Unexpected high 35 S concentration revealing strong downward transport of stratospheric air during the monsoon transitional period in East Asia

October is the monsoon transitional period in East Asia (EA) involving a series of synoptic activities that may enhance the downward transport of stratospheric air to the planetary boundary layer (PBL). Here we use cosmogenic 35 S in sulfate aerosols ( 35 SO 4 2− ) as a tracer for air masses originating from the stratosphere and transported downward to quantify these mixing processes. From 1 year 35 SO 4 2− measurements (March 2014 to February 2015) at a background station in EA we find remarkably enhanced 35 SO 4 2− concentration (3150 atoms m −3 ) in October, the highest value ever reported for natural sulfate aerosols. A four‐box 1‐D model and meteorological analysis reveal that strong downward transport from the free troposphere is a vital process entraining aged stratospheric air masses to the PBL. The aged stratospheric masses are accumulated in the PBL, accelerating the SO 2 transformation to SO 4 2− . Implications for the tropospheric O 3 budget and the CO 2 biogeochemical cycle are discussed. The highest 35 S concentration ever measured for natural sulfate aerosols is reported Strong downward transport from the FT in October brings aged stratospheric air to the PBL Temporal variations in surface O 3 in spring and autumn are in tandem with 35 SO 4 2− concentrations