Ultrafine particles in urban settings: A combined study of volatility and effective density revealed by VT-DMA-APM

Volatility and effective density of aerosol particles are complicatedly linked properties, undergoing dynamic temporal evolution in the ambient environment. However, the simultaneous investigation of these physicochemical characteristics remains unexplored in roadside environments that exhibit a significant proportion of motorcycles in the vehicle fleet - a common scenario in Southeast Asia. As such, we deployed an innovative in-situ measurement technique, specifically the Volatile Tandem Differential Mobility Analyzer and Aerosol Particle Mass Analyzer (VT-DMA-APM) system, to investigate ultrafine particles (UFPs) in an urban region of northern Taiwan. Our findings revealed that the UFPs predominant in this study area were largely soot aggregates interspersed with condensable materials. Traffic-related air pollutants (TRAPs), specifically condensable organic substances, emerged as key determinants of particle effective density and volatility. Intriguingly, the peculiar interrelation between aerosol physicochemical properties and particle size suggests that UFPs may represent an amalgamation of black carbon (BC) coated with condensable substances and non-BC-containing particles. We observed an inverse relationship between effective density and volatility, indicative of atmospheric aging processes. Moreover, concurrent measurements of Absorption Angstrom Exponent (AAE) and shape factor (Ks) provided valuable insights into variations in optical properties and morphology during aging. Peak AAE and minimal Ks values consistently occurred at noon, implying heightened particle aging during this period due to coating and morphological alterations.