Temporal and spatial variations of wet deposition flux of mineral dust in Japan

To ascertain the temporal and spatial distribution of mineral dust deposition by wet processes, weekly deposition samples were obtained at Sapporo, Toyama, Nagoya, Tottori, Fukuoka, and Cape Hedo (Okinawa) during October 2008-January 2010 using automatic wet-dry separating samplers. Mineral dust weights in water-insoluble residue mixed with pollen were estimated from Fe contents measured using an X-ray fluorescence analyzer. Highest and lowest annual dust fluxes were found respectively at Toyama (8.7 g m^-2 yr^-1) and at Cape Hedo (1.1 g m^-2 yr^-1) in 2009, although their annual levels of precipitation were almost equal (ca. 2300 mm). Wet deposition flux was high in spring and low in summer. Simultaneous wet deposition events were observed five times among the sites, but these events were limited to regional scale. Based on the spatial distribution of dust represented as a mosaic of true color images from MODIS/Terra and vertical distributions of dust concentration from lidar observations, a high wet-dust-deposition event occurring in mid-March at Toyama was attributed to a combination of high dust concentration in the atmosphere during precipitation. Higher wet dust deposition at Toyama and Tottori was ascribed to frequent precipitation during the dusty season.