Records of precipitation in Macau are collected from three local monitoring stations from 1993 to 2003. Chemical compositions of precipitation are analyzed to reveal the causes of wet deposition in Macau. The review of annual rainfall volumes and the volume weighted annual pH values over this period show that there is a slight spatial variation in the rainfall volume and the entire area of Macau is subjected to acidic precipitation. The monthly rainfall volumes and the monthly concentrations of five ionic species including H+ , nssSO4 2-, NO3 - , nssCa2+, and NH4 + are analyzed statistically and their distributions are found to have positive skewness and kurtosis. The monthly rainfall volume is found to follow the Gamma distribution which is consistent with the findings in other regions of Asia. In addition, the monthly concentration of each ionic species is found to follow the lognormal distribution. Nonetheless, the distribution of each ionic species exhibits spatial variability across the stations. Therefore, it is suspected that the sources of wet acid deposition in different parts of Macau are different and further correlation analysis on the values of [H+ ], nss[Ca2+], [NH4 + ], nss[SO4 2-], and [NO3 - ] of the precipitation is carried out.

Correlation analysis on the monthly concentrations of the ionic species shows that the linear correlation between [H+ ] and nss[SO4 2-] is higher than the linear correlation between [H+ ] and [NO3 - ] at two of the monitoring stations. The result indicates that sulfur dioxide could be a relatively important contributor of acid wet deposition with respect to nitrogen dioxide in Macau as suggested by previous preliminary studies. In addition, the eleven-year averaged ratio of [SO4 2-]/[NO3 - ] at Ka-Ho being 5.06 reinforces this conclusion. However, the dominant sources contributing to the acidity in two regions of Macau, namely Fortaleza and Taipa Grande respectively are still unknown since the anions do not show any significant positive correlation to the hydrogen ion at these two stations. It is only ascertained that the dominant source of acidity in Ka Ho can be originated from the emission of SO2 as the relatively strong linear correlation of [H+ ] and nss[SO4 2-] indicates that the free acidity is mainly contributed from the sulfuric acid. For the other stations, the wet acid deposition is then believed to be controlled by local sources of sulfuric acid, nitric acid and some other acids that are not accounted for in the present study. Finally, the monthly concentrations of the ionic species are found to be negatively correlated with the monthly rainfall volume at three monitoring stations. Hence, it is concluded that longer rainfall duration would reduce the acidity in rainwater.