UPTAKE OF GAS-PHASE ALDEHYDES BY WATER SURFACES

The uptake of gas-phase formaldehyde and acetaldehyde by liquid water droplets was measured as a function of droplet pH and gas-droplet contact time and for formaldehyde also as a function of the sulfite ion density in the droplet. The mass accommodation coefficient for formaldehyde at 267 K is 0.020 +/- 0.003. The mass accommodation coefficient for acetaldehyde at 267 K has a lower limit of 0.03, which is also our best estimate for its value. The uptake of formaldehyde is in accord with the liquid-phase solubility and reaction kinetics of the species. On the other hand, the uptake of acetaldehyde at short gas-droplet contact times is significantly greater than predicted by the calculations based on bulk-phase parameters. The time-dependent studies of the uptake suggest that the greater uptake is due to a chemisorbed surface complex at the gas-water interface. The implications of these results for atmospheric chemistry are examined. Thc small magnitudes of the uptake coefficients suggest that this parameter may be the limiting factor in the uptake rate of the aldehydes into cloud droplets. The chemisorbed surface species observed in the acetaldehyde experiments may explain the high measured concentrations of acetaldehyde in cloud water relative to the concentrations of formaldehyde.