Oxidation of biogenic and water-soluble compounds in aqueous and organic aerosol droplets by ozone: a kinetic and product analysis approach using laser Raman tweezers

Abstract

The results of an experimental study into the oxidative degradation of proxies for atmospheric aerosol are presented. We demonstrate that the laser Raman tweezers method can be used successfully to obtain uptake coefficients for gaseous oxidants on individual aqueous and organic droplets, whilst the size and composition of the droplets is simultaneously followed. A laser tweezers system was used to trap individual droplets containing an unsaturated organic compound in either an aqueous or organic (alkane) solvent. The droplet was exposed to gas-phase ozone and the reaction kinetics and products followed using Raman spectroscopy. The reactions of three different organic compounds with ozone were studied: fumarate anions, benzoate anions and alpha-pinene. The fumarate and benzoate anions in aqueous solution were used to represent components of humic-like substances, HULIS; alpha-pinene in an alkane solvent was studied as a proxy for biogenic aerosol. The kinetic analysis shows that for these systems the diffusive transport and mass accommodation of ozone is relatively fast, and that liquid-phase diffusion and reaction are the rate determining steps. Uptake coefficients, gamma, were found to be (1.1 +/- 0.7) x 10(-5), (1.5 +/- 0.7) x 10(-5) and (3.0-7.5) x 10(-3) for the reactions of ozone with the fumarate, benzoate and alpha-pinene containing droplets, respectively. Liquid-phase bimolecular rate coefficients for reactions of dissolved ozone molecules with fumarate, benzoate and alpha-pinene were also obtained: kfumarate = (2.7 +/- 2) x 10(5), kbenzoate = (3.5 +/- 3) x 10(5) and kalpha-pinene = (1-3) x 10(7) dm3 mol(-1) s(-1). The droplet size was found to remain stable over the course of the oxidation process for the HULIS-proxies and for the oxidation of alpha-pinene in pentadecane. The study of the alpha-pinene/ozone system is the first using organic seed particles to show that the hygroscopicity of the particle does not increase dramatically over the course of the oxidation. No products were detected by Raman spectroscopy for the reaction of benzoate ions with ozone. One product peak, consistent with aqueous carbonate anions, was observed when following the oxidation of fumarate ions by ozone. Product peaks observed in the reaction of ozone with alpha-pinene suggest the formation of new species containing carbonyl groups.

Kinetic multi-layer model of aerosol surface and bulk chemistry (KM-SUB): the influence of interfacial transport and bulk diffusion on the oxidation of oleic acid by ozone

“…We demonstrate the applicability of KM-SUB for a common model system of organic aerosol chemistry, the ozonolysis of oleic acid droplets (e.g., Moise and Rudich, 2002;Katrib et al, 2004Katrib et al, , 2005King et al, 2004King et al, , 2009Knopf et al, 2005;Ziemann, 2005;Hearn and Smith, 2007;Pfrang et al, 2009;Sage et al, 2009;Vesna et al, 2009), and we compare the results of our numerical simulations with the results of earlier experimental and theoretical studies of this system.…”

“…It is often difficult to discriminate surface and bulk reactions, and the relative importance of surface and bulk processes is not well understood (e.g., Moise and Rudich, 2000;Hearn et al, 2005;Pfrang et al, 2009). Resistor model formulations are widely used to describe and investigate heterogeneous reactions and multiphase processes in laboratory, field and model studies of atmospheric chemistry (Hanson, 1997;Finlayson-Pitts and Pitts, 2000;Worsnop et al, 2002;Anttila et al, 2006;King et al, 2008King et al, , 2009and references therein). The traditional resistor models, however, are usually based on simplifying assumptions such as steady-state conditions, homogeneous mixing, and limited numbers of non-interacting species and processes.…”

Light at work: The use of optical forces for particle manipulation, sorting, and analysis

“…Such droplets are of considerable interest to atmospheric chemistry; they are formed in the Earth's stratosphere by the oxidation of SO 2 followed by nucleation with H 2 O vapor. Another atmosphere-related study was conducted by King et al [413] who investigated the kinetics and product composition of oxidation of biogenic and watersoluble compounds in aqueous and organic aerosol droplets by ozone. The composition of aerosol particles was chosen to mimic the conditions in the Earth's atmosphere.…”

“…Optical trapping techniques coupled to the methods of optical spectroscopy (e.g. Raman -see Section 4.8) are especially attractive for studying the properties of aerosols as they enable to isolate an individual aerosol particle and study its composition and dynamics [58,408,412,413,471]. To this end, single-beam, dual-beam, and HOT have been used to manipulate with aerosols, perform controlled coagulation of selected aerosol particles [472], or enhance their Raman scattering signal and study the evolution of the droplet size by employing size-and refractive-indexdependent structural Mie resonances in the droplets [410,411,473].…”

Levoglucosano e íons solúveis em água no material particulado atmosférico MP10 e MP2,5. caracterização de sítios Sul-Americanos

“…Segundo o mecanismo descrito por King et al (2008), fumarato em fase líquida na presença de ozônio produz formiato. Na primeira etapa ( ).…”