Potential impact of carbonaceous aerosol on the upper troposphere and lower stratosphere (UTLS) and precipitation during Asian summer monsoon in a global model simulation

Abstract

Abstract. Recent satellite observations show efficient vertical transport of Asian pollutants from the surface to the upper-level anticyclone by deep monsoon convection. In this paper, we examine the transport of carbonaceous aerosols, including black carbon (BC) and organic carbon (OC), into the monsoon anticyclone using of ECHAM6-HAM, a global aerosol climate model. Further, we investigate impacts of enhanced (doubled) carbonaceous aerosol emissions on the upper troposphere and lower stratosphere (UTLS), underneath monsoon circulation and precipitation from sensitivity simulations.The model simulation shows that boundary layer aerosols are transported into the monsoon anticyclone by the strong monsoon convection from the Bay of Bengal, southern slopes of the Himalayas and the South China Sea. Doubling of emissions of both BC and OC aerosols over Southeast Asia (10 • S-50 • N, 65-155 • E) shows that lofted aerosols produce significant warming (0.6-1 K) over the Tibetan Plateau (TP) near 400-200 hPa and instability in the middle/upper troposphere. These aerosols enhance radiative heating rates (0.02-0.03 K day −1 ) near the tropopause. The enhanced carbonaceous aerosols alter aerosol radiative forcing (RF) at the surface by −4.74 ± 1.42 W m −2 , at the top of the atmosphere (TOA) by +0.37 ± 0.26 W m −2 and in the atmosphere by +5.11 ± 0.83 W m −2 over the TP and Indo-Gangetic Plain region • N, 80-110 • E). Atmospheric warming increases vertical velocities and thereby cloud ice in the upper troposphere. Aerosol induced anomalous warming over the TP facilitates the relative strengthening of the monsoon Hadley circulation and increases moisture inflow by strengthening the cross-equatorial monsoon jet. This increases precipitation amounts over India (1-4 mm day −1 ) and eastern China (0.2-2 mm day −1 ). These results are significant at the 99 % confidence level.

Estimates of Regional Source Contributions to the Asian Tropopause Aerosol Layer Using a Chemical Transport Model

“…The presence of an ATAL suggests that the ASM may provide a conduit for aerosols, and/or precursor gases, to reach and be dispersed in the UTLS (Neely et al, 2014;Yu et al, 2017). In addition, the proximity of ATAL aerosols to ice clouds in the ASM anticyclone (Vernier et al, 2018) indicates the potential for ATAL aerosols to alter cloud optical properties (Fadnavis et al, 2013(Fadnavis et al, , 2017Li et al, 2005), which could impact regional climate. The chemical composition and size distribution of ATAL aerosols remain poorly characterized, due to a dearth of in situ observations.…”

Origins and Spatial Distribution of Non-Pure Sulfate Particles (NSPs) in the Stratosphere Detected by the Balloon-Borne Light Optical Aerosols Counter (LOAC)

“…This layer, known as the Asian Tropopause Aerosol Layer (ATAL) [64,115], is likely to be associated with Asian emissions of anthropogenic pollutants, such as sulfur dioxide and volatile organic compounds, building a population of NSPs consisting of a mixture of sulfates and organic material both as primary and secondary organic aerosols. The ATAL is sustained by the convective activity of the Asian monsoon as indicated by global model simulations [116,117]. However, the precise composition, variability, trend, and budget of the ATAL are still largely uncertain, and are currently under investigation.…”

The impact of recent changes in Asian anthropogenic emissions of SO<sub>2</sub> on sulfate loading in the upper troposphere and lower stratosphere and the associated radiative changes

“…We refer to it as the control simulation (CTRL). In previous work, Fadnavis et al (2013Fadnavis et al ( , 2017b used the ensemble means from 6 to 10 members to analyze the variability of aerosols and associated impacts during the monsoon season. In two emission sensitivity simulations we applied (1) a flat 48 % increase in anthropogenic SO 2 emissions over India (referred to as Ind48 simulation) and (2) a flat 48 % increase in anthropogenic SO 2 emissions over India and a flat 70 % decrease in anthropogenic SO 2 emissions over China simultaneously (referred to as Ind48Chin70 simulation); the same assumptions were made for simulated years.…”

“…4c-d (enhancement ∼ 5-15 ng m −3 ; 10 %-36 %). Past studies show that the aerosols transported into the lower stratosphere by the monsoon convection are recirculated in the stratosphere by the lower branch of the Brewer-Dobson circulation (Randel and Jensen, 2013;Fadnavis et al, 2013Fadnavis et al, , 2017b. Yu et al (2017) report that ∼ 15 % of the Northern Hemisphere column stratospheric aerosol originates from the Asian summer-monsoon anticyclone region.…”