A marine biogenic source of atmospheric ice-nucleating particles.

The amount of ice present in clouds can affect cloud lifetime, precipitation and radiative properties. The formation of ice in clouds is facilitated by the presence of airborne ice-nucleating particles. Sea spray is one of the major global sources of atmospheric particles, but it is unclear to what extent these particles are capable of nucleating ice. Sea-spray aerosol contains large amounts of organic material that is ejected into the atmosphere during bubble bursting at the organically enriched sea-air interface or sea surface microlayer. Here we show that organic material in the sea surface microlayer nucleates ice under conditions relevant for mixed-phase cloud and high-altitude ice cloud formation. The ice-nucleating material is probably biogenic and less than approximately 0.2 micrometres in size. We find that exudates separated from cells of the marine diatom Thalassiosira pseudonana nucleate ice, and propose that organic material associated with phytoplankton cell exudates is a likely candidate for the observed ice-nucleating ability of the microlayer samples. Global model simulations of marine organic aerosol, in combination with our measurements, suggest that marine organic material may be an important source of ice-nucleating particles in remote marine environments such as the Southern Ocean, North Pacific Ocean and North Atlantic Ocean.

Kinetic studies of the heterogeneous oxidation of maleic and fumaric acid aerosols by ozone under conditions of high relative humidity.

In this paper, a kinetic study of the oxidation of maleic and fumaric acid organic particles by gas-phase ozone at relative humidities ranging from 90 to 93% is reported. A flow of single component aqueous particles with average size diameters in the range 2.6-2.9 µm were exposed to a known concentration of ozone for a controlled period of time in an aerosol flow tube in which products were monitored by infrared spectroscopy. The results obtained are consistent with a Langmuir-Hinshelwood type mechanism for the heterogeneous oxidation of maleic/fumaric acid aerosol particles by gas-phase ozone, for which the following parameters were found: for the reaction of maleic acid aerosols, K(O(3)) = (9 ± 4) × 10(-15) cm(3) molecule(-1) and k = (0.21 ± 0.01) s(-1); for the reaction of fumaric acid aerosols, K(O(3)) = (5 ± 2) × 10(-15) cm(3) molecule(-1) and k = (0.19 ± 0.01) s(-1). From the pseudo-first-order coefficients, apparent uptake coefficient values were calculated for which a decreasing trend with increasing ozone concentrations was observed. Comparison with previous measurements of the same system under dry conditions reveals a direct effect of the presence of water on the mechanism of these reactions, in which the water is seen to increase the formation of CO(2) and formic acid (HCO(2)H) through increased levels of hydroxyacetyl hydroperoxide intermediate.

Infrared spectroscopic studies of the heterogeneous reaction of ozone with dry maleic and fumaric acid aerosol particles.

Dicarboxylic acids, either directly emitted or formed in chemical processes, are found to be a significant component of tropospheric aerosols. To assess any potential chemical transformation of short unsaturated dicarboxylic acids in tropospheric heterogeneous chemistry, maleic and fumaric acid were selected as surrogates in this study. A novel aerosol flow tube apparatus is employed to perform kinetic studies of the oxidation of these organic compounds by gas-phase ozone. The system consists of a particle generation system, a vertically oriented glass flow tube and an infrared observation White cell with a Fourier transform infrared (FTIR) spectrometer for the detection system. A flow of single component organic aerosols with mean diameters ranging between 0.7 and 1.1 microm is introduced in a flow tube, in which the particles are subsequently exposed to a known concentration of ozone for a controlled period of time. A band assignment of infrared vibrational frequencies for dry maleic and fumaric acid aerosol spectra is presented. These studies are complemented with off-line analysis on the reaction products. The reaction exhibited pseudo-first-order kinetics on gas product formation, and the pseudo-first-order rate coefficients displayed a Langmuir-Hinshelwood dependence on gas-phase ozone concentration for both materials. By assuming a Langmuir-Hinshelwood behaviour, the following parameters were obtained: for the reaction of maleic acid aerosols, K(O3) = (3.3 + 0.5) x 10(-16) cm3 molecule(-1) and k(I)(max) = (0.038 + 0.004) s(-1); for the reaction of fumaric acid aerosols, K(O3) = (1.6 + 0.5) x 10(-16) cm3 molecule(-1) and k(I)(max) = (0.048 + 0.007) s(-1), where K(O3) is a parameter that describes the partitioning of ozone to the particle surface and k is the maximum pseudo-first-order coefficient at high ozone concentrations. Apparent reactive uptake coefficients were estimated from the pseudo-first-order rate coefficient and a trend of decreasing uptake coefficients with increasing ozone concentrations was observed, in good agreement with literature values.

A comparison of infrared spectroscopic methods for the study of heterogeneous reactions occurring on atmospheric aerosol proxies.

In this paper, the heterogeneous reaction between oleic acid and ozone has been studied using infrared spectroscopy in two distinctly different experimental configurations. The effect of the experiment on the observed products and rates of reaction is compared in order to derive a better understanding of some of the variations in oleic acid reaction rates reported by a range of researchers. One set of measurements is made using thin films of oleic acid in an attenuated total internal reflection configuration, and it is shown that a treatment in which the ATR evanescent wave is convolved with a moving reaction front is essential for the extraction of reliable kinetic data. The results are compared to similar measurements in a recently developed aerosol flow tube equipped with a cross-beam infrared spectroscopic probe. Rates of reaction in the aerosol phase are observed to be approximately 10 times faster and possible reasons for this discussed.

Infrared spectroscopic study of the effect of oleic acid on the deliquescence behaviour of ammonium sulfate aerosol particles.

In order to accurately assess the impact of fatty acids on the hygroscopic properties of atmospheric aerosol particles, (NH4)2SO4 (ammonium sulfate) and oleic acid (cis-9-octadecenoic acid) were chosen to perform this study as components of the particle phase. Micron-sized (700-900 nm) particles containing (NH4)2SO4 and oleic acid were generated by nebulising aqueous solutions of (NH4)2SO4 and sodium oleate. In this study, the effect of oleic acid on the deliquescence phase transition of particles was investigated in a room temperature aerosol flow tube (AFT) system using Fourier transform infrared (FTIR) spectroscopy. Particles morphologies and their chemical compositions were also analysed using a variety of techniques, including attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and scanning electron microscopy/energy dispersive X-ray analysis (SEM/EDX). The deliquescence relative humidity (DRH) of the (NH4)2SO4 component, determined at 81+/-2%, was slightly lowered or not affected by the presence of different thickness of oleic acid (21 nm, 44 nm and 109 nm) present in the particles. Analyses of the results presented here are consistent with earlier studies about the possible effects of water-insoluble fatty acids coatings on the phase transitions of atmospheric aerosol particles.

Ozonolysis of organic compounds and mixtures in solution. Part I: Oleic, maleic, nonanoic and benzoic acids.

In this paper, infrared spectroscopic and mass spectrometric studies of the ozonolysis of some simple proxies of precursors to organic materials found in atmospheric aerosols is reported. Oleic and maleic acids are used as proxies of reactive material, containing unsaturation which is amenable to ozonolysis. Nonanoic acid and benzoic acid are utilised as co-reactants which, although not likely to undergo direct ozonolysis themselves, are potential reaction partners to the Criegee radical intermediates formed from oleic and maleic acid ozonolysis. The precursor species are studied as single components in solution, followed by co-reaction studies. The products of the ozonolysis are followed by mass spectrometry and infrared spectroscopy. The product distributions from oleic and maleic acid are broadly in agreement with those observed in other studies. In the co-reaction studies, new evidence for cross-reaction products is obtained. Furthermore, the nature of some of the products does not fully comply with the widely accepted Ziemann scheme.

Infrared spectroscopic methods for the study of aerosol particles using White cell optics: Development and characterization of a new aerosol flow tube.

A description of a new aerosol flow tube apparatus for measurements in situ under atmospherically relevant conditions is presented here. The system consists of a laboratory-made nebulizer generation system and a flow tube with a White cell-based Fourier transform IR for the detection system. An assessment of the White cell coupled to the flow tube was carried out by an extensive set of experiments to ensure the alignment of the infrared beam and optimize the performance of this system. The detection limit for CO was established as (1.0+/-0.3) ppm and 16 passes was chosen as the optimum number of passes to be used in flow tube experiments. Infrared spectroscopy was used to characterize dry aerosol particles in the flow tube. Pure particles composed of ammonium sulfate or sodium chloride ranging between 0.8 and 2.1 mum for size diameter and (0.8-4.9)x10(6) particles/cm(3) for density number were generated by nebulization of aqueous solutions. Direct measurements of the aerosol particle size agree with size spectra retrieved from inversion of the extinction measurements using Mie calculations, where the difference residual value is in the order of 0.2%. The infrared detection limit for ammonium sulfate aerosol particles was determined as d(p)=0.9 mum and N=5x10(3) particles/cm(3) with sigma=1.1 by Mie calculation. Alternatively, Mie calculations were performed to determine the flexibility in varying the optical length when aerosol particles are sent by the injector. The very good agreement between the values retrieved for aerosol particles injected through the flow tube or through the injector clearly validates the estimation of the effective optical path length for the injector. To determine the flexibility in varying the reaction zone length, analysis of the extinction spectra as function of the position of the injector was carried out by monitoring the integrated area of different absorption modes of the ammonium sulfate. We conclude that the aerosol loss in the flow tube reactor is negligible and that the aerosol particles remain on-axis for the length of the flow tube.