Nitrogen dominates global atmospheric organic aerosol absorption.

Atmospheric organic aerosols (OAs) influence Earth's climate by absorbing sunlight. However, the link between their evolving composition and their absorptive effects is unclear. We demonstrate that brown nitrogen (BrN), the absorptive nitrogenous component of OAs, dominates their global absorption. Using a global model, we quantified BrN abundance, tracked its optical evolution with chemical aging, and assessed its radiative absorption. BrN contributes 76% of OAs' surface light absorption over the US and 61% of their global absorptive optical depth. Moreover, the observed variability of OAs' absorptive capacity is primarily driven by the sources and aging of BrN. BrN represents 18% of the global absorptive direct radiative effect of carbonaceous aerosols, with biomass burning being the largest contributor. Our research establishes a nitrogen-centric framework for attributing the climate impacts of OAs.