Acid aerosol neutralization

Consider a lake with a smaU watershed in a forest ecosystem. The forest and vegetation can be considered as an acid concentrator. SO2, NO2, and acid aerosol are deposited on vegetation surfaces during dry periods and rainfalls they are washed to the soil floor by low-pH rainwater. Much of the acidity is neutralized by dissolving and mobilizing minerals in the soil. Aluminum, calcium, magnesium, sodium, and potassium are leached from the soil into surface waters. The ability of soils to tolerate acidic deposition is very dependent on the alkalinity of the soil. The soil structure in the... 

Airstream neutralization of acid aerosols by NH3 present in the airway-lumen reduces the health risk associated with acid particles by reducing the acid concentration prior to particle deposition.- In addition, the liquid lining of the respiratory tract probably acts as a chemical buffer," further reducing the health hazard posed by inspired acid particles. Principal factors controlling airstream neutralization of acid aerosols, which is considered to be a diffusion-limited process, are particle surface area, and particle... 

Larson, T. V., Frank, R., Covert, D..S., Efolub, D., and Morgan, M.S. (1982). Measurements of respiratory ammonia and chemical neutralization of inhaled sulfuric acid aerosol in anesthetized dogs. Am. Rev. Respir. Dis. 125, 502-506. 

Cocks, A, T, and McElroy, W. J. (1984). Modeling studies of the concurrent growth and neutralization of sulfuric acid aerosols under conditions in the human airways. Eimironmefital Res. 35, 79-96. 

If the reaction is between two gas-phase species, then this reaction could be a source of cloud condensation nuclei, or simply a means to neutralize an acidic aerosol. Although there are some questions concerning the measurement of atmospheric HNO3, (Lawson, 1988) most measurements indicate that gaseous HNO3 concentrations predominate over particle NOi". 

This system was studied by Schwartz. Toluene at 10 ppm, nitric oxide at 1 ppm, and nitrogen dioxide at 1.2 ppm were irradiated with ultraviolet lamps in a 17-m batch reactor for 270 min. Collected aerosols were successively extracted with methylene chloride and then methanol. The methylene chloride extract was fractionated into water-soluble and water-insoluble material, and the latter fraction was further divided into acidic, neutral, and basic fractions. The acidic and neutral fractions were analyzed by gas chromatography and chemical-ionization mass spectrometry the compounds identified are shown in Figure 3-7. The two analyzed fractions represented only about 5.5% of the total aerosol mass. It is noteworthy that classical nitration of an aromatic ring appears to... 

Cunningham, P. T B. D. Holt, S. A. Johnson, D. L. Drapcho, and R. Kumar, Acidic Aerosols Oxygen-18 Studies of Formation and Infrared Studies of Occurrence and Neutralization, in Chemistry of Particles, Fogs, and Rain (J. L. Durham, Ed.), Acid Precipitation Series (J. I. Teasley, Series Ed.), pp. 53-130, Butterworth, Stone-ham, MA, 1984. 

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. 

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. 

If long sampling times are required to collect sufficient sample for analysis, it is desirable to remove ambient ammonia (by diffusional removal to the walls of a coated denuder tube) because increases in the concentration of this gas during sampling may result in post-collection neutralization of acidic aerosol particles. No evidence of changes in absolute [H+] caused by this procedure is reported, but further research is needed in this area. 

Further study is needed of the phenomenon of kinetic limitations to the neutralization of acidic aerosols. Simultaneous occurrences of acidic aerosols at gaseous [NH3] well above the equilibrium values have been reported (56, 67), and it is still unclear whether kinetic limits to microscale neutralization or boundary layer mixing (macroscale) kinetics (or both) are responsible for these limitations. An understanding of the extent of human exposure to acidic aerosols, as well as of the availability of acidic aerosols for wet scavenging... 

Seed aerosol neutral acidic neutral acidic neutral acidic... 

It should be noted that bathroom cleaners, while predominately trigger spray cleaners in the U.S., and either trigger sprays or in pour out bottles in Europe, still appear as aerosol sprays in some parts of the U.S. These generally use the typical aerosol propellants [289] appropriate to the types of cleaner (alkaline, acid, or neutral). 

Acid droplets absorb ammonia and become partially neutralized as they pass along the respiratory tract. Cocks and McElroy (1984) extended the work already discussed (Cocks and Fernando, 1982) on the hygroscopic growth of particles to model the growth and neutralization of acid aerosols. Before considering the results of the Cocks and McElroy (1984) study, two difficulties identified and dealt with by these authors should be examined ... 

Acid aerosols are often found in association with high concentrations of acidic gases, such as sulphur dioxide. It might, therefore, be asked whether significant depletion of nasal or oral ammonia by sulphur dioxide might occur and thus impair droplet neutralization. This was modelled by Cocks and McElroy (1984) who concluded that at a sulphur dioxide concentration of 100 qg m-3 only some 2% of the ammonia likely to be present would be removed in 10 s. The effect may be neglected. 

Data regarding exposure to mixtures of atmospheric contaminants indicate that, contrary to what might be expected, increased carbon dioxide concentration (up to 5% in air) does not alter the hyperventilatory rate induced by hyperammonemia in dogs (Herrera and Kazemi 1980). Ammonia in expired air may neutralize inhaled acid aerosols (EPA 1979 Earson et al. 1980 Utell et al. 1989). 

A third study evaluated S04 and exposure to 24 children (ages were not provided) living in Uniontown, Pennsylvania (Suh et al. 1992). This study did not focus on ammonia exposure per se, but on other airborne contaminant concentrations in aerosols found outdoors, indoors, and by personal monitors. It sought to determine how personal exposures to these aerosols correlated with indoor and outdoor concentrations. Ammonia concentrations were measured in order to assess their potential for neutralizing found in aerosols. Ammonia was found to be in highest concentrations near the children (detected by the personal monitors), followed by indoor concentrations, and were minimal outdoors. It was proposed that a large proportion of the found in indoor aerosols are neutralized by NH3, and thus would lower the children s exposure to acid aerosols. The authors noted that more research is needed to fully model the influence of factors, including NH3, on indoor acid aerosol exposure. 

EPA. 1979. Effects of endogenous ammonia on neutralization of inhaled sulfuric acid aerosols. Cininnatti, OH U S. Enviromnental Protection Agency, Health Effects Research Lab. EPA-600/1-79-045PB 80 147 978. 

Larson T, Frank R, Covert D, et al. 1980. The chemical neutralization of inhaled sulfuric acid aerosol. Am J Ind Med 1 449-452. 

This is in an unscrubbed plume. Now, concerning ammonia, you ae not really talking about ammonia as a catalyst for sulfate formation. The actual process is SO2 to sulfuric acid, followed by ammonia neutralization. The theory about SO2, water, ammonia catalysis is being questioned. The real process seems to be SO2 to sulfuric acid, then reaction with ammonia to form ammonium sulfate or ammonium bisulfate. In fact, in scavenging the sulfate, as the particle size increases, the scavenging efficiency also increases. Sulfuric acid aerosols are submicron particulates for which scavenging is very inefficient (Marsh, Atmos. Environ. 12 401-406, 1978). So what you are finding in a rain droplet is perhaps what is below the cloud or before the condensation nucleus stabilizes. 

Ammonia is removed from the atmosphere by its affinity for water and by its action as a base. It is a key species in the formation and neutralization of nitrate and sulfate aerosols in polluted atmospheres. Ammonia reacts with these acidic aerosols to form ammonium salts ... 

A further consideration in the characterization of acid aerosols is the mix of compounds present. Newly emerging information on biological responses (see below) shows that the net H concentration is not always sufficient to predict the response. This may relate to the physical behavior of the various compounds as they pass through the respiratory tract. Given the absence of specific measurement methods for the common sulfate salts, the overall molar ratio, either hVSO or NH VSO , is a convenient descriptor for the degree of aerosol neutralization. 

Waldman, J.M., P.J. Lioy, G. D. Thurston, and M. Lippmann (1990), Spatial and temporal patterns in summertime sulfate aerosol acidity and neutralization within a metropolitan area. Atm. Em. 246 115-126. 

VA/crotonates/vinyl neodecanoate copolymer is the most used polymer in aerosol hair sprays (ca 1993). Like its precursor above, it has free carboxyhc acid groups which can be neutralized to give various film properties. Recommended neutralizing agents include aminomethyl propanol, ammonium hydroxide, and dimethyl stearamine. Recommended percent neutralization is 90%, but products can be found in the 80—110% range. 

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