Characterization of surface functional groups present on laboratory-generated and ambient aerosol particles by means of heterogeneous titration reactions

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Abstract

A Knudsen flow reactor has been used to quantify surface functional groups on aerosols collected in the field. This technique is based on a heterogeneous titration reaction between a probe gas and a specific functional group on the particle surface. In the first part of this work, the reactivity of different probe gases on laboratory-generated aerosols (limonene SOA, Pb(NO3)2, Cd(NO3)2) and diesel reference soot (SRM 2975) has been studied. Five probe gases have been selected for the quantitative determination of important functional groups: N(CH3)3 (for the titration of acidic sites), NH2OH (for carbonyl functions), CF3COOH and HCl (for basic sites of different strength), and O3 (for oxidizable groups). The second part describes a field campaign that has been undertaken in several bus depots in Switzerland, where ambient fine and ultrafine particles were collected on suitable filters and quantitatively investigated using the Knudsen flow reactor. Results point to important differences in the surface reactivity of ambient particles, depending on the sampling site and season. The particle surface appears to be multi-functional, with the simultaneous presence of antagonistic functional groups which do not undergo internal chemical reactions, such as acid–base neutralization. Results also indicate that the surface of ambient particles was characterized by a high density of carbonyl functions (reactivity towards NH2OH probe in the range 0.26–6 formal molecular monolayers) and a low density of acidic sites (reactivity towards N(CH3)3 probe in the range 0.01–0.20 formal molecular monolayer). Kinetic parameters point to fast redox reactions (uptake coefficient γ0>10−3 for O3 probe) and slow acid–base reactions (γ0<10−4 for N(CH3)3 probe) on the particle surface.

Introduction

The presence of anthropogenic particulate matter in the atmosphere is nowadays considered to be a major environmental problem. Indeed, exposure to PM10 and PM2.5 (particulate matter with an aerodynamic diameter smaller than 10 and 2.5 μm, respectively) is associated with a range of adverse health effects, including cancer (Bhatia, Lopipero, & Smith, 1998), respiratory (Neuberger et al., 2004) and cardiovascular (Pope et al., 2004) diseases. So far, several mechanisms have been proposed to explain harmful effects of particulate matter. According to the most probable hypotheses, particle surface characteristics (chemical reactivity, surface area) are of prime importance for the understanding of the toxicity of particulate matter (Brown, Wilson, MacNee, Stone, & Donaldson, 2001). Surface chemistry is important, because it controls the molecular and cellular interactions with the critical parts of the respiratory-tract components, such as lung lining fluid and different cells (Kendall et al., 2004a). Therefore, physico-chemical characteristics of the particle surface impacting the lung may affect the initial physiological responses, and thus control the downstream effects. For instance, the presence of several components adsorbed on particles, such as metal ions (Park, Nam, Chung, Park, & Lim, 2006) and organics (Mauderly & Chow, 2008), has been found to generate reactive oxygen species (ROS), and thereby to cause oxidative stress in biological systems.

Moreover, particles are involved in atmospheric processes, and suspected to play a role in global climate change (Finlayson-Pitts & Pitts, 2000). They are able to scatter incoming solar radiation and, in some cases, to absorb it as well, converting absorbed energy to heat, and therefore contributing to the warming of the troposphere. Particles are also involved in the formation of clouds, and may affect the concentration of atmospheric trace gases by heterogeneous chemical reactions (Cwiertny, Young, & Grassian, 2008).

Over the past 10 years, great efforts have been placed onto the surface characterization of particulate matter. So far, experiments have been carried out mainly by means of different spectroscopic methods. X-ray photoelectron spectroscopy (XPS; Qi, Feng, Li, & Zhang, 2006), Fourier transform infra-red (FT-IR; Fermo, Piazzalunga, Vecchi, Valli, & Ceriani, 2006) and Raman (Sze, Siddique, Sloan, & Escribano, 2001) have been often used to investigate carbonaceous and inorganic particles, but these techniques do not focus exclusively on the gas-particle interface. Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy is an emerging tool in the surface characterization of carbonaceous particles (Braun, 2005). C (1 s) NEXAFS spectra have the advantage to present molecular fingerprints allowing to distinguish various kinds of carbonaceous particles, such as graphite or diesel soot. Electron energy loss spectroscopy (EELS) is a related technique which has been widely used in the analysis of carbonaceous particles (Chen, Shah, Braun, Huggins, & Huffman, 2005), often in association with transmission electron microscopy (TEM). However, these electron spectroscopies are hampered by low to modest energy resolution as well as a limited sensitivity for surface species.

The Knudsen flow reactor represents an alternative technique that allows the characterization of surface functional groups present on particles at a high sensitivity, typically in the range of 1% of a formal molecular monolayer. This method is based on a heterogeneous chemical reaction between a gas-phase probe molecule and a specific functional group on the surface of a sample. Over the past 15 years, this technique has been used especially in the field of atmospheric chemistry. Heterogeneous reactions have been investigated on different types of particle surrogates, such as soot (Stadler and Rossi, 2000), mineral dust (Karagulian & Rossi, 2005; Ullerstam, Johnson, Vogt, & Ljungström, 2003) or sea salt (Rossi, 2003). Even if the present titration reactions are undertaken in the gas-phase, we surmise that the particle surface composition will not significantly change when the particle is immersed into a liquid, such that the results of the present approach may still give useful clues for solution studies. An advantage of this technique over spectroscopic methods is that kinetic data and the identification of reaction products may be obtained. On the other hand, the identification of surface functional groups is indirect and afforded by the chemical reactivity of the surface in terms of surface composition. The results may be difficult to interpret because of several possible competing reactions.

The aim of the present research was to use the Knudsen flow reactor technique to measure functional groups present on the surface of particles sampled in occupational situations. A study was first undertaken to test the reactivity of several probe gases towards laboratory-generated aerosols. Preliminary work was already performed in our laboratory by Demirdjian and Rossi (2005), who tested the reactivity of four probe gases towards several laboratory-generated aerosols. We have extended this work in order to find other probe gases allowing the quantification of additional functional groups present on the particle surface, especially those which are suspected to play a role in adverse health effects of particulate matter (metal ions, acids). In the second part of the study, we sampled ambient fine and ultrafine particles in several bus depots in Switzerland, and investigated surface functional groups of these samples. The kinetics of titration reactions were also measured to obtain further information on the reactivity of particulate matter.

Section snippets

Knudsen flow reactor

The Knudsen flow reactor has previously been described in detail in the literature (Caloz, Fenter, Tabor, & Rossi, 1997). Briefly, this technique is used for the study of heterogeneous chemical reactions between a gas-phase probe molecule and a solid-phase sample. For each type of functional group present on the aerosol surface (such as carbonyl, acidic, basic, or oxidizable groups), the interaction of a suitable titrant molecule with the aerosol is studied. The type and number of probe

Laboratory-generated aerosols

Particle size distributions measured using the SMPS indicated that laboratory-generated aerosols had systematically a single mode (>230 nm for limonene SOA, approximately 55 nm for inorganic particles), and followed a perfect log-normal distribution. The geometric standard deviations (σg) were 1.58 for limonene SOA, and 1.75 for inorganic particles. We note that the used equipment did not allow us to check whether or not particles larger than 750 nm were generated. We make the assumption that the

Conclusion

In this paper, we report the use of a novel method allowing the quantitative characterization of functional groups on the surface of particulate matter. The Knudsen flow reactor provides an alternative approach to spectroscopic methods, focusing more on the chemical reactivity of the particle surface. We do not claim that this method allows one to study the reactivity of aerosol particles in the aqueous phase. Rather, it provides benchmark data that allows one to map the surface composition of

Acknowledgments

This research project was supported by the Swiss State Secretariat for Education and Research within the framework of the COST Action 633 “Particulate Matter—Properties Related to Health Effects”. We thank Ferdinando Storti for his support during the field campaign.

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