Air pollution turning India’s rainfall more acidic in several cities, shows 34-year study by IMD & IITM | Pune News

Pune: A comprehensive 34-year study tracking rainwater chemistry across India has found that Visakhapatnam (Andhra Pradesh), Prayagraj (Uttar Pradesh) and Mohanbari (Assam) are increasingly experiencing acidic rainfall.
Though true “acid rain” is not yet a major threat, scientists associated with the study — conducted by India Meteorological Department (IMD) and Indian Institute of Tropical Meteorology (IITM) — have detected a concerning downward trend in pH levels across most monitored locations.
The lower the pH, the higher the acidity of rain. pH is a measure that indicates how acidic or alkaline a substance is on a scale from 0 to 14, with 7 being neutral. Values below 5.65 in rainwater are considered acidic.
The research, conducted at 10 Global Atmosphere Watch (GAW) stations from 1987 to 2021, found a “general decrease in pH over time” at most locations, with significant reductions observed at multiple stations. This trend indicates rainwater is becoming more acidic in many parts of the country, particularly in the industrial and urban areas with high air pollution.
Cities such as Visakhapatnam, Mohanbari and Prayagraj showed more acidic rainwater, while places like Pune showed concerning trends that researchers warned could worsen with continued urbanisation.
The study found Pune’s rainwater pH declining at a significant rate of 0.15 per decade during wet seasons. The acidity in Visakhapatnam’s rainwater can be traced to emissions from the city’s oil refinery, power plant, fertilizer facility, and shipping yard. “In contrast, Jodhpur and Srinagar benefit from natural dust particles from nearby regions, like the Thar Desert, which help neutralize acidic components,” an IMD scientist said.
The study was conducted for 10 locations: Srinagar (Jammu & Kashmir), Jodhpur (Rajasthan), Prayagraj (earlier known as Allahabad), Mohanbari, Pune and Nagpur (Maharashtra),Visakhapatnam, Kodaikanal (Tamil Nadu), Minicoy (Lakshadweep) and Port Blair (Andaman & Nicobar Islands).
The research also found that the rain’s natural ability to neutralize acidic pollutants was weakening in several regions. “In places like Prayagraj, Jodhpur and Nagpur, the presence of calcium particles — which help cancel out acidity — is dropping over time. Some areas are witnessing a rise in other neutralisers, like ammonium. But this is not enough to fully offset the trend,” the scientist said.
“The main pollutant linked to acidic rain is nitrate (NO₃⁻), which is rising in some of the study areas because of emissions from vehicles, factories, burning of crop residue, and household sources,” the scientist said.
The data showed that Prayagraj shows a pH decline (-0.4/decade in dry months, -0.74/decade in wet months). This means that during the dry months, the pH of rainwater is decreasing by 0.4 units every 10 years. During wet months, the pH is decreasing faster – by 0.74 units every 10 years. This is significant because the pH scale is logarithmic — each whole number represents a 10-fold change in acidity. So, even small numerical changes can mean big differences in acidity. Minicoy shows a rain water pH decline (-0.53/decade in dry months, -0.3/decade in wet months).
The study concluded that city growth and industrial development significantly affect rainwater composition across India. Changes in fuel burning and farming methods influence rain chemistry more than geographic location. Despite these findings, acid rain is not currently considered an urgent threat to the region.
Experts, however, said if the trend continued, it could pose a risk in future. Bishwajit Mukhopadhyay, former scientist in IMD, said, “While our current pH levels in many Indian cities are not alarmingly low yet, we could see significant effects in the future if this acidification trend continues. Normal rain has a pH of around 5.6 because of natural factors like atmospheric carbon dioxide forming carbonic acid and other pollutant gases.”
He said, “When pH drops substantially below this level—particularly below 5—we begin to see visible damage. This includes degradation of marble structures in archaeological monuments, increased corrosion of buildings and bridges and, perhaps most concerning, the leaching of toxic heavy metals like aluminum, iron, nickel, and chromium from soil into our aquatic systems.”
Mukhopadhyay said heavy metals could thus enter the food chain and become a major health hazard. “At present levels, these effects are not widespread. But this is definitely a trend we need to closely monitor.”
Pune: A comprehensive 34-year study tracking rainwater chemistry across India has found that Visakhapatnam (Andhra Pradesh), Prayagraj (Uttar Pradesh) and Mohanbari (Assam) are increasingly experiencing acidic rainfall.
Though true “acid rain” is not yet a major threat, scientists associated with the study — conducted by India Meteorological Department (IMD) and Indian Institute of Tropical Meteorology (IITM) — have detected a concerning downward trend in pH levels across most monitored locations.
The lower the pH, the higher the acidity of rain. pH is a measure that indicates how acidic or alkaline a substance is on a scale from 0 to 14, with 7 being neutral. Values below 5.65 in rainwater are considered acidic.
The research, conducted at 10 Global Atmosphere Watch (GAW) stations from 1987 to 2021, found a “general decrease in pH over time” at most locations, with significant reductions observed at multiple stations. This trend indicates rainwater is becoming more acidic in many parts of the country, particularly in the industrial and urban areas with high air pollution.
Cities such as Visakhapatnam, Mohanbari and Prayagraj showed more acidic rainwater, while places like Pune showed concerning trends that researchers warned could worsen with continued urbanisation.
The study found Pune’s rainwater pH declining at a significant rate of 0.15 per decade during wet seasons. The acidity in Visakhapatnam’s rainwater can be traced to emissions from the city’s oil refinery, power plant, fertilizer facility, and shipping yard. “In contrast, Jodhpur and Srinagar benefit from natural dust particles from nearby regions, like the Thar Desert, which help neutralize acidic components,” an IMD scientist said.
The study was conducted for 10 locations: Srinagar (Jammu & Kashmir), Jodhpur (Rajasthan), Prayagraj (earlier known as Allahabad), Mohanbari, Pune and Nagpur (Maharashtra),Visakhapatnam, Kodaikanal (Tamil Nadu), Minicoy (Lakshadweep) and Port Blair (Andaman & Nicobar Islands).
The research also found that the rain’s natural ability to neutralize acidic pollutants was weakening in several regions. “In places like Prayagraj, Jodhpur and Nagpur, the presence of calcium particles — which help cancel out acidity — is dropping over time. Some areas are witnessing a rise in other neutralisers, like ammonium. But this is not enough to fully offset the trend,” the scientist said.
“The main pollutant linked to acidic rain is nitrate (NO₃⁻), which is rising in some of the study areas because of emissions from vehicles, factories, burning of crop residue, and household sources,” the scientist said.
The data showed that Prayagraj shows a pH decline (-0.4/decade in dry months, -0.74/decade in wet months). This means that during the dry months, the pH of rainwater is decreasing by 0.4 units every 10 years. During wet months, the pH is decreasing faster – by 0.74 units every 10 years. This is significant because the pH scale is logarithmic — each whole number represents a 10-fold change in acidity. So, even small numerical changes can mean big differences in acidity. Minicoy shows a rain water pH decline (-0.53/decade in dry months, -0.3/decade in wet months).
The study concluded that city growth and industrial development significantly affect rainwater composition across India. Changes in fuel burning and farming methods influence rain chemistry more than geographic location. Despite these findings, acid rain is not currently considered an urgent threat to the region.
Experts, however, said if the trend continued, it could pose a risk in future. Bishwajit Mukhopadhyay, former scientist in IMD, said, “While our current pH levels in many Indian cities are not alarmingly low yet, we could see significant effects in the future if this acidification trend continues. Normal rain has a pH of around 5.6 because of natural factors like atmospheric carbon dioxide forming carbonic acid and other pollutant gases.”
He said, “When pH drops substantially below this level—particularly below 5—we begin to see visible damage. This includes degradation of marble structures in archaeological monuments, increased corrosion of buildings and bridges and, perhaps most concerning, the leaching of toxic heavy metals like aluminum, iron, nickel, and chromium from soil into our aquatic systems.”
Mukhopadhyay said heavy metals could thus enter the food chain and become a major health hazard. “At present levels, these effects are not widespread. But this is definitely a trend we need to closely monitor.”