An investigation into the tropospheric chemistry of acidic aerosols and ammonia in the laboratory

This thesis describes a broad range of experiments based on an aerosol flow-tube system to probe the interactions between atmospherically relevant aerosols with trace gases. This apparatus was used to obtain simultaneous optical and size distribution measurements using FTIR and SMPS measurements respectively as a function of relative humidity and aerosol chemical composition. Heterogeneous reactions between various ratios of ammonia gas and acidic aerosols were studied in aerosol form as opposed to bulk solutions. The apparatus is unique, in that it employed two aerosol generation methods to follow the size evolution of the aerosol while allowing detailed spectroscopic investigation of its chemical content. A novel chemiluminescence apparatus was also used to measure [NH4+]. SO2.H2O is an important species as it represents the first intermediate in the overall atmospheric oxidation process of sulfur dioxide to sulfuric acid. This complex was produced within gaseous, aqueous and aerosol SO2 systems. The addition of ammonia, gave mainly hydrogen sulfite tautomers and disulfite ions. These species were prevalent at high humidities enhancing the aqueous nature of sulfur (IV) species. Their weak acidity is evident due to the low [NH4+] produced. An increasing recognition that dicarboxylic acids may contribute significantly to the total acid burden in polluted urban environments is evident in the literature. It was observed that speciation within the oxalic, malonic and succinic systems shifted towards the most ionised form as the relative humidity was increased due to complete protonisation. The addition of ammonia produced ammonium dicarboxylate ions. Less reaction for ammonia with the malonic and succinic species were observed in comparison to the oxalic acid system. This observation coincides with the decrease in acidity of these organic species. The interaction between dicarboxylic acids and ‘sulfurous’/sulfuric acid has not been previously investigated. Therefore the results presented here are original to the field of tropospheric chemistry. SHO3-; S2O52-; HSO4-; SO42- and H1,3,5C2,3,4O4-;C2,3,4O4 2- were the main components found in the complex inorganic-organic systems investigated here. The introduction of ammonia produced ammonium dicarboxylate as well as ammonium disulfite/sulfate ions and increasing the acid concentrations increased the total amount of [NH4+].

Osmotic stress tolerance mechanisms in Lactococcus lactis

The response of Lactococcus lactis subsp. cremoris NCDO 712 to low water activity (aw) was investigated, both in relation to growth following moderate reductions in the aw and in terms of survival following substantial reduction of the aw with NaCI. Lc.lactis NCDO 712 was capable of growth in the presence of ≤ 4% w/v NaCI and concentrations in excess of 4% w/v were lethal to the cells. The presence of magnesium ions significantly increased the resistance of NCDO 712 to challenge with NaCI and also to challenge with high temperature or low pH. Survival of Lc.lactis NCDO 712 exposed to high NaCI concentrations was growth phase dependent and cells were most sensitive in the early exponential phase of growth. Pre-exposure to 3% w/v NaCI induced limited protection against subsequent challenge with higher NaCI concentrations. The induction was inhibited by chloramphenicol and even when induced, the response did not protect against NaCI concentrations> 10% w/v. When growing at low aw, potassium was accumulated by Lc. lactis NCDO 712 growing at low aw, if the aw was reduced by glucose or fructose, but not by NaCI. Reducing the potassium concentration of chemically defined medium from 20 to 0.5 mM) produced a substantial reduction in the growth rate, if the aw was reduced with NaCI, but not with glucose or fructose. The reduction of the growth rate correlated strongly with a reduction in the cytoplasmic potassium concentration and in cell volume. Addition of the compatible solute glycine betaine, partially reversed the inhibition of growth rate and partially restored the cell volume. The potassium transport system was characterised in cells grown in medium at both high and low aw. It appeared that a single system was present, which was induced approximately two-fold by growth at low aw. Potassium transport was assayed in vitro using cells depleted of potassium; the assay was competitively inhibited by Na+ and by the other monovalent cations NH4+, Li+, and Cs+. There was a strong correlation between the ability of strains of Lc. lactis subsp. lactis and subsp. cremoris to grow at low aw and their ability to accumulate the compatible solute glycine betaine. The Lc. lactis subsp. cremoris strains incapable of growth at NaCI concentrations> 2% w/v did not accumulate glycine betaine when growing at low aw, whereas strains capable of growth at NaCI concentrations up to 4% w/v did. A mutant, extremely sensitive to low aw was isolated from the parent strain Lc. lactis subsp. cremoris MG 1363, a plasmid free derivative of NCDO 712. The parent strain tolerated up to 4% w/v NaCI and actively accumulated glycine betaine when challenged at low aw. The mutant had lost the ability to accumulate glycine betaine and was incapable of growth at NaCI concentrations >2% w/v or the equivalent concentration of glucose. As no other compatible solute seemed capable of substitution for glycine betaine, the data suggest that the traditional; phenotypic speciation of strains on the basis of tolerance to 4% w/v NaCI can be explained as possession or lack of a glycine betaine transport system.