Chemical ionization mass spectrometric measurements of atmospheric neutral clusters using the cluster-CIMS

This instrument is unique in that it uses a highly sensitive atmospheric chemical ionization technique combined with two neutral cluster separation methods to measure the very low concentrations of clusters formed during nucleation events. This is apparently the first time that selected-ion chemical ionization mass spectrometry has been used to identify nucleating clusters in the atmosphere.

The Cluster-CIMS was well calibrated by using an electrospray high-resolution differential mobility analyzer technique, a novel approach to generating and classifying known ion clusters. Field measurements at a moderately polluted urban site and a relatively remote forested site show that the instrument is capable of detecting neutral sulfuric acid clusters (containing up to four sulfuric acid molecules) during relatively strong nucleation events, with a concentration on the order of 10(4) molecular clusters per cubic centimeter at both sites.