10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
From Particle Counting to Aerosol Collection: Just Add Water
SUSANNE HERING, Aerosol Dynamics Inc.
Abstract Number: 1730 Working Group: Invited by Conference Chair
Abstract Ultrafine, airborne particles are important in the atmosphere, and in some industrial processes. Yet direct measurement is difficult, as most are too small to detect optically, or to collect inertially. Since the nineteenth century, aerosol scientists have circumvented this issue using condensational growth to enlarge ultrafine particles. Today, applications extend from number concentration measurements, to collection for chemical analysis, to aerodynamic concentration for mass spectrometry or exposure studies.
Although the original, adiabatic expansion methods of Aitken and Courtier used water as the condensing species, the more wide-spread method since the 1970s has been laminar-flow, alcohol-based condensational growth. The continuous flow and single particle counting capability of these methods is important to many applications, but the alcohol is not always suitable. The desire to return to water as the condensing material, without sacrificing the advantages of the alcohol systems, motivated the development of the laminar-flow water condensation particle counter (water-CPC). The challenge is the high diffusivity of water vapor. The water-CPC reverses the temperatures of the laminar flow alcohol instruments, with a cold saturator followed by a warm, wet-walled condenser. This approach recognizes that condensation is not created by a cold region, but rather is a result of the relative rates of thermal and vapor diffusion. Water, being the smaller molecule, has a mass diffusivity that is larger than the thermal diffusivity of air. As a cooled flow enters the warm, wet walled region, both heat and water vapor diffuse into the flow; yet the water vapor wins the race, creating a region of high supersaturation along the centerline. This approach readily activates condensational growth at 5 nm. However, as originally implemented, the output flow that is warm and humid, forcing optics to be operated at elevated temperature, and compromising volatile species collected for chemical analysis.
A new, “moderated” approach eliminates the warm, humid output flow. The uniformly warm, wet-walled condensing region of the original approach is replaced by a short, warm wet-walled, “initiator” section followed by a somewhat longer cold, “moderator” section. Once the vapor supersaturation is created within the wet-, warm-walled initiator region, the saturation profiles are insensitive to the temperature of the walls downstream. The subsequent moderator section can be operated the same temperature as the initiator (as done in the first water-CPCs), or at a colder temperature, with no affect on the peak supersaturation. But with the cold moderator the temperature and dew point of the output flow are below 20°C, and non-condensing at typical room temperature.
This “moderated” method opens the door to many additional applications. It is possible to collect ambient aerosol in a concentrated, dry 1-mm diameter spot without heating the sample above ambient temperatures. The approach can be used for directly imaging particles within a mobility separation column, without fogging the optics, enabling fast size distributions, or rapid determination of hygroscopic growth. With the moderated approach, the output temperature is not longer coupled the temperature of the warm initiator section, and thus the system can be operated at high temperature differences between the first two stages, enabling the detection of particles in the 1-2 nm size range. Moreover, it possible to control the output relative humidity to the same value as the that of a conditioned sampled air stream, creating a reservoir-less system that operates for weeks without replenishing the wick. Collection and counting can be combined in a single unit – making self-sustaining condensation counter-collector, such that every particle once counted is deposited on a small substrate that may be removed for microscopic or micro-chemical analysis. These are some of the various applications of water-based condensational growth to aerosol measurement.