Natural, man-made air pollutants tinkering with Guwahati’s rainwater

As rapidly-expanding Guwahati, northeast India’s largest metropolis, gears up to clean its air under the National Clean Air Policy (NCAP), a study suggests a cocktail of natural and man-made pollutants wafting through the air is tinkering with the city’s rainwater quality.

Guwahati in Assam is among the 122 ‘non-attainment’ cities identified for implementing mitigation actions under the national policy, which means it does not meet the National Ambient Air Quality Standards. Researchers said pinpointing critical sources of air pollution and their contributions to the problem are essential to crafting effective city and region-specific air pollution control plans.

Connecting the dots between the origin of raindrops, the chemical makeup of the rainwater and air pollutants, an IIT Guwahati-led study, from June 2016 to June 2017, shows that 64 percent of the rainfall during monsoon was acidic, which means the rainfall in this form is damaging the environment. Acid deposition, or acid rain, is the term used when the pH of precipitation drops below 5.6.

“We examined rainwater samples during June 2016-June 2017 and found that 75 percent of rainwater samples collected were acidic,” Rajyalakshmi Garaga, a research scholar at IIT Guwahati and study author, told Mongabay-India.

Garaga said such acidic rain events occurred throughout the year, with a frequency of 64 percent and 87 percent during monsoon and non-monsoon seasons, respectively.

The study identified particles from the sea (sea salt or marine source), dust, industrial and vehicular emissions, and coal combustion as the major sources that turn the rainfall acidic.

“Our study shows that both natural (marine source) and man-made air pollutants (emissions) are turning the rainfall acidic in the northeast Indian state of Assam. And while we can’t tweak the natural causes we can mitigate emissions stemming from human activities,” Garaga said.

Gases such as sulfur dioxide (SO2) and oxides of nitrogen (NOx), which are also PM2.5 precursors, generated from the combustion of coal, oil, and natural gas are the primary emissions responsible for acid deposition. These combustion compounds react with water, oxygen and other substances in the atmosphere to form sulfuric and nitric acid, which are transported downwind before they settle on earth. 

Acid rain can occur as wet or dry deposition. When the airborne pollutants settle on earth from the atmosphere in the form of rain or snow the phenomenon is known as wet deposition. Dry deposition is when gases and dust particles become acidic.

When too much acid accumulates, it impacts soil chemistry, plant activities and the acidity of aquatic bodies such as lakes, water, streams. Breathing in acid particles can cause lung and respiratory problems. It can damage buildings and historic monuments. One example is the yellowing of the Taj Mahal from industrialisation and subsequent acid rain.

A Greenpeace report shows India as the top emitter of sulfur dioxide in the world, contributing more than 15 percent of the global anthropogenic sulfur dioxide emissions from the point sources tracked by NASA.

Garaga explained the Assam-based project began with the collection of rainwater to analyse its chemical characteristics and to tease apart the emission sources using an air quality model for this region. To pinpoint the source origin of rain droplets, a technique called isotopic fingerprinting was used to understand whether the rain was due to evaporation of water inland or on the ocean.

Assam sits between the great Himalayas and vast flood plains of the Brahmaputra, one of India’s major transboundary rivers. Guwahati, the largest urban corridor of northeast India, lies on the south bank of the river. The city is set to get a makeover to transform into a Smart City with an aim to have healthy air quality, among other targets.

“The current levels of particulate matter (PM10 and PM2.5) in Guwahati and other cities of Assam are alarming and show increasing trends. Zeroing in on the critical sources of air pollution and their contributions to the problem is essential to building an effective air pollution control plan for the cities and regions,” explained Sharad Gokhale, study co-author, and IIT-Guwahati scientist.

Coarse dust particles (PM10) are particulate matter ranging from 2.5 to 10 micrometers in diameter. Sources include sea salt, pollen, combustion activities such as motor vehicles, industrial processes, crushing or grinding operations and dust stirred up by vehicles on roads.

PM_2.5 are fine inhalable particles, with diameters that are generally 2.5 micrometres and smaller. This means the average human hair is 30 times larger than the largest fine particle. Sources of PM_2.5 pollutants include human activity-derived particles (such as black carbon, organic carbon, sulfates, nitrates, and ammonia) and fine dust.

Besides Guwahati, four other cities in Assam (Nagaon, Nalbari, Sibsagar, Silchar) are labeled as ‘non-attainment’ cities in the NCAP for violating prescribed ambient air quality standards. These are cities where particle levels (PM10) exceed the annual average (60 micrograms per cubic meter) ambient air quality standards. Assam has 24 air quality monitoring stations, including six in Guwahati, under the National Air Quality Monitoring Programme.

Under the national policy, the state is all set to prepare city-specific interventions and action plans, targeting about 20 to 30 percent reduction of PM 2.5 and PM10 concentrations by 2024, according to an agreement between the Pollution Control Board Assam and IIT-Guwahati.

“Once we know what are the sources we can target the sectors specifically for emission reduction. This is necessary if we want cities to attain prescribed air quality standards,” said Gokhale, adding that this is where studies such as the current one come in handy.

Multiple actors at play in ruining rainwater quality

For the study, rainwater and PM 2.5 sampling was carried out in urban and non-urban areas in the city of Guwahati. Non-urban areas have relatively fewer vehicles but several small factories said Gokhale. The presence of a liquid petroleum gas (LPG) bottling plant in the city premises has further aggravated the worsening of air quality.

Running through the year (2016-2017) the study identified particles from the sea, dust, industrial and vehicular emissions, and coal combustion as the major sources that are turning the rainfall acidic. 

“Marine sources appear to have a significant impact on rainwater quality. These sources refer to the salt particles such as chloride, nitrate that are blown inland from the ocean and contribute to the acidity of the atmosphere. When the rain washes or scrubs these particles out, these particles interact with the raindrops and increase the acidity of the rainwater,” Gokhale said.

Because of the interplay of meteorological factors such as humidity, temperature, and rainfall, the part played by each of these sources (natural to man-made) in making the rainfall acidic see-saws over the year.

For example, the authors note that in rainwater samples collected during monsoon, marine source (40 percent) and vehicular emissions (36 percent) were the major contributors in lowering the pH (increasing acidity) while in the pre-monsoon phase the contributions from the marine source decreased by 16 percent and industrial emissions increased by 20 percent.

“For one thing, the decreasing difference between day and night-time temperatures may be responsible for the quick conversion of precursor gases into sulfates and nitrates,” noted Gokhale. This quick conversion could possibly explain why the gaseous pollutants (such as SO2 and NOx) have remained in the low pollution level category in the state, as documented by the Pollution Control Board Assam.

Further, the acid neutralising capacity in this region was negligible, the researchers said. “This is because the levels of sulfates and nitrates are much higher than the neutralising elements such as calcium, magnesium and potassium ions. And there is a huge imbalance of anions and cations in the atmosphere,” said Gokhale.

The other important finding is the moisture source during monsoons and non-monsoon periods. 

While the southwest monsoon arriving from the Bay of Bengal branch brings in the largest share of moisture during the summer monsoon phase, in-land water bodies in the state and transpiration from surrounding green cover result in maximum rain during the rest of the year, the researchers said.

Jaideep Baruah of Assam Science Technology and Environment Council batted for long-term monitoring and research to flesh out acid rain issues in the state.

“One of the problematic areas in Assam is the heavy load of dust stirred up vehicular movement and also from denudation of hills,” Baruah told Mongabay-India, referring to the contribution of dust particles revealed in the study.

Assam is rich in its minerals deposit and is well-known for its fertile land, contributing $46 billion to the nation’s total gross domestic product (GDP), and engaging 69 percent of the state’s population in agriculture, the study said. However, the soil of the state is acidic which restricts the productivity potential necessitating investigation to understand the cause of acidic nature, said Garaga.

Binoy K. Saikia, a scientist in CSIR-North East Institute of Science and Technology, who studies coal chemistry, and who was not associated with the current study, highlighted that the research also reveals the significance of coal combustion activities in the surrounding regions which are responsible for emission of SOx and NOx, the precursor of acid rain.

“The coke oven near Guwahati may be one of the cause for the SOx if they use the feed coals from the northeast region since coal from this part of the country are high in sulfur, particularly organic sulfur,” Saikia told Mongabay-India, stressing on regulatory control and monitoring of emissions.

Sprucing up monitoring and building onto research data

Under the agreement between IIT-Guwahati and the state’s pollution control board, one of the areas researchers are looking at is evaluating the air quality monitoring stations in the state.

“They are not scientifically stationed. For instance, you may not find the monitors in areas where there is heavy traffic congestion but where there is a relatively low volume of traffic. They are arbitrarily stationed so they do not reveal the true nature of the air quality,” said Gokhale.

“We may also restructure the network of these monitors and also convert them to automated systems,” he said.

As for the research component, Garaga said the scientists are also working on a dry deposition study, which means collecting particulate matter on a filter paper and subjecting them to a similar analysis as the wet deposition (acid rain) work. 

“Thereafter, the results will be compared and sources contributing to such high concentrations will be identified in order to inform policymakers in designing better mitigation strategies,” she signed-off.

CITATION

Garaga, R., Chakraborty, S., Zhang, H., Gokhale, S., Xue, Q., & Kota, S. H. (2019). Influence of anthropogenic emissions on wet deposition of pollutants and rainwater acidity in Guwahati, a UNESCO heritage city in Northeast India. Atmospheric Research, 104683.