Reliable VOC Monitoring in Industrial Coastal Areas: Uncertainty Quantification and Variability in the Berre Basin

Abstract. Continuous measurements of 23 volatile organic compounds (VOCs) were deployed at three sites surrounding the lake of Berre industrial basin (southern France). This dataset was used to characterize VOC variability in a complex industrial environment and to establish a robust methodological framework for long-term monitoring. The sites, located near major Seveso-classified industrial platforms, exhibit distinct chemical signatures driven by contrasting source influences, mesoscale circulations, and topographic effects. The monitoring strategy relied on dual online GC-FID systems and was supported by an extensive quality assurance and quality control (QA/QC) program, including routine calibration, assessments of linearity, repeatability, blank correction, and environmental influences. A comprehensive uncertainty calculation scheme was developed by combining elements of the NF EN 14662-3 Benzene standard with the ACTRIS network’s standard operating procedure (GAW report No.281, WMO, 2023)  and implemented within the Incert'R R-based tool to enable automated uncertainty estimation for each VOC concentration. The resulting expanded uncertainties, calculated for a reference concentration of 5 µg m^-3, ranged from 9 % to 25 % depending on the compound. Except for 1,2-dichloroethane (25 %), all VOCs exhibited uncertainties below 20 %, while benzene showed an uncertainty of 9 %, well below the 25 % limit required by the benzene standard. Overall, these uncertainty levels confirm the reliability of the dataset for interpreting concentration patterns across the three sites. Having established this, the analysis of VOC concentrations reveals pronounced spatial and temporal contrasts across the sites: Berre-l'Étang is strongly affected by land-sea breeze recirculation, leading to recurrent morning and evening peaks and exceptionally high values during industrial maintenance periods, while Lavéra shows frequent episodic enhancements dominated by chlorinated and light hydrocarbon compounds. Port-de-Bouc records lower and less structured concentrations, consistent with more intermittent plume impacts. These findings highlight the importance of combining high time-resolution measurements with harmonized QA/QC procedures and systematic uncertainty estimation to reliably detect high variability of VOC concentration in industrial regions. The resulting dataset provides a rigorous basis for subsequent source-apportionment analyses and for improving emission inventories and atmospheric modelling in one of Europe's most industrially impacted coastal areas.