Background

The Earth system is largely regulated by land-atmosphere-biosphere-climate interactions. Among those, the emissions of gaseous and particulate pollutants to the atmosphere by human activities (fossil fuel and biofuel use, agriculture, fires) and natural processes (vegetation emissions, lightning, microbial activity in soils) includes a vast array of complex processes, most of which are not well characterised, let alone quantified. Assessing the magnitude, trends and impacts of pollutant emissions on the Earth system represents a major scientific challenge that calls for the use of global observations complemented by high-quality ground-based measurements and advanced models. The integration of Earth Observation (EO) data in models has demonstrated potential for addressing key questions related to the budget and evolution of atmospheric components at global to regional to local scales. The rapid growth of atmospheric monitoring capabilities in combination with improved models hold the potential to narrow down uncertainties in land-atmosphere processes and rationalise the observed trends.

Nitrogen oxides (NOx) and volatile organic compounds (VOCs) are particularly important in tropospheric chemistry; these compounds control the formation of ozone (O₃) and hydroxyl radicals (OH), which in turn affect the oxidizing capacity of the atmosphere and the lifetime of methane, an important greenhouse gas. NOx and VOCs also contribute to the production of secondary aerosols, which affect air quality, radiation, and cloud properties, creating direct links between air pollution and climate.

HCHO, a high-yield product of VOC oxidation, is a proxy for VOC emissions thanks to its relatively short lifetime and global coverage from spaceborne instruments. While short-term variations in biogenic VOC emissions are reasonably captured by current models, their long-term response to drivers such as climate change, land use dynamics, and rising CO₂ levels remain largely uncertain. Observational constraints are limited, making it difficult to assess whether models reliably capture these processes.

Satellite datasets from the OMI (2005–) and TROPOMI (2018–) sounders offer more than two decades of information, but their interpretation is complicated due to differences in instrument design, retrieval algorithms, spatial resolution, and sensor degradation over time. Using reliable, consistent long-term records of NO₂ and HCHO and advanced models, we aim to improve our ability to assess emission changes such as the sharp declines in NOx across the United States and the rapid increases in many African cities.

Aims and objectives

The project aims to address challenges in quantifying emissions of nitrogen oxides (NOx) and volatile organic compounds (VOCs), key drivers of tropospheric chemistry that influence ozone, hydroxyl radicals (OH), methane lifetime, and secondary aerosols.

Harmonised long-term satellite data records of NO₂ and formaldehyde (HCHO) will be developed, ensuring consistent calibration, cross-sensor comparability, and robust uncertainty estimates. These tailored datasets will be designed for direct use in atmospheric models, enabling improved evaluation of long-term variability and trends in observations, but also emissions.

To bridge observational and modelling communities, PIRAMID will generate super-observations of NO₂ and HCHO, specifically adapted for both global and regional model frameworks. These products will support detailed global simulations with the TM5-MP model, providing a multi-decadal reference for assessing the evolution of reactive gases. Over Africa, detailed forward and inverse modelling will be used to disentangle the contributions of anthropogenic, biogenic, fire, soil, and lightning sources. In North America, the emphasis will be on the impact of declining NOx emissions on VOC oxidation and atmospheric chemistry, and on evaluating whether models capture the observed long-term trends.

Using the MAGRITTE model and inverse modelling techniques, PIRAMID will deliver top-down emission inventories of NOx and VOCs spanning 2005–2024, at high spatial and temporal resolution. These will be compared with bottom-up inventories to evaluate their reliability and identify systematic differences.

Finally, the project will assess the added value of satellite-constrained emissions, determining how much they improve model performance and our understanding of long-term atmospheric composition changes. Through dedicated outreach activities, results will be disseminated to the scientific community, policy stakeholders, and the wider public, providing relevant datasets that enhance our ability to track emission trends and their impacts on air quality and climate.