Reconsidering the Existence of a Trend in the Asian Tropopause Aerosol Layer (ATAL) From 1979 to 2017

An enhanced aerosol layer, known as the Asian Tropopause Aerosol Layer (ATAL), has been observed within the seasonal Asian monsoon anticyclone (AMA) since the late 1990s. Given the apparently abrupt appearance of this layer based on observations, it has been speculated that it originates from increasing human made emissions in Asia. However, the ATAL confinement is a result of a dynamical feature and does not purely consist of human made components. We herein investigate the possible existence of an ATAL earlier than the late 1990s. We exploit earliest possible, high quality space-based aerosol observations from Stratospheric Aerosol and Gas Experiment, or SAGE (1979-1981), SAGE III/ISS (2017, ongoing) and revisit SAGE II (1984-2005) data analysis. We find that seasonal averaged solar occultation aerosol measurements (past and present) can neither be used to exclude the existence of the ATAL, nor to infer a significant trend. However, first CAM5-MAM7 simulations indicate the presence of an ATAL signal for the tested years 1979 and 1980, with a human made component. We hypothesize that the human made component of the ATAL likely occurred since at least the 1970s, while the natural ATAL component (e.g., from dust) has always existed. Extended simulation based ATAL evolution studies are therefore the most reliable source for early ATAL investigations. An enhanced aerosol layer in the Upper Troposphere and Lower Stratosphere (UTLS), known as ATAL (Asian Tropopause Aerosol Layer), was discovered during the seasonal Asian monsoon. Initial observations from space-borne aerosol data suggested its first appearance in the late 1990s. However, our study reveals that the data set used (from SAGE instruments) is inadequate for ATAL evolution studies and to conclusively determine its first appearance. The limited sampling within the Asian monsoon region and cloud filtering procedure do not provide sufficient data points for definitive conclusions. Nevertheless, the dynamical structure leading to an ATAL has always been present, leading us to hypothesize that the non-human-made ATAL component has existed previously. Additionally, our first model simulations for 1979 and 1980 indicate the presence of an ATAL. Early space-borne observations (SAGE instruments) cannot be used for ATAL evolution studies, contrary to previous claims SAGE lacks sufficient data in the tropics and an effective cloud capture method to quantitatively detect or exclude the presence of ATAL First model simulations indicate an ATAL signal, without volcanic influence, in 1979 and 1980