A comparison of bulk precipitation chemistry during normal and typhoon events on subtropical Okinawa Island (Japan)

Abstract

Atmospheric acidic deposition and typhoons are prevalent environmental problems on subtropical Okinawa Island. This study was conducted to examine the chemical composition of bulk precipitation at Gesashi estuary (P1 and P2 sites) for one year, and compare the results with precipitation chemistry during four typhoon events on the Island. The acidic precipitation (pH < 5.70) was only observed during normal precipitation events with an average pH value of 5.42 at P1, while the highest mean pH of 6.11 was revealed during typhoons. The sum of the acidic minus basic concentrations (i.e. [non-sea salt (nss)–SO 4 2− + NO 3 −] − [nss–Ca 2+ +NH 4 +]) revealed a high linear correlation with pH ( P < 0.001), indicating that the pH was largely controlled by nss–SO 4 2−, NO 3 −, nss–Ca 2+ and NH 4 +. The average values of nss–SO 4 2−, NO 3 − and NH 4 + concentrations were 27.8, 14.5 and 11.5 μeqL − 1 , respectively in the Gesashi samples. They decreased by ∼ 2 fold during typhoons, contrary to nss–Ca 2+ which showed increasing trend. The Na + and Cl − ions were the dominant ions, which showed high linear correlations ( P < 0.001), which suggest that they were largely from a marine source. The average concentrations of sea salt ions increased by ∼ 4 fold during typhoons compared to Gesashi samples. A positive linear relation between electric conductivity and daily average wind speed ( P < 0.005) for the four typhoon samples may indicate high emission of seawater droplets to the atmosphere owing to typhoon winds. Dissolved Al and silica showed ∼ double average concentrations during typhoons than Gesashi samples. A significant positive linear correlation ( P < 0.001) between them suggests similar origin, probably from wind-blown soil dust. The acidic deposition during normal precipitation events may enhance solubility and transport of Al from the Okinawan acidic red soil to surface waters while typhoons tend to mitigate the acidic pollutants.