Functional characterization of the water-soluble organic carbon of size-fractionated aerosol in the southern Mississippi Valley

Abstract

The chemical content of water-soluble organic carbon (WSOC) as a function of particle size was characterized in Little Rock, Arkansas in winter and spring 2013. The objectives of this study were to (i) compare the functional characteristics of coarse, fine and ultrafine WSOC and (ii) reconcile the sources of WSOC for periods when carbonaceous aerosol was the most abundant particulate component. The WSOC accounted for 5 % of particle mass for particles with δp > 0.96 μm and 10 % of particle mass for particles with δp < 0.96 μm. Non-exchangeable aliphatic (H–C), unsaturated aliphatic (H–C–C=), oxygenated saturated aliphatic (H–C–O), acetalic (O–CH–O) and aromatic (Ar–H) protons were determined by proton nuclear magnetic resonance (1H-NMR). The total non-exchangeable organic hydrogen concentrations varied from 4.1 ± 0.1 nmol m−3 for particles with 1.5 < δp < 3.0 μm to 73.9 ± 12.3 nmol m−3 for particles with δp < 0.49 μm. The molar H/C ratios varied from 0.48 ± 0.05 to 0.92 ± 0.09, which were comparable to those observed for combustion-related organic aerosol. The R–H was the most abundant group, representing about 45 % of measured total non-exchangeable organic hydrogen concentrations, followed by H–C–O (27 %) and H–C–C= (26 %). Levoglucosan, amines, ammonium and methanesulfonate were identified in NMR fingerprints of fine particles. Sucrose, fructose, glucose, formate and acetate were associated with coarse particles. These qualitative differences of 1H-NMR profiles for different particle sizes indicated the possible contribution of biological aerosols and a mixture of aliphatic and oxygenated compounds from biomass burning and traffic exhausts. The concurrent presence of ammonium and amines also suggested the presence of ammonium/aminium nitrate and sulfate secondary aerosol. The size-dependent origin of WSOC was further corroborated by the increasing δ13C abundance from −26.81 ± 0.18 ‰ for the smallest particles to −25.93 ± 0.31 ‰ for the largest particles and the relative distribution of the functional groups as compared to those previously observed for marine, biomass burning and secondary organic aerosol. The latter also allowed for the differentiation of urban combustion-related aerosol and biological particles. The five types of organic hydrogen accounted for the majority of WSOC for particles with δp > 3.0 μm and δp < 0.96 μm.

Structural signatures of water-soluble organic aerosols in contrasting environments in South America and Western Europe

“…3, regardless of the studied environment, all aerosol WSOC samples exhibit the same major proton types; however, they differ in terms of the relative distribution of the major proton regions. Overall, the relative content of the proton functional groups obtained in this study are within the range of those published for WSOC hydrophobic acid fractions or "humic-like substances (HULIS)" from atmospheric aerosols (e.g., Graham et al, 2002;Decesari et al, 2005;Song et al, 2012;Chalbot et al, 2014Chalbot et al, , 2016Lopes et al, 2015). For the South American sites, the saturated aliphatic protons are the most important component (36e59%), followed by unsaturated (20e33%) and oxygenated (13e40%) aliphatic protons, and a less contribution from aromatic protons (2.0e10%).…”

“…Once in the atmosphere, these structures can also undergo photooxidation, originating highly oxidized aromatic SOA species (Iinuma et al, 2010). Nevertheless, other aromatic structures with a secondary origin can resonate in this NMR region, such as terephthalic acid, nitrophenyl-derived compounds, cinnamic acid and/or cinnamaldehyde, possibly formed during the oxidation of aromatic hydrocarbons from urban traffic emissions (Hallquist et al, 2009;Chalbot et al, 2014;Lee et al, 2014).…”

“…However, a different trend is observed for samples collected in PV during MBB, Medellín, Bogota, and Buenos Aires, for which the relative contribution of H-C-O structures (40%, 34%, 24%, and 27%, respectively) surpasses that of the H-C-C¼ structures (21%, 19%, 21%, and 20%, respectively). Examples of organic species that typically resonate in this region includes not only sugar alcohols (e.g., mannitol and arabitol), but also other carbohydrate-like moieties such as glucose, sucrose, and fructose (Chalbot et al, 2014), as well as anhydrosugars, such as levoglucosan and mannosan . Sugar alcohols are particularly widespread in fungi and have been proposed as molecular tracers for fungal spores in ambient aerosol (e.g., Bauer et al, 2008;Fu et al, 2010;Liang et al, 2013).…”

“…The approach chosen to fulfill this objective encompasses functional group analysis using proton nuclear magnetic resonance ( 1 H NMR) spectroscopy. This strategy has been successfully applied to different aerosol WSOC samples, namely from biomass burning aerosols, marine organic aerosols, secondary organic aerosols (SOA), urban aerosols, rural aerosols, and size-fractionated atmospheric aerosols Tagliavini et al, 2006;Decesari et al, 2007;Chalbot et al, 2014Chalbot et al, , 2016Lopes et al, 2015). In this study, the acquired 1 H NMR datasets were further used to build fingerprints for source contribution analysis, following the method of Decesari et al (2007).…”

“…In the quest for untangle the complex molecular composition of the water-soluble component of OA, it has been shown that the WSOC 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 (e.g., Duarte et al, 2007Duarte et al, , 2008Duarte et al, , 2015Timonen et al, 2013;Ng et al, 2010;Cleveland et al, 2012;Shakya et al, 2012;Pietrogrande et al, 2013;Chalbot et al, 2014;Paglione et al, 2014;Lopes et al, 2015;Chalbot et al, 2016). A common ground of the available compositional datasets on aerosol WSOC is that they refer to studies carried out in urban, rural, and remote locations predominantly at North America and Europe, besides a few urban sites in Asia.…”