Contrasting organic aerosol molecular composition between the urban and agricultural environment of the Po Valley

Abstract. The molecular composition of organic aerosol in the Po Valley remains largely unexplored, despite contributing approximately half of the fine aerosol mass. Molecularly-resolved analysis of the organic fraction is essential for understanding the sources and formation processes of organic aerosol in detail. Here, we investigated one year of PM_2.5 filter samples from a rural agricultural and an urban environment. We used liquid chromatography and high-resolution mass spectrometry with subsequent non-target analysis of 250 samples. Time-series analysis allowed for the grouping of detected organic compounds using a clustering algorithm, enabling a tentative source attribution. The most populated clusters consist mainly of CHOand CHOS-containing compounds, attributed to oxidation products of biogenic emissions. They account 31 % and 26 % of the total intensity in the urban and agricultural sites, respectively, and peak during summer. Combustion-related clusters, enriched in nitrogen-containing compounds, contribute approximately 35 % of the total intensity at both sites. A fraction of these molecules are highly conjugated compounds that appear during winter as primary contributors to light-absorbing organic aerosol. Furthermore, we identified site-specific compound clusters, either at the urban or at the agricultural site. At the latter, we found pesticides strongly influence the overall molecular composition, peaking in May when PM_2.5 concentration is at its lowest level. This suggests potential toxicological effects despite apparent good air quality conditions. Our results represent the first molecular-level characterization of organic aerosol in the Po Valley, emphasizing the need to assess its composition for a better understanding of its environmental and health impacts.