2005-2018 trends in ozone peak concentrations and spatial contributions in the Guadalquivir Valley, southern Spain

We assessed 2005-2018 data series of NO, NO2, and O-3 surface measurements, meteorological data and remote sensing of tropospheric NO2 by OMI-NASA in the Guadalquivir Valley (GV, Andalucia). The GV is one of the Spanish atmospheric basins with the most exceedances of the EU's hourly O-3 information threshold (180 mu g O-3 m(-3)). We aimed to deepen understanding of the phenomenology of O-3 episodes in the GV, quantify local and regional contributions from long-range transported O-3, and identify key episodes and atmospheric parameters to validate modeling tools for the assessment of O-3 abatement policies with acceptable uncertainty levels. A significant proportion of acute O-3 episodes (defined as episodes of 1-7 consecutive days, when very high hourly O-3 concentrations are recorded) in the GV is caused by long-range transported contributions in addition to the regional accumulation of pollution which is, in most cases, driven by the vertical recirculation of air masses, as described in several basins of the Iberian Peninsula. In these episodes, high O-3 arises from the fumigation of high reserve recirculated strata as the boundary layer grows, sea-breeze transport of O-3 and precursors, local formation, and long-range transport. The most acute episodes are recorded close to Sevilla, probably because of the mix of precursors from the urban areas and the petrochemical industry of Huelva. We quantified the average O-3 contributions to O-3 concentrations in Sevilla during these episodes as 53% background levels before the episode (defined as the concentration measured at the coastal remote sites at the beginning of the pollution episode, when the Atlantic air masses transport background O-3 from synoptic air mass transport), 22% accumulation throughout the episode, and 25% of daily local and regional contribution to the average 8h-daily maxima (157 mu g O-3 m(-3)). Thus, a maximum decreasing potential of 74 mu g O-3 m(-3) (47%) of the average 8h-daily maxima can be expected when applying abatement measures to reduce O-3 precursors in the day before the start of an episode and until the end of the episode. Our results demonstrate that it is crucial to meteorologically forecast the occurrence of such recirculation and accumulation processes in the GV basin.

Automated summary

The study found that most of the ozone episodes in the Guadalquivir Valley are caused by contributions from long-range transport, as well as local and regional pollution accumulation. These episodes are mainly due to vertical recirculation of air masses and accumulation of pollutants. Therefore, it is crucial to forecast these recirculation and accumulation processes in the GV basin in order to reduce ozone precursor emissions effectively.