Analysis of wet acid deposition in Macau

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.