Pradeep Saxena

Water-soluble organics in atmospheric particles: A critical review of the literature and application of thermodynamics to identify candidate compounds

Although organic compounds typically constitute a substantial fraction of the fine particulate matter (PM) in the atmosphere, their molecular composition remains poorly characterized. This is largely because atmospheric particles contain a myriad of diverse organic compounds, not all of which extract in a single solvent or elute through a gas chromatograph; therefore, a substantial portion typically remains unanalyzed. Most often the chemical analysis is performed on a fraction that extracts in organic solvents such as benzene, ether or hexane; consequently, information on the molecular composition of the water-soluble fraction is particularly sparse and incomplete.This paper investigates theoretically the characteristics of the water-soluble fraction by splicing together various strands of information from the literature. We identify specific compounds that are likely to contribute to the water-soluble fraction by juxtaposing observations regarding the extraction characteristics and the molecular composition of atmospheric particulate organics with compound-specific solubility and condensibility for a wide variety of organics. The results show that water-soluble organics, which constitute a substantial fraction of the total organic mass, include C2 to C7 multifunctional compounds (e.g., diacids, polyols, amino acids). The importance of diacids is already recognized; our results provide an impetus for new experiments to establish the atmospheric concentrations and sources of polyols, amino acids and other oxygenated multifunctional compounds.

Measuring and simulating particulate organics in the atmosphere: problems and prospects

Abstract Although organics constitute approximately 10–70% of the total dry fine particle mass in the atmosphere, their concentrations and formation mechanisms are less well understood than those of other components such as sulfate and nitrate. This is because particulate organic matter is an aggregate of hundreds of individual compounds whose concentrations cannot be characterized by a single analytical technique; more than half of the organic carbon mass has not yet been identified as individual compounds. Moreover, the collection process itself can alter the gas–particle equilibrium of a number of condensable organics resulting in both positive and negative sampling biases. The incomplete characterization of particulate organics coupled with the complexity of the photochemical reactions that produce particulate matter from volatile organic emissions has prevented the development of a first principle simulation approach. These limitations are providing an impetus for numerous scientific studies, proving organics to be the next frontier for particle characterization and simulation. This paper reviews the current state of organic aerosol sampling, analysis, and simulation, examines the limitations of the current technology, and presents prospects for the future. The emphasis is on distilling findings from recent atmospheric, smog chamber, and theoretical studies to provide a coherent picture of what has been accomplished, especially during the last five years, and what problems are ripe for further investigation.

Organics alter hygroscopic behavior of atmospheric particles

The optical and chemical properties of atmospheric particles and their ability to act as cloud condensation nuclei (CCN) depend strongly upon their affinity for water. Laboratory experiments have shown that water soluble substances such as ammonium sulfate, ammonium nitrate, and sodium chloride, which are major inorganic components of atmospheric particles, absorb water in an amount proportional to water vapor pressure. Analogous information about the interactions between water and organics, which are another major component of atmospheric particles, is lacking. Here we analyze concurrent observations of particle chemical composition and water content from a continental nonurban (Grand Canyon) and an urban (Los Angeles) location to determine whether the water content of atmospheric particles is influenced by the presence of organics. By comparing the observed water content with the water content expected to be associated with the inorganic fraction, we find that the aggregate hygroscopic properties of inorganic particles are altered substantially when organics are also present. Furthermore, the alterations can be positive or negative. For the nonurban location, organics enhance water absorption by inorganics. In the relative humidity (RH) range of 80–88% organics account for 25–40% of the total water uptake, on average. For the urban location, on the other hand, the net effect of organics is to diminish water absorption of the inorganics by 25–35% in the RH range of 83–93%.

Atmospheric Gas-Aerosol Equilibrium I. Thermodynamic Model

A rigorous and computationally efficient thermodynamic model that estimates the state and composition of atmospheric inorganic species between the gas and aerosol phases is presented. The estimation of important thermodynamic properties, equilibrium constants, ionic activity coefficients, water activity, and deliquescence points is described. Various sources and estimation methods for inorganic gas-liquid-solid equilibrium properties are compared and optimal approaches for the new equilibrium routine are incorporated.

Atmospheric Gas-Aerosol Equilibrium II. Analysis of Common Approximations and Activity Coefficient Calculation Methods

The gas-aerosol equilibrium model, SCAPE, presented in Part I is used to evaluate the sensitivity of thermodynamic calculations of aerosol composition to common approximations and the choice of activity coefficient estimation method. The treatment of weak electrolytes, associated ammonia, NH_3(aq), and bisulfate ion, HSO_4^−, is analyzed. Comparisons of the three multi-component activity coefficient estimation methods are carried out with a variety of data. On the basis of the sensitivity analysis results, recommendations are provided for treating these electrolytes and for selecting an activity coefficient estimation method in atmospheric gas-aerosol equilibrium calculations. The two gas-aerosol equilibrium models, SCAPE and AIM, are compared. Remaining questions in gas-aerosol equilibrium are highlighted.

Atmospheric gas-aerosol equilibrium. IV: Thermodynamics of carbonates

Data and correlations for incorporating carbonate and bicarbonate salts into a gas/aerosol equilibrium model are developed. The species considered include CO2(g), CO2(aq), HCO3 −, CO2- 2, NH2CO2 −, Na2CO3(s), NaHCO3(s), K2CO3(s), KHCO3(s), CaCO3(s), MgCO3(s), NH4HCO3(s), and NH2COONH4(s). Three activity coefficient estimation methods (Bromley, K-M, and Pitzer) are examined. Necessary parameters for these three methods are obtained. It is found that, in general, the Pitzer method performs better than the other two, and is selected as the multicomponent activity coefficient estimation method of choice. Water activity correlations for binary carbonate solutions are obtained by polynomial fitting of available data. Deliquescence points for the important atmospheric carbonate salts and their temperature dependence are given.

Concentration and Composition of Atmospheric Aerosols from the 1995 SEAVS Experiment and a Review of the Closure between Chemical and Gravimetric Measurements

ABSTRACT We summarize the results from the various measurements and the inter-sampler comparisons from Southeastern Aerosol and Visibility Study (SEAVS), a study with one of its objectives to test for closure among chemical, gravimetric and optical measurements of atmospheric aerosol particles. Sulfate and organics are the dominant components of the SEAVS fine particles (nominally, particles with aerodynamic diameter <2.5 u,m) but between 28 and 42% (range over various samplers) of the gravimetrically measured total fine particle concentration is unidentified by the chemical measurements. Estimates of water associated with inorganic components and measurement imprecision do not totally explain the observed difference between gravimetric and chemical measurements. We examine the theoretical and empirical basis for assumptions commonly made in the published literature to extrapolate total fine particle concentration on the basis of chemical measurements of ions, carbon and elements.

Mercury adsorption to elemental carbon (soot) particles and atmospheric particulate matter

Abstract An assessment of the adsorption/desorption of mercury onto/from particulate matter is presented. This assessment addresses both elemental carbon (soot) particles using published data and atmospheric particulate matter using new experimental data. Available experimental data on the adsorption of mercury onto elemental carbon particles have been reexamined in terms of their adsorption isotherms. The experimental data sets analyzed include experiments concerning mercury adsorption on activated carbon in water as well as in the gas phase. Our analysis using partition coefficients derived from published experimental data and typical atmospheric concentrations of elemental carbon particles shows that the amounts of Hg adsorbed to soot in the air or in atmospheric droplets is insignificant. However, if the particle surface/mass ratio is scaled from the experimental data to typical atmospheric conditions, an upper limit of about half the amount of Hg (primarily Hg(II)) could adsorb to elemental carbon particles, for the conditions considered here. Adsorption of Hg(0) appears to be negligible. Experiments were also conducted to investigate the partitioning of mercury between the dissolved (solution) and suspended particulate phases in rain water. The rain-water data are compared with laboratory experimental studies of Hg desorption and adsorption onto atmospheric particulate matter (APM). The results of these experiments suggest that a significant fraction of Hg(II) can be present in atmospheric particulate matter. Our analysis suggests that between 2 and 35% of the dissolved Hg species (e.g. HgCl2 and Hg(OH)2) could be adsorbed to APM, for the conditions considered here. The remainder of particulate Hg(II) consists of solid Hg species such as HgO and HgS. An analysis of the adsorption experiments as a function of time suggests that the kinetics of adsorption may need to be taken into account in atmospheric applications.

Sensitivity of estimated light extinction coefficients to model assumptions and measurement errors

Abstract The optical properties of aerosol particles, expressed in terms of scattering and absorption coefficients, determine radiation transfer and visibility in the atmosphere. In principle, given sufficiently detailed input data regarding particle concentration, morphology, size, and index of refraction, particulate scattering and absorption coefficients can be estimated. In reality, the estimation of light extinction is constrained by our limited ability to measure the physical and chemical properties of aerosol particles. To evaluate the reliability of light-extinction estimates under such constraints, we applied impactor size distribution inversion and Mie scattering models to several urban and rural U.S. aerosol data sets. The scattering algorithm includes five chemical components, nitrate, sulfate, organic carbon, elemental carbon, and geological dust in internally mixed particles. Particle composition may be treated as homogeneous or distributed between an insoluble core and an aqueous shell. Liquid water is added to dry aerosol mass in discrete size bins and a distribution number is estimated. Extinction is calculated with Mie theory. For the data sets examined, light scattering estimated with this model agreed with measured scattering to within 26% on average. We describe the sensitivity of the method to input assumptions about particle composition and morphology, liquid water as a function of relative humidity, and particle size distribution. Apportioning estimated scattering to chemical components of an ambient aerosol using species extinction efficiencies has no clear theoretical basis. The merits of several approaches for doing so are examined.

Characterization of the Southwestern Desert Aerosol, Meadview, AZ

Sulfate, organic carbon, and soil dust were the major components of the fine aerosol at Meadview, AZ, during the summer of 1992. Sulfate mass median diameters (typically 0.15-0.27 μm) were much smaller than mass median diameters for organic carbon (typically 0.43-0.83 μm). Organic carbon size distributions were broader and more varied. Intersampler comparisons show that sulfur and sulfate measurement technology provided precise and relatively accurate (within 2-22%) concentrations. However, large differences were observed between IMPROVE filter and MOUDI impactor carbon concentrations. This is indicative of the large uncertainties with which carbon concentrations are measured. The IMPROVE backup filter subtraction procedure was partially responsible for these differences. Meadview sulfate was not completely neutralized by ammonium; SO2 concentrations were comparable to sulfate concentrations; and virtually all of the nitrate was present as gas-phase nitric acid. Our estimates suggest that primary organic aerosol from urban areas accounts for no more than 24% on average of the organic aerosol found at Meadview, AZ. The remainder is most likely secondary and biogenic OC, as well as OC from local and regional anthropogenic sources.