Studying The Impact Of Aerosols On Rainfall In Western Ghats
The air in even the remote parts of the Western Ghats contains a cocktail of floating particles. Some are a result of natural geo-meteorological processes. Others are due to anthropogenic sources like vehicular pollution from nearby cities and towns (Photo by Prof tpms/Wikimedia Commons)
Off the tourist trail and away from the resorts that crowd Munnar’s lush hills in the Western Ghats is a two-storey red-brick building that overlooks tea gardens, eucalyptus plantations, mountains and forests. It is September, and the monsoons in central Kerala bear down on the landscape with ferocity.
Munnar takes a breather during the monsoons as tourist numbers drop. But at the Natural Aerosol and Bioaerosol High Altitude Laboratory, set up by the Indian Institute of Technology-Madras (IIT-M) and College of Engineering-Munnar, there is frantic activity as researchers tune-up high-tech measuring instruments. Unlike the sleepy hill station nearby, inside the laboratory, it is a rush against time.
“There is only a small window for us to get meaningful measurements of natural aerosols in the atmosphere. The monsoons are the cleanest months because the rains wash away atmospheric pollutants. By the onset of winters, there is relatively-high pollution from man-made sources, making measurements difficult,” said Sachin S. Gunthe, coordinator of the laboratory and assistant professor in the Department of Civil Engineering, IIT-Madras.
The aerosols he seeks to measure are biological – that is, solid airborne particles originating from organisms such as pollen, viruses, bacteria, fungal spores, plant debris and others. But even in this remote, tranquil location – set some 1,600 m. above sea level in the Western Ghats – the air is a cocktail of floating particles. Some are a result of natural geo-meteorological processes, such as winds carrying sand or dust from far-off deserts and plains or salt from the oceans. Others are from anthropogenic sources such as the burning of agricultural waste, bio-mass, fossil fuels from cars and industrial pollutants – all of which add black carbon particles to the atmosphere.
Different types of aerosols, in varying degrees, affect weather, human health and the forest ecosystem. Aerosols interact with clouds, affecting their formation and the amount of rainfall a region receives.
During the inauguration of Munnar laboratory in February 2021, Kerala Chief Minister Pinarayi Vijayan, said that Kerala and the Western Ghats were witnessing an increase in floods and landslides. He urged researchers to delve into the causes, and the impact anthropogenic activities have on the monsoons here.
But for now, Gunthe’s team is setting out to understand something more fundamental — an answer to the question: What is the natural concentration and composition of natural aerosols? “We don’t even have a baseline of the concentration of natural aerosols here. Without that, it is difficult to say what is natural and what is a result of anthropogenic activity,” he said.
Understanding A Complex Relationship
Any variation in rainfall in the Western Ghats, which is among the world’s top 10 biodiversity hotspots, has a domino effect on the water security of peninsular India. Perennial rivers in south India originate in these hills and are fed by the monsoons. Research already shows that in the long term, the Western Ghats will be among the places with intensely fluctuating monsoon rainfall due to global warming.
However, the role that aerosols play or will play in the hydrological cycles is yet to be fully understood. It is hotly contested and debated. Studies often contradict each other.
On one side, there are multiple studies (for instance, this study in 2019, and another in 2020) that show natural and anthropogenic aerosols significantly increased rainfall in the Western Ghats. This is in contrast with other regions of the country where aerosols have had no impact on rainfall, according to the studies.
However, many meteorologists and some ground-level observations contest this. At the Western Ghats hill station of Mahabaleshwar in Maharashtra, where the Indian Institute of Tropical Meteorology (IITM) runs a High-Altitude Physics Laboratory, researchers used ground-level observations to show that aerosols can suppress rainfall during the monsoons.
“We know that aerosols are having an impact on the local climate. Its impact will continue to increase due to anthropogenic emissions. But, right now, there are too many uncertainties on how they may be having an impact,” said M. Rajeevan, a distinguished scientist at the National Centre for Earth Science Studies, Kerala and former Secretary at the Ministry of Earth Science (MoES).
The uncertainties are due to the complex nature and chemical composition of aerosols. Some aerosols are radiative (that is, they reflect sunlight and heat) and end up causing some form of local cooling, while others, like black carbon, absorb heat and contribute to warming. Similarly, aerosols can have varied effects based on the type of clouds and the height at which they interact with these clouds.
Moreover, the concentration of these aerosols also has contrary effects on rainfall. Aerosols are particles around which moisture coalesces, and when these are heavy enough, they are pulled toward the ground as rainfall. However, when the concentration of these particles increases, the moisture is dispersed among many particles, and the smaller size of these drops can lead to a reduction in overall rainfall (a process called Twomey Effect).
“Any research based on simulation and modelling makes certain assumptions about aerosols. This leads to a lot of uncertainties. Aerosols are difficult to predict. Their processes are dynamic, and their atmospheric lifespans are short (barely a few days) compared to greenhouse gases (that last a few months). We need to have enough ground-level observations to add more information that could address some of these uncertainties in these models,” said Rajeevan. “There is a lot of research ongoing in India on aerosols. But I feel we need a scientific breakthrough to truly understand how it affects the monsoons.”
Establishing Ground-Level Stations
An attempt to establish a dense network of stations to provide this sort of ground-level measurements of aerosols is the Aerosol Radiative Forcing over India Network (ARFINET). The network is funded by ISRO’s Space Physics Laboratory (SPL) and has 42 observatories across India. It began as a concept-proving network in 1983.
“This just isn’t enough. Aerosols do play a major role in climate change, and there are too many gaps in understanding it,” said Gufran Beigh, founding project director of India’s System of Air Quality and Weather Forecasting And Research (SAFAR).
Within the ARFINET, just one observatory is in the core of the Western Ghats (Ooty). Outside this network, there are just two other aerosol monitoring stations: one at Mahabaleshwar, run by IITM, and another at Munnar, run by National Centre for Earth Science Studies.
“There is a dearth of high-altitude stations. They can make a big difference as they are often well within the clouds, which is crucial to study how aerosols interact with clouds,” said Beigh.
High-Altitude Stations To Monitor Aerosols
Shallower clouds are more common in the Western Ghats than in the plains during the monsoons (June to September). A ground station at these altitudes becomes an ideal site where aerosol concentrations and cloud microphysical properties can be studied simultaneously.
The High-Altitude Cloud Physics Laboratory (HACPL) at Mahabaleshwar, run by IITM, is surrounded by dense forests and during the monsoons, is almost always covered by clouds. Among the laboratory’s objectives, one is to test various hypotheses and validate satellite data. The tourist town sees carbon-laden aerosols from vehicular exhaust, biomass burning for domestic and agricultural purposes, and also from industries in Gujarat and western Maharashtra. Black carbon and other carbon-laden aerosols peak in summer, just before the advent of the monsoons.
“While (some) simulation and modelling-based research may show increasing rainfall in the Western Ghats, we are, in fact seeing a slow, declining trend in rainfall here,” said G. Pandithurai, senior scientist at IITM, Pune, and project director for HACPL. “But, this can’t be representative of the entire Western Ghats. There are vast differences in precipitation and local climate in the southern and northern Western Ghats,” he said.
Setting up more such high-altitude stations can complement satellite observations for aerosols. Satellites have constraints, particularly in measuring local area effects, distinguishing organic and anthropogenic aerosols, and when there is cloud cover, he said.
“However, it isn’t enough to just measure aerosol concentrations. Most laboratories that monitor aerosols in India lack advanced machinery and instrumentation that would allow us to differentiate between the various types of aerosols and how they interact with clouds,” he said.
The Search For Aerosol Data
The hurdles and challenges in setting up even rudimentary aerosol monitoring stations – in a country where funding remains a constraint – came to the fore when a monitoring station was set up at Ooty in 2008.
“The location of the monitoring station was chosen far outside the city so that local anthropogenic sources do not affect the readings. But the higher and more remote you go, the more there are challenges of road access, electricity, manpower and logistics. It took us a long time to get the station running,” said C. Udayasoorian, a retired scientist from the Tamil Nadu Agricultural University who set up the aerosol monitoring station there.
But local effects continue to seep into their aerosol readings. Though the observatory is 10 km. from Ooty, their research shows that valley winds bring significant black carbon pollution – either through industrial activity, burning of wood and other biomass and vehicular emissions – from the town. The presence of these pollutants varies with season but is the highest during the summers (March to May), which coincides with the influx of tourists to the popular hill station. “This is bound to have some sort of impact on the climate,” he said.
Researchers at ISRO’s Space Physics Laboratory, who did not want to be quoted due to their internal procedures for talking to the media, said despite the observatory running for over a decade, there was “not enough data” to come to conclusions about the effect of aerosols on the monsoons.
“You can’t isolate the effects of local pollutants from other factors. The monsoons are a vast phenomenon that is affected by greenhouse gases, deforestation, and other complex factors. We just don’t know how one type of aerosol affects the rainfall here,” said a researcher.
The constraints for expanding high-altitude stations centered around logistics and manpower. “We need suitable locations and suitable research partners. Both of these are a challenge. But there has been a gradual improvement in aerosol monitoring, and more stations have been added. It is a positive trend,” said the researcher.
