It is a known fact that ozone formation occurs when NOX, CO and VOCs react in the presence of sunlight and that more ozone is formed in tropical regions. How does this study move beyond this?

What is new here is that we demonstrate the importance of emissions near the equator in a way that has not been shown before. Over the past several decades, emissions have decreased from North America and Europe, as emission controls have been implemented. Emissions have increased rapidly near the equator, particularly in China, India and other regions. Emissions have, therefore, both increased globally and shifted towards the equator. We conducted model simulations that separate the influence of the global growth in emissions from the shift towards the equator, and we find that the shift in the location of emissions [their spatial distribution] is by far the most important cause of the growth in global ozone air pollution [tropospheric ozone]. 

What would be the implications of this finding on global strategies to reduce emissions?

Ozone is a local and regional air pollutant, but also a global one. Historically, ozone has been managed as mainly a local and regional air pollutant. Where action has been taken to reduce ozone-forming emissions, those actions have been motivated to reduce ozone air pollution nearby. But it is also true that emissions affect ozone globally. This essentially influences ozone in the ‘background’ air that flows into polluted regions, onto which regional ozone is added. 

Our work shows that by shifting emissions towards the equator, global ozone is increasing. As a result, in addition to thinking about local and regional sources of ozone pollution, it is also worthwhile to think about the causes of global ozone. In this context, international efforts might prioritise actions in regions closest to the equator to reduce global ozone – whether that is through actions of international agencies or through development assistance to promote clean energy and emission controls. 

Does it mean that countries away from the equator can continue emitting as it won’t lead to ozone creation?

Emissions from countries away from the equator have a smaller influence on ozone [per tonne of emissions], but they still cause ozone. Each region has its own incentive to reduce emissions to control their own air pollution. In fact, these emissions were highest in North America and Europe, and have already been decreasing because of concerns over air pollution in these regions. 
Rather, the important point is that air pollution management can think about global ozone as well as local and regional ozone. Our work highlights that actions to target emissions near the equator might be most effective at reducing global ozone.
Put differently, I would be concerned about a scenario in which emissions might decrease globally, but global ozone continues to increase because emissions continue to shift towards the equator. 

The study mentions south Asia and southeast Asia as vulnerable, but does not talk of equatorial regions in Africa, Australia and South America?

We think that emissions from Africa, South America and Australia also cause a high amount of ozone per tonne of emissions. But in the period (1980 to 2010) we analysed emissions in these regions did not grow as much as in south Asia and southeast Asia. Therefore, we highlight southeast Asia, south Asia and east Asia as regions that have probably had the greatest influence on the global ozone growth over this period. 

Looking to the future, emissions from Africa in particular are expected to grow rapidly, driven by high growth in population and energy use, and poor economic conditions and weak governments that may have difficulty in controlling air pollution. This growth would be expected to be of concern for increasing global ozone, in addition to increasing air pollution over Africa.