26 April 2016
By Larry O’Hanlon
Trucks idling at a rest stop or tractors plowing fields could be producing more of a hazardous pollutant than big rigs roaring down the highway, according to a new study.
New research finds exhaust from idling diesel engines produces a significant amount of isocyanic acid when photons from sunlight help it react with other compounds in the atmosphere. The amount of this secondary photochemical isocyanic acid produced by non-road, idling diesel engines, like those in tractors, loaders, and other heavy construction and farm equipment, was 50 to 230 milligrams per kilogram of diesel burned, according to the new study. That comes to about 150 to 700 milligrams per gallon burned. That’s also two to six times greater than the amount of isocyanic acid produced by diesel engines running at higher speed and under a load, like cars and trucks driving on a highway, the new research found.
These emissions are likely contributing to the high levels of isocyanic acid observed by other research groups in urban areas, said Delphine Farmer of the Department of Chemistry at Colorado State University in Fort Collins and co-author of the new study published online in Geophysical Research Letters, a journal of the American Geophysical Union.
For instance, the concentrations of isocyanic acid seen in downtown Los Angeles (0.2 parts per billion) are approaching levels thought to be harmful to humans, Farmer said. A breeze with just one part isocyanic acid per billion air molecules has been linked by other studies to biochemical reactions that can lead to cardiovascular disease, cataracts, atherosclerosis, kidney failure and rheumatoid arthritis. The new study suggests urban regions with large amounts of combustion from non-road diesel vehicles may be experiencing high concentrations of isocyanic acid that are potentially damaging to human health, Farmer said.
“We saw a lot of things we expected, but the isocyanic acid really jumped out as a huge signal,” she said.
Cigarette smoke, and smoke from wood and biomass burning are among the most common sources of isocyanic acid. The compound is also released by other forms of burning, like from cars and trucks, but the amount of isocyanic acid generated as a secondary product of exhaust has not been well studied and is not fully understood, according to Farmer. Isocyanic acid is not a significant part of diesel exhaust when it leaves the tailpipe. Rather, it’s a secondary product found after the exhaust has aged for a day and reacted with other compounds in the atmosphere with the help of ultraviolet light from the sun.
“(Isocyanic acid) hasn’t been found as a significant secondary product before because sensitive techniques for measuring the compound in real-time didn’t exist until about eight years ago,” said Jeremy Wentzell, an air quality researcher for the Canadian government’s Environment and Climate Change Canada, who was not involved in the new study.
Farmer and her colleagues discovered the increase in this photochemical isocyanic acid while they were conducting experiments studying the differences between diesel and biodiesel exhaust from a 4-cylinder, turbocharged, intercooled, heavy-duty John Deere 4045H engine. They subjected the exhaust samples to laboratory conditions that mimic the sunny, open air for one and a half days. They did not see a large difference in the amount of isocyanic acid generated by the two different types of exhaust, but they did see a big difference in the amount of isocyanic acid coming out of idling and non-idling engines.
“There were two things we learned,” said Farmer of the experiments. “We don’t see a significant difference between diesel and biodiesel, and if you idle your vehicle the combustion is incomplete.
What we think is happening is the precursors of isocyanic acid are increased in incomplete combustion.”
Further, all non-road diesels produced after 2015 have selective catalytic (SCR) systems to reduce emissions of nitrogen oxides, a precursor to ground-level ozone. SCR systems have been shown to produce isocyanic acid in the process of reducing nitrogen oxides, and engines with these systems release more isocyanic acid than those without the SCR system, according to the new study.
Farmer said the study shows the way engines are run can have big impacts on secondary pollutants. Emissions from non-road diesel engines, which have received less attention than emissions from on-road vehicles, could be a significant source of some pollutants and need to be studied more closely, she said.
It also means that she and her colleagues need to do the same experiments with some other makes and models of diesel engines to see if their exhaust follows the same pattern, Farmer added.
“This work is nice in that it confirms with laboratory results (not simply field observations) that there is a secondary photolytic source of isocyanic acid that has been hinted at by other publications,” said Wentzell. “This study pins down a secondary source, although I suspect there are also others.”
— Larry O’Hanlon is a freelance science writer and manages AGU’s blogs and social media.