Evaluating the Relation Between Ozone, NOx and Hydrocarbons: The Method of Photochemical Indicators

The method of photochemical indicators is a way to evaluate the sensitivity of O3 to its two main precursors, nitrogen oxides (NOx) and reactive organic gases (ROG), directly from ambient measurements. The method is based on identifying measureable species or species ratios that are closely associated with O3-NOx-ROG predictions in photochemical models. Several species of this type have been identified: O3/NOy (where NOy is total reactive nitrogen), O3/NOz (where NOz=NOy-NOx, O3/HNO3, H2O2/HNO3, H2O2/N0z, and H2O2/NOy’ In each case, high values of the proposed indicator are associated with NOx-sensitive chemistry in models and low values are associated with ROG-sensitive chemistry. This report presents a summary of O3NOx-ROG sensitivity in models and the chemistry that motivate the choice of species as photochemical indicators. It shows the correlation between model NOx-ROG predictions and indicator values for a variety of models, including simulations for Lake Michigan, the northeast corridor, Atlanta and Los Angeles. The indicator NOx-ROG correlation remains consistent in model scenarios with radically different assumptions about anthropogenic and biogenic emission rates and meteorology and in models with different chemical mechanisms. The report shows correlations between O3, NOz and H2O2 in models and in ambient measurements, which provide a basis for evaluating the accuracy of critical assumptions associated with the indicator method. Case studies are described for Atlanta and Los Angeles in which measured values of indicator ratios were used as a basis for evaluating model scenarios. It is shown in which model scenarios with different NOx-ROG predictions often give similar values for peak O3 but different values for indicator ratios. The case studies illustrate how comparisons between model results and measured indicator values can be used as a basis for model evaluation. Uncertainties associated with the indicator method (including measurement uncertainties, dry deposition and surface effects) are discussed.