Ozone response to precursor controls in very complex terrains: Use of photochemical indicators to assess O3‐NOx‐VOC sensitivity in the northeastern Iberian Peninsula

Abstract

The kinetics of ozone (O3) chemistry and its two main precursors, nitrogen oxides (NOx) and volatile organic compounds (VOC), represents an important field of uncertainty in atmospheric chemistry and photochemical modeling. This uncertainty affects the design of control strategies to reduce tropospheric O3 production. The effect of controlling ozone precursors on sensitivity regimes was evaluated by performing simulations with MM5‐EMICAT2000‐CMAQ model to represent O3 formation in the northeastern Iberian Peninsula with baseline emission rates for VOC and NOx, and reducing anthropogenic VOC and NOx emissions on a 35%. Three different scenarios were considered in order to assess chemical sensitivity in urban, industrial, and background domains. Areas downwind of the city of Barcelona benefit from NOx reductions (reduction of 10 ppb in ground‐level O3), while the same reduction causes an important increment of O3 in Barcelona (9 ppb) and the area downwind of Tarragona (18 ppb), with a high industrial influence. The city of Barcelona benefits from VOC reductions (10 ppb of O3) as well as the industrial zone of Alcover (20 ppb). The rest of the domain is practically insensitive to VOC reductions. Diverse photochemical species derived from the air quality model were used as indicators in order to establish the chemical sensitivity regime existing in the areas, analyzing whether O3 values reacted consistently to similar changes in emissions. Results showed that NOy (total reactive nitrogen) and O3/NOy are the indicators involving a lower uncertainty when assessing sensitivity, according to the narrow transition regime between NOx‐ and VOC‐sensitive chemistry and the low uncertainty observed. Indicators performed better in VOC‐sensitive than in NOx‐sensitive domains. H2O2‐ and HNO3‐ derived indicators entailed higher uncertainties since transition regimes between NOx and VOC sensitivity covered a wide range. The extent of reaction also performed as a good indicator to separate NOx‐ and VOC‐sensitive regimes. This sensitivity analysis provides a tool for establishing hypothetical control policies for O3 precursor emissions through a comparison of simulated correlation between the species.