Atmospheric nanoparticles (NPs) are often contained in aggregates or included in larger particles. We show that some of these structures can be crushed in water media by the application of ultrasounds, leading to a suspension of insoluble NPs. The contribution of these NPs to the total elemental concentration is evaluated as the difference between the inductively coupled plasma (ICP-MS) analysis before and after the elution of the suspension from an ion exchange cartridge. Total elemental content in PM can be therefore fractionated into three contributions - soluble species, solid NPs released from larger structures, insoluble particles - that may likely have different health and environmental effects.
The method was applied to both Certified Material NIST 1649a and size-segregated atmospheric PM samples collected by a 13-stage impactor. The results indicate that alkaline and alkaline earth metals are found in the suspension only as water-soluble species, also when they are contained in the fine fraction of PM. Instead, a significant fraction of most elements typically emitted from combustion sources (Pb, Sb, Sn, Cd, V and As) is present in fine PM as insoluble nanoparticles that are easily dispersed in water under ultrasound application.