Characterization of VOC sources in a typical urban environment (Valencia)

Tropospheric ozone (O₃) is a major concern in the atmosphere due to its adverse effects on human health and vegetation. O₃ is a secondary pollutant, formed through complex photochemical processes from its precursors like nitrogen oxides (NO_X) such as volatile organic compounds (VOCs). The Mediterranean coastline is particularly sensitive to human activities, under a climate with high insolation, under a climate with high solar radiation where a large part of the year prevails a regime of mesoscale winds (breezes) supported by a favourable orography. Large metropolitan areas, such as the city of Valencia, contribute to photochemistry with important emissions of NO_X and VOCs to the atmosphere, especially traffic, which determines high levels of ozone in the surrounding areas. However, there are other sources that can also play a relevant role in atmospheric photochemistry, which are not well known and do not have a sufficiently detailed emission profile speciation. This study deals with the analysis of the VOCs associated with three types of common emission sources (gas stations, ports and car painting shops) in a Mediterranean city, in this case Valencia. This could lead to a significant improvement in the urban VOC speciation for emission inventories used in model simulations, especially for O₃ in future studies.

10 passive samplers for VOCs were installed for two weeks during summertime when O₃ levels are typically elevated (from 29 June to 13 July 2023). Four samplers were placed at two gas stations (two each), three in areas influenced by the Port of Valencia, and the remaining three in nearby three different car painting shops. The measured VOC levels were analysed through gas chromatography-mass spectrometry (GC-MS) and liquid chromatography-mass spectrometry (LC-MS). Additionally, for more precise temporal resolution, proton-transfer-reaction mass spectrometry (PTR-MS) was installed at a gas station in another Mediterranean city (Castellon) during July 2023.

As a result, the total VOC concentrations are generally high in car painting shops with an average of 52.4 µg m^-3, followed by gas stations (44.0 µg m^-3 as an average) and ports (26.4 µg m^-3). Looking at the contribution of the functional groups, aldehydes are clearly the largest contributors for gas stations and ports (28.5% and 34.3%, respectively), while for car painting shops, the contribution of this group is comparable to aromatics (24.4% and 26.1%, respectively). Analysing the species, formaldehyde (aldehydes) is generally one of the main contributors for the three environments, accounting for 6.7%, 8.5%, and 5.3% of the total VOCs for gas stations, ports and car painting shops, respectively. 2-methylpentane (alkane) (5.3%) and acetone (ketone) (5.1%) are the second most important species for gas stations and ports, respectively, and the third one is acetaldehyde for both environments (4.6% and 4.9%, respectively). Meanwhile, butyl acetate (ester) and methylcyclohexane (aromatic) have also high levels for car painting shops (8.3% and 7.1%, respectively). These species, with the exception of acetone, have significant ozone formation potential, which could lead to elevated O₃ levels in the western vicinity of the city.