Horizontal Bubble Train Apparatus for Heterogeneous Chemistry Studies:  Uptake of Gas-Phase Formaldehyde

A new horizontal bubble train apparatus has been built to conduct time-resolved gas−liquid interaction studies of interest in atmospheric chemistry. In the apparatus, liquid is pumped horizontally through a 0.4 cm i.d. quartz tube at a controlled speed of 15−35 cm/s. A low-pressure (about 50 Torr) gas flow, carrying the trace gas of interest diluted in helium carrier gas, is injected through a moveable injector into the liquid flow forming well-defined bubbles that fill the diameter of the tube. In a controlled way, 10−25 bubbles are formed per second. The outlet gas flow is then sampled by a differentially pumped mass spectrometer. The position of the bubble injector, which is computer controlled by a stepping motor, determines the bubble travel distance and therefore the gas-liquid interaction time (typically 0.1−10 s). Modeling of the gas uptake and the validation of the apparatus performance are described. The apparatus can measure Henry's law coefficients ( H ) for non-reactive species in the range 10 - 3 to 3.0 M atm - 1 . For reactive species (first-order reaction rate k ), the apparatus measures Hk 1/2 values in the range 0.04−150 (M atm - 1 s - 1/2 ). If the Henry's law coefficient of a species is known, then first order reaction rates in the range 1−10 8 s - 1 can be measured. Using this bubble train apparatus, the uptake of gas phase formaldehyde as a function of pH (0−11) and NaCl concentration (0−4 M) has been measured. The Setchenow coefficient for formaldehyde in NaCl solution at 293 K has been determined to be K s = 0.072 ± 0.004 M -1 .