This phenomenon involves convective heavy rains that occur suddenly and locally in an urban area, which we here call urban-induced heavy rain (UHR).
Increased heavy rain events in the summer have been also reported for urban areas all over the world since the latter part of the twentieth century. Heavy rain events of more than 50 mm/h were frequently observed during the summer over two decades in heavily urbanized area in Japan. These results indicate that NO 2, which was formed secondarily by oxidants, was further oxidized by HO radicals and formed HNO 3 just before the formation of UHR, which was subsequently scavenged by UHR.
The level of air pollutants, such as NO 2, SO 2, and potential ozone, in the ambient air increased just before the formation of UHR and decreased sharply at the end of the UHR event. Hourly rainfall amounts and wet deposition fluxes of acidic components (the sum of H +, NH 4 +, NO 3 −, and nonsea-salt SO 4 2−) in UHR were 13.1 and 17.8 times larger than those in normal rainfall, respectively, indicating that large amount of air pollutants were scavenged and deposited by UHR with large amounts of rainfall. 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 in rainwater samples collected on an event basis over 5 years from October 2012 to December 2016 in Shinjuku, Tokyo.
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