The smell of sea salt at the beach is a pleasant thought for many beach goers. Wind and waves kick up spray sending salt (sodium chloride into the air. Most salt of this sort falls back into the sea or nearby beach. The bit of chloride lingering in the air can react with nitrogen oxides (NOx) to form nitryl chloride which is a forerunner of chlorine gas, the most reactive form of chlorine. Those gas can contribute to smog formation in coastal areas. However, in a surprise, researchers have found that this air chemistry thought to be restricted to sea spray occurs at similar rates in the air above Boulder, Colorado which is nearly 900 miles away from any ocean. What's more, local air quality measurements taken in a number of national parks across the United States imply similar conditions in or near other non-coastal metropolitan areas.
The smell of sea salt at the beach is a pleasant thought for many beach goers. Wind and waves kick up spray sending salt (sodium chloride into the air. Most salt of this sort falls back into the sea or nearby beach. The bit of chloride lingering in the air can react with nitrogen oxides (NOx) to form nitryl chloride which is a forerunner of chlorine gas, the most reactive form of chlorine. Those gas can contribute to smog formation in coastal areas. However, in a surprise, researchers have found that this air chemistry thought to be restricted to sea spray occurs at similar rates in the air above Boulder, Colorado which is nearly 900 miles away from any ocean. What's more, local air quality measurements taken in a number of national parks across the United States imply similar conditions in or near other non-coastal metropolitan areas.
!ADVERTISEMENT!It has been found in the last few years that NOx and smog along our coastlines may be worsened by chemical reactions that occur when NOx emissions combine with ocean air (salt).
Hans Osthoff of the University of Calgary, Canada, and colleagues for example, measured unexpectedly high levels of the pollutant nitryl chloride (ClNO2) in sea air, during a six week trip in a research vessel along the coastline of the Gulf of Mexico around Houston, Texas. The offending halogen breaks down into chlorine radicals that can produce ozone (O3) - a major component of smog.
NO2 and NO3 form dinitrogen pentoxide (N2O5), which previous studies have suggested reacts with chloride on the surface of sea salt aerosol particles to form nitryl chloride. Nitryl Chloride is highly reactive and breaks down easily under sunlight.
After sea spray, the largest global source of chlorides is coal burning, with biomass burning not far behind. Potential sources of chloride in the Boulder Denver area include smoke from fireplaces, chemicals used on icy winter roads or even air drifting in from giant salt flats in Nevada and Utah.
"It's there. We know it's there. But we don't have a good handle on where that chloride comes from," said Joel Thornton, a University of Washington associate professor of atmospheric sciences and lead author of a paper documenting the findings, published March 11 in Nature.
In February 2007, a team including Thornton prepared to set out from Boulder for a research cruise from Long Island Sound to Iceland via Norway. The plan was to sample nitryl chloride levels in marine air, which early computer models predicted would not exceed 50 parts per trillion.
Before leaving, the scientists decided to test the equipment they would use to detect airborne nitryl chloride on the cruise by sampling the air in Boulder, a mile above sea level.
"That night when we just nonchalantly stuck our tube out the window, we were getting readings of 500 parts per trillion in Boulder," Thornton recalled. Those levels turned out to be comparable to what the scientists later recorded on the research cruise, indicating the previous computer models greatly underestimate nitryl chloride in the air near the Earth's surface.
The researchers returned to Boulder in 2009 to take more comprehensive measurements from a park 150 feet above the city, away from obvious chloride sources. They confirmed their earlier observations, and they gathered further confirmation from the national park air quality monitoring systems.
The research focuses on a specific form of nitrogen oxide present only at night (during the day it is broken down by even the faintest sunlight). It is commonly thought that much of the ozone and haze forming pollutants generated in metropolitan areas during a busy weekday are removed from the air during hours of darkness. The new research calls that into question, Thornton said.
The work suggests the nighttime form of nitrogen oxides reacts with haze particles containing chloride to form nitryl chloride, which in turn forms chlorine atoms and regenerate the smog forming nitrogen oxides when the sun rises.
"Because of these impacts, we'd like to know what happens to these nitrogen oxides in nighttime air, where do they go, what do they do," Thornton said. He acknowledged that unraveling and understanding the ramifications of the findings will not be simple.
The presence of wide spread nitryl chloride indicates how little understood this phenomena is as well as how exactly it reacts. The source of the nitryl chloride may be sea mist but it may well be something else.
For further information: http://uwnews.org/article.asp?articleid=56206
