Abstract. An intensive aerosol characterization experiment was performed at the Taipei Aerosol and Radiation Observatory (TARO, 25.02° N, 121.53° E) in the urban area of Taipei, Taiwan, during July 2012. Number concentration and size distribution of aerosol particles were measured continuously, which were accompanied by concurrent measurements of mass concentration of submicron particles, PM₁ (d ≤ 1 μm), and photolysis rate of ozone, J(O^1D). The averaged number concentrations of total (N_total), accumulation mode (N_acu), Aitken mode (N_Aitken), and nucleation mode (N_nuc) particles were 13.9 × 10³ cm⁻³, 1.2 × 10³ cm⁻³, 6.1 × 10³ cm⁻³, and 6.6 × 10³ cm⁻³, respectively. Accordingly, the ultrafine particles (UFPs, d ≤ 100 nm) accounted for 91% of the total number concentration of particles measured in this study (10 ≤ d ≤ 429 nm), indicating the importance of UFPs to the air quality and radiation budget in Taipei and its surrounding areas. An averaged N_nuc / NO_x ratio of 192.4 cm⁻³ ppbv⁻¹ was derived from nighttime measurements, which was suggested to be the characteristic of vehicle emissions that contributed to the "urban background" of nucleation mode particles throughout a day. On the contrary, it was found that the number concentration of nucleation mode particles was independent of NO_x and could be elevated up to 10 times of the "urban background" levels during daytime, suggesting a substantial amount of nucleation mode particles produced from photochemical processes. Averages (± 1σ) of the diameter growth rate (GR) and formation rate of nucleation mode particles, J₁₀, were 11.9 ± 10.6 nm h⁻¹ and 6.9 ± 3.0 cm⁻³ s⁻¹, respectively. Consistency in the time series of the nucleation mode particle concentration and the proxy of H₂SO₄ production, UVB · SO₂/CS, for new particle formation (NPF) events suggested that photooxidation of SO₂ was likely one of the major mechanisms for the formation of new particles in our study area. Moreover, it was revealed that the particle growth rate correlated exponentially with the photolysis of ozone, implying that the condensable vapors were produced mostly from photooxidation reactions. In addition, this study also revealed that N_nuc exhibited a quadratic relationship with J₁₀. The quadratic relationship was inferred as a result of aerosol dynamics and featured NPF processes in urban areas.