Monsoon Rains To Be Impacted In Places With Highest Levels Of Air Pollution

Experts explain how air pollution is now also impacting monsoon rainfall in India

India as one of the world’s fastest developing economies with a billion plus people has witnessed a steep rise in particulate matter induced air pollution in the past two decades. According to the data released by The State of Global Air 2020, India saw 980,000 PM2.5 attributable deaths in 2019.

While air pollution becomes front page news during winter months, the summer time pollution trends have also been regularly and widely reported as they remain way above Central Pollution Control Board’s safety limits of 40 ug/m3 for PM2.5 in many parts of the country. While the lockdown periods gave some respite from pollution levels, analysis shows that pollution levels remained above permissible limits in many cities like Lucknow and Delhi while some cities like Mumbai witnessed a year on year increase in PM2.5 concentration during the months of March, April and May from 2019 to 2021.

While air pollution’s health and economic impacts have been established, research and experts explain that air pollution is now also impacting the Monsoon rainfall in India.

Relation between air pollution and monsoon

Air pollution is the result of suspended particles or aerosols in the atmosphere. Now, as the concentration of aerosols increases, it leads to warming of the atmosphere but cooling of the land surface.

On the other hand, Monsoon is a seasonal oceanic phenomenon which is governed by the change in the wind pattern due to differential heating of the tropics. Southwest Monsoon is driven by the temperature difference between land and ocean which occurs due to the change of winds, differential heating of tropics in the northern hemisphere summer season. These winds are predominantly easterly trade cross the equator and turn right because of rotation of earth.

According to a report ‘Anthropogenic Aerosols and the Weakening of the South Asian Summer Monsoon’, air pollution is pushing the monsoon towards erratic rainfall patterns. Air pollution may lead to a much more variable monsoon, resulting in drought one year followed by floods the next. This erratic behaviour is “more worrisome” than the overall decrease in rainfall as it increases unpredictability and tests resilience building and adaptation capability to prepare for these changes.

The concern has also been raised in the recent UN’s Intergovernmental Panel on Climate Change (IPCC) report ‘Climate Change 2021: The Physical Science Basis’. According to the IPCC, results from climate models indicate that anthropogenic aerosol forcing has dominated the recent decrease in summer monsoon precipitation. There has been a decline in the Southwest Monsoon mean rainfall observed between 1951-2019. The trend is likely to continue in coming years and air pollution is expected to play a vital contributing role.

“Air Pollution is likely to decrease the Southwest Monsoon rainfall by 10%-15% for the entire country. Meanwhile, some places might even see reduced rains as high as 50%. It would also impact the dynamics of Monsoon, for instance delay in onset. Air pollution does not allow the landmass to warm up to the required levels. Due to the presence of pollutants, heating of land takes place at a slower rate. For instance, the required surface temperature is 40°C, while the presence of air pollution will result in restricting temperature up to 38°C or 39°C,” said Dr Dilip Ganguly, Associate Professor, Centre for Atmospheric Sciences, Indian Institute of Technology (IIT) Delhi.

Citing similar views Prof S N Tripathi, Head of Civil Engineering Department, IIT Kanpur and Steering Committee Member, National Clean Air Programme, MoEFCC said, “The number might be true because there is a strong latitudinal and vertical gradient in aerosols which is present in the atmosphere. This would lead to suppression in convection and gradually resulting in reduction in Southwest Monsoon mean rainfall. The most affected places would be the areas with more pollution levels. It is very non-linear as it is the outplay of interplay between meteorology and aerosols. The Southwest Monsoon is driven by the difference between land temperatures and ocean temperatures. Presence of a large scale of aerosols over the Indian landmass would lead to dimming of the land surface. The entire process would lead to weakening of the dynamics of Monsoon, which might even include delay in onset of Monsoon.”

According to scientists, there are two kinds of effects of aerosols – direct and indirect — on precipitation. The direct impact of aerosols on precipitation can be termed as a radiative effect where aerosols directly block the solar radiation from reaching the earth surface. Meanwhile, in the second phenomenon of indirect impact on precipitation, aerosols absorb solar radiation and this indirectly interferes and changes the cloud and precipitation formation processes. However, both kinds of aerosols eventually cool down the earth’s surface, increasing the atmospheric stability and decreasing convection potential.

Dr Krishnan Raghavan, Scientist, Indian Institute of Tropical Meteorology and Lead Author on IPCC Working Group 1 report said, “Air pollution reduces the solar radiation that leads to cooling of the land surface. In the absence of heating of the earth’s surface, evaporation would decrease which would result in a drop in precipitation. Further, there are also some aerosols that tend to absorb solar radiation. But in such cases also, radiation will be reduced by the time it reaches the surface. However, such aerosols will warm the atmosphere, stabilizing it further which would weaken Monsoon circulations eventually leading to reduction in the Monsoon precipitation.”

Dimming of land surface also weakens monsoon flow and precipitation, offsetting or even overcoming the expected precipitation increase due to increased Greenhouse Gases (GHGs). The oceanic response to a weakened monsoon cross-equatorial flow can further weaken the South Asian monsoon through an amplifying feedback loop. These processes also explain (high confidence) the observed decrease of southeast Asian Monsoon precipitation during the second half of the 20th century.

Dr Roxy Mathew Koll, Senior Scientist, Indian Institute of Tropical Meteorology, Pune said, “The monsoon is driven by the temperature difference between land and ocean. Now, due to rapid Indian Ocean warming, this temperature gradient has weakened, with the ocean getting warmer at a faster rate than land. The Indian Ocean is warming at a faster rate due to greenhouse gases. The Indian subcontinent may not be warming at a fast rate probably because of the aerosols.”

Citing a similar theory, Dr V Vinoj, Assistant Professor, School of Earth Ocean and Climate Sciences, Indian Institute of Technology- Bhubaneswar said, “Air pollution can increase, decrease, or redistribute rainfall based on differences in how they interact with radiation and clouds. For example, large concentrations of anthropogenic particulates under conducive environmental conditions in the atmosphere could help clouds grow to massive thunderstorms resulting in extreme rainfall locally. On large spatial and longer time scales, air pollution is said to have led to a decrease in monsoon rainfall over India since the 1950’s. This is due to two important mechanisms, one cooling of land leading to slowing monsoon circulation leading to declining rainfall in the long term and two, warming of the atmosphere over the landmass due to absorbing aerosols leading to increased short period spells.”

“Aerosols play the role of cloud condensation nuclei and have a specific role in cloud formation as heat radiation cannot escape, leading to warming of the earth’s surface. Pollution does settle down after rainfall, but does not get completely washed away. Thereafter, convection in vertical motion might take clouds to go as high as 15 km or even further. Monsoon circulations get impacted, which in turn affects the rain intensity,” said AVM G P Sharma, President- Meteorology and Climate Change, Skymet Weather.

Air Pollution fuelling extreme weather events

Scientists are now worried that aerosols in the atmosphere would not only decrease the Monsoon rains, but also enhance extreme weather events. Intense hailstorms, thundershowers, incessant rainfall, etc. would be possible due to increase in air pollution.

“More air pollution is leading to suppression of the Monsoon rains at a longer scale, which has been noticed during the last 50 years. However, aerosols have enhanced the rainfall on a shorter time scale, which is now commonly termed as extreme weather events,” said Dr Sagnik Dey, Associate Professor, Indian Institute of Technology Delhi and Coordinator for the Centre of Excellence for Research on Clean Air (CERCA).

“Interaction of aerosols with the atmosphere during the pre-Monsoon season, especially between April and May, would result in extreme rainfall events. Some aerosols are very acute and absorbent in nature such as dust and carbon particles. This further delays thunderstorms for a longer period as cloud shelf life will increase. Thousands of cloud droplets collide together to form a bigger raindrop. They will have the probability to rise further above and freeze. If they hang around for a longer period of time and if the surface is warm, it will further push the clouds higher. As a result, we can witness hail storms. These weather activities would be intense in nature and can have great potential for damage,” said Professor Ganguly.

According to the IPCC AR6 WG1 report, absorption of solar radiation by anthropogenic aerosols such as black carbon warms the lower troposphere and increases moist static energy but also results in larger convection inhibition that suppresses light rainfall. Absorbing aerosols, however, in addition to cooling the surface, can heat the atmosphere. This would in turn lead to cloud deepening. With warm surface winds, the cloud tends to move higher and continue to grow for a longer period. Release of aerosol-induced instability, often triggered by topographical barriers, produces intense rainfall, flooding and severe convective storms (medium confidence).

Atmospheric aerosols can affect cloud formation processes via activation of cloud condensation nuclei (CCN), thereby modifying the cloud radiative forcing (CRF) as well as precipitation patterns. This aerosol-mediated change in CRF is termed as the aerosol indirect effect (AIE), which constitutes the largest uncertainty in the current climate forcing. This ambiguity in AIE is primarily due to the regional variability in aerosols, clouds, and the complexity in aerosol related changes in dynamical feedbacks at different spatiotemporal scales. Generally, polluted clouds under high aerosol loading consist of more number of smaller cloud droplets, which increases the cloud reflectance or brightness, thereby causing the cooling effect of the Earth. The greater number of smaller cloud droplets reduces the efficiency of droplet growth by collision-coalescence, which inhibits early rain formation and increases cloud lifetime. Increase in cloud lifetime can reflect more incoming solar radiation inducing a net cooling effect. However, under convective conditions, such delay in the rain formation simultaneously increases droplet mobility, leading to more water mass aloft in the atmosphere. Ice formation processes within the ascending cloud begin once the freezing level is reached and the more latent heat freezing released further invigorates the vertical development of the cloud. This phenomenon is referred to as the aerosol invigoration effect (AIvE).

Following figure explains how the presence of aerosols lead to warming of the atmosphere vs to a scenario where we have less pollution or clean air. Image Courtesy: Nature Communications

Dr Vinoj further added, “Aerosols like black carbon and dust absorb solar radiation, thereby enhancing the upper atmospheric temperature gradient, strengthening monsoon circulation resulting in more rain. This usually happens during the early part and after a long break in the monsoon when aerosols are still present over the Northern/North Western Indian region. On small spatial scales such as urban regions, pollution can have a much larger impact than we think. It can affect both the spatial distribution and intensity of rainfall. E.g., highly polluted urban areas may experience slightly reduced rainfall, while it will pour in the outskirts leading to floods. Thus anthropogenic aerosols or, more generally air pollution could both increase and decrease rainfall but at different spatial and temporal scales.”

According to a study titled, ‘Investigation of the aerosol–cloud–rainfall association over the Indian summer monsoon region’, delay in the onset of collision–coalescence processes and an enhancement in the condensation efficiency allows for more condensed mass to be lifted up to the mixed colder phases. Thus, cloud mass gets brighter, increasing the cloud shelf life further and delaying the rainfall process. Finally, these large ice phase hydrometeors result in overall higher surface rainfall. Hydrometeor is an atmospheric phenomenon which consists of solid or liquid water particles like rain, snow, hail, dew, etc. However, by the time these hydrometeors reach the earth surface, most of them convert into water unless it falls as hail.

Air Pollution Impact: Indo-Gangetic Plains to be worst affected

Although air pollution is impacting mean Monsoon rainfall for India, it is the Indo-Gangetic plains which seem to be in a tight spot. For the last few years, Delhi has repeatedly ranked among the world’s top 10 polluted cities. According to the World Air Quality Report, 2020 by IQAir, out of the top 30, 22 cities were located in India, precisely in the northwestern region. Blame it on its topography and meteorology along with factors like industrialization, urbanization and one of the most densely populated regions in the world, but all combined play a significant role in worsening the air pollution crisis in this region.

While winter is meteorologically the most favorable time for increasing air pollution levels, summers, pre-Monsoon and Monsoon have also witnessed pollution levels above WHO’s safety limit of 10 ug/m3 or even CPCB’s safe standards of 40 ug/m3 for PM2.5 levels.

Dr Vinoj explains, “Weather is one of the primary factors behind waning air quality. Air pollution over New Delhi in particular and the Indo-Gangetic Plains are frequently quoted in the media as a product of high emissions over these highly populated regions. However, the severe or extreme pollution episodes are mostly a product of emissions and unfavorable environmental conditions. For example, low dispersion or high mixing in the atmosphere due to low wind speeds or high stability of the atmosphere are frequently the reason for these extreme episodes. These factors and issues such as biomass burning and conducive transport may modulate air pollution levels in any particular location. Therefore, even if our emissions are held at present levels, such extreme environmental conditions and their changes may make pollution episodes more severe in the future.”

The Indo-Gangetic Plains are bound by the Himalayas to the north, and by the Vindhyan and Satpura ranges in the south. The western part is surrounded by the Thar Desert and Arabian Sea, whereas the eastern part is bound by the Bay of Bengal. Due to its unique topography, this region can be summarized as a type of region, where, both anthropogenic (human-led) and natural aerosols show distinct seasonal characteristics and mixing. While winter months are dominated by fine-mode aerosols, produced by various anthropogenic (human-led) sources from the region, — such as industry, vehicles, open burning, solid fuels for cooking, thermal power plants, diesel gensets, construction activity, brick kilns etc — pre-monsoon or summer months are dominated by the coarse-mode mineral dust, primarily transported from the Thar Desert region in West Rajasthan, which adds to the local and regional emitting sources. Furthermore, northwestern plains, besides being a major source region for aerosols, is densely populated and industrialized from where different aerosol species such as mineral dust, soot, nitrate, sulfate particles and organics are produced and transported to this region thus making it an ‘aerosol hotspot’.

The region itself has both, rural and urban, population and various kinds of emission sources, natural as well as industrial. In rural areas, bio-fuels such as fuel wood, dung cake and crop waste, predominantly contribute to aerosol formation. However, in urban areas, aerosol emissions from fossil fuels such as coal, petrol and diesel oil dominate. Large fluxes of absorbing aerosol emissions (black carbon and inorganic oxidized matter, which is mostly fly ash from coal-based power plants and particles from open burning of crop waste/forest-fires) were reported over the north-western plains.

“Winter weather conditions are the most favourable period for air pollution. Surface temperatures have already cooled down by that time. Meanwhile, the air in the atmosphere over the surface is warm, making it stabilised. During such conditions, an inversion layer is formed and pollutants are trapped. Talking about summertime pollution, although meteorological conditions are not favourable, there are many other kinds of aerosols that start affecting at that time. They can be transported by local aerosols. From March-May, the land surface gets heated, due to which the strength of the inversion layer reduces, bringing down the pollution levels. However, during that time, dust from western Rajasthan and the adjoining desert region of Pakistan starts transporting over the Indo-Gangetic plains. This along with local emissions leads to an increase in summertime pollution. Also, the geography of the north-western plains including Delhi-NCR is also not supportive. The entire Himalayan region guards the Indo-Gangetic plains, which does not allow pollutants to escape and act as a barrier,” said Dr Krishnan Raghavan.

The increasing air pollution levels across the region would also increase the number of dry days. “Air pollution is not only capable of decreasing the rainfall but can also increase the gap between the rainy spells. In short, the number of dry days would increase. Furthermore, cloud invigoration would make clouds more vigorous. Talking about the northwestern plains, in the absence of sufficient heating of the land surface, the water over the surface would take longer to dry up. Because of this, normal heating will take time. Thus, another spell will take more time as moist soil will take time to heat up,” said Dr Ganguly.

According to a study, ‘Pre-monsoon aerosol characteristics over the Indo-Gangetic Basin: Implications to climatic impact’, during pre-monsoon, there are different aerosol characteristics at central and eastern parts of Indo-Gangetic plains, which show significant gradients in magnitude of most of the aerosol characteristics over central to eastern sectors of the Indo-Gangetic plains. Such gradient can be due to the gradual changes in weather parameters and/or emission sources during subsequent months of the pre-monsoon season. Such gradient is, ultimately, found to impact the Earth-atmosphere system by negative radiative forcing, thus causing cooling, at the surface, and positive aerosol forcing, thus causing heating in the atmosphere.

Views expressed here are those of Dr. Seema Javed, a known Environmentalist, Journalist and Communications Expert

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