Observed decadal variations of ammonium sulfate aerosols over northern China using the Infrared Atmospheric Sounding Interferometer (IASI)

Ammonium sulfate is a key component of secondary inorganic aerosols in northern China and contributes significantly to PM2.5 pollution. Through hygroscopic growth and enhanced light extinction, it also impacts atmospheric visibility and the regional radiative balance. In recent years, China’s clean air initiatives have significantly reduced surface PM2.5 concentrations. However, a decrease in total PM2.5 or bulk aerosol optical depth (AOD) does not necessarily imply a proportional, synchronised decline in ammonium sulfate. The lack of a long-term, interannually comparable record of ammonium sulfate aerosols hinders our ability to quantitatively understand the long-term changes in ammonium sulphate on a regional scale. Hyperspectral thermal infrared remote sensing offers a unique advantage in identifying the composition of aerosols. Ammonium sulfate exhibits resolvable absorption structures in the thermal infrared atmospheric window region, with a diagnostic spectral feature near 1115 cm⁻¹, which provides a physical basis for retrieving ammonium sulfate AOD.

In this study, we use long-term hyperspectral infrared measurements from the Infrared Atmospheric Sounding Interferometer (IASI) to construct an ammonium sulfate AOD time series for the North China Plain (NCP) from 2008 to 2025, and to characterise its spatial distribution, interannual variability and multi-year trends. Our focus is on the summer months, as ammonium sulfate over the NCP typically exhibits higher and more spatially continuous regional enhancement during this period. Additionally, infrared observations are sensitive to thermal conditions, and summer daytime provides more favourable conditions for achieving stable, interannually comparable results.

We use an optimal-estimation–based retrieval algorithm to retrieve ammonium sulfate AOD for clear sky observations. The state vector also includes interfering trace gases and surface temperature. The results show a significant decreasing trend in ammonium sulfate AOD over NCP during 2008–2025, with distinct spatial patterns and pronounced interannual variability. Furthermore, the retrieval results are compared with CAMS simulation products and long-term ground-based records of sulfate and  chemical composition. Overall, this work provides a satellite-based constraint on the long-term evolution of secondary inorganic aerosols over NCP. This offers new evidence with which to evaluate the effectiveness of mitigation measures and advance our mechanistic understanding of air pollution.