Liquid aerosol-phase vapor pressure

Despite many years of interest in the phase distribution of POPs, few predictive models are available. A Langmuir-type relationship, which Junge (1977) first proposed and Pankow (1987) later reviewed and critically evaluated, is the most popular model for estimating adsorption onto aerosols. The Junge-Pankow equation relates the fraction of particulate POPs (c >) to the saturation liquid-phase vapor pressure of the compound ( P Pa) and the surface area of particles per unit volume of air (8, cm2 aerosol / cm3 air). 

Several predictive approaches are available for estimating the fraction of POPs sorbed onto aerosols. The Junge-Pankow model assumes that gaseous POPs are adsorbed onto active sites on the aerosol. Key parameters in the model are particle surface area per unit volume of air (0), the liquid-phase vapor pressure of the compound ( P ) and a factor (c) which depends on the excess heat of desorption from the particle surface. 

When estimating air-aerosol partitioning of gas phase substances such as PAHs, most of which are solids, it is usual to use the liquid state vapor pressure as the correlating parameter. This is because the PAH is effectively in a liquidlike state on or in the aerosol particle. It does not exist in crystalline form. 

FIGURE 9.63 Plots of om-phase-normalized gas-particle partitioning constant log Kp iun vs logarithm of the subcooled liquid vapor pressure, log pL, for a series of semivolatile PAHs partitioning on ( ) dioctyl phthalate (DOP) or (a) secondary organic aerosol (SOA) from photooxidized gasoline vapor. PAHs are as follows naphthalene, A acenaphthalene, B fluorene, C and C phenanthrene, D and D anthracene, E and E fluoranthene, F and F pyrene, G and G chrysene, H (adapted from Liang el al., 1997). 

Equilibrium vapor pressure is the vapor pressure of a system in which two or more phases or a substance coexist in equilibrium. In meteorology, the reference is to water substance, unless otherwise specified, If the system consists of moist air in equilibrium with a plane surface of pure water or ice, the more specialized term saturation vapor pressure is usually employed, in which case, the vapor pressure is a function of temperature only. In the atmosphere, the system is complicated by the presence of impurities in liquid or solid water substance (see also Raoult s Law), drops or ice crystals or both, existing as aerosols and, in general, the problem becomes one of nucleation. For example, the difference in vapor pressure over supercooled water... 

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... 

A chemical s Tb, the temperature at which its vapor pressure equals the ambient pressure, and Tm, the temperature at which its solid and liquid forms are in equilibrium at ambient pressure, are easily located in references and databases. As a result, many of the correlations that have been constructed for property estimations use these parameters as independent variables. The Tb of a chemical can nonetheless provide an indication of the partitioning between gas and liquid phases,53 with the higher values denoting a lower tendency to exist in the vapor phase. The surface tension, y, of a chemical, the ratio of the work done to expand the surface divided by the increase in the surface area, is often used to estimate the VP of liquids in aerosols and in soil capillaries.28 The VP of a chemical is the pressure of a pure chemical vapor that is in equilibrium with the pure liquid or solid, and... 

I) A gas-phase product of the OH-aromatic reaction simply forms a liquid particle because of its vapor pressure, in the absence of any "seed" aerosol. 

It will be remembered that the formation of a new phase by homogeneous nucleation involves first the formation of small clusters of molecules, which then may disperse or grow in size by accretion until some critical size is reached, at which point the cluster becomes recognizable as a liquid drop. The drop may then continue to grow by accretion or by coalescence with other drops to produce the final aerosol. Normally, extensive drop formation is not observed unless the vapor pressure of the incipient liquid is considerably higher than its saturation value that is, unless the vapor is supersaturated. 

As mentioned before, POP transport in the environment depends on their physicochemical properties [40-54], and these include saturated vapor pressure, solubility, Henry s law constant, octanol-water, octanol-air, and organic carbon-water partition coefficients. The saturated vapor pressure characterizes the capability of a substance to be transferred to the gaseous state. Eollowing the study of Wania and Mackay [40], the efficiency of POP condensation with subcooled liquid pressure (p°L) at 25°C above 1 Pa is very low. POPs with a vapor pressure between 1 and 10" Pa are condensed at a temperature of about -30°C and their deposition may be expected mostly in the polar latitudes. POPs with a vapor pressure of subcooled liquid from 10" to 10" Pa are condensed at a temperature above 0°C and they may reach to the middle latitudes. EinaUy, POPs of low volatility with a vapor pressure of subcooled liquid below 10" Pa are practically not vaporized and these substances may be transported and deposited as fine aerosols or coarse particles [39]. Using the vapor pressure of the subcooled liquid it is possible to characterize the partitioning of a POP between the gas phase and the solid phase of the atmospheric aerosol. The POPs having a lower vapor pressure are better bound with... 

In-cloud processes are a second major class of chemical transformations of aerosol particles (cf. Section 4.04.7.3). Clouds are technically aerosols, a special class in which the particles consist mainly of liquid or solid water and the gas phase generally exhibits slight supersaturation with respect to the condensed phase, i.e., an RH slightly greater than 100%. Clouds form in the atmosphere mainly as a consequence of air parcels being cooled below the dew-point of water, the temperature at which water vapor in a given air parcel is saturated with respect to the liquid. Generally as an air parcel rises to lower pressure. 

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