Urban air contains more sulfur dioxide than does rural air, and plants in cities typically grow more slowly than do plants in rural areas  In an experiment to see how much of the difference in growth is due to sulfur dioxide, classes in an urban and a rural school grew plants in greenhouses at their schools and filtered the greenhouse air to eliminate sulfur dioxide
Plants in the urban greenhouse grew more slowly than those in the rural greenhouse
Heavy vehicular traffic such as is found in cities constantly deposits grime on greenhouse windows, reducing the amount of light that reaches the plants inside.