Understanding the mechanisms of secondary organic aerosol formation is of critical importance for assessing aerosol radiative forcing and improving air quality control. A key quantity describing the gas-to-particle conversion is the rate at which nanoparticles of certain size are formed per unit volume and time. The quantification of nanoparticle formation rates requires precise knowledge of growth rates. Here we present a newly developed DMA train capable of providing definitive growth rates from size distribution scan frequencies of 1 Hz. This system has proven to yield statistically significant data even at ultra-low post-classification concentrations of 0.1 cm-3. A chamber study of α-pinene ozonolysis under close-to-ambient conditions demonstrates the need for particle size distribution measurements with increased time resolution.