Three Distinct Precipitation Patterns Characterized by Sulfate Analysis in Individual Raindrops

Conventional methods for analyzing components in rainwater are generally performed on samples collected in a sampler. Because the components and concentrations contained in individual raindrops differ, a rainwater collection and analysis method with higher temporal and spatial resolution is required to clarify this information. We developed a sampler to collect and quantify individual raindrops directly onto a thin hydrogel film containing barium ions, which react with sulfate ions in raindrops to form a precipitate. We estimated the volume of raindrops from droplet traces collected during 14 actual precipitation events, and calculated the number of moles of sulfate in the raindrops from the number of white pixels in the gel images. The diameter distribution of the raindrop imprints obtained on the hydrogel from several tens to several hundreds of micrometers for all precipitation events. The amount of sulfate contained in raindrops and the sulfate concentration varied greatly depending on the raindrop, ranged from 0 to several pmol and from 0 to several mmol/L, respectively. Correlation analysis of the calculated raindrop volume, sulfate mole numbers, and sulfate concentrations revealed different relationships for each precipitation event. Moreover, a tendency for one of the correlation coefficients to be higher among the three distributions was observed for each precipitation event, suggesting that the developed method will facilitate elucidation of the raindrop formation and growth process.