Odor impact associated with a refinery
Within the framework of an assignment commissioned by AECOM URS Italia S.p.A., TerrAria contributed to the modelling assessment of the odor impact associated with several emission units of the Milazzo Refinery (ME), following nuisance complaints from residents living in the areas surrounding the facility, in accordance with Italian Legislative Decree 152/2006 and related regulatory guidelines.
The analysis formed part of a broader technical verification process aimed at determining whether the meteorological conditions and the operation of the units involved could have contributed to the episodes reported during the year. The refinery, located in a territorially heterogeneous context — with residential, commercial, agricultural and industrial areas distributed within a few kilometers — represents a significant emission source, for which the assessment of odor impact requires modelling tools capable of addressing the environmental complexity of the area.
For this study, the WRF–CALMET–CALPUFF modelling chain was applied, particularly suitable for describing the dispersion of odor compounds under variable meteorological conditions and in the presence of sensitive receptors located at short distances from the sources. Meteorological data were obtained from the monitoring stations closest to the facility, while emission information — including flow rates, temperatures, concentrations and profiles of the monitored substances — was provided directly by the refinery.
The modelling considered both specific episodes, simulated at minute resolution, and the plant’s configuration at Maximum Production Capacity, in order to compare individual events with standard operating conditions.
The adopted approach involved a detailed reconstruction of atmospheric concentrations of the main odor compounds monitored by the facility (SO₂, NOX, NH₃, H₂S, benzene, butadiene and HCl). Since odor nuisance is not related to the exceedance of regulatory limits but to subjective perception, each pollutant was evaluated with respect to the odor thresholds reported in the Lombardy Region Guidelines and, where necessary, supplemented with international reference sources. The modelled concentrations were therefore compared with the corresponding thresholds to identify potential perception conditions.
For three of the selected episodes, the simulations showed that the meteorological conditions recorded during the relevant periods were compatible with a possible dispersion of emissions toward the areas where odor perceptions had been reported.
In general, the modelled concentrations were in most cases below the respective thresholds, although some hourly intervals showed localized increases in odor compound concentrations, likely associated with the meteorological conditions and the emission regime during the considered periods.
The annual assessment at Maximum Production Capacity provided a broader reference framework, useful for contextualizing acute odor events within a full operational cycle. Estimates based on the annual 98th percentile generally indicated values within reference ranges, while the analysis of all hours of the year showed that under specific combinations of atmospheric conditions some substances could reach levels close to their thresholds even outside the facility boundary. These results do not necessarily imply a direct link with the odor nuisance reported but help identify the contexts in which odor perception may be more likely.
Overall, the study contributed to objectively reconstructing the relationship between odor dispersion and local meteorological conditions, providing technical elements useful for further management analyses, contextual assessments and potential improvement actions, without drawing definitive conclusions regarding the origin of the episodes reported by the population.
THE ROLE OF TERRARIA
For this assignment, TerrAria managed the entire modelling activity, from the setup of the WRF–CALMET–CALPUFF modelling chain to the management of the emission and meteorological datasets required for the simulations. TerrAria configured the modelling domains, selected conservative assumptions and performed high temporal resolution simulations, which are particularly demanding for the modelling of acute episodes.
In addition, TerrAria interpreted the results by integrating statistical analysis, odor thresholds and territorial evaluations, producing maps, charts and indicators useful for correlating potential impacts with the timing and locations of reported odor events. The work made it possible to provide an independent technical reconstruction and specialized support in the assessment of odor impacts, contributing to the identification of possible mitigation actions and improving stakeholders’ understanding of the phenomenon.
