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When studying new particle formation, the uncertainty in determining  the "true" nucleation rate is considerably reduced when using  condensation particle counters (CPCs) capable of measuring  concentrations of aerosol particles at sizes close to or even at the  critical cluster size (1–2 nm). Recently, CPCs able to  reliably detect particles below 2 nm in size and even close to  1 nm became available. Using these instruments, the corrections  needed for calculating nucleation rates are substantially reduced compared to scaling the  observed formation rate to the nucleation rate at the critical cluster  size. However, this improved instrumentation requires a careful  characterization of their cut-off size and the shape of the detection  efficiency curve because relatively small shifts in the cut-off size  can translate into larger relative errors when measuring particles  close to the cut-off size.Here we describe the development of two continuous-flow CPCs using  diethylene glycol (DEG) as the working fluid. The design is based on  two TSI 3776 counters. Several sets of measurements to characterize  their performance at different temperature settings were carried  out. Furthermore, two mixing-type particle size magnifiers (PSM) A09  from Airmodus were characterized in parallel. One PSM was operated at  the highest mixing ratio (1 L min<sup>−1</sup> saturator flow), and the  other was operated in a scanning mode, where the mixing ratios are  changed periodically, resulting in a range of cut-off sizes. The mixing  ratios are determined by varying the saturator flow, where the aerosol flow  stays constant at 2.5 L min<sup>−1</sup>. Different test aerosols were generated  using a nano-differential mobility analyser (nano-DMA) or a high-resolution DMA,  to obtain detection efficiency curves for all four CPCs. One calibration setup included  a high-resolution mass spectrometer (APi-TOF) for the determination of  the chemical composition of the generated clusters. The lowest cut-off  sizes were achieved with negatively charged ammonium sulfate  clusters, resulting in cut-offs of 1.4 nm for the laminar flow  CPCs and 1.2 and 1.1 nm for the PSMs. A comparison of one of  the laminar-flow CPCs and one of the PSMs measuring ambient and  laboratory air showed good agreement between the instruments

Performance of diethylene glycol-based particle counters in the sub-3 nm size range