Using measured and modeled indicators to assess ozone-NOx-VOC sensitivity in a western Mediterranean coastal environment

It is widely known that the formation of ozone (O3) is chemically linked to the emissions of nitrogen oxides (NOx) and volatile organic compounds (VOC). This chemical interdependence is highly complex and gives rise to non-linear and coupled pollutant formation processes. In the present study, airborne measurements of sulfur dioxide (SO2), total reactive oxides of nitrogen (NOy), and O3 taken from an instrumented aircraft within MECAPIP-1989 project have been reexamined to asses the governing photochemical processes of ozone formation. This experiment was carried out in the Castellón urban-industrial area, located on the Mediterranean coast of the Iberian Peninsula. Noon near field (within a 25-km radius from the coast line) transects show a strong ozone titration effect downwind of the main source area. Moreover, while afternoon measurements still depict ozone consumption near the emissions area, ozone net formation is observed in the mid-field (within a 75-km radius) of the Castellón urban-industrial complex. Ozone yields have been derived from these aircraft measurements. This analysis shows that there is almost no O3 production for the noon period. Conversely, the O3 yield reaches a maximum downwind of the emissions complex for the afternoon hours. Furthermore, measured values for the O3/NOy ratio have been used as photochemical indicators to determine the effectiveness of VOC or NOx controls in decreasing O3 abundance. The concentrations of these indicator species have been calculated from a series of simulations using a lagrangian photochemical model and good agreement has been observed between modeled and measured data. The sensitivity of ozone to changes in its primary sources has been examined by simulating scenarios with varying rates of NOx and VOC emissions. The results presented here show that for this particular case the measured and modeled O3/NOy ratios seem to identify a photochemical regime in which reductions in upwind NOx sources are more effective for lowering O3 than are VOC emissions.