Chemical and Morphological Characterisation of Aerosol Particles in the Tropopause Region

Abstract: The aim of this work was to study atmospheric aerosols, focusing on the chemical composition, the morphology and the origin of the aerosol in the upper troposphere and lowermost stratosphere. An aerosol sampler was developed for this purpose. A new method for quantitative analysis of the major components of the aerosol, i.e. C, N and O was developed, and methodologies for single-particle analysis to gain information on particle morphology and chemical composition were adapted and used for the aerosol samples. Samples collected during 60 intercontinental flights from 1999 to 2002 were analysed with regard to elemental composition with particle-induced X-ray emission. The results were used to study the properties and the origin of the aerosol in the lowermost stratosphere at northern mid-latitudes, focusing on the sulphur concentration. The results of this study show that particulate potassium and iron in the lowermost stratosphere originate in the troposphere and are transported across the tropopause, whereas particulate sulphur has a strong stratospheric origin in addition to transport across the tropopause. Approximately half of the mass of particulate sulphur transported from the stratospheric overworld to the lowermost stratosphere was formed from carbonyl sulphide the remainder being dominated by particulate sulphur and sulphur dioxide that was transported across the tropical tropopause. The production of particulate sulphur in the stratosphere was estimated to be 0.066 Tg S/y. A new multi-channel aerosol sampler was developed and calibrated for the second phase of the CARIBIC project, which improved the time resolution and produced samples for quantitative and individual particle analyses. It was found that the collection efficiency of the sampler was as high as 97% for particles larger than approximately 0.2 µm in diameter, and the cut-off was 0.08 µm at the calibration conditions. The time resolution of each sample was 1.5 hours. This sampler is a powerful tool for aerosol characterization from an aircraft. A new method for the analysis of carbon, nitrogen and oxygen based on the use of PESA has been developed for use on CARIBIC samples, which are collected on an organic backing film. For example, the detection limits of C, N O, S and Fe are 3, 1, 2, 2 and 0.1 ng/m3, normalized to STP. This analytical protocol is truly unique because it offers low detection limits and high analytical capability over a wide range of elements. Moreover, it yields quantitative information. This is the first quantitative measurement of the concentration of the carbonaceous aerosol component and other components such as N and O in the upper troposphere and the lowermost stratosphere. It was found that they, together with sulphur, are the major constituents of the aerosol in this region. Individual particles? characteristics have also been studied based on classification of individual particles with respect to morphology and composition along a transect from 50º N to 30º at 10 km altitude. The classification comprised of more than 30 particle types. Their morphology is complex and varied. A dependence of morphology on the location where they were collected was found. For further investigation of these aerosol particles, chemical analysis was undertaken. Two techniques, PIXE and PESA, were used to quantitatively determine the concentration of carbon and sulphur. Then the particulate carbon to sulphur ratio was used to express the composition of the samples. This ratio varied over the aerosol samples in a range 0.5 to 3.5. The largest value of this ratio appears at around the equator, south of ITCZ, whereas the lowest one was found in the lowermost stratosphere. Further chemical investigation using EFTEM was made to reveal the distributions of carbonaceous and sulphurous matter of individual particles. The results were used to explain the complex structures and the large variation in morphology of the aerosol from different regions. Particles with satellite patterns usually are interpreted as being composed of sulfuric acid. Analyses by EFTEM presented here show that these particles have a central particle composed of both carbonaceous and sulfurous matter and the satellites are composed of carbonaceous matter. Hence a morphological analysis alone could erroneously classify these mixed particles as being pure sulfuric acid.