Ultrafine Particle Growth Rate During Biogenic Secondary Organic Matter Formation as a Function of Particle Composition, Size, and Phase

The growth rate of ultrafine seed particles by uptake of secondary organic matter (SOM) is studied with a flow tube reactor to elucidate key factors that can enhance or inhibit the rate. Through application of a time-dependent kinetic model, the molecular growth factor (GF) associated with SOM formation is determined. GF is defined as the fraction of the oxidation products from a biogenic precursor that contribute to seed particle growth. Two seed particle compositions (ammonium sulfate and ammonium bisulfate) are studied under two phase states (effloresced and deliquesced at either 10% or 60% relative humidity) for growth by two different biogenic precursors (isoprene and α-pinene). The results show: (1) ammonium sulfate seed particles give higher GFs than ammonium bisulfate seed particles, (2) increasing the water content of the seed particle increases the GF, and (3) isoprene and α-pinene show the same dependencies of GF on seed particle composition and water content. These dependencies suggest that physicochemical processes in the aqueous layer at or near the air–particle interface enhance the growth rate of particles in the 40 to 80 nm size range, which is relevant to the formation of cloud condensation nuclei.