Analysis of Aerosols in the Asian Monsoon Anticyclone as Observed by the Atmospheric Chemistry Experiment

Abstract During the Asian summer monsoon season, pollutants from the lower troposphere are transported through deep convection to the upper troposphere and lower stratosphere. Surface pollutants such as CO are transported upward and trapped in the anticyclone during this unique atmospheric phenomenon. Associated with the anticyclone is a layer of enhanced aerosols located near the tropopause often referred to as the Asian tropopause aerosol layer (ATAL). The chemical and physical properties of aerosols in the ATAL are not yet fully understood as direct observations of the aerosols are limited. The Atmospheric Chemistry Experiment (ACE) is a satellite mission that provides high‐resolution infrared observations of atmospheric trace gases and aerosols. ACE observes chemical isolation in the anticyclone and identifies three main types of aerosols in the ATAL: solid ammonium nitrate (AN), sulfate aerosols, and ice. ACE observes four main features in the spectrum of AN; the previously known 831 cm −1 feature and the first‐ever identification of the spectral bands at 1,390–1,450 cm −1 , 3,080 cm −1 , and 3,220 cm −1 . Simulated spectra reproduce the 831 cm −1 , 3,080 cm −1 , and 3,220 cm −1 features well but the 1,390–1,450 cm −1 spectral feature is poorly reproduced. In rare circumstances, ACE observes aerosols similar to AN far outside of the monsoon region. In these aerosols the 3,080 cm −1 and 3,220 cm −1 features align well with AN observed in the ATAL. The 831 cm −1 feature is suppressed and shifted to 829 cm −1 , suggesting that AN aerosols outside the ATAL are liquid, or potentially an alternative phase of AN.