Nanoparticle Chemical Composition During New Particle Formation

The nano aerosol mass spectrometer (NAMS) was deployed at a coastal site in Lewes, Delaware, to measure the composition of 21 nm mass normalized (18 nm mobility) diameter nanoparticles during new particle formation (NPF) events. NAMS provides a quantitative measure of the atomic composition of individual nanoparticles. NAMS analysis of ambient particles showed only a small change in particle composition during NPF events in Lewes compared with off-event (before and/or after the event). The N mole fraction increased 15% on-event, whereas the C mole fraction decreased 25%, suggesting an enhanced inorganic component to the aerosol during NPF. The measured changes in atomic composition constrain the possible changes in molecular composition. To explore these constraints, an apportionment algorithm was applied to the atomic composition data. This algorithm partitions the atomic composition into sulfate, nitrate, and ammonium on the basis of the atomic abundance of S, N, and O and into organic matter on the basis of C and residual O after removing contributions to inorganic species. Particles were fully neutralized both on-and off-event. The nitrate to sulfate ratio during NPF ranged from 0.7 to 1.4, suggesting that ammonium nitrate is important to particle growth in this environment. Nonetheless, nanoparticles had a significant organic fraction, and upper limits for cationic amine content were determined. The relatively small changes in total particle composition on-event versus off-event suggest that observed changes in particle hygroscopicity and volatility during NPF at other locations may be linked to subtle changes in particle composition or to shifts in the character of the organic content.