Urban aerosol composition

Lodge, J. P., Jr., A Comparison of Urban and Rural Aerosol Composition Using Dichotomous Samplers, Atmos. Environ., 19, 840 (1985). 

Table I. Average Aerosol Composition for Fine and Coarse Particles at a Rural, Forested Location (Great Smoky Mountains, Tennessee) and an Urban Location (Houston, Texas) ...

An example of the information obtainable from measurements of aerosol particle composition as a function of size is given in Figure 10. In this study of the evolution of aerosol composition in an urban air-shed, a multi-stage impactor was used to separate particles by size for the determination of size-dependent composition. The particle mass concentration increases as air moves inland from Santa Catalina Island to Long Beach, Fullerton, and finally Riverside, as do substances such as nitrate. Differences in particle composition as a function of size at a given site and from site to site are evident. 

There has been growing emphasis on the development and use of on-line measurements, which provide an abundance of data in real time. Short-term variations in aerosol composition can reveal great detail, as shown in Figure 12, which illustrates marked differences in the composition of the aerosol at an urban site over periods as short as tens of minutes. The mass concentration was measured by an oscillating microbalance whose frequency was altered by deposition of material. Ionic species were measured by subjecting the aerosol to supersaturation, which greatly increased particle sizes and permitted... 

Despite concerns over assumptions and approximations, size-resolved, multicomponent aerosol models are being employed at a variety of scales. An example at the scale of an urban air-shed is the study of Meng et al. (1998), which examined aerosol composition and size distribution for the South Coast Air Basin of California. Modeled aerosol substances included SO , NOf, NHJ, Na, CP, and secondary organics. Gas-phase... 

There have been many measurements of the elemental composition of urban aerosols stimulated by the need for large databases in aerosol source apportionment (discussed in a later section). Table 13.2 compares concentrations in the fine and coarse fractions for various U.S. cities. The results show remarkable similarities in the order of magnitude of the concentrations from city to city for each element. Soil dust is a major component of the coarse fraction as indicated by the strong enrichment in aluminum and silicon in every city. The coarse fraction is much less active chemically both with respect to its mechanisms of formation and as a site for reaction, compared to the fine fraction discussed next. 

Ultrajine Range (dp <0.1 fim) 366 Residence Time and Diy Deposition 366 Aerosol Dynamics in Power Plant Plumes 368 Chemical Composition of Urban Aerosols 370 /mmduction 370... 

Table VI summarizes aerosol mass concentrations and composition in different regions of the troposphere. It is interesting to note that average total fine particle mass (that associated with particles of diameter less than about 2 /im) in non-urban continental, i.e., regional, aerosols is only a factor of two lower than urban values. This reflects the relatively long residence time of particles (recall the estimate of a lifetime of fine particles by dry deposition of 10 days). Correspondingly, the average composition of non-urban continental and urban aerosols is roughly the same. The average mass concentration of remote aerosols is a factor of three lower than that of non-urban continental aerosols. The elemental carbon component, a direct indicator of anthropogenic combustion sources, drops to 0.3% in the remote aerosols, but sulfate is still a major compo-...

Patterson, R. K., and J. Wagman (1976). Mass and composition of an urban aerosol as a function of particle size for several visibility levels. Prepr. Pap. NatL Meet. Div. Environ. Chem., Am. Chem. Soc. 14, 102-107. 

The two most widely used receptor models in industrialized regions are the CMB and various forms of factor analysis (Hopke 1986). The CMB model requires information about the composition of the contributing aerosol sources in the model region. A great achievement of the CMB method was the identification of road dust as a major contributor to the urban aerosol mass (Cooper 1980) as well as the identification of wood combustion as an important aerosol source (Core etal. 1981). Major drawbacks of the CMB method arise when... 

The concentration of atmospheric aerosols varies considerably in space and time. This variability of the aerosol concentration field is determined by meteorology and the emissions of aerosols and their precursors. For example, the annual average concentration of PM2.5 in North America varies by more than an order of magnitude as one moves from the clean remote to the polluted urban areas of Mexico City and southern California (Figure 8.24). Sulfate dominates the fine aerosol composition in the eastern United States, while organics are major contributors to the aerosol mass everywhere. Nitrates are major components of the PM2.5 in the western United States. The EC makes a relatively small contribution to the particle mass in many areas, but because of its ability to absorb light and its toxicity, it is an important component of atmospheric particulate matter. 

Each mode has different sources, size range, formation mechanisms, and chemical compositions. Each can be described by a lognormal distribution, see Table 14.3. Figure 14.2a shows the average urban aerosol size distribution on a log-4og graph. The combined distribution, represented by the solid line, masks the contributions of the individual modes, shown by dashed lines and does not show the nature of the surface or mass distributions. 

When a liquid or solid substance is emitted to the air as particulate matter, its properties and effects may be changed. As a substance is broken up into smaller and smaller particles, more of its surface area is exposed to the air. Under these circumstances, the substance, whatever its chemical composition, tends to combine physically or chemically with other particles or gases in the atmosphere. The resulting combinations are frequently unpredictable. Very small aerosol particles (from 0.001 to 0.1 Im) can act as condensation nuclei to facilitate the condensation of water vapor, thus promoting the formation of fog and ground mist. Particles less than 2 or 3 [Lm in size (about half by weight of the particles suspended in urban air) can penetrate the mucous membrane and attract and convey harmful chemicals such as sulfur dioxide. In order to address the special concerns related to the effects of very fine, iuhalable particulates, EPA replaced its ambient air standards for total suspended particulates (TSP) with standards for particlute matter less than 10 [Lm in size (PM, ). 

The first scientists to investigate the coastal atmospheric presence of APs were Van Ry and Dachs, in a study conducted in the Hudson river estuary. GC-MS analyses showed that atmospheric NP isomer mixtures have a similar composition to technical mixtures, with relatively high total concentrations in the range of 0.0002—0.069 xg m-3 in the gas phase, and 0.0001-0.051 p,gm-3 in the aerosol phase. These concentrations are higher than those of polycyclic aromatic hydrocarbons and up to two orders of magnitude higher than polychlorinated biphenyl concentrations in impacted urban-industrial areas [32]. 

In previous studies we found that improved emission-control devices (such as hot-side electrostatic precipitaters and wet-scrubber systems) now being installed on modern pulverized-coal-fired power plants modify the quantity, chemical composition, and distribution characteristics of fine aerosol emissions (12,13). Such modifications must be understood to adequately assess human health and environmental hazards, and to apportion the contributions of sources to urban pollutant inventories. 

TABLE 10.25 Mass Concentrations of Individual Genotoxic PAHs and PACs in the Whole Sample and in Subfractionated Extracts,1 Their Absolute Mutagenic Potencies and Potencies Relative to Benzotalpyrene = 1.00, and Contributions of Each to the Overall Total Potency of a Composite Mixture of Ambient Aerosols Collected at Four Urban Sites in Southern California for One Year (1993) and Subjected to Bioassay-Directed Chemical Analysis Using the hlAlv2... 

TABLE 10.26 Concentrations, Potencies, and Percentage Contributions of Selected PAHs and PACs Tested in the hlAlv2 Human Cell Assayb and Positively Identified in the Bioassay-Directed Chemical Analysis of a Composite Whole Sample Made Up of Fine Ambient Aerosols Collected at Four Urban Sites in the Los Angeles Air Basin 1 0... 

Gschwend and Hites (1981) observed that the two closely related polycyclic aromatic hydrocarbons, phenanthrene and anthracene, occur in a ratio of about 3-to-l in urban air. In contrast, sedimentary deposits obtained from remote locations (e.g., Adirondack mountain ponds) exhibited phenanthrene-to-anthracene ratios of 15-to-l. You hypothesize that these chemicals are co-carried in aerosol droplets from Midwestern U.S. urban environments via easterly winds to remote locations (like the Adirondacks) where the aerosol particles fall out of the atmosphere and rapidly accumulate in the ponds sediment beds without any further compositional change (i.e., the phenanthrene-to-anthracene ratio stops changing after the aerosols leave the air). If summertime direct photolysis was responsible for the change in phenanthrene-to-anthracene ratio, estimate how long the aerosols would have to have been in the air. Comment on the assumptions that you make. What are your conclusions ... 

---