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Abstract To investigate the impact of mineral/road dust particles on the formation of sudden and locally distributed heavy rain in urban areas (hereafter, urban-induced heavy rain: UHR), we analyzed the trace metal elements and the stable isotope composition of hydrogen and oxygen in rainwater. Rainwater samples, which were collected in Shinjuku (Japan) from April 2014 to December 2015, were analyzed for 12 trace metal elements (Al, V, Cr, Mn, Fe, Ni, Cu, Zn, Cd, Pb, Se, and As) in three fractions: coarse suspended particles (>1.2 μm, CSP), acid-insoluble fine suspended particles (0.45–1.2 μm), and a dissolved/acid-soluble fine suspended fraction. Concentrations and wet deposition fluxes of trace metal elements in CSP were markedly higher in UHR than other types of rainfall, i.e., normal rain, typhoon heavy rain, and frontal heavy rain. There were strong positive correlations between δ18O and the total concentration of trace metal elements in UHR (r = 0.902) and specifically for Fe, Mn, Al, V, and Pb in CSP (r = 0.919, 0.883, 0.823, 0.843, and 0.820, respectively). These findings indicate that mineral/road dust particles were removed by in-cloud scavenging process under the meteorological conditions causing UHR. There is one possibility that they could play important roles as giant cloud condensation nuclei and/or effective ice nuclei for the formation of UHR. Highlights • Trace metal elements and the stable isotope ratio of water in rainwater were analyzed • Concn. and deposition fluxes of trace metal in coarse particles were highest in UHR • Mineral/road dust particles could play important roles for the formation of UHR. [ABSTRACT FROM AUTHOR]

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 NO 3 − and nss-SO 4 2 − concentrations in bulk precipitation were higher during the periods including UHR than those not including UHR. The deposition fluxes of NO 3 − and SO 4 2 − showed clear differences at the center of UHR (NO 3 − : 231 μeq/m 2 /d, SO 4 2 − : 234 μeq/m 2 /d) and its surrounding area (NO 3 − : 76.4 μeq/m 2 /d, SO 4 2 − : 86.1 μeq/m 2 /d). Our results suggest that large amounts of inorganic acidic substances are deposited locally by UHR in urban areas in summer. [ABSTRACT FROM AUTHOR]

In order to make clear the impact of air pollution on the formation of sudden and locally-distributed heavy rain in urban area (hereafter Urban Heavy Rain: UHR), we analyzed inorganic ion concentration and stable isotope ratio of water (δD and δ 18 O) in rainwater. Rainwater samples were collected in Shinjuku, which is a representative downtown of Tokyo, Japan, during four years from October 2012 to December 2015. The concentration and wet deposition fluxes of acidic components (H + , NH 4 + , NO 3 − , and nss-SO 4 2 − ) in UHR were especially higher than those in other types of rain events, i.e. normal rain, typhoon heavy rain, and frontal heavy rain. UHR had distinctly lower stable isotope ratios than those in other urban rains with same rainfall amount and summer precipitation systems. There was a high negative correlation between δ 18 O and the distances from the sampling point to the formation area of UHR within 10 km, while there were high positive correlations between δ 18 O and the concentration of acidic components in UHR. These findings indicate that UHR could effectively scavenge acidic substances within cloud and suggest the use of stable isotope ratios as tracers of an urban heavy rain's water and in-cloud scavenging process. [ABSTRACT FROM AUTHOR]