S. Decesari

Water-Soluble Organic Compounds in Biomass Burning Aerosols over Amazonia: 2. Apportionment of the Chemical Composition and Importance of the Polyacidic Fraction

[1]xa0Chemical characterization was performed on carbonaceous aerosols from Rondonia in the Brazilian Amazon region as part of the European contribution to the Large-Scale Biosphere-Atmosphere Experiment in Amazonia (LBA-EUSTACH). The sampling period (October 1999) included the peak of the burning season as well as the dry-to-wet season transition. Characterization of the carbonaceous material was performed by using a thermal combustion method. This enabled determination of aerosol total carbon (TC), black carbon (BC), and organic carbon (OC). A significant fraction of the BC material (on average about 50%) seemed to be highly refractory organic material soluble in water. A more detailed analysis of the water-soluble organic carbon (WSOC) fraction of the TC was undertaken, involving measurements of WSOC content, high-performance liquid chromatography (HPLC) separation (with UV detection) of the water-soluble components, and characterization of individual components by gas chromatography/mass spectrometry (GC/MS). The WSOC fraction accounted for 45−75% of the OC. This high WSOC fraction suggests an aerosol derived mainly from smoldering combustion. Using GC/MS, many different compounds, containing hydroxy, carboxylate, and carbonyl groups, were detected. The fraction of the WSOC identified by GC/MS was about 10%. Three classes of compounds were separated by HPLC/UV: neutral compounds (N), monocarboxylic and dicarboxylic acids (MDA), and polycarboxylic acids (PA). The sum of these three groups accounted for about 70% of the WSOC, with MDA and PA being most abundant (about 50%). Good correlations (r2 between 0.84 and 0.99) of BCwater (BC after water extraction) and levoglucosan (both indicators of biomass combustion) with the water-soluble species (i.e., WSOC, N, MDA, and PA), and their increase in concentrations during the burning period provided strong evidence that biomass burning is a major source of the WSOC. Particularly interesting is that PA and therefore, probably, humic-like substances (due to their polyacidic nature) are generated in significant amounts during biomass burning. These substances, due to their water solubility and surface tension-lowering effects, may play an important role in determining the overall cloud condensation nuclei activity of biomass burning aerosols and, consequently, could be important in cloud processes and climate forcing.

Advances in characterization of size‐resolved organic matter in marine aerosol over the North Atlantic

0.80 m gm � 3 , the remainder being non-sea-salt (nss) sulphate, 0.03 ± 0.01 m gm � 3 , and nitrate, 0.13 ± 0.04 m gm � 3 . By comparison, the mass of sea salt, nss sulphate, and nitrate in the submicron mode is found to be 0.39 ± 0.08 m gm � 3 , 0.26 ± 0.04 m gm � 3 , and 0.02 ± 0.01 m gm � 3 , respectively. Water-soluble organic carbon (WSOC) is observed in the submicron mode with a mass concentration of 0.25 ± 0.04 m gm � 3 , comparable to that of nss sulphate, and in the supermicron mode with a mass concentration of 0.17 ± 0.04 m gm � 3 . The WSOC to total carbon (TC) ratio is found to be 0.20 ± 0.12 for the submicron fraction and 0.29 ± 0.08 for the supermicron fraction, while the black carbon (BC) to TC ratio is, on average, 0.032 ± 0.001 for both aerosol modes. The remaining carbon, water-insoluble organic carbon, contributes 0.66 ± 0.11 m gm � 3 and 0.26 ± 0.06 m gm � 3 to the submicron and supermicron modes, respectively and, thus, represents the dominant submicron aerosol species. Furthermore, the WSOC chemical composition comprises mainly aliphatic and only partially oxidized species and humiclike substances, resulting in appreciable surface-active properties. The observed organic matter chemical features (size-dependent concentration, hydrophobic nature of a substantial fraction of the organic matter, and low oxidized and surface-active WSOC species) are consistent with the hypothesis of a primary marine source; bubble-bursting processes, occurring at the surface of the North Atlantic Ocean during phytoplankton blooms, effectively transfer organic matter into marine aerosol particles, particularly enriching the fine-aerosol fraction. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0315 Atmospheric Composition and Structure: Biosphere/atmosphere interactions; 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry;

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

Abstract We present here an investigation on the annual cycle of the carbonaceous fraction of the Po Valley (Italy) fine (d

Water soluble organic compounds formed by oxidation of soot

The water soluble organic compounds (WSOC) in soot samples as a function of the extent of ozone oxidation have been measured by a new methodology which utilises ion exchange chromatography, total carbon analysis and proton nuclear magnetic resonance. These results have been compared with the same analyses of various atmospheric aerosol samples. The WSOC produced from oxidation of soot particles increase rapidly with ozone exposure and consist primarily of aromatic polyacids which are found widely in atmospheric aerosols and which are frequently referred to as macromolecular humic-like substances (HULIS). This work demonstrates that the atmospheric oxidation of soot can produce HULIS in aerosols. The cloud condensation nuclei effectiveness of soot aerosol likely has its origin in these oxidation processes.

Seasonal Characteristics of the Physicochemical Properties of North Atlantic Marine Atmospheric Aerosols

The aerosol size distribution modal diameters show seasonal variations, 0.031 mm in winter and 0.049 mm in summer for the Aitken mode and 0.103 mm in winter and 0.177 mm in summer for the accumulation mode. The accumulation mode mass also showed a seasonal variation, minimum in winter and maximum in summer. A supermicron sized particle mode was found at 2 mm for all seasons showing 30% higher mass concentration during winter than summer resulting from higher wind speed conditions. Chemical analysis showed that the concentration of sea salt has a seasonal pattern, minimum in summer and maximum in winter because of a dependency of sea-salt load on wind speeds. By contrast, the non-sea-salt (nss) sulphate concentration in fine mode particles exhibited lower values during winter and higher values during midsummer. The water soluble organic carbon (WSOC) and total carbon (TC) analysis also showed a distinctive seasonal pattern. The WSOC concentration during the high biological activity period peaked at 0.2 mgC m A3 , while it was lower than 0.05 mgC m A3 during the low biological activity period. The aerosol light scattering coefficient showed a minimum value of 5.5 Mm A1 in August and a maximum of 21 Mm A1 in February. This seasonal variation was due to the higher contribution of sea salt in the MBL during North Atlantic winter. By contrast, aerosols during late spring and summer exhibited larger angstrom parameters than winter, indicating a large contribution of the biogenically driven fine or accumulation modes. Seasonal characteristics of North Atlantic marine aerosols suggest an important link between marine aerosols and biological activity through primary production of marine aerosols.

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.

Comprehensive characterization of PM2.5 aerosols in Singapore

[1]xa0A 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.

Solubility properties of surfactants in atmospheric aerosol and cloud/fog water samples

[1]xa0Organic films on deliquesced aerosols and cloud droplets lower the surface tension of water and may inhibit the exchange of water vapor and gases between the gas and the liquid phase, with important implications for aerosol and cloud microphysics and heterogeneous chemistry. This study provides an estimate of the solubility properties of surfactants in aerosol and fog/cloud water samples on the basis of the dilatational rheological properties of the surface films. The variations of surface tension induced by the fast expansion/compression of the films were measured by means of a drop shape tensiometer and were linked to the capacity of surfactants to exchange between the surface layer and bulk solution, and ultimately to their water-solubility. The results are in agreement with the properties of standards of soluble surfactants and can be interpreted by the theory of formation of hydrophilic adsorption layers. These findings suggest that the water-soluble organic compounds (WSOC) are the main contributors to the formation of films on cloud/fog droplets. It follows that the surface coverage of film-forming compounds is mainly controlled by the bulk concentration of WSOC, regardless of the available surface area. This also supports that the surface tension decreases observed under laboratory conditions actually occur in the atmosphere.

On the Origin of AMS “Cooking Organic Aerosol” at a Rural Site

A number of field observations employing aerosol mass spectrometers (AMS) have demonstrated that organic matter rich in monocarboxylic acids and aliphatic carbonyls originating from cooking activities (the COA factor) contributes significantly to ambient organic matter (OM) in urban environments. Little is known about the contribution and nature of COA in rural localities. We studied the correlation of COA with chemical tracers at a rural site in the Po Valley, Italy. Our statistical approach, based on positive matrix factorization (PMF) shows that the COA factor was clearly linked to local emissions of chloride and methanesulfonic acid (MSA), chemical tracers not associated with cooking emissions, or with combustion sources. While the association with Cl is not understood at this stage, the emission of reduced sulfur compounds, aliphatic carbonyls and monocarboxylic acids is consistent with several agricultural practices (e.g., manure storage) and waste disposal systems (e.g., landfills) which characterize the suburban and rural areas of the Po Valley and of other many populated environments. It is concluded that the nature and origins of the AMS COA factor measured at a rural site are complex and include far more than the emissions from food cooking.

Saharan Dust over Italy: Simulations with Regional Air Quality Model BOLCHEM

Saharan dust is transported over Mediterranean area, dust reaching often different regions of Italy. To the scope of predicting the advection of dust and its physical and chemical properties over Italy, a dust emission scheme has been implemented in the air quality model BOLCHEM, which solves simultaneously the chemical and meteorological equations. This study demonstrates the ability of BOLCHEM to predict the dust events over Italy and evaluates the impact of differ- rent parameterizations used in the dust production. The dust aerosols, besides of changing climate through the scattering and absor- ption of solar and thermal radiation, also affect the environment by fertilizing marine and terrestrial ecosystems that in turn influence the carbon cycle. Moreover, the dust particles contribute substantially to the total aerosol mass usually employed in the developing of the environmental policy regulations, therefore, a reliable forecast of dust events is mandatory over Italy, often affected by Saharan dust transport. This study describes the dust model implemented in the air quality model BOLCHEM and shows its ability to forecast dust events over Italy. The study also investigates the dependency of dust production on threshold friction velocity and number of dust size bins. The air quality model BOLCHEM (DIsidoro et al., 2005; Mircea et al., 2007) comprises a meteorological model, an algorithm for airborne transport and diffu- sion of pollutants and two photochemical mechanisms. The meteorology is coupled online with the chemistry. The dust model implemented in BOLCHEM was deve- lopped by Tegen et al. (2002) and is based on the soil-derived dust emission scheme designed by Marticorena and Bergametti (1995). The horizontal and vertical dust fluxes are calculated based on the location of the preferential dust sources, soil texture, surface roughness, vegetation cover, soil moisture content and surface wind velocity. The ratio between the vertical and the horizontal dust fluxes varies with the type of soil and the size of the particle mobilized. The size distribution of the mobilized dust depends on both the surface properties (soil texture) and the surface wind speed. The threshold friction velocities used to initiate the dust emissions are computed as a function of particle size following Marticorena and Bergametti (1995), but assuming constant roughness within the model grid cells (0.001 cm). Moreover, the simulation shown here was carried out with a threshold friction velocity lowered by a factor of 0.75 since lower thresholds velocities improve model results compared to observations at global level. The comparison of the dust