Last weekend I was asked about the brown haze in the sky over Gladstone.

Around this time of year there is often a light orange or brown layer in the air, usually over the ocean. This discolouration is caused by NOx, which is pronounced “Knox” as in Fort Knox. NOx is chemical shorthand for a group of several types of nitrogen oxides.

So where does NOx come from? NOx is created when fossil fuels like coal, liquid natural gas, petrol or diesel are burnt at high temperatures. When these fuels are burnt at between 1,300 – 1,500 degrees Celsius, the nitrogen and oxygen in the consumed air can react in several ways to form NOx.

Once released NOx reacts with ozone in the air and ultra violet light from the sun and this creates the telltale brown haze.

While the amount of NOx emitted in Gladstone is fairly constant, the brown haze is much more noticeable in winter. This is because in winter a temperature inversion can trap the NOx.

These inversions happen for several reasons. However, they result in a layer of warm air being caught between two cold air layers, one of which extends to the ground and the other which is above the inversion layer. When this occurs NOx rises until it strikes the warmer inversion layer and, as it cannot rise any further and cannot disperse, it forms a brown haze.

Later in the day after the sun warms the cold air, the inversion layer usually breaks up and this allows the NOx to disperse and the brown haze fades.

The majority of NOx in the Gladstone air shed comes from the Gladstone Power Station and is released through their 153 meter high stacks. This height was chosen as studies showed that these NOx emissions would disperse sufficiently not to impact ground level air quality.

To make sure that this is the case, nitrogen dioxide is one of the five contaminants that the Clean and Healthy Air for Gladstone (CHAG) project team monitors in near real-time. This information is included in the Observer’s daily Air Quality Index.

While air monitoring has shown that nitrogen dioxide levels in Gladstone are consistently less than one-third of the international air quality standard, the final CHAG report will analyse these levels in much more detail.

If you have any questions about NOx, please go to our forum topic.

If you would like to have your say online and don't know how, please click on this link which shows you how to register and make a comment, anonymously if you wish, cheers, Kurt.

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