Characteristics of trace metal concentration and stable isotopic composition of hydrogen and oxygen in “urban-induced heavy rainfall” in downtown Tokyo, Japan; The implication of mineral/dust particles on the formation of summer heavy rainfall

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.