The Arctic is experiencing rapid warming, a phenomenon known as Arctic amplification, exceeding the global average temperature increase. This amplification is primarily driven by increased anthropogenic greenhouse gases (GHGs) and feedback mechanisms. However, short-lived climate forcers, such as aerosols, also play a crucial role. Black carbon (BC) aerosols, with their positive radiative forcing (warming effect), are particularly concerning, exacerbating warming through snow darkening and reduced albedo. Conversely, sulfate aerosols exert a cooling effect. Previous research indicates significant long-range transport of carbonaceous and sulfate aerosols to the Arctic from various source regions, particularly Europe, East Asia, and South Asia. While studies have shown evidence of transport from East Asia to the Arctic, the detailed pathways of both South and East Asian aerosols during the monsoon season remain under-investigated. This study aims to fill this gap by exploring the transport and radiative impacts of South and East Asian aerosols on the Arctic climate system, focusing on the role of the Asian summer monsoon.

Past studies have highlighted the significant transport of carbonaceous (BC and organic aerosols) and sulfate aerosols to the Arctic from various sources. Multi-model simulations for 2001 showed that a 20% reduction in anthropogenic emissions led to decreased BC and sulfate levels in the Arctic, with varying contributions from different regions. Simulations using the Community Atmosphere Model with Explicit Aerosol Source Tagging showed increased sulfate and BC concentrations in the Arctic from South and East Asia, particularly in the upper troposphere. The climate impact of aerosol-radiation interactions depends on the aerosols' ability to scatter and absorb solar radiation. Sulfate aerosols generally cause cooling due to scattering, while BC aerosols cause warming due to absorption. Existing studies have quantified the cooling effect of global sulfur emissions and the warming effect of BC emissions on Arctic temperatures, with some suggesting that European and North American emissions have historically contributed more to near-surface warming than Asian emissions, although Asian emissions are increasingly important. However, most previous work has focused on near-surface effects rather than the total atmospheric impacts of aerosols. Previous model simulations have indicated two major transport branches supplying aerosols to the upper troposphere and lower stratosphere (UTLS) from South and East Asia, suggesting the potential for further transport to the Arctic. This paper builds upon this existing literature by focusing specifically on the transport during the Asian monsoon season, highlighting the importance of monsoon convection in the transport mechanism.

This study uses the state-of-the-art ECHAM6-HAMMOZ aerosol-chemistry-climate model to investigate the transport of South and East Asian aerosols to the Arctic during the monsoon season. Three sets of simulations were conducted: a control simulation (CTL) including all emissions, a simulation with South Asian anthropogenic aerosols switched off (SASO), and a simulation with East Asian anthropogenic aerosols switched off (EASO). The model simulates the formation and sinks of various aerosol types, including sulfate, BC, organic carbon (OC), sea salt, and dust. The model is coupled with a general circulation model and explicitly accounts for aerosol impacts on cloud formation. The model was run at high resolution (T63) with monthly average sea surface temperature (SST) and sea ice cover (SIC) data from the AMIP dataset used as lower boundary conditions. The simulations were conducted from January 2001 to December 2016, with the analysis focusing on the monsoon season (June-September). The differences between the CTL and SASO simulations provide information on the impact of South Asian anthropogenic aerosols, and the differences between CTL and EASO reveal the impact of East Asian anthropogenic aerosols. The transport was analyzed using isentropic surfaces to better represent long-range transport. The model results were validated against observations from various sources, including the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) and the Copernicus Atmosphere Monitoring Service (CAMS). The analysis includes the calculation of radiative forcing at the top of the atmosphere (TOA) and the surface, along with an assessment of changes in atmospheric heating rates, both shortwave and longwave, to evaluate the climatic impacts. The model's performance was further evaluated by comparing its simulation of the vertical distribution of BC aerosols with observations.

The model simulations demonstrate the crucial role of the Asian summer monsoon's deep convection in transporting aerosols from South and East Asia into the Asian summer monsoon anticyclone (ASMA). The outflow of South Asian aerosols occurs at higher altitudes (360-380 K potential temperature) compared to East Asian aerosols (330-350 K), likely due to the influence of the Himalayan orography. These aerosols are subsequently transported to the Arctic via the shallow branch of the Brewer-Dobson Circulation (BDC). East Asian aerosols (primarily sulfate) are transported to the Arctic in larger quantities than South Asian aerosols (primarily BC). The radiative forcing analysis reveals that East Asian aerosols cause a significant negative radiative forcing at the Arctic surface (-0.09 ± 0.02 Wm⁻²) and a small negative forcing at the TOA (-0.003 ± 0.001 Wm⁻²), resulting in substantial surface cooling (-0.56 K). Conversely, South Asian aerosols exert a smaller negative surface forcing (-0.07 ± 0.02 Wm⁻²) but a positive TOA forcing (+0.004 ± 0.001 Wm⁻²), leading to minimal surface cooling (-0.043 K). The analysis of heating rates shows that while both South and East Asian aerosols enhance heating in the UTLS and South Asian aerosols caused greater heating in the Arctic UTLS, the East Asian aerosols' effect on Arctic stratospheric heating is reduced by increased water vapor transport, leading to longwave cooling. The enhanced water vapor transport is attributed to increased evaporation due to warming induced by East Asian aerosols, primarily through absorption of shortwave radiation, highlighting a complex interaction between aerosol radiative properties and atmospheric dynamics. The model revealed larger amounts of East Asian aerosols (BC and sulfate) in the Arctic UTLS than South Asian aerosols, and larger amounts of South Asian aerosols in the Arctic troposphere than East Asian aerosols.

The findings highlight the contrasting impacts of South and East Asian anthropogenic aerosols on the Arctic climate. While both contribute to aerosol loading, East Asian aerosols, dominated by sulfate, cause significant surface cooling, potentially counteracting the effects of GHG-induced warming. South Asian aerosols, with their higher BC content, lead to less pronounced cooling and even some warming in the atmospheric column. The differences in aerosol composition and the subsequent effects on radiative forcing and water vapor transport explain the contrasting impacts. The study underscores the importance of considering the complex interplay between aerosol type, transport pathways, and radiative interactions in assessing the influence of Asian emissions on Arctic climate. The observed differences in the impacts of aerosols from East and South Asia emphasize the need for regional specificity in emission control strategies.

This study demonstrates the significant impact of long-range transport of South and East Asian anthropogenic aerosols on the Arctic climate system, particularly during the monsoon season. East Asian aerosols cause substantial surface cooling, potentially offsetting Arctic warming, while South Asian aerosols have a less pronounced cooling effect. The differences in aerosol composition and atmospheric dynamics drive these contrasting impacts. This work highlights the need for comprehensive modeling studies to understand the regional nuances in aerosol-climate interactions and inform effective mitigation strategies.

This study is based on simulations from a single model (ECHAM6-HAMMOZ), which may introduce uncertainties, particularly concerning transport processes and aerosol representation. The model may underestimate dust and ammonium nitrate aerosols, potentially affecting the accuracy of radiative forcing estimates. The sensitivity simulations are performed only for anthropogenic aerosols, which might limit the complete representation of the impact. Future studies could benefit from multi-model ensembles and improved aerosol representation to reduce uncertainties and refine the understanding of these complex processes.