Aqueous Aerosol Particles

Equation (12.130) is also applicable to aqueous aerosol particles, but with one significant difference. During the condensation of A and the reduction of Coo, the concentration Ceq at 

The molality of A in the aqueous phase, mA, is given (see Chapter 10) by mA = ha/(0.018 where nA and nw are the molar aerosol concentrations per m3 of air of A and water, respectively. In general, the water concentration nw changes during transport of soluble species between the two phases. For the purposes of this calculation, let us assume that this change is small (i.e., only a small amount of A is transferred) and assume that nw is approximately constant. The rate of change of the molality of A is given in this case by 

The rate of change of moles of A in the aerosol phase dnA/dt will be equal to the flux J given by (12.129), so 

In this equation, and ceq are the gas-phase concentrations of A far from the particle and at the particle surface expressed in mol m 3. The gas-phase equilibrium concentration ceq is related to the liquid-phase molality by an equilibrium constant KA (kg m 3), which is a function of the composition of the particle and the ambient temperature, such that 

Assuming that the system is open and c remains constant during the condensation of A, then we can calculate the timescale xa from (12.145). This timescale corresponds to the establishment of equilibrium between the gas and liquid aerosol phases as a result of changes in the aqueous-phase concentration of A. Following the nondimensionalization procedure of the previous section, we find 

The timescale Xa increases with increasing aerosol water content. The higher the aerosol water concentration, the slower the change of the aqueous-phase concentration of A result- 

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As the (NH4)2S04 in the particles increases compared to the ammonium nitrate, the parameter Y decreases and the equilibrium product of ammonia and nitric acid decreases. The additional ammonium and sulfate ions make the aqueous solution a more favorable environment for ammonium nitrate, shifting its partitioning toward the particulate phase. Therefore, addition of ammonium sulfate to aqueous aerosol particles will tend to increase the concentration of ammonium nitrate in the particulate phase, and, vice versa, reductions of ammonium sulfate can lead to decreases of ammonium nitrate for aqueous aerosol. 

Equation (11.130) is also applicable to aqueous aerosol particles, but with one significant difference. During the condensation of A and the reduction of Coo, the concentration c q at the particle surface changes. The timescale calculated in the previous section remains applicable, but there is an additional timescale r, characterizing the change of Ceq ... 

Most of the SOg that does get into the atmosphere reacts with oxygen to form sulfur trioxide (SO3). This compound has a strong affinity for water and dissolves in aqueous aerosol particles, forming sulfuric acid, which in turn contributes to acid rain (Section 11.2). 

H2CO3 (carbonic acid) aqueous phase HC2H3O2 (acetic acid) aqueous, gas phases HCHOz (formic acid) aqueous, gas phases H2C2O4 (oxalic acid) aerosol particles solid phase RCCX3H (many carboxylic acids)... 

Lepri L, Del Bubba M, Masi F, Udisti R, Cini R (2000) Particle size distributions of organic compounds in aqueous aerosols collected from above sewage aeration tanks. Aerosol Sci Technol 32 404-420... 

In the case of S02, oxidation in the aqueous phase, present in the atmosphere in the form of aerosol particles, clouds, and fogs, is also important. Thus S02 from the gas phase dissolves in these water droplets and may be oxidized within the droplet by such species as H202, 03, 02, and free radicals. Oxidation of S02 on the surfaces of solids either present in the air or suspended in the water droplets is also possible. On the other hand, it is believed that HN03 is formed primarily by reaction (10) in the gas phase and subsequently dissolves in droplets. 

For nebulizer and other aqueous aerosol products that use suspension systems, excipients are used to influence particle physical and chemical stability (e.g., microcrystalline cellulose for nasal sprays). The suitability of the physicochemical properties of these critical excipients should be thoroughly investigated and documented (12). Far more excipients have been included in formulations designed for nasal administration (Table 4). 

While in the air compartment, the contaminant solubilizes in the vapor-liquid phase or is associated with aerosol particles by adsorption. It is also prone to desorption from the aerosol particles into the vapor phase. Relevant properties of the air used to model transport of partitioning of a contaminant in the air compartment include temperature, turbulence, wind speed, size and composition of aerosol particles, etc.16,19 Relevant properties of the contaminant that measure its tendency to partition among the vapor, liquid, and solid phases in the air include its aqueous solubility (Saq), vapor pressure (VP), Henry s constant... 

The pH of sea salt aerosol is an important property as many important aqueous phase reactions are pH dependent. For example, oxidation of S(IV) (SO2 + HSOs + SO ) by O3 is only important for pH of more than 6. Sea salt aerosol is buffered with HC03. Uptake of acids from the gas phase leads to acidification of the particles. According to the indirect sea salt aerosol pH determinations by Keene and Savoie (1998, 1999), the pH values for moderately polluted conditions at Bermuda were in the mid-3s to mid-4s. The equilibrium model calculations of Fridlind and Jacobson (2000) estimated marine aerosol pH values of 2-5 for remote conditions during ACE-1. Using a one-dimensional model of the MBL which includes gas phase and aqueous phase chemistry of sulfate and sea salt aerosol particles, von Glasow and Sander (2001) predicted that under the chosen initial conditions the pH of sea salt aerosol decreases from 6 near... 

Another link between halogen and sulfur chemistry is the formation of S(VI) within particles. Aerosol particles grow, among other processes, by uptake of SO2 in cloud droplets where it is oxidized to sulfate. The most important aqueous phase oxidants for S(IV) are often thought to be H2O2 and O3 (e.g., Seinfeld and Pandis, 1998) with O3 being important only for pH > 6. Some authors state that oxidation by O3 is the dominant process for the formation of non-sea-salt sulfate in sea salt particles... 

Figure 5.2. Various interactions that determine the composition of a water droplet in the atmosphere (e.g., cloud, fog). Aerosol particles, which to a large extent consist of (NH4)2S04 and NH4NO3, can form the nuclei for the condensation of liquid water. Various gases can become absorbed into the aqueous phase. The atmosphere is an oxidative environment the water phase, often assisted by light, promotes oxidation reactions, for example, the oxidation of SO2 to H2SO4 and of organic matter to CO2. NH3 neutralizes mineral acids and buffers the solution phase.

Chemical Properties of Aerosols. Surface chemical properties of aerosol particles can also be tailored to improve deaggregation [273]. Hygroscopic particles absorb water when inhaled into the humid airways [282-284], increasing particle size and density in the process, as well as creating the potential for capillary bridge formation between particles. Hygroscopic growth can be reduced by the use of hydrophobic additives [285] or compounds with low aqueous solubility [286,287]. 

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