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France and the nine planetary boundaries
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Atmospheric aerosol loading

An unquantified planetary boundary with major climatic and health implications

Aerosols are solid or liquid particles suspended in the air, ranging in size from a few nanometers to tens of micrometers (μm). While the majority of aerosols are naturally occurring, a growing number have been released into the atmosphere by human activities since the pre-industrial era. This increase in aerosols in the atmosphere represents a major challenge for the climate and human health.

Some atmospheric aerosols are of primary origin: particles are released directly into the air by natural sources (soil erosion, sea spray, pollen, volcanic ash, etc.) or by anthropogenic activities (combustion of fossil fuels or biomass (see glossary), mechanical activities creating generally coarser particles from ploughing, harvesting, building sites, etc.). Another part of aerosols is of secondary origin: they are not emitted directly into the atmosphere, but are formed by chemical reactions between gases or particles already present in the air.

Aerosols are characterized by the size and chemical composition of the particles they contain. Different families of chemical compounds can be identified in particles: an organic fraction of primary origin, including soot carbon, resulting from the incomplete combustion of fossil fuels or biomass; an organic fraction of secondary origin resulting from the oxidation of volatile organic compounds released by human activities and vegetation; primary inorganic species (sea salts and mineral dust); secondary inorganic species (mainly nitrate, sulfate not emitted by sea spray and ammonium).

In the context of work on planetary boundaries, the control variable chosen is "Aerosol Optical Depth"(AOD), i.e. the degree of opacity of the atmosphere due to aerosol concentration. Given the spatio-temporal variability of particles, sources and impacts, it was not possible to identify a global threshold at this stage. Based on the case of the Southeast Asian monsoon, a phenomenon that is particularly sensitive to the presence of aerosols in the atmosphere, researchers have proposed a local boundary specific to the Southeast Asian zone (Steffen et al., 2015). The proposed AOD threshold is 0.25 (the natural AOD level for this region is between 0.15 and 0.4). In 2015, the average aerosol optical depth measured in Southeast Asia is estimated at 0.3, which places it within the uncertainty zone defined by the researchers (between 0.25 and 0.5 AOD) - (Table 10).

Table 10: Control variable and planetary boundary for the atmospheric aerosol loading

Control variable

Threshold and zone of uncertainty

Global value

Aerosol Optical Depth (AOD), but much regional variation

No global threshold defined, in the absence of sufficient knowledge


Southeast Asia zone: proposed threshold of 0.25 AOD (zone ranging from 0.25 to 0.5)

No average value, as variability is too great throughout the year and across the globe.

Average annual AOD in Southeast Asia: 0.3

Source: based on Steffen et al., 2015

Challenges for climate, human health and ecosystems

Aerosols have a direct and indirect influence on climate, by disrupting the Earth's radiation balance. Some particles, mainly sulfates or nitrates, reflect part of the sun's rays and have a cooling effect. Others, such as soot carbon, absorb the sun's rays and warm the atmosphere. Aerosols also have an indirect influence on climate by contributing to cloud formation. Despite improved models and monitoring measures, it is still difficult to estimate the global effect of aerosols on climate. Aerosols are thought to be responsible for changes in local precipitation patterns, notably rainfall in the Sahel in the 1950s-1980s and monsoons in Asia35.

35 According to the latest IPCC conclusions (report 2021): "The likely range of total global surface temperature increase due to human activity between 1850-1900 and 2010-2019 is 0.8°C to 1.3°C, with a best estimate of 1.07°C. It is likely that well-mixed GHGs have contributed to a warming of 1.0°C to 2.0°C, other human factors (mainly aerosols) have contributed to a cooling of 0.0°C to 0.8°C, natural factors changed the planet's surface temperature from - 0.1 °C to + 0.5 °C and internal variability changed it from - 0.2 °C to + 0.2 °C" (A.1.3).
"Global decreases in overland monsoon precipitation between the 1950s and 1980s are partly attributed to man-made aerosol emissions in the Northern Hemisphere, but increases since then are the result of rising GHG concentrations and decadal to multi-decadal (medium confidence) internal variability. In South Asia, East Asia and West Africa, the increase in monsoon precipitation due to warming from GHG emissions has been offset by a decrease in monsoon precipitation due to cooling from anthropogenic aerosol emissions over the 20th century (high confidence). Increases in monsoon precipitation in West Africa since the 1980s are partly due to the increasing influence of GHGs and the reduced cooling effect of man-made aerosol emissions in Europe and North America (medium confidence)". (A.3.3)

The health effects of aerosols vary according to their particle size and chemical composition. Coarse particles (between 2.5 and 10 μm in diameter) impact respiratory health and are retained in the nasopharyngeal region. Fine particles, with diameters of 2.5 μm or less (PM2.5), and in particular those smaller than 1 μm, can penetrate very deeply into the respiratory system and even pass into the bloodstream. These fine particles can cause cardiovascular disease in particular.

Coarse particles are mainly primary particles of natural origin, as well as certain types of secondary particles. Fine particles are essentially made up of primary particles of anthropogenic origin and some secondary particles.

In 2019, the World Health Organization (WHO) estimates the number of premature deaths caused by outdoor air pollution in cities and rural areas at 4.2 million. According to the latest estimate published in 2021 by Santé publique France, nearly 40,000 deaths from non-accidental causes would be attributable each year to exposure of people aged 30 and over to PM2.5 in France.

Aerosols also have an impact on buildings, by fouling facades, and on plant productivity, by hindering photosynthesis.

Air quality policies and actions

Although the effects of aerosols on climate are ambivalent, their reduction has an overall positive effect on climate, since their emissions are linked to anthropogenic activities that also emit greenhouse gases. On the other hand, their reduction has a positive impact on human and ecosystem health.

At international level, the WHO published air quality guidelines as early as 1987. These, revised in 2005 and 2021, define recommended concentration levels for suspended particulate matter, ozone, nitrogen dioxide and sulfur dioxide. These values form the scientific basis for protecting people's health from the effects of air pollution, and for helping to eliminate or reduce as far as possible air pollutants known or suspected to be harmful to human health or well-being. They are supported by medical, epidemiological and toxicological data.

As part of the Convention on Long-Range Transboundary Air Pollution, the Gothenburg Protocol to Abate Acidification, Eutrophication and Ground-Level Ozone, as amended in 2012, sets emission reduction targets for 2020 for five pollutants, including PM2.5. This protocol encourages the Parties to the Convention to give priority to implementing measures to reduce particulate emissions that would also significantly reduce soot carbon emissions.

At European level, two directives lay down air quality legislation:

  • Directive 2008/50/EC on ambient air quality and cleaner air for Europe, covering sulfur dioxide (SO2), nitrogen oxides (NOx), nitrogen dioxide (NO2), particulate matter up tonitrogen dioxide (NO2), particulate matter up to 10 μm in diameter and up to 2.5 μm in diameter (PM10 and PM2.5), carbon monoxide (CO), benzene, lead and ozone;
  • Directive 2004/107/EC relating to arsenic, cadmium, mercury, nickel and polycyclic aromatic hydrocarbons in ambient air.

These two directives provide a common framework for assessing and managing air quality, and for informing the public. They also set maximum air concentrations for certain pollutants, with the aim of avoiding, preventing or reducing their harmful effects on human health and ecosystems. As part of the implementation of the Green Pact, the European Commission has launched work to revise these directives, with the publication of a proposal for a directive in October 2022. The Commission is proposing to merge the 2004 and 2008 directives, and to define more ambitious air quality standards.

Emissions of pollutants into the air are also regulated at European level, notably by Directive 2016/2284. The latter imposes targets on member states for limiting air emissions of certain pollutants: emission ceilings are thus set for each country for five pollutants (SO2, NOx, non-methane organic compounds, PM2.5 and ammonia), to be met by 2020 and 2030. Sectoral regulations (industrial emissions, fuel quality, transport emissions, etc.) are also drawn up within this framework.

In France, the reduction in emissions that began several years ago, following the implementation of strategies and action plans such as the national plan for the reduction of atmospheric pollutant emissions (PREPA), has led to an overall improvement in air quality. Average annual concentrations of pollutants are falling, and exceedances of regulatory air quality thresholds for the protection of health are affecting fewer and smaller areas.

PREPA, which will be adopted in December 2022, sets out the French government's strategy for reducing air pollutant emissions at national level and meeting European requirements. It combines regulatory, fiscal and incentive measures, as well as planning tools for local authorities and measures to raise awareness among stakeholders. It sets out actions by major sector of activity (industry, tertiary residential, transport and agriculture).

In the transport sector, PREPA notably organizes the introduction of low-emission zones in conurbations with more than 150,000 inhabitants, an obligation enshrined in the 2021 Climate and Resilience Act. These low-emission zones, introduced by the 2019 Mobility Orientation Law, aim to reduce the circulation of the most polluting vehicles, particularly in terms of NO2. Low-emission zones have already been introduced in a dozen European countries, with encouraging results: following the introduction of a low-emission zone, reductions in concentrations of all pollutants have been recorded, ranging up to-29% for NO2,-59% for soot carbon, or-23% for PM10 (Ademe, 2020).

Local authorities can also encourage the development of non-polluting activities, notably by drawing up atmospheric protection plans, which are mandatory for urban areas with populations of over 250,000. These plans, which must be coordinated with mobility plans and territorial climate-air-energy plans (PCAET), can, for example, enable local authorities to encourage active travel such as cycling or walking, as well as public transport.

Local authorities can also contribute financially to the development of less polluting activities. For example, under the "Fonds Air Bois" scheme, certain local authorities, in collaboration with Ademe, provide financial assistance to individuals wishing to replace a non-efficient wood-burning appliance with one that is highly efficient in terms of output and fine-particle limitation.

Further information