air pollution in manhatten

Laser comb upgraded to detect all primary greenhouse gases

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Researchers at the National Institute of Standards and Technology (NIST) have upgraded their existing laser frequency-comb instrument to measure the three primary greenhouse gases in addition to the air pollutants ozone and carbon monoxide.

Previously, the NIST dual-comb technology was able to detect methane; this upgraded version can sense all primary airborne greenhouse gases – nitrous oxide, carbon dioxide and water vapour – as well as the major air pollutants ozone and carbon monoxide. This new system is suitable for monitoring and understanding emissions of these gases, as well as assessing urban air quality.

A frequency comb is an extremely precise 'ruler' equivalent which allows scientists to measure exact wavelengths of light; each 'tooth' of the comb identifies a different wavelength.

These instruments identify the unique signatures of gases by detecting the amount of light absorbed at each wavelength of the broader spectrum as beams trace a path through the air. Other applications include detecting leaks from oil and gas installations and measuring carbon-intensive emissions from livestock.

Comb systems have an advantage over conventional sensors which sample air at specific locations, as they can detect a larger number of gases, in addition to offering greater precision and longer range than similar techniques using other sources of light.

This latest advance shifts the spectrum of light analysed from the near-infrared (slightly longer wavelengths than visible red light) to the mid infrared. This opens up the possibility of identifying more gases (nitrous oxide, ozone and carbon monoxide).

Reaching the mid-infrared part of the spectrum required a bespoke crystal material (periodically poled lithium niobate) for converting light between two wavelengths. The new system split the near-infrared light from one comb into two branches, then shifted and broadened the spectrum of each branch differently with special fibre and amplifiers. Finally, the branches were combined in the crystal material.

The result was mid-infrared light at a longer wavelength that was the difference between the original wavelengths in the two branches.

The NIST researchers demonstrated the new system over round-trip paths with lengths of 600m and 2km. The light from two frequency combs was combined in optical fibre and transmitted from a telescope on top of a NIST building in Boulder, Colorado. One beam was sent to a reflector located on a balcony of another building and a second beam to a reflector on a hill; the comb light bounced off the reflector and returned to the original location for analysis. The system was precise enough to capture variations in atmospheric levels of all of the measured gases, which was affirmed by matching results from a conventional sensor for carbon monoxide and nitrous oxide.

A major advantage in detecting multiple gases at once is the ability to measure correlations between them. For instance, measured ratios of carbon dioxide to nitrous oxide agreed with other studies of emissions from traffic and the ratio of excess carbon monoxide versus dioxide agreed with similar urban studies, but was only one-third the levels predicted by the US National Emissions Inventory.

The NIST measurements, in producing similar results to other studies suggesting there is less carbon monoxide in the air than the NEI predicts, put the first “hard” numbers on the reference levels pollutants in the area.

“The comparison with the NEI shows how hard it is to create inventories, especially that cover large areas, and that it is critical to have data to feed back to the inventories,” said Kevin Cossel, the lead author. “This isn't something that will directly impact most people on a day-to-day basis; the inventory is just trying to replicate what is actually happening. However, for understanding and predicting air quality and pollution impacts, modelers do rely on the inventories, so it is critical that the inventories be correct.”

Researchers plan to extend the reach to longer distances (as already demonstrated for the simpler near-infrared system). They also plan to boost detection sensitivity by increasing the light power, to enable detection of additional gases. Finally, they are working to render the system more compact and robust.

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