Increasing the amount of nitrogen oxides increases the amount of ozone. But when the scientists looked for increased levels of smog chemistry covering the entire flight corridor, they could not find them, she said. The team flew for more than 100 hours from Bangor, Maine, Azores, and Shannon, Ireland, in a specially equipped NASA DC-8, collecting samples of the air's chemistry. What complicated the gas analyses is that ozone and nitrogen oxides have several ways of working their way six miles up into the atmosphere. The gases can come from below, when man-made smog is funneled up during a thunderstorm.
Ozone can meander down from the stratosphere, where it acts as Earth's ultraviolet shield.
And nitrogen oxides in large amounts can be produced on the spot by lightning strikes. So it becomes very difficult to pin down unequivocally where the nitrogen oxides and ozone are coming from. Thompson found that on the eastern side of the Atlantic, clean air made aircraft exhaust easy to find. But, over Maine and Canada, nitrogen oxides from summer-like thunderstorms and lightning swamped the aircraft signal, leaving scientists to wonder how typical their sampling period was. According to Thompson, when ozone gets that high up, it starts to act like a greenhouse gas and can contribute to global warming.