in midlatitudes which is separated from the tropical reservoir by an approximately zonal band of lower aerosol values. The ISAMS aerosol data are compared with output from the off-line three-dimensional transport model SLIMCAT. Integrations are performed with isentropic transport only, diabatic transport only, isentropic and diabatic transport, and isentropic, diabatic and sedimentary transport of aerosol. Integrations using winds and temperatures from UKMO and ECMWF are compared. It is shown that even in the lower stratosphere, cross-isentropic transport over the timescale of a month can have a significant effect on the distribution of aerosol. The SLIMCAT integrations with isentropic transport only do not form a ring of high aerosol in midlatitudes on the 450 K isentrope. Inclusion of diabatic transport in the SLIMCAT integration shows that this ring forms from diabatic descent of air containing higher aerosol concentrations from aloft. Isentropic transport of high-aerosol air from the tropics into midlatitudes on the 500 K isentrope, followed by diabatic descent, further increases the values in the midlatitude ring on the 450 K isentrope. Inclusion of the sedimentation of aerosol in SLIMCAT integrations significantly offsets the diabatic ascent in the tropics. The SLIMCAT integrations, particularly those run using UKMO analyses, produce too much transport of midlatitude air into the tropics. The implications for accurate modeling of the tropical reservoir are discussed.