Particulate matter from activities such as transport, industry and agriculture, as well as from wildfires and wind-blown desert dust remains a major health hazard.
For the first time, the 2025 edition of the Bulletin includes estimates from three different models in its overview of Particulate Matter 2.5 microns in diameter or smaller (PM2.5) anomalies in 2024 (compared to a 2003-2024 reference period).
PM2.5 levels continued to decline in eastern China thanks to sustained mitigation measures. By contrast there were pollution hotspots in northern India. Wildfire activity led to above-average PM2.5 levels in Canada, Siberia and central Africa. The highest anomaly, however, was in the Amazon basin, as part of record wildfires in the western Amazon region and drought-fuelled fires in northern South America.
Wildfires are a big contributor to particle pollution and the problem is expected to increase as the climate warms, posing growing risks for infrastructure and ecosystems and human health.
PM2.5 anomaly in 2024 (reference period 2003-2024)
Third-party maps. These maps were provided by CAMS (left), NASA Global Modelling and Assimilation office (centre) and the Finnish Meteorological Institute on 28 July 2025 (right) and may not fully align with United Nations and WMO map guidance.
Aerosols, or tiny airborne particles, are complex. They can both warm and cool the atmosphere depending on their composition. Darker ones, such as black and brown carbon, warm the atmosphere and melt the ice or glaciers that they land on.
Conversely, brighter aerosols, such as sulphates, reflect solar radiation back to space, providing temporary cooling before depositing as acid rain and snow.
Concentrations of aerosols in the atmosphere increased globally from the 1950s to 1980s but have come down substantially since then due to concerted efforts in North America, Europe and, later, East Asia. They continue to rise in some regions such as South Asia, South America and the northern latitudes due, in part, to increasing wildfires.
International regulations reducing sulphur emissions in shipping fuels have improved air quality and successfully cut premature deaths and childhood asthma. But they also had a measurable impact on reducing the cooling effect of sulphate aerosols, slightly accelerating global warming.
Disentangling complex interplay between aerosols, reactive gases and long-lived greenhouse gases is a difficult task and a proper understanding of these interactions will help in developing better mitigation measures for both climate and air quality. In this context, it is important to take an integrated approach to managing emissions, not only to protect the climate but also ecosystem and human health.
The Indo-Gangetic Plain, home to over 900 million residents, is among the most densely populated and agriculturally active regions in the world and has experienced a marked rise in air pollution as well as winter fog episodes. Although fog is a seasonal occurrence, its growing frequency and duration are increasingly linked to pollution from vehicles, construction, heating, cattle, and burning of vegetation.
“Persistence of fog is no longer a simple, seasonal weather event – it is a symptom of escalating human impact on the environment. Addressing this requires comprehensive strategies, such as enforcing regulations on agricultural residue burning, and promoting cleaner energy for cooking, heating, lighting and public transport systems,” says the Bulletin.
Another article looks at PM2.5 emissions from wildfires in the amazon basin and how these emissions resulted in a measurable degradation in air quality in far-away, densely populated urban centres in Brazil, underscoring the significant consequences of the 2024 wildfire season.
From ozone studies in South America to improved pollen forecasting in Europe, to assessing atmospheric deposition in Africa, the Bulletin underscores the importance of a global, fit-for-purpose in situ atmospheric monitoring infrastructure, especially in developing regions. Observations are foundational. Satellites provide critical insights, but ground-based monitoring networks are essential for calibration and validation, particularly in developing countries where infrastructure remains sparse.
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