Real-Time Measurements of Biomass-Burning and Secondary Organic Aerosol Composition in the Po Valley using Ultra-High Resolution (Orbitrap) Mass Spectrometry

Determining the chemical composition of organic aerosols (OA) is still challenging and requires accurate and precise mass spectrometric measurements. Understanding the chemical composition of these OA can provide crucial insight into the origins, formation, and characteristics of OA.  A particularly demanding environment for the characterization of OA is the Po-Valley (Itay) since it is one of the European hotspots for air pollution. The main reasons for the air pollution here are intensive agriculture, livestock breeding and industrial areas in combination with unfavorable topographic and meteorological conditions. This combination can lead to long, stagnant weather conditions, resulting in the accumulation of atmospheric pollutants in the valley. The interaction of these factors was investigated during the ALFA (Aerosol Loadings of the Future Atmosphere) measurement campaign in Schivenoglia (Lombardy, Italy) from September to November 2023. During this campaign, we deployed an ultrahigh-resolution mass spectrometry system on an agricultural field site for the first time to measure in real-time how agricultural activities influence OA's formation and chemical composition. For a comprehensive investigation, we equipped a measurement container capable of assessing particle phase composition. The chemical analyses of aerosols were conducted by measuring with an atmospheric pressure chemical ionization Orbitrap mass spectrometer (APCI-Orbitrap-MS), which operated continuously in either positive or negative full-MS mode or was selectively used for targeted MS2 fragmentation experiments. Additionally, we deployed an Aerosol Chemical Speciation Monitor (ACSM) in tandem with the Orbitrap measurements to reinforce the online measurement results and providing quantitative information. With this mode of operation, we obtained time series for diurnal cycles of various OA and recorded individual events of biomass-burning. Especially worth mentioning are strong diurnal cycles of organic nitrates (C8H13O8N, C8H11O8N and C10H17O7N), which reach their peak concentration at night and are completely depleted during the day. On the contrary, and mainly influenced by daytime photochemistry, diurnal cycles of MBTCA (C8H12O6) and shikimic acid (C7H10O5), which peak during the day and decrease at night, were detected. Additionally, biomass-burning events were detected in real-time, during which typical biomass-burning markers such as levoglucosan (C6H10O5), vanillin (C8H8O3), galactosan (C6H12O6) increased significantly. Overall, during this measurement campaign, we successfully deployed an APCI-Orbitrap-MS into a field-side measurement station for the first time without the need for complicated infrastructure. The use of this method has demonstrated many benefits, which will be presented in this work.