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 ¿m2 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.