Once inside the leaf, however, ozone is highly reactive and can destroy the leaf cells, which can substantially reduce crop yield. Open-topped enclosures and ZAPS are useful for detailed research studies, but they do not provide information on O 3 damage over large areas. Networks of instruments that continuously record O 3 concentrations exist in many populated regions but are sparse in rural areas. They are set out throughout a region and then inspected regularly for flecks of dead-tissue, which are symptoms of injury from O 3 . The biomonitors therefore provide an estimate of O 3 absorption by leaves and indicate potential damage to other less-sensitive crops, even though these may show no visual signs of stress.
Passive monitors are simply filter papers treated with indigo dye.
The degree of color change provides an index of the total exposure to O 3 during the period. Diagram of O 3 flowing into a leaf via a stomate and causing damage by oxidizing cell walls and mesophyl Observations from biomonitors and passive monitors can be related to potential crop effects by placing these monitors inside a ZAPS along with other crop plants. Using these relationships, scientists can use biomonitors and passive monitors placed throughout a region to estimate yield effects of O 3 absorption throughout that area.