Vehicle emissions from high traffic roads are recorded in roadside weeds - and show that traffic pollution is concentrated in poorer neighborhoods
When you collect ecological data in cities you quickly observe that neighborhood demographics like race, ethnicity, and income are correlated with all kinds of variables. For example, both tree cover and biodiversity are greater in wealthier neighborhoods. Researchers can often trace the underlying causes of these relationships, like racist redlining housing policies that have left a legacy of inequitable urban tree amenities.
Sometimes though, we find relationships that are difficult to explain. A few years ago, my former Ph.D. student La'Shaye Cobley found that the ratio of heavy to light isotopes of nitrogen in the leaves of urban trees was correlated with household income in two different U.S. cities. There were several possible explanations for this finding. Most inorganic fertilizer has a distinctive isotopic signature, and different amounts of fertilizer applied to urban gardens or found in stormwater runoff could explain neighborhood differences in leaf isotopes. Atmospheric pollution also has a unique isotopic "fingerprint," so greater pollution in poorer neighborhoods could also be an explanation. It's long been known that race and class demographics are correlated with the location of point sources of pollution, like factories, incinerators, and toxic waste.
La'Shaye's data suggested that the distribution of air pollution was the likely explanation for the relationship between household income and leaf isotopes. But the original study of tree leaves wasn't designed to test for this. So she created a new study of high and low income neighborhoods in the Salt Lake City metropolitan area. To keep the sampled plant species the same in all neighborhoods, she measured nitrogen isotopes in a common species of weed: Convolvulus arvensis, a ubiquitous bindweed found throughout the urban area. To account for the effects of high and low traffic roads on air pollution, she obtained traffic data from the Utah Department of Transportation and sampled both and high and low trafficked roads in each neighborhood. She also measured air pollution directly with a mobile NOx and ozone sensor mounted in a vehicle.
The results showed that there was a relationship between leaf chemistry, average neighborhood income, and air pollution. This time, the cause was clearer: low income neighborhoods had considerably more high traffic roads than high income neighborhoods. So it isn't just point sources of pollution that are inequitably located. The urban road network is a significant cause of environmental injustice, and air pollutants will be concentrated in low income neighborhoods until inequities in road infrastructure and emissions are addressed.
La'Shaye has since finished her Ph.D. and is now working on emissions reductions from the freight sector at the California Environmental Protection Agency. Her paper was published by the Journal of Geophyical Research - Atmospheres.