Bowen says the new technique for measuring the sources of combustion water in urban air “is in the early stages. We have a lot of ideas about potential applications.”
The study didn’t measure how much water vapor came from vehicles versus furnaces. But “our hope is that we will be able to distinguish emissions from different sources based on their contribution to water in the atmosphere over the city,” he adds. The researchers also hope to apply their method to measuring greenhouse gas emissions from wood stoves or industrial combustion sources that produce water vapor, and “we might be able to use it to look at the efficiency of combustion – to diagnose combustion problems in car engines or industrial processes,” Bowen says.
Water vapor makes inversions cold and damp Bowen conducted the University of Utah study with technicians Galen Gorski and Ryan Bares; Courtenay Strong, assistant professor of atmospheric sciences; Stephen Good, postdoctoral researcher in geology and geophysics; and James Ehleringer, distinguished professor of biology. Funding came from the National Science Foundation, U.S. Department of Energy and National Oceanic and Atmospheric Administration. The study dealt with water vapor in the “boundary layer” – the cold, smoggy layer from the ground up to the warmer air layer that caps and traps the inversion. Bowen says water vapor from burning fuels “is not as scary as other stuff that comes out of our tailpipes. But it affects our local environment during these inversions.” “In many or most inversion events humidity increases in the lower atmosphere, and that contributes to the apparent temperature and overnight frost formation,” he adds. “These events feel gray, gloomy and damp compared with the rest of our winter. Part of the reason is combustion-emitted water trapped in the boundary layer at the surface.”
Cocaine, evolution, murder – and now water vapor The new study represents the latest use for stable isotope analysis, a technique that looks at ratios of rare to common weights or isotopes of elements such as hydrogen, carbon, oxygen and nitrogen. The isotopes are stable; they don’t decay radioactively.
University of Utah researchers have used the method to help identify sources of cocaine and counterfeit currency, the diets of early human ancestors and the routes traveled by elephants in Africa, and even to help identify a murder victim based on isotope analysis or hair that pointed police to the region where the victim had lived. In natural water, the ratio of heavier, rare oxygen-18 to lighter, common oxygen-16 is low because the heavier isotope falls out first as rainstorms move inland. The same is true of deuterium, rare hydrogen-2, compared with the common isotope, hydrogen-1. Water produced from burning or combustion is different. The ratio of hydrogen-2 to hydrogen-1 is very low, because hydrogen in fuel comes from ancient plants and microbes that preferred hydrogen-1. But the ratio of oxygen-18 to oxygen-16 is much higher compared with natural water. That’s because burning fuel uses oxygen in air – oxygen produced by plant leaves from which heavier oxygen-18 evaporated more slowly than lighter oxygen-16. Bowen and colleagues used this unusual signature to devise a scale on which they can estimate the amount of combustion-derived vapor in any air sample. The approach works best in inversions during Utah winters, but the researchers believe they may also be able to detect water from combustion with careful measurements during times without inversions. From Dec. 3, 2013 to Jan. 31, 2014 – a period with four inversions – Bowen and colleagues measured carbon dioxide and water vapor concentrations and water vapor isotope ratios an average of every five minutes. They found the amount of combustion water vapor in the air tracked closely with the amount of carbon dioxide. During each inversion, both increased. During three of the inversions, both gases leveled off or fell when the inversions mixed somewhat with cleaner air. Combustion water vapor and carbon dioxide climbed from 7 to 10 a.m. due to traffic, began dropping at 10:30 a.m., then rose during the evening rush, peaked by 8 p.m. and remained level until midnight as furnaces worked. The vapor and gas levels dropped by 3 a.m., due to canyon winds and as dew and frost reduced water in the air. The researchers estimated oxygen and hydrogen isotope composition of water from burning natural gas and gasoline, confirming their estimates for gasoline by testing tailpipe water from an inefficient old SUV and younger, more efficient sedan. They calculated how much water from tailpipes must be added to the air to produce the levels seen in the air during inversions. They estimated conservatively that up to 13 percent of water vapor in inversions comes from burning fuels. That is a large percentage given that combustion water is only 0.004 percent of the global water cycle. Co-author Strong created a computer model to simulate how water enters a Salt Lake City inversion from wind, rain, snow and combustion emissions, how those contribute to air moisture. The model predicted what researchers saw, including the daily patterns of traffic and furnace water vapor.
The peak in combustion water vapor during the overnight home-heating period was about half that as after morning and evening rush hours, largely because modern furnaces condense water as they extract heat.
“We might use this new tool to understand where the carbon dioxide emissions are coming from,” Bowen says. “Emissions of carbon dioxide from different sources will produce different amounts of water vapor.” The same may prove true of other combustion pollutants such as fine particulates and nitrous oxides, eventually allowing water vapor to be used to better track their sources as well.
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Proceedings of the National Academy of Science, online week of March 2, 2015; National Science Foundation, EF-01241286,EF-01240142; U.S. Dept. of Energy DE-SC-001-0624; National Oceanic and Atmospheric Administration NA14OAR4310178