Effects of the VACES particle concentrator on secondary organic aerosol and ambient particle composition

The effects of concentrating particles using the Versatile Aerosol Concentration Enrichment System (VACES) were investigated with a focus on the organic particle composition. The VACES uses water condensation to grow particles and allow enrichment in particle concentrations by virtual impaction, followed by diffusion drying to remove excess liquid water. Aerosol mass spectrometry was used to compare the submicron organic composition before and after the VACES for laboratory-generated secondary organic aerosol (SOA) from ozonolysis of α-pinene as well as ambient particles. Size distributions were retained after the VACES for both types of samples. SOA particles showed evidence of high molecular weight compounds forming after enrichment, suggesting that some oligomerization occurs during the water condensation and/or evaporation stages, but with only slight changes in the elemental composition. Oligomerization was also observed for ambient particles, especially those with higher O/C in the accumulation mode. Ambient ultrafine particles with low O/C were not as efficiently enriched. Such composition changes in the organic fraction may be relevant to other methodologies where particles are grown through water uptake and then dried, as well as to atmospheric processes involving water uptake and evaporation. In the context of particle mass, the extent of oligomerization is small. However, given the widespread use of particle concentrators in health effects and particle composition studies, whether this impacts interpretation of health effects and particle composition data when using particle concentrators needs further exploration.