Using surface and satellite data, a team led by scientist M.V. Ramana of the Hyderabad-based National Remote Sensing Centre has found that aerosol plumes from ships produced severe pollution along the international shipping route in the Bay of Bengal.
According to a paper published on August 2 in the journal Scientific Reports , emissions from ships along the corridor (5-6 degrees North latitude) has directly heated the lower troposphere by two-and-half times compared to surrounding areas and also created a temperature gradient of around 0.1K/day on either side of the shipping route. The troposphere is the lowest region of the atmosphere.
Satellite measurement confirmed the presence of high levels of nitrogen dioxide (NO 2 ) along the shipping corridor, which is 100 km wide. The measurements were made during 2011-2012.
Compared with the surrounding regions, the concentration of NO 2 was five times higher along the shipping corridor.
While the NO 2 value in the surrounding region was 2x10 14 molecules per cm 2 , along the shipping route it was 10 x 10 14 molecules per cm 2 . The NO 2 concentration along the shipping route over Bay of Bengal has been increasing at a steady rate of 0.08 x 10 14 molecules/cm 2 /year, notes the paper. There is a five to six per cent increase in shipping trade along the corridor per year.
Since NO 2 can absorb solar radiation, the increased levels of this gas along the shipping route led to more heating of the atmosphere.
In addition to gaseous emissions, ship exhaust also contains particulate matter such as black carbon.
Along the shipping route, the black carbon concentration was elevated by a factor of four compared to surrounding regions; the lower troposphere solar heating rate was elevated by 0.1°C in cloud-free conditions.
“We need to examine for cloudy conditions,” said Dr. Ramana, the first author of the paper, who was with the Indian Institute of Space Science and Technology, Thiruvananthapuram, at the time of the study. The heating of the lower troposphere can have huge implications in terms of temperature profile and cloud formation, to name a few factors.
The emissions also led to increased concentration of cloud condensation nuclei (CCN). The CCN (at 0.4 per cent supersaturation) concentration is one order of magnitude (about 10 times) greater in the shipping corridor than the pristine regions of Bay of Bengal.
Increased levels of CCN can “disrupt organised convection in the monsoon depressions”, which means the more the concentration of nuclei in the atmosphere the more the number of water droplets that can form. However, “if the water vapour content remains the same and you have more CCN then it may not help cloud formation,” explained Dr. Ramana.
“The effects [of CCN] seen [along the corridor] may have significant impact on the monsoon activity over southern Bay of Bengal and implications for climate change mitigation strategies,” the authors write.
