Hygroscopicity Prediction Impacted by Particle Mixing in Sub-urban Environment (HIMS)

SHRAVAN DESHMUKH, Laurent Poulain, Birgit Wehner, Silvia Henning, Jean-Eudes Petit, Olivier Favez, Hartmut Herrmann, Mira L. Pöhlker, Leibniz Institute for Tropospheric Research, Leipzig,Germany

     Abstract Number: 13
     Working Group: Chemicals of Emerging Concern in Aerosol: Sources, Transformations, and Impacts

Abstract
Hygroscopicity and particle mixing state strongly influence aerosol properties and multiphase chemistry, which plays an essential role in estimating the direct radiative effect of aerosols on Earth’s climate. Although CCN (cloud condensation nuclei) properties are commonly measured, sub-saturation hygroscopicity measurements remain rare. Within the ACROSS campaign, which took place in the Paris region, France, during the summer of 2022, the particle’s hygroscopic growth at 90 % relative humidity (RH) and chemical composition were concurrently measured at sub-urban site SIRTA using a Hygroscopicity Tandem Differential Mobility Analyser (HTDMA) and Aerodyne High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) respectively.

The growth factor probability density function (GF-PDF) revealed two distinct modes—hydrophobic and hygroscopic—suggesting a combination of internal and external particle mixing. The prevalence of the hygroscopic mode increased with particle size, as reflected in the averaged values of hygroscopicity. The classical Zdanovskii-Stokes-Robinson (ZSR) mixing rule was applied (Stokes and Robinson 1966) to the chemical composition to make accurate quantitative predictions of the mean GF of atmospheric aerosol. The closure between measured and chemical-derived hygroscopicity is approximately 50% for 100nm particle size, but this level of agreement is not observed for 250nm. Our study found that chemically derived mean growth factor values do not accurately identify the growth factors of multiple modes. Furthermore, this approach proves unreliable in predicting the wide spread of growth for aerosol particles that are externally mixed. The mixing state of aerosol strongly influences the prediction of the aerosol hygroscopicity. Whereas during this measurement, 80-90% of particles were externally mixed and influenced by fresh emission, which impacted the prediction of the hygroscopicity by a factor of 2. These findings highlight the complex interplay between aerosol properties, chemical composition, and atmospheric processes, providing valuable insights for future research. Moreover, laboratory studies are conducted on pure chemical compounds to develop new hygroscopicity prediction method.