Variability of indicator values for ozone production sensitivity: a model study in Switzerland and San Joaquin Valley (California)

The threshold values of indicator species and ratios delineating the transi tion between NOx and VOC sensitivity of ozone formation are assumed to be u niversal by various investigators. However, our previous studies suggested that threshold values might vary according to the locations and conditions. In this study, threshold values derived from various model simulations at two different locations (the area of Switzerland by UAM Model and San Joaqu in Valley of Central California by SAQM Model) are examined using a new app roach for defining NOy and VOC sensitive regimes. Possible definitions for the distinction of NO, and VOC sensitive ozone production regimes are given . The dependence of the threshold values for indicators and indicator ratio s such as NOy, O-3/NOz, HCHO/NOy, and H2O2/HNO3 on the definition of NOx an d VOC sensitivity is discussed. Then the variations of threshold values und er low emission conditions and in two different days are examined in both a reas to check whether the models respond consistently to changes in environ mental conditions. In both cases, threshold values are shifted similarly wh en emissions are reduced. Changes in the wind fields and aging of the photo chemical oxidants seem to cause the day-to-day variation of the threshold v alues. O-3/NOz and HCHO/NOy indicators are predicted to be unsatisfactory t o separate the NOx and VOC sensitive regimes. Although NOy and H2O2/HNO3 pr ovide a good separation of the two regimes, threshold values are affected b y changes in the environmental conditions studied in this work. (C) 2001 El sevier Science Ltd. All rights reserved.