Particles larger than 50-100 nm in diameter have been considered to be effective cloud condensation nuclei (CCN) under typical atmospheric conditions. We studied the growth of newly formed particles (NPs) in the atmosphere and the conditions for these particles to grow beyond 50 nm at a suburban coastal site in Hong Kong. Altogether, 17 new particle formation events each lasting over 1 h were observed in 17 days during 8 Mar-28 Apr and 1 Nov-30 Dec 2011. In 12 events, single-stage growth of NPs was observed in daytime when the median mobility diameter of NPs (Dp) increased up to ∼40 nm but did not increase further. In three events, two-stage particle growth to 61-97 nm was observed at nighttime. The second stage growth was preceded by a first-stage growth in daytime when the Dp reached 43 ± 4 nm. In all these 15 events, organics and sulfuric acid were major contributors to the first-stage growth in daytime. Ammonium nitrate unlikely contributed to the growth in daytime, but it was correlated with the second-stage growth of ∼40 nm NPs to CCN sizes at nighttime. The remaining two events apparently showed second-stage growth in late afternoon but were confirmed to be due to mixing of NPs with pre-existing particles. We conclude that daytime NP growth cannot reach CCN sizes at our site, but nighttime NP growth driven by organics and NH4NO3 can.

Grown nucleated particles >50 nm in diameter are an important source of cloud condensation nuclei (CCN) and when the size is >100 nm, they can also have direct influence on the climate. In this study, the nucleation and growth of new particles in the atmosphere in Hong Kong were investigated during dry season (monthly averaged RH < 75%). The maximum size of grown nucleated particles was generally less than 40 nm during new particle burst and growth events. The exception, accounting for ∼20% of all burst and growth events, was those induced by strong photochemical reactions, in which subsequent particle shrinkage occurred. Temporal particle and gas concentration variability and meteorological conditions support the occurrence of particle shrinkage. The shrinkage rate calculated (∼8 nm h -1 ) was close to the growth rate. The observation of particle shrinkage sheds new light on particle transformation dynamics and it would add to the understanding of particle behavior in the atmosphere.

An Engine Exhaust Particle Sizer was used on a mobile platform to measure ultrafine particle (5.6-560 nm) concentration profiles in streets and tunnels. Particle size distribution data along the lengths of the tunnels showed that number concentration profiles of <30 nm particles were highly nonlinear. Conventional point measurement studies inside tunnels to determine, for instance, emission factors of ultrafine particles in this range are therefore location dependent and may not be representative. The particle concentration profiles obtained in this experiment allowed the study of the transformation of 10 nm particles to 50 nm particles. Outside of the tunnels and on heavily traveled highways, particles exhibited a dominant mode at 10 nm and a minor mode at 50 nm. Inside the tunnels, 10 nm particles gradually decreased after an abrupt increase, while 50 nm particles increased from entrance to exit. The 10 nm particles decreased at 2-4 times the rate of increase of the 50 nm particles, therefore, condensational growth of the 10 nm particles alone cannot explain the decrease. The estimated hetero-coagulation rate between 10 nm particles and particles >30 nm inside the tunnel was substantially higher than that outside due to the increase in the concentration of particles >30 nm. The hetero-coagulation process is proposed to play a key role in the transformation of particle modes in the tunnel. This study provides new insight on the transformation of ultrafine particle modes in vehicular plumes.