Role of concentration and time of day in developing ozone exposure indicesfor a secondary standard

Evidence from exposure-response studies and a turbulent transfer model demo nstrate that plant response is differential to concentration, duration, tem poral pattern, and time of day of exposure. Reductions in productivity of c rops and trees as seedlings are greater when plants are exposed to higher d aytime ozone (O-3) concentrations (0800-2000 hr standard time) or for longe r durations. Primary evidence on the greater role of concentration comes fr om exposure-response experiments where plants are exposed to a series of po llutant concentrations in open-top chambers under field conditions. These s tudies demonstrate that the integrated exposure indices that give preferent ial weight to higher concentrations are better predictors of response than mean or peak indices. Evidence suggesting that midrange O-3 concentrations (0.05-0.09 parts per million, ppm) play a greater role than higher concentr ations (>0.09 ppm) in biological response could not be justified. The time of day when O-3 concentrations and atmospheric and stomatal conductances of gas exchange are optimal is a key to understanding plant response because plants respond only to O-3 entering the leaf via stomata. A turbulent trans fer model that describes the resistance of pollutant gas exchange from the atmosphere to the boundary layer of a forest canopy, as a function of micro meteorological variables, is developed to determine when flux of O-3 is opt imal. Based on meteorological and ambient air quality monitoring data at re mote forest sites in the United States, it appears that O-3 flux densities to the forest boundary layer are optimal during the 0800-2000 hr window. It is concluded that descriptors of ambient air quality for use in setting a federal standard should (1) cumulate hourly O-3 concentrations, (2) give pr eferential weight to daytime concentrations between 0800 and 2000 hr, and ( 3) give preferential weight to higher O-3 concentrations.