Abstract The concentrations of ammonium sulfate, an important component of secondary inorganic aerosols, have dropped substantially in the North China Plain (NCP) over the past decade. However, current understanding of its variations is limited by the small number of ground‐based measurement sites. Using long‐term satellite hyperspectral infrared spectra, we developed a full‐physics retrieval of ammonium sulfate aerosol optical depth (AOD) and produced the first long‐term data set from 2008 to 2024. These data reveal a significant 50% decrease in NCP from 2008 to 2024, with a more pronounced decrease from 2014 to 2015 than in reanalysis data sets. In addition, the ratio of observed ammonium sulfate to total AOD, a proxy for the ammonium sulfate fraction of inorganic aerosols, decreased before 2015 and rebounded afterward. Unexpectedly, modeled data sets show opposite trends. These inconsistencies may undermine models’ ability to project future secondary aerosol changes and evaluate their impacts on public health and climate.