The upper troposphere/lower stratosphere (UTLS) is a crucial region for Earth's climate, where changes of aerosol loading and composition can have a direct impact on the amount of radiation absorbed and emitted. The proposed UTLS model experiments has the following objectives: (1) Compare and evaluate the model simulated aerosol and precursors in the UTLS regions in recent decades, (2) examine the pathways of aerosols in the UTLS region (e.g., roles of convective transport, chemistry, and direct injection), (3) Assess the contributions of anthropogenic and volcanic emissions to the decadal variations of UTLS aerosols. It will be coordinated with and benefited from other community projects, such as the IGAC/SPARC Atmospheric Composition and Asian Monsoon (ACAM), and the SPARC Stratospheric Sulfur and its Role in Climate (SSiRC). These objectives will be achieved by designed model experiments described here: A3_UTLS_2019-11-26.pdf