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Intake Fraction

Air pollution is regulated and controlled primarily to protect human health. Accordingly, emissions sources that cause high human air pollution exposures are of special concern. Emissions sources vary dramatically in the degree to which they influence exposures. For example, one tonne of vehicle emissions in a crowded city would result in much higher exposure than the same quantity of emissions from a ship in mid-ocean.

In general, the efficiency with which a population inhales a pollution source's emissions is a strong function of several properties, including the size and proximity of the exposed population and the microenvironmental and/or atmospheric persistence of the emitted pollutant. These considerations can be summarized with a single metric, intake fraction (iF), which represents the fraction of a source's emissions that are ultimately inhaled by all exposed individuals.

Global modeling of iF for urban vehicle emissions

Intake fraction map(with Bill Nazaroff and Julian Marshall)

Among key sources of outdoor air pollution, urban motor vehicles have especially high iF, since they emit at ground level in close proximity to populations. Previous studies have demonstrated that iF for primary urban vehicle emissions varies among cities owing to differences in urban form and meteorology. However, the vast majority of existing iF data are available only for cities in high income regions (e.g., North America, Europe).

To understand gobal patterns of iF for urban vehicle emissions, we modeled iF for primary, distributed ground-level air pollutant emissions in 3,646 worldwide urban areas. This dataset includes nearly all cities with year-2000 population ≥ 100,000, with a combined total of > 2 billion inhabitants. The analysis employed a scalable single-compartment Eulerian model, which we configured for each city using global meteorological and demographic datasets.

The analysis shows that the global average intraurban iF for vehicle emissions of primary, conserved pollutants (39 ppm) is roughly twice as large as the mean iF in US cities (21 ppm) and in other "land-rich" high income countries (16 ppm). The intraurban iF is high in rapidly motorizing countries such as China and India (mean iF ~ 44 and 51 ppm). We found very high iF for the 11 "megacities" in Asia, such as Beijing, New Delhi, and Dhaka (mean iF: 110 ppm, range: 67 - 260 ppm).

A policy implication of this research is that efforts to reduce vehicle emissions in high-iF cities (such as many cities in Asia) may yield especially high population exposure benefits per unit of mitigation.

Journal article: Apte JS, Bombrun E, Marshall JD, Nazaroff WW, 2012. Global intraurban intake fractions for air pollutants from vehicles and other distributed sources. Environmental Science & Technology 46, 3415-3423. [link, open access]
Data available for download
High resolution map of intraurban intake fraction [2.2Mb]
Entire global dataset (700 kb) available upon request via email: jsapte {at} utexas {dot} edu.