Interactions between aerosols, clouds, and radiation remain a major source of uncertainty in effective radiative forcing (ERF), limiting the accuracy of climate projections. This study aims to quantify parametric uncertainties in aerosol-cloud and aerosol-radiation interactions using a perturbed parameter ensemble (PPE) of 221 simulations with the ECHAM6.3-HAM2.3 climate model, varying 23 aerosol-related parameters that control emissions, removal, chemistry, and microphysics.The resulting global mean aerosol ERF is -1.24 W m-2 (5-95 percentile: -1.56 to -0.89 W m-2). Uncertainty in ERF is dominated by sulfate-related processes, biomass burning, aerosol size, and natural emissions. For aerosol-cloud interactions, dimethyl sulfide (DMS) and biomass burning emissions are key drivers, whereas sulfate chemistry and dry deposition exert the strongest influence on aerosol-radiation interactions. Structural uncertainty is difficult to characterize, and this study focuses primarily on evaluating parametric uncertainty. The leading sources of ERF parametric uncertainty identified here are consistent with those found in other PPE studies, highlighting common sensitivities across climate models.Comparison with POLDER-3/PARASOL satellite retrievals reveals persistent model biases in aerosol optical depth (AOD), & Aring;ngstr & ouml;m exponent (AE), and single-scattering albedo (SSA), many of which fall within the parametric uncertainty range. Sulfate-related processes account for over 40 % of AOD uncertainty, while AE and SSA are most sensitive to DMS, sea salt, and black carbon parameters. Correlation analysis between key parameters and observables indicates that several biases may be reduced by tuning through physically consistent parameter adjustments for bias reduction. Our results highlight the need for combined efforts in parameter optimization and structural model development to improve confidence in aerosol-forcing estimates and future climate projections.

To investigate optical properties, sources, and radiative effects of brown carbon (BrC), we conducted synchronous field campaigns in the Qinghai-Tibet Plateau (Yangbajing) and urban Guangzhou in July 2022, using multi-wavelength Aethalometer (AE33) and aerosol mass spectrometer (AMS) measurements. Total aerosol and BrC light absorption coefficients at 370 nm (Abs(total): 1.6 +/- 1.6 M m(-1); BrC: 0.2 +/- 0.3 M m(-1)) in Tibet were an order of magnitude lower than Guangzhou, attributed to extremely low aerosol/organic aerosol (OA) mass concentrations. However, BrC fractions in Abs(total) (15 % vs. 21 % at 370 nm) correlated with primary OA (POA) ratios, highlighting anthropogenic emission impacts even in this clean background. Diurnal variations (morning/evening peaks) of source-specific BrC absorption were regulated by local emissions (e.g., biomass burning, traffic emission) and regional secondary formation. Source apportionment revealed primary sources (biomass burning OA (BBOA), hydrocarbon-like OA (HOA)) dominated BrC absorption (> 75 %). The mass absorption cross- (MAC) of HOA (2.08 m(2) g(-1) in Tibet; 2.57 m(2) g(-1) in Guangzhou) was comparable to that of BBOA (1.11-2.54 m(2) g(-1) in Tibet; 1.91 m(2) g(-1) in Guangzhou), indicating the high light absorption capacity of BrC from fossil fuel. Integrated simple forcing efficiency (370-660 nm) showed primary emissions contributed > 98 % of total radiative forcing at both sites. This study advances understanding of BrC dynamics and sources in diverse environments, underscores primary sources' critical role in BrC absorption, and emphasizes the need for source-specific OA optical parameterization.

The impact of aviation soot on natural cirrus clouds is considered the most uncertain among the climate impacts of the aviation sector. In this study, a global aerosol-climate model equipped with a cirrus parametrisation is applied to quantify the impact of aviation soot on natural cirrus clouds and its resulting climate effect. For the first time, the cirrus parametrisation in the model is driven by novel laboratory measurements specifically targeting the ice nucleation ability of aviation soot, thus enabling an experimentally-constrained estimate of the aviation-soot cirrus effect. The results indicate no statistically significant impact of aviation soot on natural cirrus clouds, with an effective radiative forcing of -6.9 +/- 29.8 mWm-2 (95 % confidence interval). Sensitivity simulations conducted to investigate the role of other ice nucleating particles (INPs) competing with aviation soot for ice supersaturation in the cirrus regime (soot from sources other than aviation, mineral dust and ammonium sulphate) further show that the impact of aviation soot remains statistically insignificant also when the impact of these other INPs on cirrus is reduced in the model. Acknowledging that the complexity of the soot cirrus interaction is associated with uncertainties, the model results supported by dedicated laboratory measurements suggest that the climate impact due to the aviation soot cirrus effect is likely negligible with no statistical significance.

The Svalbard Archipelago, highly sensitive to rapid environmental changes, offers an ideal physical laboratory to investigate how environmental drivers can shape the seasonal chemical composition of snow in a warming climate. From 2018 to 2021, sampling campaigns at the Gruvebadet Snow Research Site in Ny-& Aring;lesund, in the North-West of the Svalbard Archipelago, captured the interannual variability in ionic and elemental impurities within surface snow, reflecting seasonal differences in atmospheric and oceanic conditions. Notably, warmer conditions prevailed in 2018-2019 and 2020-2021, contrasting with the relatively colder season of 2019-2020. Our findings suggest that impurity concentrations in the 2019-2020 colder season are impacted by enhanced sea spray aerosol production, likely driven by a larger extent of sea ice, and drier, windy conditions. This phenomenon was particularly evident in March 2020, when extensive sea ice was present in Kongsfjorden and around Spitsbergen due to an exceptionally strong, cold stratospheric polar vortex and unusual Arctic Oscillation (AO) index positive phase. This study provides a detailed characterization of how snow chemistry in this area responds to major environmental conditions, with particular attention to sea-ice extent, atmospheric circulation, synoptic conditions, and Arctic climate variability.

Mineral dust significantly affects the downwelling and upwelling shortwave (SW) and longwave (LW) radiative fluxes, and changes in dust can therefore alter the Earth's energy balance. This study analyses the dust effective radiative forcing (DuERF) in nine CMIP6 Earth System Models (ESMs) using the piClim-2xdust experiment from AerChemMIP. The piClim-2xdust experiment uses a global dust emission tuning factor to double the emission flux. The DuERF is decomposed into contributions from dust-radiation (direct DuERF) and dust-cloud (cloud DuERF) interactions. The net direct DuERF ranges from -0.56 to 0.05 W m-2. Models with lower (higher) dust absorption and a smaller (larger) fraction of coarse dust show the most negative (positive) direct DuERF. The cloud DuERF is positive in most models, ranging from -0.02 to 0.2 W m-2, however, they differ in their LW and SW flux contributions. Specifically, NorESM2-LM shows a positive LW cloud DuERF attributable to the effect of dust on cirrus clouds. The dust forcing efficiency varies tenfold among models, indicating that uncertainty in DuERF is likely underestimated in AerChemMIP. There is a consistent fast precipitation response associated with dust decreasing atmospheric radiative cooling (ARC). Models with strongly absorbing dust show reduced precipitation, explainable by decreased clear-sky ARC (up to 3.2 mm yr-1). In NorESM2-LM, this decrease is associated with a cloudy sky ARC due to an increase in cirrus clouds (up to 5.6 mm yr-1). Taken together, these findings suggest that the fast precipitation response induced by dust alone may be significant and comparable to that caused by anthropogenic black carbon.

During the dry season, the Amazonian atmosphere is strongly impacted by fires, even in remote areas. However, there are still knowledge gaps regarding how each aerosol type affects the aerosol radiative forcing. This work characterizes the chemical composition of submicrometer aerosols and source apportionment of organic aerosols (OAs) and equivalent black carbon (eBC) to study their influence on light scattering and absorption at a remote site in central Amazonia during the dry season (August-December 2013). We applied positive matrix factorization (PMF) and multilinear regression (MLR) models to estimate chemical-dependent mass scattering efficiency (MSE) and extinction efficiency (MEE). Mean PM1 aerosol mass loading was 6.3 +/- 3.3 mu g m-3, with 77 % of organics, grouped into 3 factors: biomass burning OA (BBOA), isoprene-epoxydiol-derived secondary OA (IEPOX-SOA) and oxygenated OA (OOA). The bulk scattering and absorption coefficients at 637 nm were 17 +/- 10 and 3 +/- 2 Mm-1, yielding a single scattering albedo of 0.87 +/- 0.03. Although eBC represented only 6 % of the PM1 mass loading, MSE was highest for the eBC (13.58-7.62 m2 g-1 at 450-700 nm), followed by BBOA (7.96-3.10 m2 g-1) and ammonium sulfate (AS, 4.79-4.58 m2 g-1). The MEE was dominated by eBC (30.8 %), followed by OOA (19.9 %) and AS (17.6 %). The dominance of eBC over light scattering, in addition to absorption, plays a remarkably important role for this important climate agent, with potentially broad implications for more precise radiative forcing quantification, increasing climate modeling precision and representing deep contributions to Earth's climate system comprehension.

Black carbon (BC) and brown carbon (BrC) are the dominant light-absorbing carbonaceous aerosols (LACs) that contribute significantly to climate change through absorbing and scattering radiation. We used GEOS-Chem integrated with the Rapid Radiative Transfer Model for General Circulation Models to estimate LAC properties and direct radiative forcings (DRFs) in China. Primary BrC (Pri-BrC) and secondary BrC (Sec-BrC) were separated from organic carbon and modeled as independent tracers. LAC Chinese anthropogenic emissions and refractive indexes were updated. Additionally, we investigated the impacts of LAC properties and atmospheric variables on LAC DRFs based on principal component analysis. It was shown that BC exerts a warming effect at the top of the atmosphere, while Pri-BrC and Sec-BrC induce a cooling effect. At the surface, they collectively lead to surface cooling, whereas within the atmosphere, they all can contribute to atmospheric heating, with 1.848 +/- 1.098, 0.146 +/- 0.079 and 0.022 +/- 0.008 W m-2, respectively. The atmospheric shortwave DRFs of BC and Pri-BrC were proportional to their corresponding concentrations, aerosol optical depth (AOD) and absorption aerosol optical depth (AAOD), and they were inversely proportional to the single scattering albedo, surface albedo and ozone concentration in most regions. The surface longwave DRFs for the LACs showed negative correlations with water vapor in most areas. The highest atmospheric warming effect of LACs was observed in Central China, followed by East China, owing to the high LAC concentrations, AOD and AAOD as well as the low surface albedo and ozone concentration. This study enhances our understanding of the climatic impacts of LACs.

Uncertainties persist in estimating the radiative forcing of black carbon (BC) due to an incomplete understanding of its microphysical properties. This study investigated the physical properties of refractory black carbon (rBC) at the central European background site Melpitz during summer and winter, using a single-particle soot photometer coupled with a thermodenuder. Different air masses associated with distinct rBC properties were identified in both seasons. In summer, rBC exhibited a similar mass concentration (similar to 0.16 mu gm-3) among different air masses, with the smallest mass median diameter (MMD) of rBC observed in the long transportation from the northwest (140 nm), while in winter, the highest concentration (1.23 mu gm-3) and largest MMD (216 nm) were both observed in the air mass influenced by the easterly winds. Thickly coated rBC fractions increased during the daytime in summer, indicating that the photochemical processes significantly influence the rBC mixing state. In winter, a higher fraction (27 %) of rBC, with thick coatings in the cold air mass compared to the warm air mass (14 %), suggests the contribution of residential heating emissions to the mixing state. Most rBC retained a low-volatility coating in the thermodenuder samples (63 % mass fraction). In summer, photochemical processes also contribute to coating volatility, showing a higher fraction of rBC particles containing low-volatility coatings during the daytime. In winter, low-volatility coatings showed no significant diurnal variation and were more dependent on ambient temperature. Therefore, rBC coating volatility in winter is more influenced by emission sources, particularly residential heating, rather than atmospheric processes.

The radiative forcing of black carbon (BC) is subject to many complex, interconnected sources of uncertainty. Here we isolate the role of the refractive index, which determines the extent to which BC absorbs and scatters radiation. We compare four refractive index schemes: three that are commonly used in Earth system models and a fourth more recent estimate with higher absorption. With other parameterizations held constant, changing BC's spectrally varying refractive index from the least- to most-absorbing estimate commonly used in Earth system models (m550nm=1.75-0.44i to m550nm=1.95-0.79i) increases simulated absorbing aerosol optical depth (AAOD) by 42 % and the effective radiative forcing from BC-radiation interactions (BC ERFari) by 47 %. The more recent estimate, m532nm=1.48-0.84i, increases AAOD and BC ERFari by 59 % and 100 % respectively relative to the low-absorption case. The AAOD increases are comparable to those from recent updates to aerosol emission inventories and, in BC source regions, up to two-thirds as large as the difference in AAOD retrieved from MISR (Multi-angle Imaging SpectroRadiometer) and POLDER-GRASP (Polarization and Directionality of the Earth's Reflectances instrument with the Generalized Retrieval of Atmosphere and Surface Properties algorithm) satellites. The BC ERFari increases are comparable to previous assessments of overall uncertainties in BC ERFari, even though this source of uncertainty is typically overlooked. Although model sensitivity to the choice of BC refractive index is known to be modulated by other parameterization choices, our results highlight the importance of considering refractive index diversity in model intercomparison projects.

Brown carbon (BrC) is a fraction of organic aerosol (OA) that absorbs radiation in the ultraviolet and short visible wavelengths. Its contribution to radiative forcing is uncertain due to limited knowledge of its imaginary refractive index (k). This study investigates the variability of k for OA from wildfires, residential, shipping, and traffic emission sources over Europe. The Multiscale Online Nonhydrostatic Atmosphere Chemistry (MONARCH) model simulated OA concentrations and source contributions, feeding an offline optical tool to constrain k values at 370 nm. The model was evaluated against OA mass concentrations from aerosol chemical speciation monitors (ACSMs) and filter sample measurements, as well as aerosol light absorption measurements at 370 nm derived from an Aethalometer (TM) from 12 sites across Europe. Results show that MONARCH captures the OA temporal variability across environments (regional, suburban, and urban background). Residential emissions are a major OA source in colder months, while secondary organic aerosol (SOA) dominates in warmer periods. Traffic is a minor primary OA contributor. Biomass and coal combustion significantly influence OA absorption, with shipping emissions also notable near harbors. Optimizing k values at 370 nm revealed significant variability in OA light absorption, influenced by emission sources and environmental conditions. Derived k values for biomass burning (0.03 to 0.13), residential (0.008 to 0.13), shipping (0.005 to 0.08), and traffic (0.005 to 0.07) sources improved model representation of OA absorption compared to a constant k. Introducing such emission source-specific constraints is an innovative approach to enhance OA absorption in atmospheric models.