Delhi flagged in IMD study as acid rain risk rises across Indian cities
Delhi: A 34-year-long study conducted by the India Meteorological Department (IMD) and the Indian Institute of Tropical Meteorology (IITM) has flagged rising acidity in rainfall across multiple Indian cities, including Delhi, raising concerns about long-term environmental effects of air pollution. The research, which tracked rain chemistry across ten Global Atmosphere Watch (GAW) stations from 1987 to 2021, found a consistent drop in pH levels, suggesting a trend toward more acidic rainfall, particularly during the dry season.
Rain is considered acidic when its pH falls below 5.65. While the study did not find immediate evidence of acid rain harming residents or ecosystems, cities such as Visakhapatnam, Prayagraj, and Mohanbari frequently recorded pH values below this threshold. In Delhi, high emissions of nitrogen dioxide (NO₂) and sulfur dioxide (SO₂) — from vehicles, coal combustion, and crop burning — make the capital particularly susceptible to the formation of nitric and sulfuric acids that can trigger acid rain during precipitation events.
Researchers observed that the first rains of the dry season tend to be slightly more acidic than those in the wet season, as pollutants suspended in the atmosphere are washed out in initial rainfall episodes. This early rain flushes out concentrated acidic particles, reducing pH levels and marking a seasonal dip in rainwater quality.
Data from stations in Srinagar, Jodhpur, Kodaikanal, Pune, Nagpur, Minicoy, and Port Blair also pointed to a gradual decline in pH levels over decades. The acidity primarily stems from nitrate (NO₃⁻) and sulfate ions formed through atmospheric chemical reactions involving industrial emissions, vehicular pollutants, and biomass burning. These pollutants mix with water and oxygen, forming sulfuric and nitric acids which are then carried down in rain.
While the study notes that acid rain in its current form does not pose a major and immediate threat, researchers caution that continued exposure over time can have toxic effects on aquatic life, crops, soil health, and even historical monuments. Acid rain, whether acidic or slightly alkaline, can still disrupt ecological balances when persistent.
In Delhi’s case, earlier records dating back to 1995 already showed monsoonal rain pH dipping below 5.6 later in the season — a sign of growing acidity exacerbated by the city’s lack of neutralising agents like airborne calcium and magnesium. Although there is no official documentation yet of acid rain harming the capital’s population, the worsening AQI and evolving climate patterns underline the need for urgent mitigation strategies.
Experts say the findings should serve as a wake-up call for tighter control on air pollution sources and enhanced monitoring of rainwater chemistry. As the capital continues to struggle with chronic air quality issues, acidification of rainfall could become an increasingly visible symptom of the broader environmental toll of unchecked urban emissions.
