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We present information regarding the relative performance of five TSI particle sizing instruments when presented with several log-normally distributed particle populations that vary in terms of composition, concentration, and modal mean diameter (in the range of 10-100 nm) in a controlled laboratory environment. In experiments conducted with NaCl, NaNO 3 , and organic aerosols, across a total particle concentration suite ranging from approximately 1 × 10 4 cm −3 to 1 × 10 6 cm −3 , total number concentrations of sub-100-nm diameter particles from four SMPS systems and an FMPS all fall within ±50% of each other (and generally are within ±30%). However, larger discrepancies are evident in the particle size distribution, particularly for the NaCl particles, with an SMPS operated with a water-based CPC exhibiting large negative bias in modal peak concentrations relative to the isobutanol-based SMPS systems and the FMPS. Much closer agreement is found for NaNO 3 particles, although the SMPS systems tended to exhibit higher modal peak concentrations, and a slight shifting toward lower modal peak diameter than the FMPS.

We report physical particle size distribution and chemical composition data over a Beech forest in Denmark during May and June of 2004 with a focus on four days characterized by high concentrations of ultra‐fine particles. Highest number concentrations of sub‐30 nm particles were observed when net radiation >300 W m−2 and ambient particle surface area <100 μm2 cm−3. The 10–30 nm particles show growth rates of 1.5–4.5 nm hr−1 and are comprised predominantly of ammonium, sulfate and associated water.

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