The use of NO_<i>y</i>, H₂O₂, and HNO₃ as indicators for ozone‐NO_<i>x</i>‐hydrocarbon sensitivity in urban locations

TLDR

The study develops methods to evaluate how well model predictions of NOx‑hydrocarbon sensitivity match indicator species values. The authors use photochemical simulations and theoretical analysis of odd‑hydrogen chemistry to derive indicator species correlations and criteria for NOx‑ versus hydrocarbon‑sensitive ozone regimes, extending rural correlations to urban sites. The study finds that afternoon concentrations of NOy, O3/(NOy‑NOx), HCHO/NOy, and H2O2/HNO3 correlate with model‑predicted O3‑NOx‑hydrocarbon sensitivity, with hydrocarbon‑sensitive chemistry linked to NOy > 20 ppb, O3/(NOy‑NOx) < 7, HCHO/NOy < 0.28, and H2O2/HNO3 < 0.4, and the correlation remains robust under doubled emission rates and varying hydrocarbon/NOx ratios.

Abstract

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 &gt; 20 ppb, O 3 /(NO y ‐ NO x ) &lt; 7, HCHO/NO y &lt; 0.28, and H 2 O 2 /HNO 3 &lt; 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.