Heterogeneous nucleation measurements in a sheathed planar diffusive condensation particle counter

While the lack of efficient tools yielding controllable uniform saturation ratios (S) has delayed basic experimental heterogeneous nucleation studies, common diffusive condensation particle counters (DCPCs) could fill this gap if their S-variation were minimized by increasing the proportion of sheath gas (σ) surrounding a central core of purified clusters.

We measure the activation probability P of Tetraheptylammonium Bromide cluster cations (THA-Br)_n-1THA⁺ in Kanomax's fast CPC while controlling S through the saturator and condenser temperatures (T_s, T_c), varying σ, and changing the size (n) of purified salt clusters via high resolution mobility selection.

Experimental curves P(T_s,n) obtained in 1-butanol/air at fixed T_c (13 °C) and variable n and T_s (3 ≤ n ≤ 16; 30 ≤ T_s ≤ 40 °C) rise sharply versus both n and T_s. Their steepness increases five-fold with increasing σ to about σ = 75%, with little effect thereafter. Measurements changing S would yield size distributions of unknown aerosols at fairly high resolution. Comparing P(T_s,n) data with predictions from capillary theory suggests that basic heterogeneous nucleation measurements can be carried out, but instrument improvements are still needed.