Organic aerosols phase partitioning

Thus, if a particle secondary oxidation product does not get partitioned efficiently into the condensed phase (i.e., KtmJ is small) or the available organic condensed phase for uptake of the semivolatile product is small, Eq. (LL) reduces to Y M(lE ,/fnlI1/ and the secondary organic aerosol yield is proportional to the amount of condensed phase available for uptake of the low-volatily gaseous products. On the other hand, if KomJ and M are large, Eq. (LL) becomes Y Ea, independent of the amount of condensed phase available for product uptake. 

In short, the same types of aerosol organic products have been identified both in model systems and in polluted urban ambient air and can generally be rationalized based on the oxidation of known constituents of air. The measured yields of organics in the particles can depend on the amount of particle phase available for uptake of the organic if it is semivolatile and partitions between the gas and condensed phases. This partitioning, and its dependence on the amount of condensed phase available, may be at least in part responsible for discrepancies in the yields of secondary organic aerosol reported in a number of studies. 

Figure 9.63, for example, plots log K against log pL for the partitioning of a series of PAH (see Chapter 10) between the gas phase and particles of either dioctyl phthalate (DOP) or secondary organic aerosol (SOA) generated from the photooxidation of gasoline vapor (Liang et al., 1997). The slope of the plot for uptake into DOP is 1.09 and that for uptake into SOA is... 

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). 

Qriffin, R. J., D. Dabdub, J. H. Seinfeld Development and initial evaluation of a dynamic species-resolved model for gas phase chemistry and size-resolved gas/particle partitioning associated with secondary organic aerosol formation, J. Geophys. Res. 110 (2005) doi 10.1029/2004JD005219... 

In the aromatic chamber experiments aerosol particles are formed and chemistry may take place on the surface of these particles. For example, NO2 could partition to the condensed phase and be reduced to HONO by the surface bound species on the secondary organic aerosol formed in the aromatic chamber experiments. Such heterogeneous processes would provide a sink for NOx and a source of radicals via the photolysis of HONO). 

FoUcers M., T.F. Mentel, H. Henk, R. Tillmanu, A. Wahner, R.P. Otjes, M.J. Blom and H.M. Ten Brink Partitioning of organic aerosol components between gas phase and particulate phase, Eos. Trans. AGU 84 (2003b) Fall Meet. Suppl., Abst. A51F-0746. 

Several chemical compounds (water, ammonia, nitric acid, organics, etc.) can exist in both the gas and aerosol phases in the atmosphere. Understanding the partitioning of these species between the vapor and particulate phases requires an analysis of the thermodynamic properties of aerosols. Since the most important solvent for constituents of atmospheric particles and drops is water, we will pay particular attention to the thermodynamic properties of aqueous solutions. 

The first section of this chapter is a review of fundamental chemical thermodynamic principles focusing on the chemical potential of species in the gas, aqueous, and solid phases. Further discussion of fundamentals of chemical thermodynamics can be found in Denbigh (1981). Chemical potentials form the basis for the development of a rigorous mathematical framework for the derivation of the equilibrium conditions between different phases. This framework is then applied to the partitioning of inorganic aerosol components (sulfate, nitrate, chloride, ammonium, and water) between the gas and particulate phases. The behavior of organic aerosol components will be discussed in Chapter 14. 

The thermodynamic principles introduced in Chapter 10 can be used to investigate the partitioning of organic compounds between the gas and aerosol phases. We will first focus on the analysis of a series of idealized scenarios and then discuss their applicability to the atmosphere. 

Partitioning of Noninteracting SOA Compounds Find the mass fraction of the secondary organic aerosol species i that will exist in the particulate phase, Xp as a function of reacted ROG. Plot the XPii versus AROG curves for a, = 0.05, T = 298 K, and Mrog = lSOgmol-1 for saturation mixing ratios equal to 0.1, 1, and 5 ppb. 

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... 

Fig. 1 Partitioning behavior of organics for 1 pg m of total organic aerosol (Cqa) shown as the fraction in the condensed phase (, height of bars and curve) vs saturation concentration (C )...

Asa-Awuku A, Miracolo MA, Kroll JH, Robinson AL, Donahue NM (2009) Mixing and phase partitioning of primary and secondary organic aerosols. Geophys Res Lett 36 L15827. doi 10.1029/2009gl039301... 

The ozonolysis of the monoterpenes, biogenic species of molecular formula CioHm (e.g., a- and /3-pinene), provides a significant source of secondary organic aerosol. A number of multi-functional organic acids, such as norpinic acid from j8-pinene ozonolysis, and pinonic acid and pinic acid from a-pinene ozonolysis, have been conclusively identified as components of the organic aerosol formed (e.g., Presto et al., 2005 and references therein). As summarized in table VI-F-1, these acidic species possess very low vapor pressures (typically < 10 " Pa at 298 K, Bilde and Pandis, 2001), and thus have a strong tendency to partition to the condensed phase, where they are removed via depositional processes. The low vapor pressures preclude any studies of their gas-phase kinetics. However, the structure-activity relations (SARs) of Kwok and Atkinson (1995) can be used to estimate rate coefficients of about (1-5) xl0 cm ... 

Furthermore, air-organic solvent partition constants, in particular the air-octanol partition constant, are widely used to evaluate and/or predict the partitioning of organic compounds between air and natural organic phases. Such organic phases are present, for example, in aerosols or soils (Chapters 9 and 11) or as part of biological systems (Chapter 10). 

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