When it comes to air pollution, almost anything you can measure is worse in cities, including oxides of nitrogen (NOx). And, even if you didn’t know before, you surely know now that one of the main sources of urban NOx is diesel vehicles.
Scientists at Cornell University in New York asked: what does this do to plants? To find out, in three consecutive years they grew cuttings of eastern cottonwood, a fast-growing tree, at sites in New York City and in the surrounding countryside – the Hudson Valley, north of New York, and Long Island, east of the city.
The findings were simple, but surprising: in all three years the trees in the city grew twice as big as those in the rural sites. At least three possible causes of this unexpected result seemed likely. First, soils in cities receive much higher inputs of nitrogen (in the form of NOx and also ammonia), mostly from vehicle exhausts.
Nitrogen is a major plant fertiliser, so maybe the soils in cities are more fertile. To test this, the scientists grew some cuttings in the same compost in both locations. They also transplanted some soil and grew some cuttings in the countryside in city soil and vice versa. None of this made any difference – the city trees persisted in growing twice as big.
Another possibility is that cities are warmer (the “heat island” effect) and the air in cities has more CO2. Both have the potential to make plants grow faster. To test this idea, they grew plants in chambers where they could control both temperature and CO2 concentration. Some plants were given city temperature and CO2 and some were given country temperature and CO2. Again the results were plain: plants grown in the different conditions were identical, ruling out CO2 and temperature as the cause of the difference in growth.
Other, more remote possibilities were examined and quickly discarded. Rainfall was the same in the city and in the country, and none of the plants were nibbled much by herbivores. By now the scientists were really puzzled, so they had another look at the air pollution data, and noticed that one pollutant went in the “wrong” direction: there was more ozone in the country. Ozone is toxic to plants in quite low concentrations, so could this be the cause of the difference?
The original growth data suggested it might be, since ozone concentrations at the test sites correlated perfectly with growth: high ozone, low growth. To find out for sure, they grew some more cuttings in chambers at typical city and rural ozone concentrations and – bingo! Plants in low city ozone grew twice as big as plants in high rural ozone.
So it’s clear that ozone is the answer, but what is going on here? Well, ozone chemistry is complicated; nothing in the urban environment releases ozone.
Instead, ozone is a secondary pollutant, formed by the oxidation of other primary pollutants (NOx and hydrocarbons) in the presence of sunlight. But once formed, ozone is removed by reacting with nitric oxide (NO), more abundant in urban areas as a result of traffic fumes. So although less ozone may be produced in the country, it persists for far longer, because the clean air doesn’t contain the chemicals that would destroy it.
Consequently, ozone usually occurs in higher concentrations in summer than winter, and in rural rather than urban areas. Plants are very sensitive to ozone, and ozone pollution causes crop losses worth several hundred million pounds every year in the UK alone. So although the main problem with NOx is that it kills people, that’s not all it does.