Is air pollution turning India's rainfall more acidic? With its high AQI how will Delhi fare? IMD study flags rising risk

The India Meteorological Department (IMD) and Indian Institute of Tropical Meteorology (IITM) have discovered from a 34-year-long study (1987-2021) that rainwater is becoming acidic across multiple cities in India. When the pH value becomes lower, it indicates higher acidity. Rain with a pH below 5.65 is considered acidic. Locations like Visakhapatnam (Andhra Pradesh), Prayagraj (Uttar Pradesh), and Mohanbari (Assam) have frequently exceeded this threshold, according to a report in TOI.

The rain chemistry at 10 Global Atmosphere Watch (GAW) stations were traced by the researchers, which includes: Srinagar (Jammu & Kashmir), Jodhpur (Rajasthan), Prayagraj, Mohanbari, Visakhapatnam, Kodaikanal (Tamil Nadu), Pune and Nagpur (Maharashtra), Minicoy (Lakshadweep), and Port Blair (Andaman & Nicobar Islands). The findings in these cities noted that there was a general decrease in pH value over time.

Why is rain acidic?

The main drivers are nitrate (NO₃⁻) and sulphate ions formed from pollutants such as vehicle emissions, factory releases, power plants, and crop residue burning. It reacts with water, oxygen and other chemicals to form sulfuric and nitric acids, then mixes with water and other materials before falling to the ground.

As per the PTI report, the study indicated that atmospheric conditions and local emissions can affect the chemistry or pH of rainfall. It was discovered that both acidic and alkaline rain can have toxic effects on the environment. It could affect aquatic and plant life. However, the author was surprised that "acid rain does not currently pose a major and immediate threat to our region".

The research team added that rainfall during the dry season tends to be slightly more acidic compared to the wet season, as most of the acidic particles in the air would be washed out at the first rainfall event, resulting in a decrease in pH values.

Does acid rain affect Delhi?

The air pollutants with high NO₂ and SO₂, primarily released from vehicle emissions, coal combustion, and stubble burning, contribute to the formation of nitric and sulfuric acids in rain droplets over Delhi. It plays a key role in keeping the city's Air Quality Index (AQI) consistently poor, with conditions deteriorating even further during the winter months.

These pollutants can easily combine with water, resulting in acid rain in the city. However, according to the author’s report, it does not currently pose any significant threat. While prolonged acid rain may have some impact over time, however, researchers have not come up with any reports of acid rain or its harmful effects on the residents.

As early as 1995, Delhi’s rain showed pH above 5.6 early in the monsoon, but fell below that later on, indicating acidity due to insufficient neutralising alkaline particles like calcium and magnesium, according to a report in the National Library of Medicine.