O₃-NO_x-VOC sensitivity and NO_x-VOC indicators in Paris: Results from models and Atmospheric Pollution Over the Paris Area (ESQUIF) measurements

A three-dimensional photochemical model has been used to interpret aircraft measurements from the Atmospheric Pollution Over the Paris Area campaign near Paris, with special attention to measurements that are related to predicted O₃-NO_x-volatile organic compound (VOC) sensitivity. The model (CHIMERE) includes a representation of ozone formation over Europe and a more detailed spatial representation of the region around Paris. A series of model scenarios were developed with varying wind speeds and emission rates. Comparisons are shown with measured O₃, total reactive nitrogen (NO_y), summed VOCs, and isoprene. Results show that model NO_x-VOC sensitivity predictions are correlated with the ratio O₃/NO_y but not with O₃/peroxyacetyl nitrate. Measured O₃ and NO_y on high-ozone days tends to agree with model values when models predict NO_x-sensitive or transitional chemistry but not when models predict VOC-sensitive chemistry. Model values for O₃/NO_y and the O₃-NO_y slope are lower than measured values, suggesting the possibility of missing, unmeasured VOCs in the Paris plume. Standard performance tests for ozone models, such as normalized bias, show good agreement between models and measurements, even in cases when significant differences appear in the O₃-NO_y correlation. Model predictions shift slightly toward NO_x-sensitive chemistry when model wind speeds are increased. Isoprene represents 20% of total VOC reactivity-weighted carbon in the center of the Paris plume and 50% in the surrounding rural area during high-ozone events.