Figure 1 Measurements of trichloroethane, an industrially- produced, ozone-depleting chemical, in the atmosphere at four different sites. A marked downturn following the 1987 Montreal Protocol that banned use of this chemical substance is clearly apparent at all stations, at different latitudes from North to South. The measurements are shown in parts per trillion (10¹²) of the volume of air. From R. Prinn et al. , Science, vol 269, pp 187-192, 1995.