Climate change is rapidly transforming Arctic landscapes where increasing soil temperatures speed up permafrost thaw. This exposes large carbon stocks to microbial decomposition, possibly worsening climate change by releasing more greenhouse gases. Understanding how microbes break down soil carbon, especially under the anaerobic conditions of thawing permafrost, is important to determine future changes. Here, we studied the microbial community dynamics and soil carbon decomposition potential in permafrost and active layer soils under anaerobic laboratory conditions that simulated an Arctic summer thaw. The microbial and viral compositions in the samples were analyzed based on metagenomes, metagenome-assembled genomes, and metagenomic viral contigs (mVCs). Following the thawing of permafrost, there was a notable shift in microbial community structure, with fermentative Firmicutes and Bacteroidota taking over from Actinobacteria and Proteobacteria over the 60-day incubation period. The increase in iron and sulfate-reducing microbes had a significant role in limiting methane production from thawed permafrost, underscoring the competition within microbial communities. We explored the growth strategies of microbial communities and found that slow growth was the major strategy in both the active layer and permafrost. Our findings challenge the assumption that fast-growing microbes mainly respond to environmental changes like permafrost thaw. Instead, they indicate a common strategy of slow growth among microbial communities, likely due to the thermodynamic constraints of soil substrates and electron acceptors, and the need for microbes to adjust to post-thaw conditions. The mVCs harbored a wide range of auxiliary metabolic genes that may support cell protection from ice formation in virus-infected cells.IMPORTANCE As the Arctic warms, thawing permafrost unlocks carbon, potentially accelerating climate change by releasing greenhouse gases. Our research delves into the underlying biogeochemical processes likely mediated by the soil microbial community in response to the wet and anaerobic conditions, akin to an Arctic summer thaw. We observed a significant shift in the microbial community post-thaw, with fermentative bacteria like Firmicutes and Bacteroidota taking over and switching to different fermentation pathways. The dominance of iron and sulfate-reducing bacteria likely constrained methane production in the thawing permafrost. Slow-growing microbes outweighed fast-growing ones, even after thaw, upending the expectation that rapid microbial responses to dominate after permafrost thaws. This research highlights the nuanced and complex interactions within Arctic soil microbial communities and underscores the challenges in predicting microbial response to environmental change. As the Arctic warms, thawing permafrost unlocks carbon, potentially accelerating climate change by releasing greenhouse gases. Our research delves into the underlying biogeochemical processes likely mediated by the soil microbial community in response to the wet and anaerobic conditions, akin to an Arctic summer thaw. We observed a significant shift in the microbial community post-thaw, with fermentative bacteria like Firmicutes and Bacteroidota taking over and switching to different fermentation pathways. The dominance of iron and sulfate-reducing bacteria likely constrained methane production in the thawing permafrost. Slow-growing microbes outweighed fast-growing ones, even after thaw, upending the expectation that rapid microbial responses to dominate after permafrost thaws. This research highlights the nuanced and complex interactions within Arctic soil microbial communities and underscores the challenges in predicting microbial response to environmental change.

Reducing the uncertainty in aerosol radiative forcing requires a comprehensive understanding of the factors affecting black carbon (BC) light absorption. In this study, the characteristics and influencing factors of light absorption enhancement (Eabs) of refractory BC (rBC) were investigated by conducting intensive measurements at an urban site in northwest China during the early summer of 2018. On average, the absorption of rBC was enhanced by 34% as a result of the internal mixing of rBC with other aerosol components. Secondary inorganic aerosols (SIAs) were found to have considerable effects on the Eabs of rBC. The Eabs showed a robust linear relationship with the bulk nitrate/rBC mass ratio in fine particles, with an increase of 3% per nitrate/rBC ratio unit. A notable increase in Eabs from dusk to the next morning was observed, in accordance with the diurnal variations in nitrate and sulfate, indicating the excellent contribution of non-photochemical formation of SIAs to Eabs. This fact was further supported by the positive correlation of the nitrate/rBC and sulfate/rBC ratios with relative humidity (RH) rather than photochemical indicators. This study indicates that the aqueous and/or heterogeneous formation of SIAs is likely the dominant aging pathway leading to the high Eabs of rBC.

Permafrost degradation is altering biogeochemical processes throughout the Arctic. Thaw-induced changes in organic matter transformations and mineral weathering reactions are impacting fluxes of inorganic carbon (IC) and alkalinity (ALK) in Arctic rivers. However, the net impact of these changing fluxes on the concentration of carbon dioxide in the atmosphere (pCO(2)) is relatively unconstrained. Resolving this uncertainty is important as thaw-driven changes in the fluxes of IC and ALK could produce feedbacks in the global carbon cycle. Enhanced production of sulfuric acid through sulfide oxidation is particularly poorly quantified despite its potential to remove ALK from the ocean-atmosphere system and increase pCO(2), producing a positive feedback leading to more warming and permafrost degradation. In this work, we quantified weathering in the Koyukuk River, a major tributary of the Yukon River draining discontinuous permafrost in central Alaska, based on water and sediment samples collected near the village of Huslia in summer 2018. Using measurements of major ion abundances and sulfate (SO42-) sulfur (S-34/S-32) and oxygen (O-18/O-16) isotope ratios, we employed the MEANDIR inversion model to quantify the relative importance of a suite of weathering processes and their net impact on pCO(2). Calculations found that approximately 80% of SO42- in mainstem samples derived from sulfide oxidation with the remainder from evaporite dissolution. Moreover, S-34/S-32 ratios, C-13/C-12 ratios of dissolved IC, and sulfur X-ray absorption spectra of mainstem, secondary channel, and floodplain pore fluid and sediment samples revealed modest degrees of microbial sulfate reduction within the floodplain. Weathering fluxes of ALK and IC result in lower values of pCO(2) over timescales shorter than carbonate compensation (similar to 10(4) yr) and, for mainstem samples, higher values of pCO(2) over timescales longer than carbonate compensation but shorter than the residence time of marine SO42- (similar to 10(7) yr). Furthermore, the absolute concentrations of SO42- and Mg2+ in the Koyukuk River, as well as the ratios of SO42- and Mg2+ to other dissolved weathering products, have increased over the past 50 years. Through analogy to similar trends in the Yukon River, we interpret these changes as reflecting enhanced sulfide oxidation due to ongoing exposure of previously frozen sediment and changes in the contributions of shallow and deep flow paths to the active channel. Overall, these findings confirm that sulfide oxidation is a substantial outcome of permafrost degradation and that the sulfur cycle responds to permafrost thaw with a timescale-dependent feedback on warming.

Mixing state of black carbon (BC) with secondary species has been highlighted as a major uncertainty in assessing its radiative forcing. While recent laboratory simulation has demonstrated that BC could serve as a catalyst to enhance the formation of sulfate, its role in the formation and evolution of secondary aerosols in the real atmosphere remains poorly understood. In the present study, the mixing of BC with sulfate/nitrate in the atmosphere of Guangzhou (China) was directly investigated with a single particle aerosol mass spectrometer (SPAMS). The peak area ratios of sulfate to nitrate (SNRs) for the BC-containing particles are constantly higher than those of the BC-free particles (defined as particles with negligible BC signals). Furthermore, the seasonal SNR peak is observed in summer and autumn, and the diurnal peak is found in the afternoon, consistent with the trends of radiation-related parameters (i.e., solar radiation and temperature), pointing to the BC-induced photochemical production of sulfate. Such hypothesis is further supported by the multilinear regression and random forest analysis, showing that the variation of SNRs associated with the BC-containing particles could be well explained (R-2 = similar to 0.7-0.8) by the radiation-related parameters (>30% of the variance) and the relative BC content (similar to 20%) in individual particles, but with limited influence of precursors (SO2/NOx: <5%). Differently, the radiation-related factors only explain <10% of the SNR variation for the BC-free particles. These results provide ambient observational evidence pointing to a unique role of BC on the photochemical formation and evolution of sulfate, which merits further quantitative evaluations.

Source attribution of Arctic sulfate and its radiative forcing due to aerosol-radiation interactions (RFari) for 2010-2014 are quantified in this study using the Community Earth System Model equipped with an explicit sulfur source-tagging technique. The model roughly reproduces the seasonal pattern of sulfate but has biases in simulating the magnitude of near-surface concentrations and vertical distribution. Regions that have high emissions and/or are near/within the Arctic present relatively large contributions to Arctic sulfate burden, with the largest contribution from sources in East Asia (27%). Seasonal variations of the contribution to Arctic sulfate burden from remote sources are strongly influenced by meteorology. The mean RFari of anthropogenic sulfate offsets one third of the positive top of the atmosphere (TOA) RFari from black carbon. A 20% global reduction in anthropogenic SO2 emissions leads to a net Arctic TOA forcing increase of +0.019Wm(-2). These results indicate that a joint reduction in BC and SO2 emissions could prevent at least some of the Arctic warming from any future SO2 emission reductions. Sulfate RFari efficiency calculations suggest that source regions with short transport pathways and meteorology favoring longer lifetimes are more efficient in influencing the Arctic sulfate RFari. Based on Arctic climate sensitivity factors, about -0.19K of the Arctic surface temperature cooling is attributed to anthropogenic sulfate, with -0.05K of that from sources in East Asia, relative to preindustrial conditions.

Despite growing evidence of light-absorbing organic aerosols (OAs), their contribution to the Earth's radiative budget is still poorly understood. In this study we derived a new empirical relationship that binds OA single scattering albedo (SSA), which is the ratio of light scattering to extinction, with sulfate + nitrate aerosol optical depth (AOD) and applied this method to estimate OA SSA over the tropical biomass burning regions. This method includes division of the attribution of black carbon (BC) and OA absorption aerosol optical depths from the Aerosol Robotic Network (AERONET) observation and determination of the fine-mode ratio of sea-salt and dust AODs from several atmospheric chemistry models. Our best estimate of OA SSA over the tropical biomass burning regions is 0.91 at 550 nm. Uncertainties associated with observations and models permit a value range of 0.82-0.93. Furthermore, by using the estimated OA SSA and comprehensive observations including AERONET, Moderate Resolution Imaging Spectroradiometer (MODIS) and Multi-angle Imaging Spectroradiometer (MISR), we examined the first global estimate of sulfate + nitrate AOD through a semi-observational approach. The global mean sulfate + nitrate AOD of 0.017 is in the lower range of the values obtained from 21 models participated in AeroCom phase II. The results imply that most aerosol models as well as climate models, which commonly use OA SSA of 0.96-1.0, have so far ignored light absorption by OAs and have overestimated light scattering by sulfate + nitrate aerosols. This indicates that the actual aerosol direct radiative forcing should be less negative than currently believed. (C) 2016 Elsevier Ltd. All rights reserved.

Stratospheric aerosols cool the Earth by scattering sunlight. Although sulfuric acid dominates the stratospheric aerosol, this study finds that organic material in the lowermost stratosphere contributes 30-40% of the nonvolcanic stratospheric aerosol optical depth (sAOD). Simulations indicate that nonvolcanic sAOD has increased 77% since 1850. Stratospheric aerosol accounts for 21% of the total direct aerosol radiative forcing (which is negative) and 12% of the total aerosol optical depth (AOD) increase from organics and sulfate. There is a larger stratospheric influence on radiative forcing (i.e., 21%) relative to AOD (i.e., 12%) because an increase of tropospheric black carbon warms the planet while stratospheric aerosols (including black carbon) cool the planet. Radiative forcing from nonvolcanic stratospheric aerosol mass of anthropogenic origin, including organics, has not been widely considered as a significant influence on the climate system.

In this paper, the RIEMS 2.0 model, source emission in 2006 and 2010 are used to simulate the distributions and radiative effects of different anthropogenic aerosols over China. The comparison between the results forced by source emissions in 2006 and 2010 also reveals the sensitivity of the radiative effects to source emission. The results are shown as follows: (1) Compared with those in 2006, the annual average surface concentration of sulfate in 2010 decreased over central and eastern China with a range of -5 to 0 mu g/m(3); the decrease of annual average aerosol optical depth of sulfate over East China varied from 0.04 to 0.08; the annual average surface concentrations of BC, OC and nitrate increased over central and eastern China with maximums of 10.90, 11.52 and 12.50 mu g/m(3), respectively; the annual aerosol optical depths of BC, OC and nitrate increased over some areas of East China with extremes of 0.006, 0.007 and 0.008, respectively. (2) For the regional average results in 2010, the radiative forcings of sulfate, BC, OC, nitrate and their total net radiative forcing at the top of the atmosphere over central and eastern China were -0.64, 0.29, -0.41, -0.33 and -1.1 W/m(2), respectively. Compared with those in 2006, the radiative forcings of BC and OC in 2010 were both enhanced, while that of sulfate and the net radiative forcing were both weakened over East China mostly. (3) The reduction of the cooling effect of sulfate in 2010 produced a warmer surface air temperature over central and eastern China; the maximum value was 0.25 K. The cooling effect of nitrate was also slightly weakened. The warming effect of BC was enhanced over most of the areas in China, while the cooling effect of OC was enhanced over the similar area, particularly the area between Yangtze and Huanghe Rivers. The net radiative effect of the four anthropogenic aerosols generated the annual average reduction and the maximum reduction were -0.096 and -0.285 K, respectively, for the surface temperature in 2006, while in 2010 they were -0.063 and -0.256 K, respectively. In summary, the change in source emission lowered the cooling effect of anthropogenic aerosols, mainly because of the enhanced warming effect of BC and weakened cooling effect of scattering aerosols.

Aridity index (AI), defined as the ratio of precipitation to potential evapotranspiration (PET), is a measure of the dryness of terrestrial climate. Global climate models generally project future decreases of AI (drying) associated with global warming scenarios driven by increasing greenhouse gas and declining aerosols. Given their different effects in the climate system, scattering and absorbing aerosols may affect AI differently. Here we explore the terrestrial aridity responses to anthropogenic black carbon (BC) and sulfate (SO4) aerosols with Community Earth System Model simulations. Positive BC radiative forcing decreases precipitation averaged over global land at a rate of 0.9%/degrees C of global mean surface temperature increase (moderate drying), while BC radiative forcing increases PET by 1.0%/degrees C (also drying). BC leads to a global decrease of 1.9%/degrees C in AI (drying). SO4 forcing is negative and causes precipitation a decrease at a rate of 6.7%/degrees C cooling (strong drying). PET also decreases in response to SO4 aerosol cooling by 6.3%/degrees C cooling (contributing to moistening). Thus, SO4 cooling leads to a small decrease in AI (drying) by 0.4%/degrees C cooling. Despite the opposite effects on global mean temperature, BC and SO4 both contribute to the twentieth century drying (AI decrease). Sensitivity test indicates that surface temperature and surface available energy changes dominate BC- and SO4-induced PET changes.

Aerosols play important roles in the climate system and one of key issues is to quantitatively investigate their radiative forcing (RF). Anthropogenic RF due to black carbon (BC) and sulfate relative to 1850 s is therefore investigated in this study using an atmospheric general climate model (AGCM) and the aerosol dataset simulated by an atmospheric chemistry transport model. To calculate the instantaneous aerosol RF, meteorological fields are simulated, by the AGCM. The AGCM used in this study is developed by the State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences. The long-term three-dimensional black carbon and sulfate fields are taken from the simulation from the NCAR CAM Chemistry model. The dataset covers the period from the 1850s to 2100, and each 10-year set of results is averaged to produce the decadal mean. The decades of 1850-1859 and 2000-2009 represent the pre-industry (PI) and present day (PD), respectively. Cloud albedo forcing (CAF) is calculated from a diagnose scheme. The anthropogenic aerosols and the associated direct RF are estimated as the differences between a specified decade and the PI. The aerosol RF is obtained using a double radiation call method, in which the radiation scheme is called twice at each radiation time step. The GHGs, ozone, and solar forcing are fixed at present levels. The sea surface temperature and sea ice are from the prescribed climatology. This study shows that the present global annual mean anthropogenic sulfate all-sky direct and cloud albedo RF is estimated to be -0.37 and -0.98 W.m(-2), respectively; BC RF at all-sky top of atmosphere (TOA) and in the atmospheric column is calculated to be 0.16 and 0.47 W.m(-2), respectively. The present strongest RF due to above aerosols occurs in Eastern China, where sulfate direct and indirect RF exceeds -2.0 and -4.0 W.m(-2), respectively, and BC RF at TOA and column atmosphere is up to 2.0 and 5.0 W.m(-2), respectively. Furthermore, the estimated aerosol RF over East Asia still continuously increases and the maximum values are projected to occur in the 2010s. The projected stronger RF over Eastern China will even last until the 2030s. Thus, sulfate and BC from East Asia is projected to contribute more proportion to the global aerosol RF under future middle and high emission scenarios. The analysis in this study also indicates that the stronger summer atmospheric moisture over East Asia tends to intensify aerosol optical depth and direct clear-sky RF due to hydrophilic sulfate aerosol; moreover, cloud effects not only strengthen BC direct RF at all-sky TOA but also influence seasonal features of sulfate cloud albedo forcing over East Asia. Climatological characteristics over East Asia lead to the corresponding differences in aerosol RF compared to European and Northern American regions. The BC and sulfate RF and their possible time evolution are investigated in this paper. Many valuable results are obtained as mentioned above. However, the estimation of aerosol RF in the AGCM is determined by many factors and still faces large uncertainties. The first uncertainty arises from aerosol loading. Compared to surface measurements in Eastern China, the simulated BC and sulfate surface concentrations are much weaker and spatial correlations are also not high. These biases lead to our estimated present RF due to BC and sulfate likely lower than the actual values in East Asia. Other uncertainties are caused by simulated model meteorological fields and aerosol radiative parameterizations, such as atmospheric moisture, clouds and aerosol optical properties. So, it is suggested in our work that improvements of current climate models associated with aerosol processes and meteorological fields, which help to obtain more reasonable East Asian aerosol RF.