Field observations and quantum chemical calculations have shown that organic amine compounds may be important in new particle formation processes involving H<sub>2</sub>SO<sub>4</sub>. Here, we report laboratory observations that investigate the effect of trimethylamine (TMA) on H<sub>2</sub>SO<sub>4</sub>-H<sub>2</sub>O nucleation made under aerosol precursor concentrations typically found in the lower troposphere ([H<sub>2</sub>SO<sub>4</sub>] of 10<sup>6</sup>–10<sup>7</sup> cm<sup>−3</sup>; [TMA] of 180–1350 pptv). These results show that the threshold [H<sub>2</sub>SO<sub>4</sub>] needed to produce the unity nucleation rate ([H<sub>2</sub>SO<sub>4</sub>] of 10<sup>6</sup>–10<sup>7</sup> cm<sup>−3</sup>) and the number of precursor molecules in the critical cluster (<i>n</i><sub>H<sub>2</sub>SO<sub>4</sub></sub> = 4–6; <i>n</i><sub>TMA</sub> = 1) are surprisingly similar to those found in the ammonia (NH<sub>3</sub>) ternary nucleation study (Benson et al., 2010a). At lower RH, however, enhancement in nucleation rates due to TMA was up to an order of magnitude greater than that due to NH<sub>3</sub>. These findings imply that both amines and NH<sub>3</sub> are important nucleation species, but under dry atmospheric conditions, amines may have stronger effects on H<sub>2</sub>SO<sub>4</sub> nucleation than NH<sub>3</sub>. Aerosol models should therefore take into account inorganic and organic bases together to fully understand the widespread new particle formation events in the lower troposphere.