Nanoparticle Growth Mechanisms During New Particle Formation (Invited)

Atmospheric new particle formation (NPF) is a key source of ambient ultrafine particles that may contribute substantially to the global production of cloud condensation nuclei (CCN). While NPF is driven by atmospheric nucleation, its impact on CCN concentration depends strongly on atmospheric growth mechanisms since the growth rate must exceed the loss rate due to scavenging in order for the particles to reach the CCN size range. We have made chemical composition measurements of 20 nm diameter particles during NPF in urban and remote locations using the Nano Aerosol Mass Spectrometer (NAMS). These measurements provide a quantitative assessment of important growth channels. We find several common features among NPF events in different locations and different times of year. These features will be discussed in the context of our most recent field campaigns. At the onset of NPF, we find that the composition of 20 nm particles changes even though the total aerosol mass concentration does not. These changes provide insight into the nucleation process since the chemical species that condense onto pre-existing 'large' particles are likely to be similar to those that come together to form new particles. In this regard, the species found to condense onto pre-existing particles during this time period include sulfuric acid, ammonia/amine and high O/C ratio carbonaceous matter. As the mode diameter increases and the aerosol mass concentration grows through the NAMS-measured size range, other chemical species can increase relative to those involved in nucleation. The sulfur content of particles during this time period is adequately described by condensation of gas phase sulfuric acid molecules. The nitrogen content is not as well understood, and there is evidence for nitrogen species in addition to the cations (ammonium/aminium) that neutralize sulfate. Furthermore, there is evidence for an activation barrier for uptake of ammonia/amines into the particles. The carbonaceous matter associated with particle growth has an O/C ratio characteristic of fresh and/or slightly aged SOA.