In order to clarify the impact of air pollution on the formation of sudden and locally distributed heavy rain in urban areas (hereafter UHR = urban-induced heavy rain), we analyzed inorganic ions concentration, dissolved and suspended fraction of trace metals, and stable isotope ratio of water (δD and δ18O) in rainwater samples collected from 2008 to 2016 in Tokyo. Acidic substance-derived components (H+, NH4 +, NO3 –, nss-SO4 2–) have high concentrations in UHR, and acidic deposition increased locally along with rainfall amount near urban centers when UHR occurs. In addition, UHR has stable isotopes of hydrogen and oxygen smaller than normal rainfall, indicating that the influence of marine-derived water vapor was small. The concentration of air pollutants before the UHR was high, and secondary reaction rapidly progressed just before the UHR. Based on these results, we proposed the following UHR formation mechanism: (1) Air pollutants accumulate from the surrounding area into the area with the low atmospheric pressure due to the development of upflow by urban warming. (2) Due to oxidants formed by photochemical reactions, sulfate and nitrate that are produced secondarily from SO2 and NOx become cloud condensation tuberculosis (CCN). (3) These CCNs generate many small cloud particles using water vapor evaporated from the urban area. (4) When the updraft is further developed, moist sea breezes containing sea salt particles, which are giant cloud condensations, flow in to form large cloud particles, which rapidly grow into raindrops due to collection with small water droplets.
ASJC Scopus subject areas