Aerosol strong acid content

Is this re-equilibration phenomenon important for the measurement of aerosol strong acid content, as it is for gaseous nitric acid Generally, nitric acid is taken up into aerosol particles (solid or liquid droplets) only if the particles have been nearly completely neutralized by ambient ammonia, because the nitrate-nitric acid equilibrium favors the gas phase in the presence of significant particulate strong acid (62). Most concern, however, has been expressed concerning sampling of acidic aerosols in the presence of ambient ammonia. 

Measuring the Strong Acid Content of Atmospheric Aerosol Particles... 

Methods used to determine the strong acid content of aerosol particles in the ambient atmosphere are reviewed. These methods include those for generic determination of strong acid content and those in which the concentrations of individual strong acid species are determined. Difficulties in sampling these species due to their reactivity and occurrence under non-steady-state atmospheric conditions are discussed, and the methods currently used for resolving these difficulties are critically evaluated. 

Al fundamental question about the interpretation of acidic aerosol data is whether researchers can characterize past and current atmospheric concentrations and distributions (spatial and temporal) with sufficient accuracy for studies of their effects on ecosystems and human health. Part of the answer to this question can be provided by a review of the methods that have been used to measure the strong acid content of aerosol particles collected from the atmosphere. This chapter serves as such a review, and, in evaluating analytical procedures, it specifically assesses the ability of each procedure to overcome sampling artifacts, to distinguish between strong and weak acids, to properly partition strong acidity between gas-phase and aero-sol-phase species, and to quantitate strong acidity at the levels at which it is found in the ambient atmosphere. 

Errors may occur in the Gran titration procedure if weakly acidic species with dissociation constants (expressed as pKd) in the range of the extract pH are present. In particular, curvature or reduction (or both) of the slope of the Gran exponential plot results (24), because weak acid dissociation and titration of released free acidity take place during the portion of the titration used for end-point determination. Fortuitously, some of the common, weak carboxylic acids (e.g., formic and acetic) are not stable toward microbial decomposition when collected in aerosol samples from the atmosphere, so much of the historical data base on strong acid content of aerosols does not suffer from this positive error source, unless of course the microbial processes produce additional strong acids. 

Surface Interactions. Loss of strong acid content of atmospheric aerosols was observed and attributed to reaction with basic sites in the glass or cellulose filter matrices commonly used for high-volume sampling of atmospheric aerosols (46, 50). These filter materials, and glass fiber filters of... 

One procedure that is widely used to circumvent these complications is to remove ambient ammonia from the sampled air without removing particles by inserting one of several types of diffusion denuders upstream from the filter(s). In fact, in a recent Environmental Protection Agency (EPA)-sponsored intercomparison of methods for determination of strong acid content of aerosols, all but one protocol utilized an ammonia denuder (63), and all used an impactor or cyclone to remove coarse particles. The presence of this denuder clearly prevents neutralization of acidic aerosols by ammonia but also disturbs the gas-aerosol equilibrium between sulfate-nitrate aerosols and gaseous species. Ammonia and nitric acid are released from the depositing particles (64, 65) and must be collected downstream if accurate particulate ammonium and nitrate determinations are to be made. If equal amounts of ammonia and nitric acid are released, then the absolute [H+] (neq/m3) will not be altered. No specific evidence is available in the literature to demonstrate alteration of the observed [H+] as the result of reequilibration, but this area deserves further study. 

Particle-Particle Interactions. Loss of strong acid content of aerosol particles can also occur because of reactions between co-collected acidic and basic particles impacted together on the collection surface. This phenomenon most frequently occurs as the result of interaction of coarse (>2.5 xm diameter), alkaline, soil-derived particles with fine (<2.5 xm diameter) acidic sulfate particles (66). Particle-particle interactions with net neutralization can be reduced in many cases by sampling with a virtual impactor or a cyclone to remove coarse particles, although this procedure does not prevent the effect if external mixtures of fine particles of different acid contents are sampled. In situ methods with shorter sampling times can be used such that these topochemical reactions are less likely to occur. 

Descriptions of analytical methods for strong acid and acidic sulfate content of atmospheric aerosols have been reviewed (6-10). Methods for acidic aerosol determination are reviewed in this chapter according to the measurement principle either filter collection and post-collection extraction, deriv-atization or thermal treatment, and analysis or in situ collection (real-time or stepwise) and analysis. 

Chemically, diatomite consists primarily of silicon dioxide, SiOs -nHtO, and is essentially inert. It is attacked by strong alkalies and by hydrofluoric acid hut is virtually unaffected by other acids. The silicon dioxide has a unique structure, resulting from the intricate form of the diatom skeletons. The chemically combined waler content varies from 2 to 10%. Impurities that arc often found mixed with the diatomite are other aquatic fossils such as sponge residues, Radiolaria, siiicoflagcllata, sand, clay, volcanic ash, mineral aerosols, calcium carbonate, magnesium carbonate, soluble salts, and organic matter. 

Fatty acid film formation and the associated surface tension depression has been studied extensively for idealized systems (i.e., pure water subphases) by the colloid science community, starting with the work of Langmuir nearly a century ago [211, 212]. Since it is known that fatty acid phase behavior depends strongly on factors such as pH, the presence of salts, and mixed organic content [6, 213, 214], it is not possible to extrapolate measurements performed in pure water or very dilute systems to the relevant aerosol conditions. Here we focus on studies which move towards more atmospherically relevant systems. The results of several studies are summarized in Table 4. 

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