Towards a concentration closure of sub-6 nm aerosol particles and sub-3 nm atmospheric clusters

Atmospheric clusters play a key role in atmospheric new particle formation and they are a sensitive indicator for atmospheric chemistry. Both the formation and loss of atmospheric clusters include a complex set of interlinked physical and chemical processes, and therefore their dynamics is highly non-linear. Here we derive a set of simple equations to estimate the atmospheric cluster concentrations in size ranges of 1.5-2 nm and 2-3 nm as well as 3-6 nm aerosol particles. We compared the estimated concentrations with measured ones both in a boreal forest site (the SMEAR II station in Hyytia center dot la center dot, Finland) and in an urban site (the AHL/BUCT station in Beijing, China). We made this comparison first for 3-6 nm particles, since in this size range observations are more reliable than at smaller sizes, and then repeated it for the 2-3 nm size range. Finally, we estimated cluster concentrations in the 1.5-2 nm size range. Our main finding is that the present observations are able to detect a major fraction of existing atmospheric clusters.