Modeling and Characterization of a Particle-Into- Liquid Sampler (PILS) ∗
2.3 Modified PILS
The basic principle of the PILS instrument is to grow aerosol particles into droplets sufficiently large to be collected by inertial impaction for subsequent chemical
analysis (Weber et al., 2001). Many of the instrumental features of the modified PILS design presented here are the same as those described by Orsini et al. (2003). As shown in Figure 2.1, the modified PILS samples ambient air at approximately 12.5 L min-1, with a critical orifice maintaining a constant volumetric flow rate. A single-stage pre-impactor (D50 = 1 μm) removes super-micrometer sized particles. An automated three-way valve (Swagelok) either directs flow through (filter mode) or bypasses (sample mode) a high- efficiency particulate air (HEPA) filter to allow for background level testing during experiments. A series of three denuders remove gases that can bias aerosol measurements; inorganic vapors are removed by two denuders with annular glass surfaces (URG-2000-30x242-3CSS), and a third denuder (Sunset Laboratory Inc.), composed of 15 thin carbon filter paper sheets (3.15 cm x 20.32 cm x 0.04 cm thick) with 0.2 cm gaps between them, removes organic gases. The glass denuders are placed upstream of the carbon denuder so that detailed extraction tests can be carried out following operation.
The ambient air mixes with steam that is introduced through a tube (0.04 cm ID), the tip of which has a thermocouple attached to provide accurate steam injection tip temperature readings. An algorithm is used to account for pressure fluctuations (specifically for airborne operation) and air flow over the tip, to provide real-time control of the steam injection tip temperature at 100 + 2°C. Rapid adiabatic mixing of the steam and the cooler ambient air produces a high supersaturation of water vapor that allows droplets to grow large enough (Dp > 1 μm) to be collected by inertial impaction onto a wettable quartz or polycarbonate impactor plate. The condensation chamber is kept at a slight angle (15°) to allow wall condensate to be removed by a drain line at the end of the
chamber’s main body. Those particles too small (< 1 μm) to be collected by inertial impaction leave the system with the exhausted air flow.
Impacted droplets are transported down the impactor by a wash-flow (0.15 to 0.20 mL min-1) supplied by two syringe pumps with 250 μL syringes (Kloehn Ltd. Model V3), which work together using a “handshaking” technique. Using this technique, one syringe aspirates sample from a wash-flow stock bottle in a downward stroke, while the other is dispensing already-collected wash-flow to the top of the impactor plate in an upward stroke. A stainless steel mesh wick on the perimeter aids in drawing the liquid down the impactor plate. Two syringes use the “handshaking” technique to extract the liquid sample from the bottom of the impactor plate (0.13 mL min-1) and deliver it to a debubbler; volumes of 250 and 50 μL were used for these syringes (these syringes, regardless of volume, are operated at the same speed, 2.17 μL s-1), with the lower volume reducing the mean liquid residence time in the PILS. The liquid flow rates are controlled by the programmed syringe pump speeds. After air bubbles are removed by the debubbler, the liquid sample is drawn into one of two syringes (at any one time, one syringe aspirates liquid from the debubbler and the other dispenses liquid to the vials);
the debubbler and each syringe behave like two continuously stirred tank reactors (CSTRs) in series. These types of vessels are characterized by perfect mixing, such that the concentration is uniform throughout the vessel. The sample then travels to a computer-controlled injection needle that is inserted into individual vials, the caps of which have septa to minimize contamination. Seventy-two vials are held on a rotating carousel; typically 0.65 mL of sample was collected over a 5-min period in each vial for results presented here. The sample volume and filling time for each vial can be adjusted
to optimize the level of dilution and time resolution needed. The addition of syringe pumps and the vial collection technique are beneficial for aircraft sampling since the liquid sample in each vial can be partitioned into fractions for different analytical tests after each flight. Furthermore, the use of a vial collection system avoids the space, power, weight, and down-time of an analytical device while promoting autonomous operation.
Dilution occurs when the impacted droplets are washed from the impactor plate.
In addition to the wash-flow, dilution takes place as a result of water vapor condensation on the impactor wall. The dilution factor is determined by spiking the wash-flow of Milli-Q water with non-interfering ions (LiF and LiBr were used for this study); this factor, which typically ranges from 1.1 to 1.4 during airborne flights, is calculated by taking the ratio of the Li+ concentration in the wash-flow supply bottle to the Li+ concentration in each vial. The pressure changes that occur during aircraft flights have been shown to have no influence on the dilution factor since the syringe pumps allow for precise liquid sample flow control.
To make meaningful intercomparisons with other instruments sampling the same air stream simultaneously, it is necessary to know the residence time of an air parcel and liquid sample inside the PILS system (Figure 2.2). The debubbler and the syringes complicate this measurement since they act as CSTRs and mix each incoming liquid parcel with their entire liquid volume. An estimate of the residence time in the mixing vessels is made by observing the time needed to empty them when they are completely filled with liquid. All other plumbing residence times are calculated using measured tube volumes and flow rates. The time elapsed from the moment a parcel of air enters the inlet
until it impacts onto the droplet impactor plate is estimated to be 3.6 s; this time will vary between different designs based on the plumbing length between the inlet and the droplet impactor (combined 3 m in this design with tube IDs varying between 0.35 and 1.65 cm).
The time for each syringe to complete a full round-trip (downward stroke followed by an upward stroke) is measured to be 46 and 230 s using 50 and 250 μL syringes, respectively. The last liquid parcel to enter a syringe at the end of a downward stroke exits the syringe at the beginning of the next upward stroke, which results in very little time spent in the syringe. As will be shown later, model predictions show that there is a minimum time delay of 301 s and 304-313 s to overcome a step change in concentration for the original and modified PILS (assuming a 66 s debubbler liquid residence time), respectively; the time delays for the modified PILS are used here as a time-offset to intercompare with DMA measurements. The time-offset can be reduced by lowering the debubbler residence time; this involves optimizing the ratio between the flow rate of the wash-flow at the top of the droplet impactor and the speed of the syringes aspirating liquid from the debubbler.