A comprehensive 34-year scientific investigation into the chemistry of rainwater across India has revealed a concerning trend: a progressive increase in acidity levels in several key regions, notably Visakhapatnam in Andhra Pradesh, Prayagraj in Uttar Pradesh, and Mohanbari in Assam.
The long-term study, conducted collaboratively by the India Meteorological Department (IMD) and the Indian Institute of Tropical Meteorology (IITM), tracked rainwater composition at 10 Global Atmosphere Watch stations spanning from 1987 to 2021. While true “acid rain,” defined by severely low pH levels posing an immediate ecological threat, is not yet a widespread phenomenon across the nation, the researchers have detected a significant and consistent downward trend in pH values across the majority of monitored locations. The pH scale, ranging from 0 to 14 with 7 being neutral, serves as a crucial indicator of a substance’s acidity or alkalinity. Rainwater with a pH value below 5.65 is classified as acidic. The findings of this extensive research indicate a “general decrease in pH over time” at most of the ten monitoring stations, with particularly significant reductions observed in areas experiencing rapid industrialisation and urban expansion, which are often associated with elevated levels of air pollution. This gradual but persistent decline in pH signifies that rainwater across India is becoming increasingly acidic, raising concerns about potential long-term environmental consequences.
The study pinpoints specific local factors contributing to this growing acidity in different regions. For instance, the increasing acidity observed in Visakhapatnam’s rainwater has been directly linked to emissions emanating from the city’s significant industrial infrastructure, including its oil refinery, thermal power plant, fertiliser manufacturing facility, and the bustling shipping yard. These industrial activities release pollutants into the atmosphere that subsequently react with water vapour to form acidic compounds, which then fall to the earth as rain. In stark contrast, the study highlights the relative resilience of rainwater quality in Jodhpur and Srinagar. According to an IMD scientist associated with the research, these locations benefit from the presence of natural alkaline dust particles originating from nearby arid regions, such as the expansive Thar Desert. These airborne dust particles possess the inherent ability to neutralise acidic components present in the atmosphere, thus mitigating the impact of pollutants on rainwater pH levels in these areas.
Furthermore, the research has uncovered a worrying weakening of rainwater’s natural capacity to neutralise acidic pollutants in several other regions across India. The study reveals that in places like Prayagraj, Jodhpur, and Nagpur, the concentration of calcium particles – which play a crucial role in counteracting acidity – has been declining over time. While some areas are witnessing a concurrent rise in other neutralising agents, such as ammonium compounds, the scientists caution that this increase is currently insufficient to fully offset the overall trend towards greater acidity. The primary culprit identified in contributing to this increasing rainwater acidity is nitrate, a pollutant whose atmospheric concentration is steadily rising due to a multitude of anthropogenic sources. These sources include emissions from the ever-growing fleet of vehicles on India’s roads, industrial emissions from factories and manufacturing plants, the widespread practice of burning agricultural crop residue, and various household sources of combustion. The atmospheric transformation of nitrogen oxides from these sources into nitric acid, which then dissolves in rainwater, is a key driver of the observed downward trend in pH levels.
The overarching conclusion of this comprehensive 34-year study underscores the significant impact of urban growth and industrial development on the fundamental chemical composition of rainwater across India. As cities expand and industrial activity intensifies, the delicate balance of atmospheric chemistry is being altered, leading to a gradual acidification of precipitation. This trend necessitates a closer examination of pollution control measures and a concerted effort to transition towards more sustainable industrial and urban development practices to safeguard the quality of India’s vital water resources and mitigate potential long-term ecological consequences. The findings serve as a critical reminder of the interconnectedness between human activities, air quality, and the natural environment.
Urban Growth Impacts Rain Chemistry Across India
