Overall analysis of precipitation chemistry of the Indochina Peninsula in the last two decades from 2003 to 2022

The Acid Deposition Monitoring Network in East Asia (EANET) was launched in the year 2000 with a view to collecting data in order to observe, analyze and monitor air, water, and land pollution in Asia. The following study focuses on the precipitation data from the Southeast Asian countries of Cambodia, Laos, Myanmar, Thailand, and Vietnam, which constitute the Indochina Peninsula. Urban, rural, and remote sites are selected from each country with an aim to analyze spatial distribution in pH, ion concentration, Nitrogen/Sulfur ratios and urban to rural ion precipitation ratios. The region is further divided into four groups (Southeast, North, Mid, and Southwest) based on geography, and it is seen that all four groups well characterize the spatial and temporal distributions in terms of the decadal means and anomalies of precipitation, pH and ionic concentrations. In general, crustal and sea salt cations (Ca2+, Mg2+, K+, and Na+) are rich in the North and Southeast groups, which is different from the other groups in the Western Peninsula showing high NH4+. The most dominant anion is SO42− in the North and Southeast groups. Nitrogen/Sulfur ratios in rainwater increase along with those in emissions in the Mid and Southwest groups, but opposite trends in Nitrogen/Sulfur ratios are found in emissions and rainwater in the North group. Precipitation chemistry at the urban sites seemed to be influenced by local emissions as urban to rural ratios for the major ions are mostly over one.