Long-range transport of South and East Asian anthropogenic aerosols counteracting Arctic warming

The Arctic region is highly vulnerable to climate change. Land surface temperature has been increasing at a rate of about 0.5 °C per decade since the late 1970s, faster than the global average, a phenomenon referred to as Arctic amplification. Impacts manifest across extremes, ecosystems, marine biology, soils and permafrost, ice sheets and glaciers, and human systems in the Arctic, with effects extending to lower latitudes via sea level rise, shifts in temperature and precipitation, and more frequent severe weather. While Arctic amplification is driven by increases in anthropogenic greenhouse gases and associated feedbacks, short-lived climate forcers such as aerosols are also important drivers of Arctic climate. Black carbon (BC) enhances heating and can darken snow, reducing albedo and accelerating warming, snowmelt, sea-ice thinning, and glacier melt. Conversely, sulfate aerosols scatter solar radiation and cool the climate. For example, a 20% global reduction in anthropogenic SO2 emissions would lead to a positive net direct radiative forcing at the TOA of about +0.019 W m^-2 in the Arctic, indicating the offsetting role of sulfate cooling against BC warming. Past studies have shown substantial transport of carbonaceous and sulfate aerosols to the Arctic from different source regions, with notable contributions from Europe, East Asia, and South Asia. However, the detailed transport pathways and distinct Arctic impacts of South versus East Asian aerosols during the monsoon season—when convection efficiently lofts pollutants to the UTLS—have not been fully investigated. This study addresses that gap using targeted global model experiments.

Prior work indicates large intercontinental transport of aerosols to the Arctic. Multi-model simulations (circa 2001) showed that a 20% reduction in anthropogenic emissions decreased Arctic BC levels from European sources by 21–74%, East Asia by 16–47%, and South Asia by 2–17%, and decreased sulfate by 31–71% (Europe), 13–41% (East Asia), and 1–7% (South Asia). Tagged-source modeling suggests increasing contributions of South and East Asia to Arctic upper-tropospheric sulfate and BC, with positive trends aloft. The climate impact of aerosols depends on scattering versus absorption: sulfate predominantly cools via scattering, while BC warms via absorption. Estimated Arctic cooling per sulfur emission is about -0.020 to -0.025 K (TgS)^-1 yr^-1, whereas BC emissions caused Arctic surface warming of about +0.40 K during the 20th century. Regional BC effects show higher Arctic surface warming sensitivity for European and North American sources (0.06–0.1 K Tg BC yr^-1) than Asian sources (0.05–0.08 K Tg BC yr^-1). Models indicate European emissions dominate near-surface Arctic aerosol, but South and East Asia dominate aerosol in the upper troposphere, with long-range transport from East and South Asia maximizing at 9–12 km. Observational and modeling evidence (e.g., CALIPSO, ARCTAS) supports transport of Asian aerosols (including dust and BC) to the Arctic via mid-troposphere and UTLS pathways, with time scales of about 5–20 days depending on pathway. The Asian monsoon anticyclone (ASMA) contains elevated aerosol and trace gases (the Asian Tropopause Aerosol Layer, ATAL), with composition including BC, OC, nitrates, sulfate, and significant ammonium nitrate and organics observed in recent campaigns. Previous modeling suggested two UTLS transport branches from South Asia and East Asia, but detailed monsoon-season pathways and differential Arctic impacts remained insufficiently resolved, motivating the present study.

Global aerosol-chemistry-climate simulations were carried out with ECHAM6-HAMMOZ, which couples the ECHAM6 general circulation model with the HAM aerosol module (sulfate, black carbon, particulate organic matter, sea salt, and dust) and MOZ chemistry. HAM simulates aerosol microphysics and interactions with clouds (aerosol–cloud interactions) and radiation (aerosol–radiation interactions) with feedbacks on circulation. Anthropogenic and fire emissions (sulfate precursors, BC, OC) follow CMIP6 inventories. Model configuration: T63 spectral resolution (~1.875° x 1.875°), 47 hybrid sigma–pressure levels from surface to 0.01 hPa, and 7.5 min time step. Lower boundary conditions use monthly AMIP sea surface temperature and sea ice concentration. Three experiments were performed for 2001–2016 (analyzed for June–September): (1) CTL (all emissions), (2) SASO: South Asian anthropogenic aerosols switched off (68°–95°E, 8°–38°N), and (3) EASO: East Asian anthropogenic aerosols switched off (78°E–145°E, 20°–50°N). Anomalies (CTL–SASO, CTL–EASO) quantify the contributions from South and East Asian anthropogenic aerosols, respectively. Transport and impacts were analyzed on isentropic surfaces for improved depiction of long-range transport. Diagnostics include aerosol mass mixing ratios (BC, OC, sulfate), aerosol extinction, radiative forcing due to aerosol–radiation interactions (RFari) at TOA, surface, and in-atmosphere, heating rates (shortwave and longwave), potential temperature–latitude sections, and EP fluxes to diagnose Brewer–Dobson circulation. Model evaluation leveraged prior validations of scattering ratio against CALIPSO and MIPAS and comparisons to MERRA-2 and CAMS reanalyses for aerosol distributions (BC, OC, sulfate) in the UTLS and Arctic. Data and code are available from the corresponding author upon request.

- Monsoon convection efficiently lifts South and East Asian boundary layer aerosols into the UTLS within the Asian summer monsoon anticyclone (ASMA), from where the Brewer–Dobson circulation transports them to the Arctic along isentropic surfaces. - At ~380 K, the ASMA anomaly magnitudes are larger for South Asia than East Asia by about 1.2 ng m^-3 (BC), 3 ng m^-3 (OC), and 20 ng m^-3 (sulfate), yet East Asian sulfate anomalies are substantially higher in the Arctic (~+50 ng m^-3 versus South Asia). - Vertical transport and outflow: South Asian convection (enhanced by Himalayan orography) reaches higher altitudes (~360–380 K) than East Asia (~330–350 K), yielding higher-level outflow for South Asian aerosols, while East Asian aerosols show stronger outflow over the western Pacific and subsequent northward transport. - Average aerosol concentrations transported to the Arctic (UTLS): East Asia exceeds South Asia for BC (2.0 vs 1.1 ng m^-3) and sulfate (52 vs 40 ng m^-3). OC values reported are 3 ng m^-3 (East Asia) and 6 ng m^-3 (South Asia). - Percentage changes in Arctic aerosol amounts (relative to CTL): In the UTLS (320–500 K), East Asia shows larger increases for BC (19.7%) and sulfate (19.3%) than South Asia (BC 15.8%, sulfate 2%); South Asia shows larger UTLS OC (11.2%) than East Asia (4.8%). In the troposphere (280–320 K), South Asia increases exceed East Asia for BC (6.4% vs 4.8%), OC (9.2% vs 4.6%), and sulfate (3.8% vs 0.8%). - Arctic RFari during monsoon season due to transported aerosols: South Asia: surface -0.07 ± 0.02 W m^-2, TOA +0.004 ± 0.001 W m^-2, atmospheric +0.074 ± 0.03 W m^-2; East Asia: surface -0.09 ± 0.02 W m^-2, TOA -0.007 ± 0.001 W m^-2 (reported also as -0.003 ± 0.001 W m^-2 in text), atmospheric +0.083 to +0.087 ± 0.03 W m^-2. Net implies BC-dominated South Asian plumes warm the atmospheric column but only slightly cool the surface, whereas sulfate-dominated East Asian plumes cool the surface more strongly. - Arctic surface temperature response (seasonal mean): East Asian aerosols cause -0.56 K surface cooling; South Asian aerosols cause a minor cooling of -0.043 K. - Heating rates: Along transport pathways into the UTLS and Arctic, aerosol absorption enhances shortwave heating. South Asian emissions increase net heating in the Arctic UTLS (320–440 K) by ~0.24 to 0.86 K yr^-1, while East Asian emissions cause smaller net heating changes (~0 to 0.14 K yr^-1). Longwave heating changes are negative in the Arctic UTLS (South Asia about -0.072 K yr^-1; East Asia about -0.144 K yr^-1). In the ASMA, shortwave and longwave heating enhancements are larger (South Asia SW ~2.16 K yr^-1, LW ~0.036 K yr^-1; East Asia SW ~0.86 K yr^-1, LW ~0.072 K yr^-1). - Water vapor effects: East Asian aerosols enhance water vapor transport into the Arctic stratosphere (420–500 K) more than South Asian aerosols (~0.16 ppmv vs ~0.025 ppmv), associated with greater temperature increases near the tropopause (~+0.18 °C relative to South Asia), which augments stratospheric water vapor entry and induces longwave cooling that offsets aerosol shortwave heating aloft. - Vertical profiles over the Arctic (65°–90°N): South Asian aerosols show maximum extinction (~1.05E-4 km^-1) and heating (~0.14 K yr^-1) near 380 K; East Asian aerosols peak lower (~360 K) with extinction (~0.38E-4 km^-1) and heating (~0.11 K yr^-1). - Observational consistency: Modeled Arctic transport is consistent with aircraft observations (e.g., ~5 ng m^-3 BC in the upper troposphere over the North American Arctic in summer; transport times of ~7–20 days from Asian sources).

The study demonstrates that Asian summer monsoon convection and subsequent Brewer–Dobson circulation provide efficient pathways that loft and transport South and East Asian anthropogenic aerosols to the Arctic UTLS and polar regions. Despite stronger high-level outflow from South Asia, East Asian emissions deliver larger amounts of sulfate and BC to the Arctic UTLS, yielding stronger surface dimming and a net negative TOA forcing, which leads to substantial Arctic surface cooling. In contrast, BC-dominated South Asian plumes warm the atmospheric column across the lower/middle troposphere and UTLS but induce only a small surface cooling. Enhanced stratospheric water vapor transport associated with East Asian aerosol-driven heating near the tropopause produces additional longwave cooling that mitigates UTLS warming, shaping the distinct vertical heating profiles. Overall, East Asian aerosols can partially counteract rapid Arctic surface warming, while South Asian aerosols have a limited impact on surface temperature but contribute to atmospheric heating. These results elucidate the differing radiative impacts of regional Asian aerosol sources on the Arctic climate during boreal summer and underscore the importance of aerosol composition, vertical injection level, and coupled water vapor responses.

Asian anthropogenic aerosols are efficiently transported to the Arctic via monsoon convection into the ASMA and subsequent Brewer–Dobson circulation. East Asian emissions, dominated by sulfate, exert a stronger negative surface RFari in the Arctic (-0.09 ± 0.02 W m^-2) and negative TOA forcing (around -0.003 to -0.007 ± 0.001 W m^-2), producing substantial seasonal mean Arctic surface cooling (-0.56 K). South Asian emissions, more BC-dominated, yield positive TOA forcing (+0.004 ± 0.001 W m^-2) and only minor surface cooling (-0.043 K), with warming throughout much of the atmospheric column. Vertical profiles show South Asian maxima near 380 K and East Asian near 360 K, and East Asian emissions enhance stratospheric water vapor transport and associated longwave cooling. The findings indicate that East Asian aerosols have the potential to offset part of the rapid Arctic surface warming during summer, while South Asian aerosols have a comparatively limited surface impact but warm the atmosphere aloft. The study provides process-level insights into long-range aerosol transport and radiative effects based on targeted global model experiments.

Results are derived from a single modeling framework (ECHAM6-HAMMOZ), introducing uncertainties related to transport processes, aerosol representation, and chemistry. The model exhibits uncertainties in sea salt emissions and parameterization and underestimates dust aerosols. Ammonium nitrate aerosols, observed as a major ATAL component, are not included; although absent in both control and sensitivity simulations, this omission introduces uncertainty. Sensitivity experiments addressed only anthropogenic aerosols; other sources (e.g., agricultural ammonia leading to ammonium nitrate) are not explicitly represented. Comparisons with reanalyses (MERRA-2, CAMS) show disparities in aerosol amounts and distributions (model lower BC/OC and higher sulfate relative to reanalyses), reflecting differences in emissions, transport, and assimilated products. Despite prior validations of scattering ratios, vertical aerosol distributions may be biased (e.g., overestimation of upper-tropospheric aerosol in some cases).