The Formation and Growth of Marine Aerosols and the Development of New Techniques for their In-situ Analysis.

摘要

Marine aerosols have attracted increasing attention over the past 15 years because ofududtheir potential significance for global climate modelling. The size distribution of theseududaerosols extends from super-micrometer sea salt mode particles down through 150 nmududaccumulation mode particles, 40 nm Aitken mode particles and nucleation modeududparticles which extend from 25 nm right down to clusters of a few molecules. Theududprocess by which the submicrometer modes form and grow and their compositionududhave remained topics of debate throughout this time in large part because of theududdifficulties associated with determining their composition and relating it to proposedududmodels of the formation process.ududududThe work compared the modality of marine aerosol influencing the South-east-Queensland region with that of other environmental aerosols in the region. The aerosol was found to be consistent with marine aerosols observed elsewhere withududconcentrations below 1000 cm-3 and frequently exhibiting the distinct bimodal structure associated with cloud processing, consisting of an Aitken mode at approximately 40 nm, an accumulation mode in the range 100-200 nm and a coarse mode attributed to sea salt between 600 and 1200 nm.ududududThis work included the development of two new techniques for aerosol research. The first technique measures aerosol density using a combination of aerosol size distribution and gravimetric mass concentration measurements. This technique was used to measure the density of a number of submicrometer aerosols includingududlaboratory generated NaCl aerosol and ambient aerosol. The densities for the laboratory generated aerosols were found to be similar to those for the bulk materials used to produce them. The technique, extended to super-micrometer particle size range may find application in ambient aerosol research where it could be used to discriminate between periods when the aerosol is dominated by NaCl and periodsududwhen the density is more representative of crustal material or sulfates. The techniqueududmay also prove useful in laboratory or industrial settings for investigating particleududdensity or in case where the composition is known, morphology and porosity.ududududThe second technique developed, integrates the existing physicochemical techniquesududof volatilisation and hygroscopic growth analysis to investigate particle compositionududin terms of both the volatilisation temperatures of the chemical constituents and theirududcontribution to particle hygroscopic behaviour. The resulting volatilisation and humidification tandem differential mobility analyser or VH-TDMA, has proven to be a valuable research tool which is being used in ongoing research.ududududFindings of investigations relating the composition of the submicrometer marineududaerosol modes to candidate models for their formation are presented. Sea salt was notududfound in the numerically dominant particle type in coastal nucleation mode or marineududAitken and accumulation modes examined on the Southeast Queensland coast duringududperiods where back trajectories indicated marine origin. The work suggests that allududthree submicrometer modes contain the same four volatile chemical species and anududinsoluble non-volatile residue. The volatility and hygroscopic behaviours of theududparticles are consistent with a composition consisting of a core composed of sulfuricududacid, ammonium sulfate and an iodine oxide coated with a volatile organic compound.ududThe volume fraction of the sulfuric acid like species in the particles shows a strongududdependence on particle size.