The role of humic-like substances as atmospheric surfactants in the formation of summer-heavy rainfall in downtown Tokyo

In order to study the impact of organic aerosols in the formation of UHR (hereafter UHR = urban-induced heavy rain), we analyzed humic-like substances (hereafter HULIS) that serve as major atmospheric surfactants as well as the major inorganic ions in rainwater collected on an event basis at the Nishi-Waseda Campus of Waseda University in Shinjuku, Tokyo. Fulvic and humic acid fractions in HULIS was determined by the DEAE (Diethylaminoethyl cellulose) -UV method. The total wet deposition flux of HULIS in UHR was 33.5 times higher than that found in a normal rainfall and 57.3 times higher than that found in THR (hereafter THR = typhoon heavy rain). The accumulation of HULIS to acidic anions such as NO3− and SO42− in UHR compared to other types of rain indicates that HULIS in UHR accelerated the growth of cloud droplets via reduction of surface tension, which likely led to the rapid development of cumulonimbus clouds. •HULIS concentration was the highest in urban-induced heavy rain (UHR).•HULIS tended to be most accumulated compared to acidic substances in UHR.•HULIS enrichment in the UHR may promote cumulonimbus development.