Observation of nucleation mode particle burst and new particle formation events at an urban site in Hong Kong

Particle number (PN) concentrations and particle size distributions (PSD) in the size range of 5.5-350 nm were continuously measured from 22 December 2010 to 20 January 2011 at an urban site in Hong Kong when northeastern monsoon prevailed. Apart from the PN peaks appeared in traffic rush hours (i.e. 08:00-09:00 and 17:00-18:00), a distinct peak of PN concentrations in the afternoon (11:00-16:00) was observed during the sampling period. Concurrent measurement data of PSD, ozone (O3) and proxy sulfuric acid (H2SO4) concentrations revealed that the afternoon peaks observed were likely due to new particle formation (NPF) via photochemical reactions. These NPF events were frequently observed under a clean and dry weather in Hong Kong. The occurrence of NPF was closely associated with high solar radiation (SR), low relative humidity (RH) and low condensation sink (CS) in the atmosphere. Besides the NPF events, we also found four nucleation mode particle burst events, typically with increased number concentrations of nucleation mode particles (Nnuc) without growth to larger size particles. These burst events were generally accompanied by high-level primary air pollutants, i.e. sulfur dioxide (SO2), nitrogen oxide (NOx) and carbon monoxide (CO), low SR and high CS conditions. The very different characteristics of the burst events from those of the NPF events indicated that these nucleation mode particle burst events were not caused by the photochemical reactions, but by the primary emission from the local combustion source(s).