India is the global leader in sulphurous air pollution. The US held the honour through the 1970s and 80s, till it was surpassed by China. China’s performance peaked in 2006 but has been slipping since then. India, a non-starter at the time, has made rapid progress and is now ranked number one.

Can Li, Chris McLinden, Vitali Fioletov, Nickolay Krotkov, Simon Carn, Joanna Joiner, David Streets, Hao He, Xinrong, Zhanqing Li and Russell R Dickerson, from the University of Maryland, Nasa’s atmospheric chemistry and dynamics laboratory and the Air Resources Laboratory at Maryland, and from laboratories in Canada, Michigan and Illinois, explain in their paper in the journal, Scientific Reports (a Nature group publication), that the switch is because of measures China has taken like emission controls in the generation of power.

The CO2 emission when coal and petroleum are burned is inevitable so long as we need the energy. Coal and mineral oil, which are mainly carbon and hydrocarbons, contain sulphur and this gives rise to sulphur dioxide or SO2. While SO2 is a greenhouse gas too, it is of concern even in low concentration because it is poisonous and a health hazard leading to respiratory failure.

SO2 is the principal pollutant that causes the infamous “London Fog”, which could reduce visibility to an arm’s length on cold and wet evenings. It is again the cause of the serious haze problem faced by China and India and the Scientific Reports paper says SO2 pollution leads to over a million premature deaths every year.

Apart from effects on health, high levels in the atmosphere lead to “acid rain”, which degrades the soil and impacts plant, insect and aquatic life as well as steel and stone structures. The first legal action in respect of SO2, in fact, was in 1929, when the House of Lords upheld the claim of a landowner against the Barton Electricity Works of the Manchester Corporation for damages to his land because of SO2 emissions. There are practically no natural sources of sulphur dioxide, except for volcanic eruptions.

There was hence negligible average SO2 in the atmosphere before the industrial revolution. The steam engine and the burning of coal started the build-up with rising releases from industry — for generation of electricity, the railways to power machinery, steel making and chemical processes. And then, there has been an increase in the number of petrol and diesel-driven vehicles, which emit SO2 mainly inside crowded cities.

The US rapidly became the largest source, peaking at 28.3 million tonnes of SO2 every year in 1970. Steps to clean fuels of sulphur content and to wash gas emissions to reduce SO2 release then started having their effects. The rapid industrialisation of China, mostly with coal based electricity, however, pushed up emissions and in 2006, China became emitter number one at 23.1 million tonnes. That is where the US was in 1980. In 2006, the emissions from India were less than a fifth of that from China.

The Scientific Reports paper says that while China has taken measures that have reduced emissions, those from India have doubled since 2006 and levelled with China in 2015. There are many drivers of emissions from burning fossil fuels in a growing economy, market forces that demand energy and the rise in motor vehicles being the most important.

While there is now the awareness to control generation of SO2, the Scientific Reports paper says the plurality of sources makes it difficult to know where the emissions are coming from and to exercise control. “To predict and mitigate air pollution, air quality models require accurate information on the emissions,” the paper says. The conventional approach has been to create inventories of the related activities and the emission factors to arrive at estimates. The data available, however, is generally a few years old and is far from accurate.

Estimates of pollution are hence highly uncertain, particularly in China and India, where the economy is changing by the hour and the mechanisms to regulate what is being done and to collect data are barely developed. The researchers hence took recourse to satellite measurements and data that have now become available. The way SO2 acts to create fog is by first combining with water to form sulphuric acid vapour. The vapour then condenses on particles of dust or soot.

This suspension of particles, an aerosol, scatters light and is the reason for the fog in wintry mornings where there is dust in the air. SO2 aerosols can also play a useful role, albeit not enough to compensate for the harm they cause, by reflecting sunlight in the upper atmosphere to cause “global dimming”. This reflection, however, helps satellite sensors work out the levels of SO2 aerosols, as distinct from other particulate reflectors, in the atmosphere.

Data has been collected since the “Total Ozone Mapping Spectrometers”, aboard Nasa’s satellites, provided daily information of the total ozone column, from 1978 to 2006. In 2006, the Toms series was replaced by the “Ozone Monitoring Instrument” aboard Nasa’s research satellite, Aura. The Scientific Reports paper says the superior ground resolution of the OMI has been particularly useful in observing distribution and levels of SO2 pollution.

It was this data, the paper says, that uncovered the evidence that SO2 pollution was reducing in China, which had had installed devices that removed the sulphur content of flue gases in coal-based power plants. The data also showed the major reduction of emissions from power plants in the US, the paper says. It is the OMI data, along with “bottom-up” field data, that has enabled assessment of the pollution levels in China and India. Rapid industrial progress in China led to pollution and smog and haze in cities, particularly Beijing, which drew attention worldwide.

China itself took measures to address particulate and toxic emissions from power plants and motor vehicles but still struggles with the problem in cities. The measures, however, have brought the SO2 pollution levels substantially down — from 35 million tonnes a year in 2007 to below 10 million tonnes in 2016. The 2000s have also seen great industrialisation in India and power generation capacity has increased from 74 GW in 2002 to 218 GW in 2017.

As for motor vehicles, against the million vehicles registered during 1993, 19 million vehicles were registered in 2014. While there has been some movement towards containing pollution, it is nowhere near what China has done and the SO2 pollution in India is now higher than that of our neighbouring country. A comparison of the levels of SO2 over China and India, as of 2005 and 2016, is shown in the maps in the picture.

What’s indicated in the maps are only the SO2 levels and there are other components that China still needs to take care of. The message for India, however, is clear — to match steps with China, control the use of fossil fuels and reduce all emissions, rather than move towards levels of China and the US in years past.

(The writer can be contacted at response@simplescience.in)