Rueful Rainfall Linked To Aerosols


Study from IIT Bombay finds that airborne pollutants affect the water available for  agriculture.

Nearly two-thirds of the Indian population depends on agriculture for their livelihood.  Disruptions in rainfall pattern affect them all as most of the agricultural land is rain-fed. Over  the second half of the 20 th century, levels of grain production have fallen due to deficiencies  in rainfall. Are the errant monsoon rains linked to the rising levels of pollution? A study  from the Indian Institute of Technology Bombay says so. The researchers, Mr Prashant Dave,  Prof. Mani Bhushan and Prof. Chandra Venkatraman have discovered that aerosols cause  aggravated breaks in the monsoon, which could significantly influence drought and impact  agriculture.

Aerosols are fine solid particles, liquid droplets or a mixture of solid-liquid particles  suspended in the air. Dust, sea-salt, particles emitted from burning fossil-fuels,  vehicular  exhaust—all contribute to the rise in aerosol levels in the atmosphere. Black carbon or soot,  which absorbs sunlight, and compounds such as sulfates and nitrates, which scatter light, are  the significant components of atmospheric aerosols.

Although previous studies have attributed increases or decreases in monsoon rainfall to  changes in atmospheric aerosols, the relationship has not been examined, specifically in terms  of causes and effects, with observational data. “The question we asked was, can we go  beyond correlation and establish causation in a system as complex as the atmosphere to  understand linkages between aerosols and monsoon rainfall changes using mainly  observational data?” said Prof. Mani Bhushan.

In this study, published in Scientific Reports (from the Nature group), the researchers have  used aerosol levels and cloud properties derived from satellite data, and rainfall measured  using ground-based instruments, during the years 2000 to 2009. They analysed this data to  study the regional differences between the presence of aerosols and the corresponding  precipitation.

Changes in weather patterns, though seemingly chaotic, are the result of a few visible and  invisible fundamental processes repeating themselves. Meteorologists divide the study of the  atmosphere into four groupings---micro-, meso-, synoptic and global---depending on the  scale and the duration of these processes. Microscale events such as cloud puffs or gusts  occur in an area within the radius of 1 km or less, while global scale events span over 1000  km and last at least a month.

This study examines processes at the mesoscale, spanning 10 to 1000 km, in regions over  India experiencing high levels of aerosol particles and low levels of rainfall. On this scale, the  interplay between moving air and water is the driving force behind precipitation. The sun’s  radiation heats-up the earth's surface, which then heats the air just above it. Warm air rises,  carrying evaporated water with it, and cools to form cloud droplets. These droplets  conglomerate over time and fall to the ground under their weight.

However, the increase in the aerosol levels in the atmosphere disturbs the delicate  equilibrium. Since aerosols absorb or scatter sunlight, the amount of sunlight reaching the  ground decreases and the ground surface remains cool, while the layer in which absorbing  aerosols are present gets heated up. As a consequence, instead of rising to form clouds, the  water vapour diverges horizontally and gets dispersed. Along with this, the air stagnates  leading to a loss of vertical motion of air and water; thus suppressing the rainfall.

The researchers of the study observed frequent rainfall breaks, lasting longer than seven days  several times in a season, in areas with high levels of aerosols. If the pattern continues, the  pauses could merge into a drought and prolong an existing drought condition. At the micro  scale, a single drop of rain is formed by vapours around a nucleus such as a speck of dust.  The researchers also studied if an increase in the levels of aerosols changed the size of  individual raindrops in the cloud, leading to subsequent suppression of rainfall. However,  they did not find a strong correlation between the two.

Thus, in regions of high levels of aerosols, the more likely reason for the repeated occurrence  of low rainfall, according to the study, is the absorption of solar radiation by aerosols that  prevents the upward movement of water vapour and air and increased horizontal divergence  of moisture. Within a day of increase in aerosol levels, these effects manifest and last for two  or more days.

“The study makes important linkages between air pollution and climate change, in regard to  regional monsoon rainfall. It is important to understand how such disruptions would evolve,  with expected future increases in aerosol emissions driven by human activities” concluded  Prof. Chandra Venkataraman.