Permafrost is a potential mercury (Hg) pool released by thawing, which can raise the risk of Hg pollution under global warming. Tree rings are useful archives of environment-specific Hg exposure over long periods. We determined Hg concentrations in tree rings of two dominant tree species (Larix gmelinii Rupr. and Pinus sylvestris var. mongolica) at permafrost sites in northeastern China. The biweighted mean Hg concentrations ranged from 0.36 to 3.96 ng g(-1) from 1840 to 2014. The tree-ring width had no significant influence on the Hg concentration. Larch Hg increased slightly before the 1970s and peaked in the 1990s. However, the pine Hg concentration increased continuously until the 1930s, decreased rapidly until the 1970s, then rose to a peak in the late 1980s. The change of Hg concentrations in larch and pine revealed a time offset of 4 to 5 years, which implied possibly high mobility of Hg in pine tree rings. Higher Hg concentrations from 1920 to 1960 and subsequent decreases in isolated permafrost forests revealed the local geographical Hg cycling history. Lower Hg concentrations and faster increases in larch suggest the role of additional winter Hg loading for the evergreen pine and species-specific differences in root absorption in response to melting permafrost. Our results highlight possible geographical impacts on tree-ring Hg records, improve understanding of Hg cycles in permafrost forest, and suggest a need to sample additional species in a range of permafrost environments.

Long-term variations in aerosol optical properties, types, and radiative forcing over the Sichuan Basin (SCB) and surrounding regions in Southwest China were investigated based on two-decade data (2001-2020) from the Moderate Resolution Imaging Spectroradiometer, Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation, and the Santa Barbara DISORT Atmospheric Radiative Transfer model. The results showed that the aerosol optical depth (AOD550nm) in the SCB, a major polluted region in Southwest China, experienced an increasing tendency at a rate of +0.052 yr-1 during 2001-2006; thereafter, it decreased speedy up from -0.020 to -0.058 yr-1 over recent years, whereas the interannual variation in angstrom ngstrom exponent (AE470-660nm) presented a persistently increasing trend during 2001-2020, with a rate of +0.014 yr-1. An improved atmospheric environment but an enhanced fine particle contribution to regional aerosols in the SCB was observed. Over the polluted SCB region, the dominant aerosol types were biomass burning/urban industrial and mixedtype aerosols with the proportions of 80.7%-87.5% in regional aerosols, with a higher frequency of clean aerosols in recent years, reflecting an effect of controlling anthropogenic emission in the SCB owing to governmental regulation. By contrast, few changes were observed in the aerosol types and amounts in the eastern Tibetan Plateau (ETP), where clean continental aerosols dominate with high proportion of 93.7% in the clean atmospheric environment. A significant decline in polluted anthropogenic aerosols was observed below 3 km over the SCB, resulting in the regional aerosol extinction coefficients at 532 nm (EC532nm) were declined by -0.22 km-1 from 2013 to 2020. Notably, the decreases in aerosol radiative forcing within the atmosphere were found in