Developing an on-line parameterization approach for predicting the ambient organic aerosol hygroscopicity based on High- Resolution AMS measurements

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Several atmospheric processes are driven by water take-up ability (hygroscopicity) of the atmospheric aerosol particles such particle light scattering, cloud droplet formation, activation of cloud condensation nuclei (CCN), altering the hydrological cycle as well as the cloud radiative forcing. Despite its crucial role on the atmosphere and climate, there is still a large disparity of knowledge regarding the contribution of the organic aerosol, which constitutes a larger part of the sub-micrometer particle mass concentration, to the overall hygroscopicity. The following research proposal propounds an original approach of the parameterization of the hygroscopic properties of organic aerosol particles, using an online chemical function concept based on the organic mass spectra analysis from High Resolution-Time of Flight-Aerosol Mass Spectrometer (HR-ToF-AMS) measurements. The development of this parameterization will be based on a combination of Humidified Hygroscopic Tandem Differential Analyzer (HTDMA) and HR-ToF-AMS measurements in a dual, but complementary approach. For this, an intensive laboratory screening of chemical compounds with targeted functional groups and a mixture of varied organic standards will be conducted. Simultaneously, a machine learning approach based on previous TROPOS field campaigns, which integrated both the instruments will be carried out. Comparison between the two approaches will be made for final validation in the study. This parameterization will be then validated in two field campaigns each of them dedicated to a specific type of organic aerosol: a biogenic aerosol dominated environment at Melpitz (Germany) and an urban aerosol dominated environment at SIRTA (France), where both instruments will be deployed within this project. The ambient aerosol online hygroscopicity will be estimated by combining HR-ToF-AMS (organic and inorganic aerosol) and optical measurements from aethalometer (equivalent black carbon) and then be compared to the one measured by HTDMA. Taking the advantage of the High-resolution and Unit mass spectra resolution of the HR-ToF-AMS, and the presence of Aerosol Chemical Speciation Monitor (ACSM) at both selected field sites, where the method will be also optimized for the ACSM. Consequently, an automatic routine will be developed for both instruments (HR-ToF-AMS and ACSM) to be further implemented on the Aerosols, Clouds, and Trace gases Research Infrastructure Network (ACTRIS) ACSM network to provide a unique opportunity for a near-real-time and long-term bulk aerosol hygroscopicity measurement over Europe.