Secondary aerosol oxidation products

Finally, atmospheric chemical transformations are classified in terms of whether they occur as a gas (homogeneous), on a surface, or in a liquid droplet (heterogeneous). An example of the last is the oxidation of dissolved sulfur dioxide in a liquid droplet. Thus, chemical transformations can occur in the gas phase, forming secondary products such as NO2 and O3 in the liquid phase, such as SO2 oxidation in liquid droplets or water films and as gas-to-particle conversion, in which the oxidized product condenses to form an aerosol. 

Review of the literature provides ample evidence that aerosol formation is an important part of the atmospheric chemistry linked with photochemical-oxidant production. The important chemical constituents of concern include sulfate, nitrate, and secondary organic material. 

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. 

Indoor Chemistry Various terpenes and terpenoids are emitted from household products and building materials. Ozone that has entered from outdoors or has been generated indoors can react with these compounds, either in the gas phase or on the surface of materials. The resulting oxidation products will contribute to the production and growth of meaningful quantities of secondary organic aerosols (SOA). The formation and growth of SOA can be studied under controlled conditions in test chambers (see also Chapter 13). 

Several compounds were also found to have a seasonal distribution. Kubatova et al. (2002) found that concentrations of lignin and cellulose pyrolysis products from wood burning were higher in aerosol samples collected during low-temperature conditions. On the other hand, concentrations of dicarboxylic acids and related products that are believed to be the oxidation products of hydrocarbons and fatty acids were highest in summer aerosols. PAHs, which are susceptible to atmospheric oxidation, were also more prevalent in winter than in summer. These results suggest that atmospheric oxidation of VOCs into secondary OAs and related oxidative degradation products are key factors in any OA mass closure, source identification, and source apportionment study. However, additional work is much desirable to assess the extent and seasonal variation of these processes. 

Claeys, M., Wang, W., Ion, A. C., Kourtchev, I., Gelencsdr, A., and Maenhaut, W. (2004). Formation of secondary organic aerosols from isoprene and its gas-phase oxidation products through reaction with hydrogen peroxide. Atmos. Environ. 38, 4093 1098. 

Formation of combustion particles also involves nucleation and condensation of vapors, although the processes occur at elevated temperatures inside the combustion source and during cooling of the plume. Like secondary aerosols, combustion particles have a major semivolatile component composed of sulfates from sulfur dioxide oxidation and organic oxidation products, and of unburned fuel and oil as well. Furthermore, they contain a large non-volatile component consisting of soot, metals, and metal oxides. 

Secondary organic aerosol material is formed in the atmosphere by the mass transfer to the aerosol phase of low vapor pressure products of the oxidation of organic gases. As organic gases are oxidized in the gas phase by species such as the hydroxyl radical (OH), ozone (03), and the nitrate radical (N03), their oxidation products accumulate. Some of these products have low volatilities and condense on the available particles in an effort to establish equilibrium between the gas and aerosol phases. 

Primary particles (e.g., dust, pollens, plant waxes) and secondary oxidation products are the main components of remote continental aerosol (Deepak and Gali, 1991). Aerosol number concentrations average around 2000 to 10,000 cm and PM lo concentrations are... 

Particulate matters are classified in different types, depending on their origins. While marine aerosols are formed of sea salt particles, remote continental aerosols are of primary particles (Uke dust, pollens and plant waxes), as well as secondary oxidation products. Moreover, desert aerosols, which resemble remote continental aerosols in their shape and size, are found over deserts and adjacent regions and strongly depend on the wind velocity. Urban aerosols are then considered a... 

In addition to the important role biogenic terpenes play in gas-hase chemistry, their impact also extends to heterogeneous air chemistry. Although Went (1960) linked the formation of the blue haze over coniferous forests to the biogenic emission of 20 monoterpenes over 40 years ago, it was not until recently that terpenes received their due attention with respect to their role in secondary organic aerosol (SOA) formation. O Dowd et al. (2002) reported that nucleation events over a boreal forest were driven by the condensation of terpene oxidation products. Formaldehyde (HCHO) is a high-yield product of isoprene oxidation. The short photochemical lifetime of HCHO allows the observation of this trace gas to help constrain isoprene emissions (Shim et al. 2005). 

The presence of hydrocarbons, therefore, can accelerate the oxidation of nitric oxide to NO2, which in turn reacts in the light to produce more ozone. Atmospheric oxygen is the source of oxidation capacity for both this reaction and the transformation of propene to more oxidized compounds. While the ultimate fate of much of the hydrocarbon is oxidation to carbon dioxide, it is equally important to note that this complex web of reactions involves many volatile intermediates, such as formaldehyde (HCHO), acetaldehyde (CH3CHO), and PAN (peroxyacetyl nitrate, not shown in Fig. 4.40), all of which can cause human health effects and/or damage to materials. The oxidation process also leads to the production of secondary aerosols, which are responsible for the decreased visibility caused by smog. 

Secondary organic aerosol yields have been measured in smog chamber experiments. The SOA yield for a given oxidation product i, Yi, is the amount of aerosol mass of species / that is produced from the oxidation of the parent reactive organic gas (ROG) and is defined by... 

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