When size matters : size-selective chemistry in the heterogeneous processing of organic aerosols

Abstract

New particle formation and growth in the atmosphere critically influence cloud properties and climate, as well as human health through the presence of ultrafine particles. While the early stages of particle nucleation are increasingly understood, the mechanisms driving the subsequent growth of nanoparticles, particularly the role of organic compounds, remain unclear. This study investigates the heterogeneous ozonolysis of 5-norbornene-2-endo,3-exodicarboxylic acid (NDA) in size-selected aerosol particles (30–110 nm) under varying relative humidity and ozone conditions. NDA serves as a model compound representative of unsaturated monoterpene derivatives with low volatility and high particle-phase partitioning. Using real-time mass spectrometry, we explore how particle size influences both reactivity and product distribution. Our results reveal pronounced particle-size and humidity dependencies: smaller particles favor dimerization and oligomer formation, likely due to increased Laplace pressure, whereas larger particles promote hydrolysis and decomposition reactions. The findings suggest that nanometer-sized particles provide a distinct chemical microenvironment that alters the course of multiphase reactions, thereby affecting particle aging, volatility, and growth potential. These results underscore the importance of considering particle-size-dependent chemistry in models of atmospheric aerosol formation and evolution.