Effects of quasi-biweekly oscillation of the South Asian high on the Asian Tropopause Aerosol Layer

Based on the MERRA-2 and CAMS reanalysis data from 2011 to 2020, the east-west oscillation of the South Asia high (SAH) is described by South Asian High Index at the sub-seasonal scale. Under TP mode and IP mode the effects of SAH on pollutants in the Asian Tropopause Aerosol Layer (ATAL) are compared and analyzed. The results show that the distribution of CO, NO_X and PAN in ATAL has a good correspondence with the spatial structure of SAH. According to the average distribution and transport over the years, the three pollutants all have a strong transport channel over the Qinghai-Tibet Plateau. In the TP and IP modes, obviously different are the horizontal distribution and vertical transport of CO, NO_X and PAN. In the horizontal distribution, the positive anomaly regions of the three pollutants in the two modes are corresponding to the anticyclonic circulation centers with different wind fields. The regions with negative anomalies (or low values) correspond to cyclonic circulations with wind field differences. In the process of SAH from TP mode to IP mode, the concentration of CO and NO_X decreases, while the concentration of PAN increases significantly, indicating that in addition to transport from bottom to top, the possibility of new PAN formation occurs at ATAL.