ANALYSIS OF RAINFALL AND THROUGHFALL CHEMISTRY AT A SWAMP FOREST IN COMPARISON WITH URBAN RAINFALL PARAMETERS IN SINGAPORE

Abstract

Globally, rainwater acidification affects many urban areas. In Singapore, scrutiny of rainfall over forest ecosystems lags urban environments in recent years. Urban climate concepts suggest rainwater chemistry differences can exist at the micro-scale. Thus, accurate information on rainwater inputs into Singapore’s forest ecosystems is lacking. This study established the concentrations of H+, Na+, K+, Ca2+, Mg2+, NH4+, NO3-, SO42- and Cl- in direct rainfall and throughfall, at a freshwater swamp forest and an urban site in Singapore, between June and October 2019. For direct rainfall, the volume-weighted mean pH at the urban site was 4.32 and forest site 4.38. H+ and Cl- concentrations were higher at the urban site, while K+, Ca2+, non-sea-salt Ca2+ and NH4+ lower at 10% significance level. Through principal component analysis, both sites yielded two input sources, i) sea salt, construction and biomass, and ii) industrial pollution. Two throughfall collectors were established at each site. The volume-weighted mean pH at the urban site collectors were 4.13 and 4.99 and forest site collectors 5.09 and 4.60. The two urban throughfall collectors had cations enriched 4.17 and 4.58 times, anions 4.42 and 4.57 times, relative to direct rainfall. The two forest throughfall collectors had cations enriched 1.44 and 2.10 times, anions 1.94 and 1.41 times. Through PCA, one urban site yielded two sources, i) H+ neutralization and ii) a mix of tree leaching and dry deposition. The other yielded one source that removed H+ but enriched all other ions. Both forest sites yielded two sources, i) vehicular emissions and ii) a mix of tree leaching and dry deposition. Overall, rainwater was found to be acidic at both sites and may pose ecological risks to the freshwater ecosystem. Factors affecting rainwater components range from local sources to regional fluxes, underscoring the complexity of urban atmospheric chemistry parameters.