The sequential spot sampler consists of a water condensation growth tube followed by a single-jet impactor collector, and is designed to be operated at a flow rate between 1.0 to 1.5 L/min. The growth tube uses the three-stage, moderated laminar flow condensation approach described by Hering et al. (2014) (link). This provides for sufficient supersaturation to activate particles in the nanometer size range, but moderates the flow temperatures and exiting dew point. Particles as small as 8 nm grow through water condensation to form 1–3 μm droplets, and are readily collected as a concentrated spot. The moderator stage reduces the dew point of the flow exiting the growth tube to less than 18°C, thus avoiding vapor condensation on room temperature surfaces. The active well is heated slightly to evaporate water from the droplets during collection, forming a dry spot.
The system is shown inFigure 1 . The first two stages of the growth tube, referred to as the “conditioner” and “initiator”, are 154 mm and 73 mm long, and are lined with a 4.8 mm ID wick. The wick is formed by rolling 0.45 μm pore size nylon filter media (Whatman 10416194). A thermoelectric device (TED) mounted between the two stages acts as a heat pump, cooling the conditioner and warming the initiator. A set of fans on the initiator stage dissipate the heat generated by the TED. This upper wick is mounted on a standpipe and wetted by means of a small interior water reservoir at the bottom of the initiator. The third stage, called the “moderator” has a separate wick, 100 mm long and 4.8 mm ID, made from the same nylon filter material, held by a standpipe at the bottom. The moderator stage is cooled by means of a second TED. A parasitic flow of 0.05 L/min is extracted from the system at the base of the moderator to remove water condensate that accumulates around the standpipe.
The flow exits the growth tube through a single, 1.2 mm diameter nozzle and impinges into one of the wells of the multiwell collection plate. The multiwell plate contains 24 evenly spaced wells, each measuring 6 mm diameter, 3 mm depth, and capable of holding 100 μL of solution. A spring loaded mount pushes the well-plate up against a donut-shaped Teflon gasket mounted on the ceiling of the collection chamber that covers all but the active well. This shields the nonactive wells from the flow, and minimizes the head space for volatilization of already-deposited samples. The well-plate has a small, spring-loaded heater that touches the bottom side of the multiwell plate immediately underneath the active well. Its temperature is selected to prevent condensation, and to evaporate droplets as they collect, and its setting depends on the details of the multiwell plate material and thickness. There is also a small heater on the nozzle mount to prevent water condensation in the nozzle. A small stepper motor advances the plate to the next well at the end of each collection period.
A microprocessor controls five temperatures, the conditioner temperature, the initiator-conditioner temperature difference, and the moderator, the impaction nozzle and the multiwell plate temperatures. Under normal operation the conditioner is operated at 2°C– 5°C, the initiator is set to 30°C warmer than the conditioner (ie 32°–35°C), and the moderator is set to 10– 12°C. Maintaining a constant temperature difference between the initiator and conditioner provides the same supersaturation, and hence the same particle activation and growth, regardless of slight variations in the absolute temperatures (see Lewis and Hering, 2013 (link)). The temperature controllers for the conditioner and moderator regulate the power to their respective TEDs, while the controller for the initiator cycles the initiator heat sink fans.
The microprocessor also controls the well-plate position, and logs the instrument status. The user may select the duration for each sample (in minutes or hours) and further has the option for either “synchronized” or “unsynchronized” modes. When “synchronized” the sampling schedule aligns with midnight (e.g., with hourly samples starting at the top of the hour, twice daily samples starting at noon and midnight), while “unsynchronized” sampling starts immediately upon receiving the command. The sample and parasitic water transport flows are controlled by means of a small valve and an external pump. For ambient air sampling an external cyclone, with transport flow as needed, is used to provide an inlet precut, typically 2.5 μm.
The system is shown in
The flow exits the growth tube through a single, 1.2 mm diameter nozzle and impinges into one of the wells of the multiwell collection plate. The multiwell plate contains 24 evenly spaced wells, each measuring 6 mm diameter, 3 mm depth, and capable of holding 100 μL of solution. A spring loaded mount pushes the well-plate up against a donut-shaped Teflon gasket mounted on the ceiling of the collection chamber that covers all but the active well. This shields the nonactive wells from the flow, and minimizes the head space for volatilization of already-deposited samples. The well-plate has a small, spring-loaded heater that touches the bottom side of the multiwell plate immediately underneath the active well. Its temperature is selected to prevent condensation, and to evaporate droplets as they collect, and its setting depends on the details of the multiwell plate material and thickness. There is also a small heater on the nozzle mount to prevent water condensation in the nozzle. A small stepper motor advances the plate to the next well at the end of each collection period.
A microprocessor controls five temperatures, the conditioner temperature, the initiator-conditioner temperature difference, and the moderator, the impaction nozzle and the multiwell plate temperatures. Under normal operation the conditioner is operated at 2°C– 5°C, the initiator is set to 30°C warmer than the conditioner (ie 32°–35°C), and the moderator is set to 10– 12°C. Maintaining a constant temperature difference between the initiator and conditioner provides the same supersaturation, and hence the same particle activation and growth, regardless of slight variations in the absolute temperatures (see Lewis and Hering, 2013 (link)). The temperature controllers for the conditioner and moderator regulate the power to their respective TEDs, while the controller for the initiator cycles the initiator heat sink fans.
The microprocessor also controls the well-plate position, and logs the instrument status. The user may select the duration for each sample (in minutes or hours) and further has the option for either “synchronized” or “unsynchronized” modes. When “synchronized” the sampling schedule aligns with midnight (e.g., with hourly samples starting at the top of the hour, twice daily samples starting at noon and midnight), while “unsynchronized” sampling starts immediately upon receiving the command. The sample and parasitic water transport flows are controlled by means of a small valve and an external pump. For ambient air sampling an external cyclone, with transport flow as needed, is used to provide an inlet precut, typically 2.5 μm.
