The impacts of “urban-induced heavy rains” on the distribution of deposition fluxes of inorganic acidic substances in the Tokyo metropolitan area in summer

We analyzed the trends in rainfall amounts and the number of heavy rain events with hourly rainfall amounts over 30 mm at the inland section of the 23-ward Tokyo metropolitan area (nine wards and ten sites) in the warm season (July to September) from 1978 to 2008. Heavy rain events in the Nerima Ward occurred nine times during decade I (1978–1987; 0.90 times/yr), 13 times during decade II (1988–1997; 1.3 times/yr), and 23 times during decade III (1998–2008; 2.1 times/yr). The annual average number of heavy rain events was 1.43 times/yr in Nerima from 1978 to 2008, while it was 0.98 times/yr on average at nine locations excluding Nerima. The frequency of extremely strong heavy rain events with hourly rainfall amounts greater than 50 mm during decade III was the highest in Nerima (six times) among ten sites. These data suggest that the frequency of heavy rain events has increased at Nerima during the past 30 years. Bulk precipitation data were collected biweekly by filtration-type collectors at 14 sites around Nerima from 2008 to 2010 to determine the impacts of urban-induced heavy rains (hereafter UHR), defined as a heavy rain event other than typhoons and frontal heavy rains, on the distribution of deposition fluxes of inorganic acidic substances. The NO₃ ⁻ and nss-SO₄ ^{2 −} concentrations in bulk precipitation were higher during the periods including UHR than those not including UHR. The deposition fluxes of NO₃ ⁻ and SO₄ ^{2 −} showed clear differences at the center of UHR (NO₃ ⁻: 231 μeq/m²/d, SO₄ ^{2 −}: 234 μeq/m²/d) and its surrounding area (NO₃ ⁻: 76.4 μeq/m²/d, SO₄ ^{2 −}: 86.1 μeq/m²/d). Our results suggest that large amounts of inorganic acidic substances are deposited locally by UHR in urban areas in summer.