A Field Intercomparison of Three Cascade Impactors

ABSTRACT

Cascade impactors separate aerosol particles inertially and collect them for later analysis. While laboratory calibrations typically indicate performance close to design specifications, during field operation impactors are subject to a number of sampling artifacts, including particle bounce, inlet and internal losses, and particle size changes as pressure drops within the impactor.

To test the vulnerability of some commonly used impactors to these problems under Held conditions, we participated in a shipboard intercomparison off the coast of Washington state between a micro-orifice uniform deposit impactor (MOUDI), a Berner low-pressure impactor, and a Sierra high-volume slotted impactor. Since there were some inconsistencies in the results, a second intercomparison was performed at Bellows Beach, Hawaii, between two MOUDIs and the Berner impactor.

Impactor samples were analyzed for soluble inorganic ions including Na⁺, K⁺, Cl⁻, and NO⁻ ₃, primarily from large (>1 μm) sea salt particles and NH⁺ ₄, nonsea salt sulfate (NSS), and methanesulfonate (MS⁻), found primarily in smaller aerosols.

The Sierra collected sea salt particles far more efficiently than the other impactors, which had severe inlet losses for 7 μm and larger particles. The MOUDI and Berner showed insignificant differences in the mass median diameter of accumulation mode particles (∼0.34 μm), whereas the Sierra indicated almost twice the diameter (0.58 μm) of the others.