CHART: a novel system for detector evaluation against toxic chemical aerosols

Abstract

Concern over the possibility of deliberate dispersion of chemical warfare agents and highly toxic pharmaceutical based agents as persistent aerosols has raised the need for experimental assessment of current and future defensive capabilities of armed forces and law enforcement agencies. Therefor we herewith present the design, realization and validation of the Chemical Hot Aerosol Research Tool (CHART) as a validated and safe experimental set-up for performance evaluation of chemical detection and identification equipment against chemical warfare agents and other highly toxic compounds. In the CHART liquid and solid compounds in solution or suspension are being dispersed as aerosols in a nebulization chamber. A broad dynamic particle size range can be generated, including particles known to be able to reach the lower respiratory tract. The aerosol generated is presented to the detection system-under-test while being monitored and characterized in real-time, using an optical particle counter and a time-of-flight aerosol analyzer, respectively. Additionally, the chemical composition of the aerosol is ex situ measured by analytical chemical methods. Evidently, in the design of the CHART significant emphasis was placed on laboratory safety and containment of toxic chemicals. The CHART presented in this paper has proven to be an indispensable experimental tool to study detectors and fieldable identification equipment against toxic chemical aerosols.

Figure 1

Left: The three main parts of the CHART: Generation, Conditioning and Exposure in the aluminum containment casing. Right: The CHART as realized in the laboratory. The external volume of the CHART is 7.50 m³.

Figure 2

Schematic representation of the virtual impactor used in the CHART. Particles flow from the top through the acceleration nozzle with a flow of 9 L/min and are collected at the collection nozzle with a flow of 1 L/min or let through a filter via the Major flow orifice with a flow of 8 L/min.

Figure 3

(Left) Cumulative normalized particle size distribution of TOP with and without employment of the virtual impactor with a nozzle spacing of 2.93 mm. (Right) The relation between the d₅₀ and nozzle spacing.

Figure 4

Overview of the zones in the CHART. The hot zone comprises of the space where the aerosol is generated, conditioned and exposed and is kept at a negative pressure (25 to 50 mbar below the laboratory air pressure). The safety zone consist of an extracted enclosure which houses all the auxiliary equipment that does not come in direct contact with the hot agent.

Figure 5

Schematic view of the components of the CHART, where the flows constituting the flow balance are indicated with Q̇.

Figure 6

Left: Mass concentration and the moving average over 1 min in the exposure chamber of the CHART in a single dynamic run using the PI Control loop at 6 setpoints and the time resolved number particle geometric mean. Right: the corresponding particle size number distribution as function of time.

Figure 7

Two step concentration profile of a VX exposure for detector evaluation at 0.75 mg/m³ and 1 mg/m³ where the concentration is slowly built to the first step and rapidly increased to the second concentration.