The formation and behaviour of exhaust emissions is affected by environmental, traffic and meteorological conditions. The understanding of the governing processes and dependency between particles and other relevant parameters, as well as the magnitude of the impacts, is still limited and mostly based on a few laboratory studies. The focus of this work is the effect of temperature (TEMP) and relative humidity (RH) and their interaction on traffic emission particles in the size range of 15-850 nm. The relationship was assessed using a large data set collected over a period of six months at two road sites in Brisbane. A sequence of statistical analyses were designed and applied in order to quantify the relationships, comprising exploratory correlation analysis to identify pairwise linear associations, factor analysis to assess multivariate effects and nonparametric regression tree methods to more carefully explore interactions. The results show that total particle number concentration was dominated by traffic flow rate and wind speed and to a lesser degree by RH and TEMP. In general, an inverse relationship between TEMP and concentration and a direct relationship between RH and concentration was observed. While TEMP was a dominant parameter for particle concentrations in the size range 15-30 nm, its role diminished and RH emerged as a stronger influence as particle size increased. The observed increase for particle concentrations in the size range 50-150 nm could be associated with particle transfer from a smaller to larger size group due to coagulation and condensation induced growth, as well as an increase in primary (engine) emissions. The significant influence of RH on particles in 150-880 nm size range could be related to particle growth, changes in hygroscopic properties of traffic emissions and particles originating from sources other than traffic. Decreased combustion efficiency may also contribute to higher emissions of particles in this size range.