Reconciling volume- and mass-based hygroscopicity parameters: Insights from coupled H-TDMA–APM measurements

Abstract

This study investigates aerosol hygroscopicity by reconciling volume-based (κᵥ) and mass-based (κ_m) hygroscopicity parameters through a dual-system approach combining a Hygroscopic Tandem Differential Mobility Analyzer (H-TDMA) and a Hygroscopic Differential Mobility Analyzer–Aerosol Particle Mass Analyzer (H-DMA-APM). The integrated system enables simultaneous measurement of size and mass growth factors for pure and internally mixed sub-100 nm NaCl and (NH₄)₂SO₄ particles under varying relative humidities (RH). Results show that κᵥ exhibits strong size dependence, declining by over 25 % as particle diameter decreases from 51.4 nm to 20.2 nm, attributed to curvature effects and morphology-induced artifacts. In contrast, κ_m shows a more stable trend across sizes, varying by less than 15 %, reflecting its robustness against morphological and curvature influences. However, κ_m did not consistently outperform κᵥ in predictive accuracy, likely due to the broader classification resolution of the APM system. While κ_m remains a theoretically superior parameter, κᵥ, when derived from high-resolution HTDMA systems, remains an effective predictor of hygroscopic growth. These findings highlight the importance of particle size, morphology, and instrumentation in characterizing aerosol hygroscopicity and advocate for the complementary use of κᵥ and κ_m in climate-relevant aerosol studies.