Uptake rate measurements of methanesulfonic acid and glyoxal by aqueous droplets

The uptake kinetics of methanesulfonic acid (CH3SO3H, MSA) and glyoxal (CHOCHO, ethanedial) by aqueous solutions were studied as a function of temperature using the droplet train technique combined with mass spectrometry and FTIR detection. The measured uptake kinetics for MSA were shown to be independent of the composition of the aqueous phase for NaCl concentrations in the range from 0 to 2 M. The mass accommodation coefficient was determined as a function of temperature between 261 and 283 K. The measured values decreased from 0.16 to 0.1 in this temperature range. The uptake kinetics of glyoxal were studied as a function of temperature between 263 and 283 K and were very close to our detection limit in pure water (i.e., uptake coefficient gamma close to 10(-3)) but were strongly affected by the pH (in the range from 1 to 14) or by sulfite ions (gamma increasing to similar to 0.02). The rate constant of the reaction between nonhydrated glyoxal and sulfite ions was determined to be similar to 7.6 x 10(6) M-1 s(-1) at 283 K. The mass accommodation temperature dependence was beyond the sensitivity of the technique employed in this study; therefore, we report an average value of alpha = 0.023 for the studied temperature range. The uptake kinetics of CHOCHO were shown to be in agreement with bulk properties for temperatures larger than 273 K but deviate from it below. A surface reaction where glyoxal is protonated prior to accommodation was discussed as a possible explanation for an increased uptake rate in acidic solutions.