Emissions Could Be Making Rains More Acidic, Study Shows

Air pollution could be making rainfall more acidic in cities such as Vishakhapatnam, Allahabad, Mohanbari (Assam), while dust from the Thar could be making rain more alkaline in Jodhpur, Pune and Srinagar, according to an analysis.

The study by the India Meteorological Department (IMD) and the Indian Institute of Tropical Meteorology (IITM), Pune, analysed the pH value -- which measures acidity or basicity -- of rainfall of ten cities in India.

The data recorded at GAW stations from 1987 to 2021 were analysed.

The findings are published by the IMD as a 'met monograph', a comprehensive analysis of a meteorological topic.

The study suggests that atmospheric conditions and local emissions can affect the chemistry or pH of rainfall.

Both acidic and alkaline rain can have toxic effects, affecting aquatic and plant life, even as the study's authors said "acid rain does not currently pose a major and immediate threat to our region".

They attributed acidity in rain in Vizag to emissions from the oil refinery, power plant and fertiliser facility, and that in Mohanbari to acidic nature of soil and charged particles coming from vegetation.

However, dust from the Thar desert could be countering the acidic nature of rainwater of Jodhpur and Srinagar, thereby raising the pH value of rains in these cities, the team said.

Season-to-season changes too could play a role, the authors said, as they found that rainfall during dry season tends to be slightly more acidic compared to that during wet season.

This is because most of the acidic particles in the air would be washed at the first rainfall event, resulting in lower pH values, the team explained.

However, rainfall across most of the ten cities was found to become more acidic over time.

Further, nitrates were found to be the most dominant charged particles in cities with vehicular and industrial activities, while charged particles of calcium were dominant in Jodhpur, Pune and Srinagar, suggesting an influence of dust and soil.

"The analysis of precipitation chemistry across India shows that urbanisation and industrialisation have a significant impact on the composition of rainwater," the authors wrote.

"Stations such as Vizag, Mohanbari and Allahabad exhibited more acidic mean pH values, while Jodhpur and Srinagar had more alkaline values. This suggests variability in atmospheric conditions and local sources affecting rainwater chemistry," they wrote. 

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