Spatial dynamics of volatile organic compounds (VOCs) in an urban environment and associated impacts on air quality

Abstract. Urban air pollution, affecting residents and commuters alike, depends considerably on volatile organic compounds (VOCs) due to their toxicity and potential to form secondary pollutants. The distribution and composition of urban VOCs reflect their emission as well as transport and atmospheric removal and formation processes. Hence, the VOC burden can differ substantially on small spatial scales. Aiming at investigating whether and how a large street canyon with dense traffic influences the air quality of adjacent neighbourhoods, we conducted trailer-based mobile measurements of nitrogen oxides, ozone, and VOCs in the city of Munich, Germany in September 2023. VOCs were measured using a highly resolved proton transfer reaction time-of-flight mass spectrometer (PTR-ToF-MS). To support different statistical analyses, the study area was divided into grid cells. A distinct separation of the VOC pool into traffic-related, biogenic, and functionalized compounds was achieved by hierarchical cluster analysis, ultimately assisting non-target analyses. The spatial variability was characterized by a combination of hierarchical cluster analysis, k-means analysis, and non-metric multidimensional scaling (NMDS). We show for exemplary locations that small-scale variations of some of the less abundant but highly reactive VOCs emitted by traffic (e.g. aromatics, alkenes) induce statistically significant spatial differences in terms of oxidant reactivities and the potential for the formation of aerosols or ozone. Finally, the variability-lifetime relationships of the VOC groups were analysed, revealing that the variability of the traffic-related VOCs was more tightly linked to local source strength compared to the biogenic and functionalized VOCs, and supporting their respective allocation.