Real-Time Studies of Chemical Conversions across the Air–Water Interface of a Single Droplet: Generating Secondary Pollutants under Visible Light

Pollutant conversions at the air–water interface of a single droplet are crucial for obtaining accurate predictions on environment quality. Herein, droplet ESI MS was carried out to understand the photochemical conversions of pollutants across the water–air interface of a single droplet. A single droplet was kept on a Y-shaped Cu electrode, realizing the in situ electrospray ionization for the examinations. By applying high voltage, in situ MS detection of a droplet was carried out layer-by-layer, from the water–air interface to the inner droplet. By droplet ESI MS, secondary pollutants in a single droplet were observed under visible-light irradiation, while no obvious conversion was recorded in the bulk solution, indicating the complicated and active conversions in droplets. More significantly, different pollutant conversions at the water–air interface and inner droplet were recorded by the dynamic monitoring via droplet ESI MS detections layer-by-layer. By examining the observed dimer and radical intermediates, photochemical mechanisms across the water–air interface were proposed. By providing an effective pathway for single-droplet examinations, this work enlarges applications of ambient MS into environment studies.