Atmospheric nanoparticles formed from heterogeneous reactions of organics

Atmospheric aerosols directly and indirectly affect the radiative balance of the Earth’s atmosphere1. Nanoparticles are a key component of atmospheric aerosols, growing rapidly under ambient conditions2–4. Organic species are thought to lead to the growth of nanoparticles smaller than 20 nm (refs 5, 6), but the identity of these species and the underlying chemical mechanisms remain elusive. Here we exposed nanoparticles to a range of organic vapours—2,4-hexadienal, glyoxal and trimethylamine—and monitored particle size to determine the contribution of organic vapours to nanoparticle growth. We show that organic species enhance the growth of nanoparticles, producing non-volatile oligomers, polymers and alkylaminium sulphates in the particle phase. Nanoparticle growth increased with relative humidity in the presence of glyoxal and trimethylamine, but decreased at higher relative humidities in the presence of 2,4-hexadienal, dependent on the reaction mechanism of the organic species involved. Oligomerization and polymerization were largely suppressed in particles smaller than 4 nm and nanoparticle growth increased with particle size. Our findings help to explain the presence of previously measured, but unidentified non-volatile compounds in atmospheric nanoparticles and to improve model simulations of new particle formation. Atmospheric aerosols impair visibility, alter the radiative balance of the Earth’s atmosphere andmodify themicrophysical properties, abundance and lifetime of clouds1. In addition to their climatic significance, aerosols are associated with adverse effects on human health. Considerable efforts have been devoted to understanding the processes governing new particle formation and growth to form cloud condensation nuclei. Whereas sulphuric acid has been commonly identified as an important atmospheric nucleating species2, a variety of other species have been implicated in aerosol nucleation, including ammonia, organic acids and ion clusters7–10. One area of considerable uncertainty in new particle formation concerns the mechanism responsible for the growth of newly nucleated nanoparticles, which increases the particle number concentration by preventing the coagulation frompre-existing particles. Although organic compounds are believed to have a key role in the formation of secondary aerosols, the processes leading to aerosol growth from exposure to organic vapours are highly complex, representing one of the most poorly understood topics in atmospheric aerosol research. Organic aerosol growth includes condensation of low-volatility organic compounds, gas– particle partitioning of semi-volatile organic compounds and heterogeneous reactions of organics. However, particles in the nucleation mode (<20 nm) are subject to a large curvature effect as the particle size (Dp) decreases (the Kelvin effect). The highly elevated equilibrium vapour pressure over freshly nucleated particles implies that condensation of low-volatility organics, such