Strong acid content, atmospheric aerosol

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. 

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

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... 

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. 

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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