Aiming to reduce ammonia emissions may be a more cost-effective way to mitigate air pollution than focusing on nitrogen oxides alone.

Fine particulates less than 2.5 micrometres in diameter are formed when ammonia reacts with nitrous oxides (NOx) and sulphur dioxide. These particulates, known as PM2.5, can pass from the lungs into the bloodstream and cause illnesses such as asthma, coronary heart disease and chronic obstructive pulmonary disease.

“Nitrogen is an important precursor that can lead to air pollution, so if we want to control air pollution, we have to control nitrogen emissions to the air,” says at Zhejiang University in China. “But there are too many different types of nitrogen emitted to air.”

Gu and his colleagues developed a new way to calculate the contribution of nitrogen compounds to PM2.5 pollution called the N-share. They estimate that, in total, nitrogen emissions caused roughly 23.3 million years of lost life in 2013, with an economic cost of $420 billion.

The team found that targeting ammonia emissions – the majority of which come from agricultural sources such as livestock production – would be a more cost-effective way to reduce PM2.5 pollution than focusing on NOx emissions, which are produced chiefly by combustion, such as in car engines.

Updating the way we produce meat, for example via changes to animal housing and diet, could help reduce ammonia emissions, since about 80 per cent comes from agriculture, says Gu.

Currently, most places around the world focus on reducing sulphur dioxide and nitrogen oxides to tackle fine particulate air pollution. While ammonia reductions have been suggested as a focus for the European Union, other countries, including China and the US, have no policies on ammonia emissions.

“Globally, around 5 million people die each year due to [ambient] air pollution,” says Gu. “We want to change policymaking to focus on not only NOx, but ammonia for PM2.5 pollution.”

Hundreds of ancient Mesoamerican ceremonial sites built over a period of about 600 years have been uncovered. This comes after the discovery of the first such site – Aguada Fénix – was announced in 2020

at the University of Arizona and his colleagues used a form of remote sensing called lidar, which uses airborne lasers to form a 3D picture of the surface of the ground. They used this to discover 478 sites in an area covering 84,516 square kilometres in southern Mexico, some of which was covered by dense jungle.

The sites were made up of rectangular complexes, which the Maya and Olmec probably used for ceremonial gatherings. “People just come and go… sort of like a pilgrimage centre,” says Inomata.

They consisted of a central open plaza, where people might have gathered, with a series of low earthen mounds along the edges where there might have been built structures. And they were probably constructed between 1050 and 400 BC by two ancient civilisations – the Olmec, which was the earliest known civilisation in the region, and the Maya, which may have learned from the Olmec and whose culture collapsed around AD 800.

“Nobody knew about those rectangular ceremonial sites until we found Aguada Fénix, so all these new findings are a revelation about this early period,” says Inomata. “That really made us rethink about the origin of Mesoamerican civilisation. Many of those complexes were built by people without too much hierarchical organisation.”

The sites were all relatively flat with a few small pyramids compared with later constructions in the region such as Chichen-Itza that typically contain large pyramids.

“We don’t know exactly what they might have looked like in life, but one could argue that they were small cities,” says at University College London, who wasn’t involved with the study. “When I first started work in the 70s it was still a time where everyone thought cities developed in the Classic period around 200 AD. Now, steadily the period in which I guess you could say urbanism developed has been pushed back and pushed back.”