The WMO Air Quality and Climate Bulletin includes a special focus on wildfires. It also looks at global and regional concentrations of particulate matter pollution and its harmful effects on crops in 2023. The WMO bulletin was released for Clean Air for Blue Skies Day on 7 September. This year’s theme is Invest in Clean Air Now. Ambient air pollution causes more than 4.5 million premature deaths annually and wreaks a high economic and environmental cost. The bulletin, the fourth in an annual series, explores the intricate relationship between air quality and climate. The chemical species that lead to a degradation in air quality are normally co-emitted with greenhouse gases. Thus, changes in one inevitably cause changes in the other. Air quality in turn affects ecosystem health as air pollutants settle from the atmosphere to Earth’s surface. Deposition of nitrogen, sulfur and ozone reduces the services provided by natural ecosystems such as clean water, biodiversity, and carbon storage.