Aerosol Longwave Radiative Forcing in the Arctic: Forty Years of Change Under Reducing Global SO₂ Emissions
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This study presents a comprehensive assessment of the longwave radiative effects of Arctic tropospheric aerosols based on unique measurements collected at the North Pole drifting station SP 28 in 1987. The primary objective is to compare these historical observations with modern datasets from the Surface Heat Budget of the Arctic Ocean (SHEBA, 1997–1998) and the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC, 2019–2020) to evaluate long term changes in the Arctic radiation regime. Continuous longwave radiation measurements were obtained using high precision spectral pyrgeometers, and a new diagnostic approach was introduced that employs the standard deviation of longwave radiation together with the normalized longwave aerosol effect (NLAE) to identify haze inversion layers. The results show that in 1987, sub inversion haze layers enhanced the downward longwave flux by 15–20 W/m² and increased atmospheric emissivity. In contrast, MOSAiC observations reveal emissivity values that closely match aerosol free model calculations, indicating a substantial decline in Arctic haze and the disappearance of radiatively significant aerosol layers. This shift is consistent with the long term reduction of sulfur dioxide emissions across the Northern Hemisphere and suggests that gaseous components now dominate the formation of inversion radiative properties in the modern Arctic.