  Ambient exposure to fine particulate matters of diameters smaller than
2.5{\mu}m (PM2.5) has been identified as one critical cause for respiratory
disease. Disparities in exposure to PM2.5 among income groups at individual
residences are known to exist and are easy to calculate. Existing approaches
for exposure assessment, however, do not capture the exposure implied by the
dynamic mobility of city dwellers that accounts for a large proportion of the
exposure outside homes. To overcome the challenge of gauging the exposure to
PM2.5 for city dwellers, we analyzed billions of anonymized and
privacy-enhanced location-based data generated by mobile phone users in Harris
County, Texas, to characterize the mobility patterns of the populations and
associated exposure. We introduce the metric for exposure extent based on the
time people spent at places with the air pollutant and examine the disparities
in mobility-based exposure across income groups. Our results show that PM2.5
emissions disproportionately expose low-income populations due to their
mobility activities. People with higher-than-average income are exposed to
lower levels of PM2.5 emissions. These disparities in mobility-based exposure
are the result of frequent visits of low-income people to the industrial
sectors of urban areas with high PM2.5 emissions, and the larger mobility scale
of these people for life needs. The results inform about environmental justice
and public health strategies, not only to reduce the overall PM2.5 exposure but
also to mitigate the disproportional impacts on low-income populations. The
findings also suggest that an integration of extensive fine-scale population
mobility and pollution emissions data can unveil new insights into inequality
in air pollution exposures at the urban scale.
