Aerosol Water-soluble Organic Compounds Research Articles

Effects of pH on light absorption properties of water-soluble organic compounds in particulate matter emitted from typical emission sources

Water-soluble organic compounds (WSOC) have a significant impact on aerosol radiative forcing and climate change, and there is considerable uncertainty in predicting and mitigating their climate and environmental effects. Here, the effects of pH on the light absorption properties of WSOC in particulate matter from different typical emission sources and ambient aerosols were systematically investigated using UV−vis spectrophotometer. pH (2−10) had an important impact on the light absorption properties of WSOC. The absorption, aromaticity, and the light absorption capacity of WSOC increased significantly with increasing pH for all samples. The difference absorbance spectra (∆absorbance) showed that the change of light absorption properties with pH was related to the deprotonate of carboxyl and phenolic groups resonating with aromatic and conjugated systems, with the most likely structures being carboxylic acids and phenols. Coal combustion and summer samples exhibited much higher susceptibility of light absorption properties to pH variation (increased by 27.0% and 65.9% relative to the pH 2 level, respectively). Absorption indices of almost all samples were significantly correlated with pH, indicating that the light absorption properties of WSOC may be quantitatively related to pH. The pH-dependent light absorption properties may have profound implications for evaluating the climate impacts of aerosol WSOC such as radiative forcing.

Two-Dimensional NMR Studies of Water-Soluble Organic Matter in Atmospheric Aerosols

The potential and the limitations of combining two-dimensional nuclear magnetic resonance (2D NMR) techniques to deliver new qualitative information on the substructures present in water-soluble organic compounds (WSOC) from fine atmospheric aerosols collected during Winter and Spring/Summer seasons are highlighted in this study. The WSOC fractions were extracted from quartz filter samples and further separated into hydrophobic acids and hydrophilic acids fractions by using a XAD-8/XAD-4 isolation procedure. Despite different spectral resolution, it was found that the aliphatic material of both samples includes mostly long-chain (carbons greater than 3 or 4) and branched mono- and dicarboxylic acids, carbonyl, and ester structural types. The presence of such structural fragments could be associated to secondary organic aerosol formation. It has been further demonstrated that spectral signatures typical of anhydrosugars from cellulose and methoxyphenols from lignin can be identified among the carbohydrate and aromatic moieties of the Winter sample. Their presence highlights the major influence of biomass burning on atmospheric particulate matter collected during colder periods. The data reported here demonstrate the success of 2D NMRtechniquesto investigate aerosol WSOC composition, as well as to evaluate aerosol source fingerprints.

Thermogravimetric characteristics of water-soluble organic matter from atmospheric aerosols collected in a rural–coastal area

The influence of chemical composition on the thermal degradation behaviour of water-soluble organic compounds (WSOC) from fine atmospheric aerosols collected over a period of 12 months was investigated by using thermogravimetric (TG) analysis. The obtained results were further compared to those of Suwannee River Humic Acid (SRHA) and Fulvic Acid (SRFA) standards, which were obtained from the International Humic Substances Society (IHSS). Prior to the TG analyses, the WSOC samples were separated into hydrophobic acids and hydrophilic acids fractions by using a XAD-8/XAD-4 isolation procedure. The data reported in this study were illustrative of the very complex nature of aerosol WSOC hydrophobic acids samples (three to six peaks of thermal degradation) in comparison to that of SRHA and SRFA standards (two degradation peaks). A pseudo-first order mathematical approach was applied to estimate the apparent activation energy of the different multi-step degradation regions. It was found that the degradation processes of the SRHA and SRFA standards occurred with lower apparent activation energies than those obtained for the aerosol WSOC hydrophobic acids samples, although these latter samples appear to exhibit the most thermally labile structural component. The WSOC hydrophobic acids showed a trend towards high apparent activation energies at high temperatures, thus suggesting the decomposition of very stable structural units with relatively strong bond energies above ca. 460 °C. The thermal profiles presented in this study are unique in showing the annual evolution of the thermal-oxidative properties of the fine aerosol WSOC hydrophobic acids samples, and in providing important supplementary information on the structural stability of the bulk aerosol WSOC.

Comparison of structural features of water-soluble organic matter from atmospheric aerosols with those of aquatic humic substances

Elemental analysis, Fourier transform infrared coupled to attenuated total reflectance (FTIR-ATR) and solid-state cross polarization with magic angle spinning- 13C-nuclear magnetic resonance (CPMAS 13C NMR) spectroscopies were used to compare the chemical features of water-soluble organic compounds (WSOC) from atmospheric aerosols with those of aquatic humic and fulvic acids. The influence of different meteorological conditions on the structural composition of aerosol WSOC was also evaluated. Prior to the structural characterisation, the WSOC samples were separated into hydrophobic acids and hydrophilic acids fractions by using a XAD-8/XAD-4 isolation procedure. Results showed that WSOC hydrophobic acids are mostly aliphatic (40–62% of total NMR peak area), followed by oxygenated alkyls (15–21%) and carboxylic acid (5.4–13.4%) functional groups. Moreover, the aromatic content of aerosol WSOC samples collected between autumn and winter seasons is higher (∼18–19%) than that of samples collected during warmer periods (∼6–10%). The presence of aromatic signals typical of lignin-derived structures in samples collected during low-temperature conditions highlights the major contribution of wood burning processes in domestic fireplaces into the bulk chemical properties of WSOC from aerosols. According to our investigations, aerosol WSOC hydrophobic acids and aquatic fulvic and humic acids hold similar carbon functional groups; however, they differ in terms of the relative carbon distribution. Elemental analysis indicates that H and N contents of WSOC hydrophobic acids samples surpass those of aquatic fulvic and humic acids. In general, the obtained results suggest that WSOC hydrophobic acids have a higher aliphatic character and a lower degree of oxidation than those of standard fulvic and humic acids. The study here reported suggests that aquatic fulvic and humic acids may not be good models for WSOC from airborne particulate matter.

Importance of the organic aerosol fraction for modeling aerosol hygroscopic growth and activation: a case study in the Amazon Basin

Abstract. The aerosol in the Amazon basin is dominated throughout the year by organic matter, for the most part soluble in water. In this modeling study, we show how the knowledge of water-soluble organic compounds (WSOC) and the associated physical and chemical properties (e.g. solubility, surface tension, dissociation into ions) affect the hygroscopic growth and activation of the aerosol in this area. The study is based on data obtained during the SMOCC field experiment carried out in Rondônia, Brazil, over a period encompassing the dry (biomass burning) season to the onset of the wet season (September to mid-November, 2002). The comparison of predicted and measured cloud condensation nuclei (CCN) number concentration shows that the knowledge of aerosol WSOC composition in terms of classes of compounds and of their relative molecular weights and acidic properties may be sufficient to predict aerosol activation, without any information on solubility. Conversely, the lack of knowledge on WSOC solubility leads to a high overestimation of the observed diameter growth factors (DGF) by the theory. Moreover, the aerosol water soluble inorganic species fail to predict both DGFs and CCN number concentration. In fact, this study shows that a good reproduction of the measured DGF and CCN concentration is obtained if the chemical composition of aerosol, especially that of WSOC, is appropriately taken into account in the calculations. New parameterizations for the computed CCN spectra are also derived which take into account the variability caused by chemical effects (surface tension, molecular composition, solubility, degree of dissociation of WSOC).

Application of Non-Ionic Solid Sorbents (XAD Resins) for the Isolation and Fractionation of Water-Soluble Organic Compounds from Atmospheric Aerosols

A detailed procedure using non-ionic macropourous XAD-8 and XAD-4 resins is presented for the isolation and fractionation of aerosol water-soluble organic compounds (WSOC) from aerosol samples. The procedure entails adsorption of WSOC fraction onto XAD-8 and XAD-4 resins, desalting of the adsorbed organic material with ultra-pure water, elution of the retained organic matter with 40% MeOH solution and freeze-drying. Due to resin’s different properties and to certain hydrophobic/hydrophilic interactions between the resin polymers and the organic matter, two major fractions were obtained; namely the XAD-8 and the XAD-4 eluates. The XAD-8 eluate, which accounts for 55–60% of total aerosol WSOC, is represented by partially acidic compounds with significant hydrophobic moieties. The XAD-4 fraction holds few conjugated systems and a higher content of hydrophilic structures with low molecular size, and accounts for 9% of total WSOC. The isolated WSOC sub-fractions were nearly free from inorganic species, and successful recoveries of organic matter from the resins were accomplished. With this procedure the XAD-8 eluate yields a mixture representative of those WSOC that are highly conjugated compounds in atmospheric aerosols. It also allows a successful characterisation of the organic material by advanced analytical techniques without the interference of inorganic species present in the original sample of atmospheric particles.

Comprehensive characterization of PM2.5 aerosols in Singapore

A comprehensive characterization of PM2.5 aerosols collected in Singapore from January through December 2000 is presented. The annual average mass concentration of PM2.5 was 27.2 μg/m3. The atmospheric loading of PM2.5 was elevated sporadically from March through May, mainly due to advection of biomass burning (deliberate fires to clear plantation areas) impacted air masses from Sumatra, Indonesia. Satellite images of the area, trajectory calculations, and surface wind direction data are in support of the transport of pyrogenic products from Sumatra toward Singapore. Aerosol samples collected during the dry season were analyzed for water‐soluble ions, water‐soluble organic compounds (WSOC), elemental carbon (EC), organic carbon, and trace elements using a number of analytical techniques. The major components were sulfate, EC, water‐soluble carbonaceous materials, and water‐insoluble carbonaceous materials. Aerosol WSOC were characterized based on a combination of chromatographic separations by ion exchange chromatography, functional group investigation by proton nuclear magnetic resonance, and total organic carbon determination. The comprehensive chemical characterization of PM2.5 particles revealed that both non‐sea‐salt sufate (nss‐SO42−) and carbonaceous aerosols mainly contributed to the increase in the mass concentration of aerosols during the smoke haze period. Using a mass closure test (a mass balance), we determined whether the physical measurement of gravimetric fine PM concentration of a sample is equal to the summed concentrations of the individually identified chemical constituents (measured or inferred) in the sample. The sum of the determined groups of aerosol components and the gravimetrically determined mass agreed reasonably well. Principal component analysis was performed from the combined data set, and five factors were observed: a soil dust component, a metallurgical industry factor, a factor representing emissions from biomass burning and automobiles, a sea‐salt component, and an oil combustion factor.

A simplified model of the water soluble organic component of atmospheric aerosols

The properties of atmospheric aerosol organic compounds are often simulated in models by arbitrarily choosing model compounds which have, in most cases, little contact with the physical reality. We propose here a procedure with which to derive a simplified model of the water soluble organic fraction of the atmospheric aerosol. Quantitative measurements of organic carbon concentration and proton concentration of the main functional groups contained in the aerosol water soluble organic compounds (WSOC) mixture are used to formulate a set of a few model compounds, representative of the whole WSOC mass, which can simulate in models the chemical and physical properties of aerosol WSOC.

Chemical features and seasonal variation of fine aerosol water-soluble organic compounds in the Po Valley, Italy

We present here an investigation on the annual cycle of the carbonaceous fraction of the Po Valley (Italy) fine ( d<1.5 μm) aerosol. Water-soluble organic compounds (WSOC) characterisation was accomplished using a simplified procedure, described in the paper, of a previously proposed methodology. The new simplified procedure allows the analysis of large sets of samples, which was difficult to achieve with the previous complex and labour-intensive method. The results of this study show that the aerosol total carbon (TC) concentration follows the same annual trend as the aerosol inorganic ion constituents, characterised by lower concentration values during the summer and higher concentrations in winter. The total aerosol organic carbon (OC) represents from 90 to 97% of TC, the rest being accounted for by elemental carbon. Within this study, we studied in particular the aerosol WSOC, a class of chemical compounds for which present knowledge is rather limited. In our samples, WSOC account for between 38% (in winter) and 50% (in summer) of OC, in terms of carbon. WSOC were fractionated into three main classes: (a) neutral/basic compounds; (b) mono- and di-carboxylic acids; (c) polyacidic compounds. The three fractions together accounted for an average 87% of total aerosol WSOC. The acidic compounds (mono- and di-carboxylic acids+polyacidic compounds) were found to be far more abundant than the neutral ones in all seasons, with polycarboxylic acids being the most abundant class of WSOC in spring, fall, and winter, while the mono/di-acids are dominant in summer. The main structural features of aerosol WSOC, revealed by proton nuclear magnetic resonance, suggest that WSOC are composed of highly oxidised species with residual aromatic nuclei and aliphatic chains. In particular, neutral compounds are mainly polyols or polyethers, mono/di-acids are mainly hydroxylated aliphatic acidic compounds, and polyacids are highly unsaturated compounds of predominantly aliphatic character, with a minor content of hydroxy groups.