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1.2.1. Introduction

Although there are hundreds of sources of outdoor air pollution, the source categories that are the largest contributors to most air pollutants in many locations are: vehicle emissions; stationary power generation; other industrial and agricultural emissions; residential heating and cooking; re-emission from terrestrial and aquatic surfaces; the manufacturing, distribution, and use of chemicals; and natural processes (Unger et al., 2010). Given the large differences in the number and density of these sources as well as in their design, fuel source, and effectiveness of emission control technology, the relative contribution of these sources to air pollution concentrations and exposures varies considerably across locations.

Daily, weekly, and seasonal changes in source activity, as well as meteorological factors, can also lead to very large changes in the temporal trends in atmospheric pollutant concentrations and the relative contributions from different sources.

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Sources of air pollutants can be divided into several types. These can be helpful in understanding the spatial and temporal distribution of source emissions, which has a large impact on exposures to emissions from different sources. Sources are commonly classified into three broad groups: primary, secondary, and re-emission sources. A primary source results from the direct emissions from an air pollution source. In contrast, a secondary source results from the formation of a pollutant in the atmosphere from the chemical reaction of precursors emitted from air pollution sources. Finally, a re-emission source results from primary or secondary pollutants depositing on the Earth’s terrestrial or aquatic surfaces, followed by re-emission to the atmosphere.

Not all pollutants fall exclusively into one group, but in many locations, the classification of a pollutant into these categories can provide insight into exposure gradients. Secondary and re-emission sources tend to have smaller temporal and spatial concentration gradients than primary sources, due to the physical processes controlling their emissions. Primary sources can be further subdivided into point sources, mobile sources, and area sources. Point sources’ emissions are from emissions stacks and tend to lead to very large spatial and temporal gradients in concentration. Mobile sources are associated with transportation and tend to have large spatial gradients close to roadways but tend to be more homogeneous away from roadways in urban areas. Area sources are sources with relatively dispersed emissions over large areas and lead to relatively constant source contributions over space but can have very large temporal changes in emissions. In addition, fugitive sources, including VOCs and dust, result from the leakage of gases from storage and handling facilities and the resuspension of dust, respectively. The nature of these source categories leads to source contributions and exposures that can be parameterized with physical and statistical models to represent pollutant concentrations, given knowledge of emission factors.

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Estimates of the source contribution to pollutant concentrations in the atmosphere and to exposures can be obtained with transport models, receptor models, or hybrid models that integrate aspects of transport models and receptor models. Transport models use emissions inventories along with mathematical representations of wind speed and direction to estimate pollutant concentrations over time and space. Receptor models use measurements of pollutants at a given location or from personal exposure measurements to elucidate the sources of the pollutants (EPA, 2014; European Commission, 2014). Reasonable confidence in source apportionment models usually requires agreement between transport and receptor models, but this is not always achieved if the applied models are not adequately developed.

In locations or scenarios where transport and receptor models have not been developed, the use of emissions inventories and source-specific intake fractions can provide reasonable estimates of exposures and the sources of the exposures.

Table 1.3 provides a global anthropogenic emissions inventory of key global pollutants by sector in 2000. On a global average, the power and industry sectors were the two major anthropogenic sources of SO2 emission. These two sectors together with biomass burning and on-road transportation also contributed greatly to NOx emission. Biomass burning, household biofuel, on-road transportation, and industry were the most important sources of carbonaceous emissions, including CO, BC, OC, and VOCs (Unger et al., 2010). It is important to note that the relative source contribution and absolute source contribution to these pollutants vary considerably across different regions of the world, across urban areas, and across seasons.

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