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

Phys. Chem. Chem. Phys.

Horn

2009

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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.

Section: Methodsmentioning

confidence: 99%

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

Phys. Chem. Chem. Phys.

Horn

2009

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“…In particular, it should be noted that the number concentration value obtained from TOPAS was corrected after taking the aerosol dilution factor in the AFT into account. The final spectrum shown here at 3.3 seconds exposure time represents reaction of approximately 65% of the oleic acid remaining in the particles at 0.7 seconds of exposure time (which is the minimum measurable contact time in this apparatus 46 ). To determine the ozonolysis rate constant by oleic acid aerosols in this study, the decrease in the n(C-H) band at 3024-2989 cm À1 was integrated in the measured spectra.…”

Section: Kinetics Studies Of Oleic Acid Ozonolysis In the Ftir-aftmentioning

confidence: 99%

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

Last

Phys. Chem. Chem. Phys.

et al. 2009

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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.

“…The AFT apparatus and experimental kinetic procedure employed in this work have been described in detail previously . A temperature‐stabilized lab‐made ultrasonic nebulizer was used to generate microdroplet aerosols of sodium oleate from 0.084 M bulk aqueous solutions at 40°C .…”

Section: Methodsmentioning

confidence: 99%

Infrared Spectroscopic Evidence for a Heterogeneous Reaction between Ozone and Sodium Oleate at the Gas–Aerosol Interface: Effect of Relative Humidity

Int J of Chemical Kinetics

Horn

2015

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The heterogeneous ozonolysis of sodium oleate aerosols in an aerosol flow tube is reported under different relative humidity (RH%) conditions. Submicron sodium oleate particles were exposed to a known ozone concentration and the consumption of sodium oleate was monitored by infrared spectroscopy. When the experimental results are treated as a surface‐mediated reaction (i.e., following a Langmuir–Hinshelwood type mechanism), the following parameters are obtained: at low RH%, KnormalO3 = (3 ± 1) × 10−16 cm3 molecule−1 and k max I = (0.046 ± 0.006) s−1; at high RH%, KnormalO3 = (6 ± 2) × 10−16 cm3 molecule−1 and k max I = (0.21 ± 0.05) s−1. From these pseudo–first‐order coefficients, the reactive uptake coefficients for dry and aqueous sodium oleate aerosols are calculated as (1.5 − 0.5) × 10−7 and (1.7 − 0.7) × 10−6, respectively. Hydrated oleate aerosols display both an increase in the ozone trapping ability and an increase in the effective rate reaction at the droplet surface compared to dry aerosol surfaces. These observations may provide an explanation for some of the variability observed between lab studies of dry ozonolysis and real‐world, atmospheric lifetimes of oleic acid–related species.