Nitrogen oxides – collectively known as NOx compounds – react with hydrocarbons to form a variety of nitrogen-containing pollutants, including nitric acid, the cause of acid rain.
Until recently, however, as much as half of the nitrogen oxides resulting from emissions have been unaccounted for in the atmosphere, leaving air pollution models incomplete.
Now though, chemists led by Professor Ronald Cohen at the University of California, Berkeley, think they have found the missing nitrogen oxides with the aid of the most sensitive detector of nitrogen dioxide (NO2) in the world.
The new sensor works by flash heating an air sample at the tip of an inlet to convert NO-compounds to nitrogen dioxide (NO2), and then the amount of NO2 is measured by hitting the sample with a tunable dye laser and measuring the amount of fluorescence.
By flash heating the air at different temperatures, the new device takes advantage of the fact that different NO compounds decompose to NO2 at different temperatures. Thus it is possible to measure separately the levels of nitric acid, peroxynitrates, and the sum of all alkyl and hydroxyalkyl nitrates.
The technique, which Cohen refers to as thermal dissociation-laser induced fluorescence (TD-LIF), can monitor NO compounds continuously with sensitivity down to 30 parts per trillion – 1000 times more sensitive than needed for today’s pollution monitoring.
Deploying the detector in downtown Houston and in a remote Sierra Nevada forest, Cohen and his colleagues detected large amounts of organic nitrogen oxide (NO) compounds, alkyl nitrates, that were thought to be only a minor constituent of smog. In the forest, these alkyl nitrates included chemicals such as isoprene nitrate, which could only have come from combining hydrocarbons emitted by trees with tailpipe emissions of NOx, presumably from the city of Sacramento, which is upwind of the forest. In Houston, other alkyl nitrates are formed by combining NOx with industrial hydrocarbon chemicals.
‘Nitrogen oxide radicals are the major species controlling production of photochemical smog and subsequent chemical reactions in the atmosphere,’ said Proifessor Cohen.
‘Until now, though, no one had really looked at what are, in some places, the most abundant NO-containing chemicals in the atmosphere.’
Cohen is now trying to make a more compact and cheaper nitrogen oxide detector for routine air pollution monitoring in urban areas.