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 0.1 K/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 (NO2) along the shipping corridor, which is 100 km wide. The measurements were made during 2011-2012.

Since NO2 can absorb solar radiation, the increased levels of this gas along the shipping route led to more heating of the atmosphere.Compared with the surrounding regions, the concentration of nitrogen dioxide  was five times higher along the shipping corridor.

While the NO2 value in the surrounding region was 2 x 10¹⁴ molecules per cm², along the shipping route it was 10 x 10¹⁴ molecules per cm². According to the paper, the NO2 concentration along the shipping route over Bay of Bengal has been increasing at a steady rate of 0.08 x 10¹⁴ molecules/cm²/year. There is a five to six per cent increase in shipping traffic along the corridor per year.

Since NO2 can absorb solar radiation, the increased levels of this gas along the shipping route led to more heating of the atmosphere.

Black carbon emissions

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 degree C in cloud-free conditions.

“We need to examine for cloudy conditions,” says 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.

Cloud condensation nuclei

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”. In theory, 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,” says Dr. Ramana. The increased cloud condensation nuclei seen along the shipping route has the “potential to disrupt organised convection in the monsoon depressions”.

“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.

“If you have good monsoon, then the pollutants and gases get washed away. But if you have a weak monsoon, then these gases and pollutants can stay in the atmosphere and cause further delay in monsoon activity and intensity,” Dr. Ramana says. “We have to study how the pollutants and gases from the ships impact the large-scale monsoon activity.”

Regulations are in place to limit SO2 and NO2 emissions by ships. “But these are not sufficient. We should look at black carbon emissions too. The regulations do not include black carbon emissions,” he says.

Serendipity

The original Continental Tropical Convergence Zone (CTCZ) campaign in 2012 on board the research vessel Sagar Nidhi was to be stationed at 8 degree N latitude and study the atmosphere and ocean parameters before the active monsoon phase, during and after. “Nothing happened for about 20 days. So we thought of going in for an explorative study. As we approached the shipping corridor we encountered high levels of pollution,” Dr. Ramana recalls. The team returned and studied satellite data and found the presence of high levels of greenhouse gases along the international shipping corridor.

Published in The Hindu on August 10, 2016