Spatial-temporal analysis of recent trends in nitrogen dioxide (NO2) and volatile organic compounds (VOCs) in the Mediterranean metropolitan area of Valencia city

Nitrogen dioxide (NO₂) and volatile organic compounds (VOCs) are of concern in urban environments as primary pollutants and as main precursors of tropospheric O₃, which has adverse effects on both human health and vegetation. Furthermore, as a secondary pollutant, its complex nature due to its non-linear chemistry makes it difficult to reduce with the reduction in the precursors.

This study is carried out to spatially characterise the urban air pollution, using NO₂ and VOCs, and to evaluate the recent temporal trends of these compounds, which are closely related to the O₃ formation in the metropolitan area of Valencia. Being the third largest Spanish city, with one of the Mediterranean's largest ports, the complex emission sources contribute to high ozone levels in the surrounding areas.

A total of 97 passive samplers for NO₂ were used in the study area, covering an area of 11 km x 10 km including the urban centre with a resolution of about 1 km x 1 km. The temporal resolution of the measurement covers the winter and summer seasons (one exposure week every February and July, respectively) for the last 8 years, from 2017 to 2024. Meanwhile, the passive samplers for VOCs were installed at 10 selected points through the urban centre among the 97 points, following the main wind direction of the region. The measurement period is two years shorter than those for NO₂ from 2019 to 2024 but covers the same seasons. The measured NO₂ levels were determined using ultraviolet-visible (UV-VIS) spectroscopy, while the VOCs levels were analysed through gas chromatography-mass spectrometry (GC-MS) and liquid chromatography-mass spectrometry (LC-MS).

To characterise the spatial patterns of the city, the k-mean clustering is used to group all points where NO₂ levels are measured. As a result, the city is divided into overall two clusters. Cluster 1 is close to road traffic emissions and follows the prevailing wind direction, resulting in relatively high levels of NO₂. Cluster 2 represents the rest of the points, which have lower levels. As for VOCs, the analysis is performed at two specific points, where NO₂ has the highest and the lowest among the given 10 points, in the city centre and in the harbour area, representing Cluster 1 and Cluster 2, respectively. In the city centre, the aromatic hydrocarbons are more abundant, while in the harbour area, the contribution of the aldehydes is greater.

The Theil-Sen method is used for the temporal analysis of each cluster. NO₂ shows a decreasing trend in both clusters. The reduction is more pronounced in Cluster 1 where the levels tend to be greater than the other cluster, especially in winter. However, total VOCs levels seem to be increasing overall. In particular, there is a tendency to increase in winter, while VOCs decrease slightly in summer.

This result shows that the ozone formation regime of this area has been changing as NO₂ levels are decreasing while VOCs are generally increasing. Therefore, ozone levels may be locally increasing.