Size- and Composition-Dependent Hygroscopic Growth of Sub-20 nm Atmospherically Relevant Particles: Implications for New Particle Survival

Abstract

New particle formation (NPF) is a significant source of atmospheric particle number concentration and cloud condensation nuclei. The fate of nascent particles is dependent on their ability to take up water, a process influenced by particle size, composition, and morphology. However, there is a lack of comprehensive characterization of the hygroscopic properties of sub-20 nm particles that are compositionally similar to those of atmospheric new particles, leading to uncertainties in predicting their growth and survival. In this study, we examine the hygroscopic properties of particles composed of both inorganic (NaCl and gas-phase reaction products of sulfuric acid and ammonia) and organic components (derived from α-pinene oxidation) by tandem differential mobility analysis. For NaCl-organic particles, we find that the organic coating reduces the particle deliquescence relative humidity (DRH), efflorescence relative humidity (ERH), and hygroscopic growth factor (GF). These reductions are size-dependent, but the Zdanovskii-Stokes-Robinson (ZSR) rule successfully predicts the particle GF once the Kelvin effect is considered. In contrast, particles containing sulfuric acid-ammonia reaction products exhibit no well-defined DRH or ERH. These particles, which are rich in acids, progressively approach the behavior of ammonium sulfate ((NH₄)₂SO₄) as their size increases. The ZSR rule underestimates the particle GF, possibly due to partial mixing of the organic and aqueous phases. Based on our experimental data, simulations of particle growth show that incorporating hygroscopic growth is essential for accurately estimating particle survival probabilities, even at moderate relative humidity. Our findings enhance the understanding of the factors governing water uptake by atmospheric new particles and contribute to the development of more accurate models for simulating new particle growth.

Keywords: hygroscopicity; new particle formation; particle growth; secondary organic aerosol; survival probability.