Sub-3 nm Particles Detection in a Large Photoreactor Background: Possible Implications for New Particles Formation Studies in a Smog Chamber

New particles formation experiments have been conducted in the experimental multiphasic atmospheric simulation chamber (CESAM) smog chamber. The nucleation events generated during the dark ozonolysis of sabinene were monitored using a neutral cluster and air ions spectrometer in the size range 0.8-42nm under simulated atmospheric conditions. The measurements show that a significant amount of pre-existing clusters exist in the 1-2nm size range even in very clean controlled conditions. Numerical modeling experiments indicate that a range of 9-67% of pre-existing clusters can explain, through activation by organic vapors, the number of new particles formed when sabinene is oxidized, with an increasing contributing fraction for increasing condensable vapor concentrations. These findings suggest that atmospheric simulation chambers backgrounds have to be carefully characterized for their sub-3-nm neutral cluster concentration content before nucleation parameterizations can be derived.