How does aerosol grow in a real atmosphere? Measurement of aerosol precursor gases during a highly complex urban air simulation campaign SAPHIR-CHANEL

Atmospheric secondary organic aerosol (SOA) formation is a tremendously complex phenomena which details keep eluding the researchers. The overwhelming combinatorics in gas-phase organic oxidation demands that in any real atmosphere the aerosol formation and growth occur by a network of interactions likely involving at least thousands of chemical compounds, making development of molecular level description a formidable task. By extension, any single component study will not be able to describe the inherent complexity of ambient gas-phase, necessitating the usage or evermore complex gas mixtures in order to bridge the gap to the real world processes.

In the present work we have performed investigations on probably the most complete gas mixture representing ambient urban conditions. The experiments were performed during the CHANEL measurement campaign during Summer 2024 in the large atmospheric simulation chamber SAPHIR in Forschungszentrum Jülich. The evolving gas mixture and aerosol formation were followed with an unusually large suite of instruments with around 10 chemical ionization mass spectrometers equipped with state-of-the-science detection techniques, including practically all the latest nanoparticle thermal desorption sampling instrumentation, and several nanoparticle sizing methods. The  data presented here were mainly obtained with a high-resolution orbitrap nitrate (NO₃^-) chemical ionization mass spectrometry (CIMS) with up to 120 000 mass resolution. The nitrate ionization is highly selective towards very polar and highly functionalized gas-phase compounds and thereby the data includes most oxygenated reaction products that are expected to play an important role in the generation of secondary aerosol. The experiment philosophy was to investigate very complex gas mixtures by including or excluding select mixture components to determine their individual impacts. Significant differences in the measured aerosol precursors were observed as a function of the complexity of the oxidized hydrocarbon pool.