Have we exceeded the planetary boundary for air pollution?
Human activity causes the emission of small particles such as soot from combustion into the atmosphere. These particles, also called aerosols, have a growing impact on the climate system and negative effects on human health. Further research is needed to conclusively determine whether we have already overstepped the global boundary for this kind of air pollution. However, adverse effects can already be observed at the regional level.
Our industries, transport systems, agriculture and heating all emit aerosols into the air. These anthropogenic aerosols have a different origin than naturally occurring aerosol particles like dust and sea salt. Typical anthropogenic particles include soot from combustion and aerosols containing sulfur and nitrogen from transport and agriculture. They differ in particle size and chemical composition, and that affects their water solubility in clouds.
Aerosols affect the Earth’s radiation budget. Depending on their characteristics and the conditions under which they occur, they reflect incoming solar radiation back into space. This has a cooling effect on Earth’s climate. But some particles can absorb radiation, which warms the air. Since they also serve as nuclei for condensation, aerosols also directly affect Earth’s water cycle. Water vapor can condense onto aerosol particles to form cloud droplets; this has a significant influence on the characteristics of clouds and the distribution of precipitation, and thus on our weather and climate.
In urban areas where smog prevails, meaning there is an especially large amount of particles in the air, scientists have observed changes. There is more precipitation in some regions and less rainfall in others. And cloud formation, which is influenced by aerosols acting as condensation nuclei, is changing monsoon circulation. In addition, increased air pollution also affects human health. For example, exposure to particulate aerosols smaller than 2.5 microns in diameter (PM 2.5) can cause cardiopulmonary disease and lead to increased mortality because such small particles can penetrate further into the respiratory tract.
The planetary boundaries concept developed by Johan Rockström also takes air pollution into account since, in addition to the health hazard it poses, it also has a negative impact on parts of the Earth system.
How aerosol levels in the air are measured
There are various ways to quantify the air’s aerosol content. One is to measure what is called the aerosol optical depth (AOD). We can draw conclusions about aerosol levels from the degree of atmospheric turbidity. Particles in the air weaken sunlight as it travels through the atmosphere. The higher the aerosol level, the greater the air turbidity and the higher the AOD.
How can we return to the safe zone?
The planetary boundary for air pollution by aerosols has not yet been completely quantified. However, it has become apparent that there are already high levels of aerosol pollution in the air in some regions. This makes it very important that we reduce emissions of aerosols that are hazardous to our health, for example by expanding our use of renewable energy sources or using modern filter technology in industrial facilities. In addition, low-emission zones in city centers can help to reduce air pollution locally.
An AOD of 0.25 has been specified as a boundary for limiting the negative impact on people and the environment to a level that is still marginally acceptable. This boundary should only be exceeded in exceptional situations. However, depending on location and season, it is exceeded by a considerable margin in some regions – particularly in southern Asia but also in other parts of the world. The reason: our anthropogenic aerosol emissions.
Using the AOD as a boundary only gives us information about local conditions. Other processes act globally, processes that have not yet been adequately researched. Because of the diversity, complexity, and spatial and temporal dynamics of atmospheric aerosols, our understanding of the relationships and processes involved is still incomplete. With what we know now, it is not yet possible to establish a complete and globally certain framework for atmospheric aerosol pollution.
What consequences can be expected for the climate?
In addition to its effects on human health, exceeding the planetary boundary for air pollution in the long term will also strain the climate system and the weather. This is because the particles that act as condensation nuclei change the process of cloud formation and the hydrological balance, especially at the regional level. Overall, the global concentration of aerosols has doubled since pre-industrial times.
Scientific editing: Markus Rex, Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research.
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