It turns out the filters put on vehicle exhausts to remove fine particulate material have also unbalanced the chemistry behind ozone formation.

BBC: London's clue to stubborn ozone levels

He acknowledged that any soil used to grow nitrogen-fixing plants or were fertilised for agricultural reasons would result in an increase in gases involved in the formation of ozone.

BBC: Invasive plant 'increases ozone pollution'

However, Dr Hickman was keen to express that the scenario was created to illustrate the plants' ability to contribute to the formation of ozone, rather than being based on projections of what was expected to happen in the future.

BBC: Invasive plant 'increases ozone pollution'