Sources and vertical distribution of VOCs and their oxidized products across rural, suburban and industrial environments in the Western Mediterranean

Volatile organic compounds (VOCs) play a central role in atmospheric chemistry, yet their vertical distribution and transformation in the lower troposphere remain insufficiently characterized. VOCs originate from diverse natural and anthropogenic sources, and their accumulation, dispersion, and photo‑oxidation are strongly influenced by meteorological conditions and boundary‑layer dynamics. Some VOCs are carcinogenic (e.g., benzene) and others are neurotoxic (e.g., toluene). In addition to their toxicological relevance, many VOCs act as key precursors of tropospheric ozone and secondary organic aerosol (SOA).

For organic contaminants, the air above the daytime mixing layer and within the nocturnal residual layer remain poorly characterized. The photolysis of VOCs can release radicals that promote ozone formation aloft, while their photooxidation and transformation into lower-volatility products may contribute to SOA. The accumulation of these secondary pollutants in the nocturnal residual layer can increase its oxidative capacity, and once vertical mixing resumes, they may contribute to photochemical reactions and to the secondary pollutant burden at the surface. Beyond evaluating the oxidative state of air aloft, an important question is whether oxidized compounds result from local emissions undergoing rapid transformations.

This work investigated the origins, composition, and vertical distribution of VOCs and its oxidized products across rural, suburban, and industrial environments in the Western Mediterranean. Active offline sampling of VOCs and total suspended particles was conducted using multiple sorbent cartridges and quartz filters, followed by GC‑MS and HPLC analysis. To resolve vertical gradients, ground‑level observations were complemented with tethered‑balloon measurements reaching 350 meters above ground level, which allowed sampling both within the surface layer and above the nocturnal residual layer.

The results obtained from the vertical profiles showed a consistent decrease in VOC concentrations with altitude due to dilution and oxidation. Primary VOCs concentration declined by roughly 30% at balloon height, while secondary VOCs showed a smaller decrease, and even some carbonyl species exhibited nearly uniform vertical distributions. Air masses aloft were found consistently more oxidized than those near the surface, particularly in winter under strong stratification, and they contained higher levels of long‑lived VOCs and secondary products, including SOA. Diagnostic ratios, such as benzene to toluene or SOA tracers to isoprene and α-pinene, confirmed that aged compounds predominated at higher altitudes. Multivariate analysis showed that local photooxidation of freshly emitted compounds contributed substantially to this ageing and accounted for up to 41% of aged VOCs aloft.

Overall, these findings highlight the importance of incorporating vertical pollutant gradients and source apportionment analysis to better understand the origin of compounds accumulated in the residual layer and tackle their influence on surface photochemical pollution the following day.