Encyclopedia
Water-soluble organic aerosols (OA) are an important component of atmospheric particulate matter and one of the key drivers that impact both climate and human health. Understanding these processes involving water-soluble OA depends on how well the chemical composition of this aerosol component is decoded. Yet, obtaining such a detailed chemical information faces several challenges, of which the complexity of the sample matrix is one of the most demanding issues. A number of different advanced multidimensional analytical techniques are available today with the potential to tackel the complex chemical nature of water-soluble OA, allowing the untargeted profilling of new chemical structures without the need for use of databases or libraries. This critical review is aimed at nonspecialists who are interested in learning more about the potential and impact of such multidimensional non-targeted analytical strategies in water-soluble OA research.
- organic aerosols
- air particles
- water-soluble organic matter
- 2D NMR spectroscopy
- high-resolution mass spectrometry
- EEM fluorescence spectroscopy
- 2D chromatographic separation
1. Introduction
The study and characterization of atmospheric aerosols, as well as their effects on radiative climate forcing, atmospheric chemistry, air quality and visibility, and human health, are some of the most predominant research topics in atmospheric chemistry [1][2][1,2]. Atmospheric particulate matter (PM) can be directly emitted into the atmosphere (primary aerosols) from a diversity of natural and anthropogenic sources, including biomass burning, incomplete combustion of fossil fuels, volcanic eruptions, and wind-driven or traffic-related suspension of road, soil, and mineral dust, sea salt, and biological materials [1]. Nonetheless, atmospheric PM can also be formed in the atmosphere (secondary aerosols) through gas-to-particle conversion processes of gaseous species (i.e., nucleation, condensation, and heterogeneous and multiphase chemical reactions) [1][3][1,3]. Furthermore, primary and secondary aerosols may undergo chemical and physical transformations, being subjected to transport, cloud processing, and removal from the atmosphere [4]. This multitude of emission sources and formation/processing mechanisms contribute to the diversity and complexity of the chemical composition (i.e., carbonaceous and inorganic components) and physical properties (i.e., concentration, size distribution, and surface area) of atmospheric PM, which in turn influences climate and health effects, further adding a layer of complexity.
Although the composition of the inorganic aerosol component has been thoroughly described in the literature, in-depth knowledge of its organic counterpart is particularly more difficult to achieve due to the structural diversity and complexity of this aerosol component. The organic aerosols (OA) fraction can be the predominant component of suspended PM mass, accounting for ~20–50% of the total fine particulate mass at continental mid-latitudes and reach 90% in tropical forested areas [5][6][5,6]. The interest of the atmospheric research community on this OA component has been fueled by the realization that an important fraction of the OA is water-soluble. In Northern Hemisphere midlatitudes, the ubiquitous water-soluble organic matter (WSOM) represents 10% to 80% of the total particulate organics [7][8][9][10][7,8,9,10], whereas lower percentage values (up to 13%) have been reported for Southern Hemisphere locations [11]. The aerosol WSOM plays a key role in cloud formation and properties [12][13][12,13], Earth’s radiative balance [14][15][14,15], and atmospheric chemistry [14][16][14,16]. Atmospheric deposition of aerosol WSOM can also affect carbon and nitrogen biogeochemical cycles in aquatic ecosystems [17][18][19][17,18,19]. Aerosol WSOM in fine inhalable air particles may also exert adverse health effects by generating reactive oxygen and nitrogen species [20][21][20,21] or by promoting a moderate pro-inflammatory status [7]. Understanding these dynamic processes involving aerosol WSOM depends on how well one can identify its organic constituents.
2. Coping with Water-Soluble OA Complexity
As mentioned above, the water-soluble OA fraction covers a huge variety of molecular structures with different physicochemical properties and sources. Nevertheless, according to Nozière et al. [22], not all atmospheric issues require the identification of all organic compounds present in OA samples. Depending on the purpose of the investigation, different levels of organic compositional information can be distinguished [22][23][22,24]: (i) functional group analysis is typically employed when interested in understanding specific properties of OA (e.g., chemistry, optical properties or structural average parameters of OAs); (ii) resolve the chemical composition of OA into different organic components (e.g., hydrocarbon-like OA (HOA), oxidized OA (OOA), and biomass-burning OA (BBOA)) in real time is usually chosen for capturing the characteristic chemical and physical properties of OA in a rapidly changing environment; (iii) target analysis of molecular markers is typically used when monitoring known OA formation processes or sources; and (iv) identification of up to three specific organic compounds when studying unknown OA formation processes or sources. For over two decades, these different levels of OA analysis have been dedicated to the quantification and characterization of the aerosol WSOM component. These efforts have been accompanied by numerous laboratory and field studies focused on the characterization of WSOM sources, as well as its mechanisms of formation and rates of its atmospheric transformations. Currently, these issues are still poorly known, although the major sources of these compounds are considered to be biomass burning and secondary formation (involving both anthropogenic and biogenic volatile organic compounds). To further enhance the diversity and complexity of this OA fraction, the atmospheric WSOM and their precursor gases can also be modified in the atmosphere by oxidative processes and, thereby, becoming less volatile, more hygroscopic and, consequently, more water-soluble [24][25][25,26].
Several procedures and methodologies have been developed to study the chemical composition of the aerosol WSOM [8][9][10][11][26][27][28][29][30][31][32][33][34][8,9,10,11,23,27,28,29,30,31,32,33,34]. These studies are usually carried out using a combination of total organic carbon analysis, isolation and fractionation procedures, and characterization of molecular fragments and intermolecular bonds by different analytical techniques. Such studies have demonstrated that this fraction consists of a highly diverse suite of oxygenated compounds, including dicarboxylic acids, keto-carboxylic acids, aliphatic aldehydes and alcohols, saccharides, saccharide anhydrides, aromatic acids, phenols, but also amines, amino acids, organic nitrates, and organic sulfates [11][28][30][33][35][36][37][38][39][40][41][42][11,28,30,33,35,36,37,38,39,40,41,42]. Despite the advances in understanding the atmospheric importance of the aerosol WSOM component, there are still several fundamental questions related to the complexity of its chemical composition, mechanisms of formation, atmospheric fate, and reactivity [15][22][15,22]. Determining the molecular composition and structure of aerosol WSOM is warranted to further increase the current understanding of its role in various atmospheric processes. Furthermore, due to its dynamic nature, the establishment of general models for the structure of the water-soluble fraction of OA still is far from being fully accomplished [26][23], and further studies are needed to understand their actual impact on a regional and global scale. As reviewed by Nozière et al. [22] and Duarte and Duarte [43], a universal technique for OA analysis does not exist. Several different offline and online analytical methodologies have been developed and applied to mitigate the complexity of OA, as well as to unravel the structure and composition of this aerosol component. The following Section 3 addresses the state-of-the-art multidimensional analytical strategies applied thus far in the characterization of OA. Analytical strategies combining one- or two-dimensional chromatographic separation and a detector (e.g., diode array, fluorescence, nuclear magnetic resonance (NMR), or mass spectrometry (MS)) or involving two or more stages of MS (i.e., tandem MS) or frequency (i.e., NMR spectroscopy) or wavelength (i.e., excitation–emission matrix (EEM) fluorescence spectroscopy) dimensions are here defined as multidimensional.
