Particle hygroscopicity in an urban background environment during the ACROSS campaign

Hygroscopicity strongly influences aerosol particle properties and multiphase chemistry, which also plays an essential role in several atmospheric processes. 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 using a Hygroscopicity Tandem Differential Mobility Analyser (HTDMA, scanning at 100, 150, 200, and 250 nm) and Aerodyne High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) respectively, at the ACTRIS (the European Aerosol, Clouds and Trace gases Research Infrastructure) SIRTA near facility, a peri-urban site representative of the Greater Paris background conditions. Growth factor probability density distributions (GF-PDF) show two distinct modes: hydrophobic and hygroscopic, indicating that the particles are internally and externally mixed. The hygroscopic mode is always more prominent in the GF-PDF, indicating a change in the particles' mixing state. The dominance of the hygroscopic mode becomes more pronounced with increasing particle size. The mean hygroscopicity parameter values, κ of 100, 150, 200, and 250 nm particles derived from hygroscopicity measurements are respectively 0.23, 0.29, 0.36, and 0.38 during the sampling period. The size dependence is reflected in the averaged values of κ and the GF distributions. A diurnal pattern was observed with an average daytime κ higher than during night-time, which could be linked to aging processes and secondary aerosol formation during the day. The Zdanovskii-Stokes-Robinson (ZSR) mixing rule was applied on the particle chemical to make accurate quantitative predictions of the mean GF of mixed atmospheric aerosol particles.

Keywords: Hygroscopicity, Growth factor, hygroscopicity parameter κ.