Observation of aerosol size distribution and new particle formation at a mountain site in subtropical Hong Kong

Abstract. In order to investigate the formation and growth processes of nucleation mode particles, and to quantify the particle number (PN) concentration and size distributions in Hong Kong, an intensive field measurement was conducted from 25 October to 29 November in 2010 near the mountain summit of Tai Mo Shan, a suburban site approximately the geographical centre of the New Territories in Hong Kong. Based on observations of the particle size distribution, new particle formation (NPF) events were found on 12 out of 35 days with the estimated formation rate J_5.5 from 0.97 to 10.2 cm⁻³ s⁻¹, and the average growth rates from 1.5 to 8.4 nm h⁻¹. The events usually began at 10:00–11:00 LT characterized by the occurrence of a nucleation mode with a peak diameter of 6–10 nm. Solar radiation, wind speed, sulfur dioxide (SO₂) and ozone (O₃) concentrations were on average higher, whereas temperature, relative humidity and daytime nitrogen dioxide (NO₂) concentration were lower on NPF days than on non-NPF days. Back trajectory analysis suggested that in majority of the NPF event days, the air masses originated from the northwest to northeast directions. The concentrations of gaseous sulfuric acid (SA) showed good power-law relationship with formation rates, with exponents ranging from 1 to 2. The result suggests that the cluster activation theory and kinetic nucleation could potentially explain the observed NPF events in this mountainous atmosphere of Hong Kong. Meanwhile, in these NPF events, the contribution of sulfuric acid vapor to particle growth rate (GR_5.5–25) ranged from 9.2 to 52.5% with an average of 26%. Measurement-based calculated oxidation rates of monoterpenes (i.e. α-pinene, β-pinene, myrcene and limonene) by O₃ positively correlated with the GR_5.5–25 (R = 0.80, p < 0.05). The observed associations of the estimated formation rate J_5.5 and the growth rate GR_5.5–25 with gaseous sulfuric acid and volatile organic compounds (VOCs) suggested the critical roles of sulfuric acid and biogenic VOCs (e.g. α-pinene and β-pinene) in these NPF events.