To Track An Oxidant

The hydroxyl radical (^•OH) packs a powerful oxidative punch in the atmosphere, but chemists don't fully understand its chemistry in your home or office. The reactive species may produce irritants and play a role in "sick building syndrome." Now researchers report the first measurements of ^•OH in two real-life indoor settings (Environ. Sci. Technol., DOI: 10.1021/es901699a).

In the outdoors, ultraviolet light from the sun fractures ozone to produce ^•OH. Meanwhile, inside where windows block most UV light, ozone reacts with other molecules such as terpenes in air fresheners or cleaning products to produce the radical. Scientists haven't studied ^•OH in indoor settings, so its role in human health is unclear: The radical may scavenge harmful chemicals from the air or it could oxidize organic molecules to produce irritants. 

Also ^•OH is difficult to detect, because its reactivity makes the radical short-lived. As a result, researchers indirectly detect the species by monitoring its reactions with organic tracer molecules.  But this technique has never been applied to indoor environments without tinkering with a room's normal ventilation conditions.

So atmospheric chemist Dudley Shallcross of the University of Bristol in the UK and his colleagues adapted the method to more real-life indoor settings by synthesizing a new tracer, d₅-isoprene. With the deuterium labels, the scientists could distinguish the non-toxic, reactive tracer's products from other chemicals present in the indoor air.

The researchers tested their method by releasing d₅-isoprene into two seminar rooms at the University of Bristol. They then collected air samples in each room and monitored d₅-isoprene’s degradation over time by gas chromatography/mass spectrometry. Based on how much tracer disappeared and the room ventilation rate, the researchers could calculate ^•OH concentrations in each room.

The scientists then used a model of ^•OH formation based on indoor ozone levels to verify their data. After they measured ozone concentrations in each room, they found that the model's estimates matched their tracer calculations in one room. But in the other room, their data exceeded the model's predicted concentrations by more than 10-fold. The discrepancy suggests other chemistry was at work in the room, the researchers concluded: Unexpected organic molecules reacted with ozone to produce more radical, another source of ^•OH such as nitrous acid was present, or their new method measured another oxidant in the air.

Still Charles Weschler of the University of Medicine and Dentistry of New Jersey calls the new method clever and well-conceived, as well as bringing attention to indoor free radical chemistry.