Variability of Indicator Values for Ozone Production Sensitivity: A Model Study in Switzerland

The threshold values of indicator species and ratios delineating the transition between NOx and VOC sensitivity of ozone formation are assumed to be universal by various investigators. However, our previous studies suggested that these values may vary according to the locations and conditions. In this study, threshold values derived from various model simulations in Switzerland by the UAM Model are examined using a new approach. Possible definitions for the distinction of NOx and VOC sensitive O3 production regimes are given. The dependence of the threshold values of indicators and indicator ratios such as NOy, O3/NOz, HCHO/NOy, and H2O2/HNO3 on the definition of NOx and VOC sensitivity is discussed. Then, the variation of threshold values under two different meteorological and emission conditions is examined to check how the model responds to changes in environmental conditions. Both perturbed cases – i.e. low emissions and less-stagnant meteorology- lead to similar shifts in threshold values towards more NOx sensitive chemistry. O3/NOz and HCHO/NOy are predicted to be unsatisfactory as indicators. Although H2O2/HNO3 provides a good separation of NOx and VOC sensitive regimes, threshold values for this indicator ratio are affected by emission and meteorological perturbations studied in this work