Theoretical Analyses of Aerosol Aging on a Substrate without Wall-Effects by a Cross-Flow

ABSTRACT

Long time (~1day) aging or reactions of aerosol is typically studied using either large aerosol chambers (>10 m3) or particles supported on a substrate to minimize wall effects. To avoid wall effects in the latter, it is often essential that the wall reactivity be extremely small (<10-5 reactions per encounter) and that the particle loadings be very small (<1 pg/cm2) to eliminate transport-limited trace gas depletion near the particles and substrate. We evaluate here a cross-flow approach, which greatly reduces these constraints. Particles are to be supported on a micromesh (~50% or more open area) through which the reactive gas is drawn at around a few hundred cm/s. The analysis shows how the competitions between flow and diffusion establishes a “zone of isolation” several microns wide around each reactive particle, outside of which the reactivity of other particles or the substrate is irrelevant to the local reactions. This cross-flow approach reduces the effects of substrate and collective particle reactivity typically orders of magnitude, and will facilitate aging studies of supported aerosols.