Investigation of air–water exchange of formaldehyde using the water surface sampler: Flux enhancement due to chemical reaction

Laboratory (n=30n=30) and field (n=89n=89) experiments were conducted using the water surface sampler (WSS) to investigate the air–water exchange of formaldehyde (HCHO). Average gas-phase overall mass transfer coefficients (Kg) calculated using concurrently measured gas-phase fluxes and air concentrations were 0.58±0.21 and 0.25±0.12 cm s−1 for laboratory and field experiments, respectively.Kg values were compared to the predictions of two different models, one previously developed based on experiments performed with the WSS (considering no enhancement, Model I) and, one previously published by others (accounting the enhancement due to chemical reaction, Model II). The predictions of Model I (0.17±0.07 cm s−1) significantly underestimated the laboratory determined Kg while the predictions of Model II (0.58±0.17 cm s−1) were in excellent agreement with those measured. Under the laboratory conditions, the flux enhancement of HCHO mass transfer due to chemical reaction ranged between 2.8 and 4.1 times (average±SD, 3.6±0.4).For field studies, the average measured Kg was significantly lower than the average predictions of Model I (0.44 cm s−1) and the Model II (0.90 cm s−1). It was shown that the difference between the modeled and experimental Kg values was due to the propagated effect of interfering factors with the measured gas-phase flux (i.e., decreased deposition due to non-zero water concentration, sulfite/bisulfite interference, and losses due to chemical degradation or transformation).The results of this study indicated that the formaldehyde transfer into the surface waters is significantly enhanced due to chemical reaction and air–water exchange may be an important removal mechanism for atmospheric HCHO. The results also suggest that transformation and degradation of aqueous HCHO may be significant under field conditions.