Even as aerosol loading is increasing at a rate of 3% annually, the quantum of dust aerosol over north India during the pre-monsoon period has reduced during the period 2000-2015. Reduced dust aerosol loading during pre-monsoon period might be reducing rainfall during early monsoon over north India.
Though the aerosol burden over north India is three times more than the global mean value and has been increasing at about 3% per year for the past few decades, the amount of dust aerosol during the pre-monsoon period has decreased by 10-20% during the period 2000 to 2015.
“Past studies have shown that whenever pre-monsoon dust aerosol is more over north Indian region, the early part of monsoon rainfall is higher,” says Dr. V. Vinoj from the School of Earth, Ocean and Climate Sciences at the Indian Institute of Technology (IIT) Bhubaneswar, Odisha. “Our study shows that dust aerosol loading is declining during pre-monsoon period, but the bad news is that rainfall may be reducing during early monsoon.”
In a study published in March 2014 in the journal Nature Geoscience, Dr. Vinoj and other researchers found that monsoon rainfall over central India showed strong correlation with desert dust aerosol levels over West Asia, North Africa and the Arabian Peninsula.
Dust particles absorb heat and as so heat the atmosphere; they also act as nuclei for cloud liquid drop and ice crystal formation. As a result, dust aerosols induce large-scale convergence over North Africa and the Arabian Peninsula, leading to increased flow of moisture over India. Any reduction in dust aerosol leads to decreased rainfall over central India within a week.
Using the Community Atmosphere Model (CAM5) to simulate the effect of different types of aerosols on monsoon, Dr. Vinoj and other researchers found that in pre-industrial emissions, removal of dust aerosol alone result in decreased rainfall over central India. Removal of sea salt, which acts a very good nucleation sites, alone show only slight reduction in rainfall over central India.
Summer monsoon rainfall over coastal southern peninsular and central Indian region is differently affected by dust and sea salt. While increased sea salt aerosol emissions lead to decreased precipitation, increased dust aerosol emissions lead to increased rainfall. “Though sea salt is a good nucleation site, we think increased sea salt does not lead to increased rainfall over central India as it results in increased precipitation over the Arabian Sea and so does not reach India,” Dr. Vinoj says.
Decreased dust loading
Data from ground-based stations (AERONET sites) and satellite-based measurements from different platforms during the period 2000 to 2005 show decreased aerosol loading during the pre-monsoon period from Karachi to kanpur.
Largest decrease has been over Jaipur and the least reduction was in eastern Uttar Pradesh. Since the amount of aerosol loading has been increasing in this region on an annual basis, the reduction registered at these stations must be due to decrease in dust aerosols, the researchers say. The results were published in the journal Scientific Reports.
“Maximum reduction [in terms of quantity] in total particulate loading during pre-monsoon period is seen in northwest part of India. However, the eastern parts of India have witnessed the greatest percentage reduction in particulate loading,” says Dr. Vinoj. “This indicates that the source of observed changes is towards the west.”
Proof of dust reduction
The satellite-based measurements too indicate a reduction in aerosol loading during the pre-monsoon period over a large swath of area over northwest India. Generally, satellite-based methodologies are not very good at distinguishing between aerosol types. However, the Ozone Monitoring Instrument (OMI) on board the Aura satellite, which is sensitive to absorbing aerosols, shows a decreasing trend. This indicates that the changes are related to dust and/or black carbon, both of which are more absorbing in nature.
Ground-based stations in Karachi, Lahore and Kanpur, which have the longest available data, shows “significant reduction” in dust loading during 2000-2015. The decreases are 10-20% over all the sites.
The decrease in aerosol has been most pronounced in the areas west of the Indo-Gangetic Plain, with Jaipur registering about 3% drop per year with respect to the year 2000 while Kanpur showing a relatively lower reduction of about 0.5%.
Based on aerosol size and absorption information collected from ground-based stations, the researchers have been able to confirm that the decreasing trend is due to dust particulates. MERRA2, a more sophisticated, model-based analysis too shows similar trends. “This is proof that it is dust which is reducing the total particulate loading during recent times,” Satyendra K. Pandey from IIT Bhubaneswar and the first author of the paper.
Why less dust loading
The reason for a reduction in dust loading during the pre-monsoon period is due to increased rainfall, with maximum increase seen over Pakistan region and Thar desert, which is a dust-source region. The pre-monsoon rainfall makes the soil wet thereby reducing the amount of dust that gets emitted and also increases the removal of dust present in the atmosphere.
In addition, there has been a gradual slowdown in wind speed in the vicinity of Thar desert. “These two factors might be contributing to reduced dust loading during pre-monsoon period over north India,” he says.
“In the last 10-15 years, the area under irrigation in Rajasthan has increased and so is the area under vegetation,” says Prof. Vimal Mishra from the Civil Engineering department at IIT Gandhinagar, who is not part of this study.