Relative abundance of new particles competing against the growth of preexisting particles during isoprene ozonolysis

Oligomeric hydroperoxides, including stabilized Criegee intermediates generated during isoprene ozonolysis, play an important role in new particle formation (NPF). In this study, we experimentally determined the relative abundance (ΦNPF) of new particles formed during isoprene ozonolysis, competing against the growth of preexisting particles. The number concentration of newly formed particles (NNPF) during isoprene ozonolysis was derived by comparing the size distribution of secondary organic aerosols (SOAs) in the presence of seed particles with that under humid conditions (relative humidity (RH) > 20%) at the same reaction time. The number concentration of particles that took up semi-volatile organic compounds (Nuptake) was estimated from the difference in the size distribution between particle wall loss (PWL)-considered seed particles and SOAs with seed particles under humid conditions. The ΦNPF was then calculated using the formula: NNPF/(NNPF + Nuptake) under different conditions. The methodology to determine the NNPF was generally successful, whereas the determination of Nuptake was complicated due to the instability of PWL in the small Teflon bag experiments. The ΦNPF can be represented as a product of the rNPF(RH), the relative abundance of new particles formed during isoprene ozonolysis as a function of RH, and the ϕNPF(dry), the ΦNPF value obtained under dry conditions. The obtained rNPF(RH) values suggested that NPF can occur only under very limited RH conditions (RH < 10%) of isoprene ozonolysis in the atmosphere, but the products from the reaction of isoprene with O3, probably Criegee intermediate oligomerization products, were found mainly to contribute to NPF.