Abstract. Ambient aerosols play an important role in atmospheric processes affecting the human and natural environments. They affect air quality, reduce visibility, and induce climate change by directly scattering and/or absorbing the incoming solar radiation (Charlson et al., 1992; Kim et al., 2006), or indirectly by acting as cloud condensation nuclei (Hobbs, 1993). Aerosol particles are emitted from a variety of anthropogenic and natural sources either directly into the atmosphere or as secondary particles by gas-to-particle formation process.

There is growing interest in studying and analysing the mechanisms of formation of secondary particles. The development of new instruments during the 1990s to measure the particle size distribution of nanoparticles (<50 nm) has enabled scientists to observe the formation and growth of new particles (see Kulmala et al. (2004) for review). Nucleation events, that is, the appearance of a mode below 25–30 nm in the particle number size distribution, known as "nucleation mode" (e.g. Dal Maso et al., 2007; Tunved et al., 2003), usually in very large numbers, have been observed around the world. For example, they have been reported in remote (e.g. Tunved et al., 2003), urban (e.g. Jeong et al., 2004; Zhang et al., 2004) and coastal areas (e.g. Vaattovaara et al., 2006) and at various latitudes in the upper troposphere and the lower stratosphere (Lee et al., 2003).

It has been shown that the probability of nucleation was increased by elevated sulphur dioxide (SO₂) concentrations (Stanier et al., 2004). This gas is mainly emitted from anthropogenic sources such as the combustion of sulphur-containing fossil fuel (Stern, 2005). Therefore, aerosol nucleation in the atmosphere would be expected to be enhanced by human activities (see also Curtius (2006) for discussion). In urban air, morning nucleation events have been found to be consistent with peaks in traffic (Jeong et al., 2004). In contrast, in coastal environments, higher concentrations of nucleation mode particles have been observed during entries of clean air rather than of polluted air (O'Dowd et al., 2002). This is also confirmed by a Finish study (Spracklen et al., 2006), which found that particle concentrations in remote continental regions are dominated by nucleated particles whereas in polluted continental regions are dominated by primary particles.

This paper aims to analyse the frequency of and the atmospheric conditions favourable for nucleation events at coastal urban location in Brisbane, Australia, with a focus on the contribution of vehicle emissions. Monitoring was conducted during four campaigns of two weeks duration each, and a campaign of four weeks duration, covering a total period of 13 months. The objective was to investigate which meteorological conditions enhanced the probability of nucleation and to investigate any patterns in gaseous concentrations leading to the events to determine whether the local traffic was a major source of secondary particles in the study area.