Reflectance was important to the researchers as they photographed the birds because they measured the light that was reflected off of the specimens. Black carbon, one of the major components of soot, is a light-absorbing compound, so the images precisely revealed how dirty — or not — the birds tended to be during particular times.
The scientists charted the information, then compared the observed trends to the history of coal burning in the areas where the birds were collected. The findings, reported Oct. 9 in the journal Proceedings of the National Academy of Sciences, reveal that the soot on the birds closely tracks the usage of coal in the US.
The bird images suggest that there was a slight improvement in air quality after 1910. Fuldner said this was “because the reform movements in American cities to combat the problem were starting to mature.”
He shared that the first municipal anti-smoke ordinances, which date to the early 1880s, were mostly symbolic pieces of legislation. They typically exempted domestic chimneys, for example, and were rarely enforced in practice.
“Starting in the 1910s, we start to see a shift in approach, which emphasized education — teaching more efficient burning techniques, for example, rather than simply prosecuting parties who violated a local ordinance,” he said. “Combine that with some incremental technology improvements, such as mechanical stokers for steam boilers, and you start to see modest improvements.”
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The next observed soot reduction happened during the Great Depression (1929–1939), when the burning of coal plummeted across the US. Levels of black carbon on the birds then began to sharply rise during World War II (1939–1945), when wartime manufacturing drove up coal use.
The levels then dropped again after the war, when people in the Rust Belt began to heat their homes with natural gas piped in from the West. Before then, residential furnaces, such as coal-burning stoves, were estimated to produce approximately half of all atmospheric black carbon in the US.
As for how the birds often accumulated soot over the decades, the researchers suspect that it came from both particulate matter in the air and from direct contact with the material in the birds’ environments. The bird specimens included in the study came from numerous types of locations, including those near coal-burning factories like steel mills, residential urban areas, and rural sites.