Comparison of daytime and nighttime new particle growth at the HKUST supersite in Hong Kong.

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.