Aerosol chemistry and particle formation

As part of the biogeochemical cycle, the injection of iodine-containing gases into the atmosphere, and their subsequent chemical transformation therein, play a crucial role in environmental and health aspects associated with iodine - most importandy, in determining the quantity of the element available to the mammalian diet. This chapter focuses on these processes and the variety of gas- and aerosol-phase species that constitute the terrestrial iodine cycle, through discussion of the origin and measurement of atmospheric iodine in its various forms ( Sources and Measurements of Atmospheric Iodine ), the principal photo-chemical pathways in the gas phase ( Photolysis and Gas-Phase Iodine Chemistry ), and the role of aerosol uptake and chemistry and new particle production ( Aerosol Chemistry and Particle Formation ). Potential health and environmental issues related to atmospheric iodine are also reviewed ( Health and Environment Impacts ), along with discussion of the consequences of the release of radioactive iodine (1-131) into the air from nuclear reactor accidents and weapons tests that have occurred over the past half-century or so ( Radioactive Iodine Atmospheric Sources and Consequences ). 

The composition of single aerosol particles in the micrometer range is important for assessing their environmental health hazard and for studying the atmospheric chemistry of particle formation and transformation during atmospheric transport. Both... 

Fig. 1 Schematic of surface-active organic material in a deliquesced aerosol particle. Surface organics can potentially inhibit the uptake of gas-phase species to the particle, enhance ice nncle-ation, and depress particle surface tension, with important implications for aerosol heterogeneous chemistry and cloud formation...

As we have seen, a great deal is known about emission sources and strengths, ambient levels, and mutagenic/carcinogenic properties of the particle-phase PAHs in airborne POM. However, because of the tremendous physical and chemical complexity of the aerosol surfaces on which photolysis, photooxidations, and gas-particle interactions take place in real polluted ambient air, much less is known about the structures, yields, and absolute rates and mechanisms of formation of PAH and PAC reaction products, especially for the more polar PACs. This is one area in which there exists a major gap in our knowledge of their atmospheric chemistry and toxicology. 

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. 

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... 

The net effect of these two links between sulfur and halogen chemistry is to decrease the gas phase concentration of SO2 via a reduced yield of SO2 from the oxidation of DMS and the stronger aqueous phase sink for S(IV) which results in enhanced uptake of SO2 by droplets and aerosols. A critical prerequisite for new particle formation in the marine troposphere is the reaction chain ... 

Pirjola L, O Dowd CD, Brooks IM, Kulmala M (2000) Can new particle formation occiu in the clean marine boundary layer J Geophys Res 105 26531-26546 Pitchford ML, McMurry PH (1994) Relationship between measitred water vapor growth and chemistry of atmospheric aerosol for Grand Canyon, Arizona, in winter 1990. Atmos Environ 28 827-839 Pope CA (2000) What do epidemiologic findings tell us about health effects of environmental aerosols J Aerosol Med 13 335-354... 

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... 

A wide range of structurally diverse compounds is produced during incineration. These include PAHs and related compounds, azaarenes, and chlorinated PAHs from combustion of fossil fuels and natural wildfires. Organic compounds in the atmosphere may exist both in the free (gaseous) state or on particles of various dimensions. Recent concern has been directed to the occurrence in aerosols both of the compounds themselves and of their transformation products (secondary aerosols) (1) for their role in atmospheric chemistry and as determinants of climate (Andreae and Crutzen 1997) and (2) due to health risks since aerosol formation facilitates the transport into and sorption by the lungs. 

Measurements of the urban aerosol mass distribution have shown that two distinct modes often exist in the 0.1 to 1.0 pm diameter range (Hering and Friedlander 1982 McMurry and Wilson 1983 Wall et al. 1988 John et al. 1990). These are referred to as the condensation mode (approximate aerodynamic diameter 0.2 pm) and the droplet mode (aerodynamic diameter around 0.7 pm). These two submicrometer mass distribution modes have also been observed in nonurban continental locations (McMurry and Wilson 1983 Hobbs et al. 1985 Radke et al. 1989). Hering and Friedlander (1982) and John et al. (1990) proposed that the larger mode could be the result of aqueous-phase chemical reactions. Meng and Seinfeld (1994) showed that growth of condensation mode particles by accretion of water vapor or by gas-phase or aerosol-phase sulfate production cannot explain existence of the droplet mode. Activation of condensation mode particles, formation of cloud/fog drops, followed by aqueous-phase chemistry, and droplet evaporation were shown to be a plausible mechanism for formation of the aerosol droplet mode. 

Fig. 20.26 Schematic for particle formation mechanisms during flame-assisted spray pyrolysis (FASP), FSP, euid vapor-fed aerosol flame synthesis (VAFS) (Reprinted with permission from Strobel and Pratsinis 2(X)7, Copyright 2(X)7 Royal Society of Chemistry)...

The essential elements of urban aerosol chemistry are shown in Figure 2, in which we have represented the chemistry in terms of the conversion of SO2, NO and hydrocarbons to particulate sulfate, nitrate, and organics, respectively. Table VII summarizes the key unknown aspects of the processes depicted in Figure 2. There are many feat ires of atmospheric aerosol chemistry that must be elucidated before we understand fully the formation and growth of atmospheric particles. 

Bricard, J., M. Cabone, G. Madeleine, and D. Vigla, Formation and Properties of Neutral Ultrafine Particles and Small Ions, in Aerosols and Atmospheric Chemistry (G.M. Hidy, ed.) pp. 27-43 Academic Press (1972). 

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