Droplet Train Flow Reactor:  In the droplet train apparatus, a fast-moving monodisperse, spatially collimated train of droplets is produced by forcing a liquid through a vibrating orifice located in a separate chamber.  The number of droplets produced per second is determined by the frequency applied to the piezo- ceramic that drives the orifice.  Droplet formation frequencies range from 8 to 60 kHz.  Depending on the pressure of the gas forcing the liquid through the orifice, and the orifice diameter, the speed of the water droplets is in the range 1600-4400 cm s^-1.  The droplet train is passed through a ~30 cm long, 1.4 cm diameter, longitudinal low pressure (6-20 torr) flow reactor that contains the trace gas species at a number density between 10¹³ and 10¹⁴ cm^-3.  The trace gas is entrained in a flowing mixture of an inert gas (usually helium) and water vapor at equilibrium pressure with the water droplets.  Depending on experimental conditions the gas volume flow rate is in the range 80 to 600 cm³ s^-1 corresponding to a linear speed of 50 to 400 cm s^-1.  The flowing carrier gases are introduced at the entrance of the reactor.  The flowing trace gas is introduced through one of three loop injectors located along the flow tube.  By selecting the gas inlet port and the droplet velocity, the gas-droplet interaction time can be varied between about 2 and 15 ms. The density of the species is monitored downstream of the flow tube either via infrared absorption or with a differentially pumped quadrupole mass spectrometer.  A measured decrease in the trace gas signal (Dn^g) resulting from an increase in the exposed droplet surface area, corresponds to an uptake of the gas by the droplet surface.  The uptake coefficient g_meas is obtained from the Dng measurement.

Bubble Train Flow Reactor: A schematic of the apparatus is shown in Figure below. Liquid is pumped through the 0.4 cm i.d. quartz tube at a controlled speed of 15-35 cm/s. The system is designed to operate with aqueous solutions as well as with concentrated sulfuric acid. The liquid is temperature controlled by a coolant flowing through a glass jacket surrounding the reservoir flask. A low-pressure (about 50 Torr) gas flow, carrying trace gas of interest diluted in helium carrier gas, is injected into the liquid flow via 1/12"  stainless steel tubing. Well-defined bubbles are formed filling the diameter of the tube. The bubbles are non-spherical with length between 0.4 and 1.2 cm depending on the gas and liquid flow conditions. Ten to 25 bubbles are formed per second at a controlled rate. The liquid flow carries the bubbles to the end of the flow tube.  The position of the bubble injector, that is computer controlled by a stepping motor, determines the bubble travel distance and therefore the gas-liquid interaction time. The frequency, size, and speed are monitored in the apparatus by two light-emitting diodes (LED) positioned along the flow tube and separated by a fixed distance of 2 cm. After the bubble breaks at the end of the flow tube, the trace species is sampled by a differentially pumped mass spectrometer. Other methods of gas species detection can be easily coupled to the apparatus.

Aerosol Mass Spectrometer (AMS)/Aerosol Flow Reactor:

In the AMS apparatus poly-disperse aerosols are produced in a constant output atomizer from a solution and are entrained in a flow of dry air at atmospheric pressure.  After passing through a silica gel diffusion dryer, where the solvent evaporates from the particles, the organic aerosols are carried into a differential mobility analyzer (DMA).  Within the DMA particles are electrostatically classified by their mobility diameter.  Most of the aerosols carry either one, two or three charges.  Therefore, up to three known sizes are pre-selected and emerge simultaneously from the DMA.  Aerosols are injected into the flow tube through a movable injector.  Here the gas flow is controlled and set to be either in laminar or turbulent flow condition for measuring slow (> 1sec) and fast (<1sec) aerosol processing rates, respectively. After a known aerosol-trace gas interaction time, the aerosols are sampled through a 120 mm pinhole into the aerosol mass spectrometer.  The AMS consists of three differentially pumped chambers.  The sampled aerosols enter the first chamber through an aerodynamic focusing lens which focuses the particles into a narrow beam (~1/2 mm diameter).  The pressures at the lens entrance and exit are about 2 torr and 10^-3 torr respectively.  The pressure difference accelerates the aerosols through the lens, imparting velocities as a function of aerosol aerodynamic diameter.  Typically, velocities range between 150 m/s and 90 m/s for 150 nm and 1 mm particles respectively.  The pressures in the next two chambers are 10^-5 torr and 10^-7 torr respectively.  At these low pressures each particle maintains its velocity as it traverses the three differentially pumped vacuum chambers.  In the third chamber the particle strikes a resistively heated specially shaped surface where the aerosol vaporizes in 20 to 40 microseconds depending on particle composition and heater temperature.  The vapor plume is ionized by electron impact and is analyzed with a quadrupole mass spectrometer. 

        A slitted chopper wheel positioned as shown in the figure intercepts the particle beam producing pulses of particles for time of flight (TOF) mass spectrometric analysis. The chopper transmission period is 1.2 x 10^-4 s.  The time of flight is determined by the interval between the trigger pulse derived from the chopper opening and ion pulse due to the vaporized aerosol.  The ion signal is stored in a time-calibrated bin with a 10 ms resolution.  The time of flight, which is in the millisecond range, can be converted to particle size or more specifically, particle aerodynamic diameter