The use of NOy, H2O2, and HNO3 as indicators for ozone‐NOx‐hydrocarbon sensitivity in urban locations

Correlations are presented between model predictions for O 3 ‐NO x ‐hydrocarbon sensitivity and afternoon concentrations of four “indicator species”: NO y , O 3 /(NO y ‐NO x ), HCHO/NO y , and H 2 O 2 /HNO 3 . The indicator species correlations are based on a series of photochemical simulations with varying rates of anthropogenic and biogenic emissions and meteorology. Hydrocarbon‐sensitive chemistry in models is shown to be linked to afternoon NO y > 20 ppb, O 3 /(NO y ‐ NO x ) < 7, HCHO/NO y < 0.28, and H 2 O 2 /HNO 3 < 0.4. Lower NO y and higher ratios correspond with NO x ‐sensitive ozone. The correlation between NO x ‐hydrocarbon sensitivity and indicator species remains, even when model emission rates and hydrocarbon/NO x ratios are changed by a factor of 2. Methods are developed for evaluating the goodness of fit between model NO x ‐hydrocarbon sensitivity and indicator values. Ozone chemistry is also analyzed in terms of fundamental properties of odd hydrogen, and theoretical criteria for the transition between NO x ‐ and hydrocarbon‐sensitive regimes are derived. A theoretical correlation between O 3 and H 2 O 2 + NO y ‐ NO x is developed as a way to extend rural O 3 ‐NO y correlations into urban locations. Measured indicator values during pollution events in Los Angeles, Atlanta, and rural Virginia are used to illustrate the range of observed values under different environmental conditions.