Humidity Dependence of the Condensational Growth of α-Pinene Secondary Organic Aerosol Particles

The influence of relative humidity (RH) on the condensational growth of organic aerosol particles remains incompletely understood. Herein, the RH dependence was investigated via a series of experiments for a-pinene ozonolysis in a continuously mixed flow chamber in which recurring cycles of particle growth occurred every 7 to 8 h at a given RH. In 5 h, the mean increase in the particle mode diameter was 15 nm at 0% RH and 110 nm at 75% RH. The corresponding particle growth coefficients, representing a combination of the thermodynamic driving force and the kinetic resistance to mass transfer, increased from 0.35 to 2.3 nm(2) s(-1). The chemical composition, characterized by O:C and H:C atomic ratios of 0.52 and 1.48, respectively, and determined by mass spectrometry, did not depend on RH. The Model for Simulating Aerosol Interactions and Chemistry (MOSAIC) was applied to reproduce the observed size- and RH-dependent particle growth by optimizing the diffusivities D-b within the particles of the condensing molecules. The D-b values increased from 5 alpha(-1) x 10(-16) at 0% RH to 2 alpha(-1) x 10(-12) cm(-2) s(-1) at 75% RH for mass accommodation coefficients alpha of 0.1 to 1.0, highlighting the importance of particle-phase properties in modeling the growth of atmospheric aerosol particles.

Automated summary

The influence of relative humidity on the condensational growth of organic aerosol particles was investigated in this study. Higher RH resulted in faster particle growth, with the particle growth being influenced by factors such as diffusivities within the particles.