The use of experimental data and their uncertainty for assessing ozone photochemistry in the Eastern Iberian Peninsula

Observation-based methods are useful tools to explore the sensitivity of ozone concentrations to precursor controls. With the aim of assessing the ozone precursor sensitivity in two locations: Paterna (suburban) and Villar del Arzobispo (rural) of the Turia river basin in the east of Spain, the photochemical indicator O₃/NO_y and the Extent-of-Reaction (EOR) parameter have been calculated from field measurements. In Paterna, the O₃/NO_y ratio varied from 0 to 13 with an average value of 5.1 (SD 3.2), whereas the averaged value for the EOR was 0.43 (SD 0.14). In Villar del Arzobispo, the O₃/NO_y ratio changed from 5 to 30 with a mean value of 13.6 (SD 4.7) and the EOR gave an averaged value of 0.72 (SD 0.11). The results show two different patterns of ozone production as a function of the location. The suburban area shows a VOC-sensitive regime whereas the rural one shows a transition regime close to NO_x-sensitive conditions. No seasonal differences in these regimes are observed along the monitoring campaigns. Finally, an analysis of the influence of the measurement quality of NO_y, NO_x and O₃ on the uncertainty of the O₃/NO_y ratio and the EOR was performed showing that the uncertainty of O₃/NO_y is not dependent on either its value or the individual values of O₃ and NO_y but just on the quality of O₃ and NO_y measurements. The maximum uncertainty is 26% as long as the combined uncertainties of O₃ and NO_y remain below the 7.5%. The case of the EOR is different and its uncertainty depends on both the value of the EOR parameter and the individual concentration values of NO_y and NO_x. The uncertainty of the EOR estimation can be very high (>200%) if the combined uncertainties of both NO_y and NO_x are high (>7.5%), or especially, if u(NO_y) and u(NO_x) differ considerably from each other (>3.5%).