Functional group analysis of high-molecular weight compounds in the water-soluble fraction of organic aerosols

Abstract

In this study, we present the analysis of functional groups of humic-like substances (HULIS) isolated from atmospheric aerosols (<1 μm). Aerosol samples were collected in an urban area (Dübendorf, Switzerland) both in summer and winter. Quantification of carboxylic, arylic, phenolic and aliphatic functional groups of HULIS was performed by a specially adapted and optimized H-NMR method. The concentrations of carboxylic, arylic, phenolic and aliphatic functional groups were between 9×10⁻¹¹ and 6×10⁻⁸ mol/m³ for all samples, corresponding up to 14% of the total HULIS mass. A good correlation between the H-NMR results and the potentiometric titration of carboxylic groups was observed for all winter and summer samples. The pK distributions of carboxylic groups of HULIS were calculated from potentiometric titration data. pK spectra showed that pK values of most carboxylic groups is between 3 and 5. The H-NMR data show that the content of aromatic groups is higher in winter than in summer. This may either be due to emission of aromatic compounds by wood burning or to slower degradation reaction of aromatics in winter.

Introduction

Atmospheric aerosol particles play a crucial role in many aspects of the earth's climate system due to the light scattering and light absorption properties of the particles and their role in cloud formation (Seinfeld and Pandis, 1988). In addition, epidemiological studies showed that ambient concentrations of aerosols are associated with morbidity and mortality (Delfino et al., 2005; Dockery et al., 1993). For many of these processes, the chemical composition of the particles is an important parameter. Although up to 50% or more of the total mass of ambient aerosols is organic material, only little is known about the chemical composition of this aerosol fraction, and only 10–20% of the organic mass can be resolved on a molecular level (Likens et al., 1983; Puxbaum et al., 2000; Seinfeld and Pandis, 1988).

In recent years, high-molecular weight compounds have been identified in ambient and laboratory-generated particles, which may explain up to 60% of the total organic aerosol mass (Facchini et al., 1999; Havers et al., 1998; Kalberer et al., 2004). Several studies investigating this class of compound in ambient samples showed that these complex compound mixtures have physico-chemical characteristics similar to those of humic and fulvic acids (Graber and Rudich, 2006) and therefore this compound class is often called humic-like substances (HULIS). The chemical characterization of HULIS is only at its very beginning.

One important aspect of the characterization of HULIS to estimate their role in atmospheric chemistry and to get insight into their formation mechanisms is the qualitative and quantitative determination of functional groups in HULIS. The average concentrations of functional groups in the complex HULIS compound mixture may, for example, influence the water solubility or toxicity of an aerosol particle.

Several studies have investigated functional groups of HULIS such as sulfates (Liggio et al., 2005; Reemtsma et al., 2006; Romero and Oehme, 2005) and sulfonates, nitrate groups (Reemtsma et al., 2006) with mass spectrometric methods. UV–vis spectroscopy and titration methods have been applied for the quantitative analysis of arylic, phenolic and acidic functional groups of HULIS (Dinar et al., 2006). H-NMR was used by several authors to estimate the quantity of a variety of functional groups such as aliphatic, arylic, carboxylic, acetalic, vinylic, alcoholic, ether or ester groups, as well as hydrogens in α-position to unsaturated carbons (Decesari et al., 2006; Havers et al., 1998). Time consuming derivatization reactions were used to specifically quantify carboxylic acids with H-NMR, which are otherwise not detectable with H-NMR when D₂O is used as solvent (Tagliavini et al., 2006).

In the present study, H-NMR and acid titration was used in parallel to quantify several functional groups (acids, phenols, arylic H's, aliphatic H's) in the water-soluble fraction of HULIS isolated from urban atmospheric aerosols samples. The quantitative results for carboxylic acids obtained from H-NMR and potentiometric titrations were found to be in good agreement. In addition, pK distributions of carboxylic groups of HULIS were calculated from the titration measurements.