Under environment with various water contents, the variations in the mixing state and particle size of coated black carbon (BC) aerosols cause changes in optical and radiative effects. In this study, fractal models for thinly, partially, and thickly coated BC under six relative humidities (RHs 1/4 0-95%) are constructed and optically simulated at 1064 and 532 nm. Differential scattering cross-sections are selected to retrieve the mixing state (Dp/Dc) of BC to investigate the possible retrieval errors caused by the nonspherical morphology when using the single-particle soot photometer (SP2). Furthermore, the radiative forcing of BC aerosols at different RHs are analyzed. Results showed that the retrieval errors (REs) of Dp/Dc are negative for coated particles with BC volume fraction smaller than 0.10, indicating that the mixing states of coated fractal BC are underestimated during the hygroscopic growth. The partiallycoated BC has the best retrieval accuracy of the mixing state, followed by the closed-cell and coatedaggregate model, judging from averaged REs. Radiative forcing enhancements for partially-coated aerosols with different BC volume fractions exponentially increase to opposite values, resulting in a warming or cooling effect. This study helps understand the uncertainties in Dp/Dcof BC aerosols retrieved by SP2 and their radiative forcing at different RHs. (c) 2025 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights are reserved, including those for text and data mining, AI training, and similar technologies.

Global warming has caused the gradual degradation of permafrost, which may affect the vegetation water uptake from different depths. However, the water utilization strategies of different vegetation species during the thawing stages of permafrost regions need further study. To elucidate these differences, we selected the permafrost region in Northeast China as study area. We analyzed the water uptake from different depths of Larix gmelinii, a deciduous coniferous tree, Pinus sylvestris var. mongolica, an evergreen tree, and Betula platyphylla, a deciduous broadleaf tree, using stable isotopes of xylem water, soil water, and precipitation from June to October 2019. The results showed that L. gmelinii primarily used shallow soil water (0-40 cm) with the highest proportion at 64.1%, B. platyphylla generally used middle soil water (40-110 cm) with the highest proportion at 55.7%, and P. sylvestris mainly used middle (40-110cm) and deep soil water (110-150 cm) with the highest proportion at 40.4% and 56.9%. The water sources from different depths exhibited more frequent changes in P. sylvestris, indicating a higher water uptake capacity from different soil depths. L. gmelinii mainly uptakes water from shallow soils, suggesting that the water uptake of this species is sensitive to permafrost degradation. This study revealed the water uptake strategies from different depths of three tree species in a permafrost region, and the results suggested that water uptake capacity of different tree species should be considered in the prediction of vegetation changes in permafrost regions under a warming climate.

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.

Aerosol mixtures, which are still unclear in current knowledge, may cause large uncertainties in aerosol climate effect assessments. To better understand this research gap, a well-developed online coupled regional climate-chemistry model is employed here to investigate the influences of different aerosol mixing states on the direct interactions between aerosols and the East Asian summer monsoon (EASM). The results show that anthropogenic aerosols have high-level loadings with heterogeneous spatial distributions in East Asia. Black carbon aerosol loading accounts for more than 13% of the totals in this region in summer. Thus, different aerosol mixing states cause very different aerosol single scattering albedos, with a variation of 0.27 in East Asia in summer. Consequently, the sign of the aerosol instantaneous direct radiative forcing at the top of the atmosphere is changed, varying from - 0.95 to + 1.50 W/m(2) with increasing internal mixing aerosols. The influence of aerosol mixtures on regional climate responses seems to be weaker. The EASM circulation can be enhanced due to the warming effect of anthropogenic aerosols in the lower atmosphere, which further induces considerable aerosol accumulation associated with dynamic field anomaly, decrease in rainfall and so on, despite aerosol mixtures. However, this interaction between aerosols and the EASM will become more obvious if the aerosols are more mixed internally. Additionally, the differences in aerosol-induced EASM anomalies during the strongest and weakest monsoon index years are highly determined by the aerosol mixing states. The results here may further help us better address the environmental and climate change issues in East Asia.

The addition of coating to the black carbon (BC) enhances its absorption as more light is focused by the coating lens. The absorption enhancement factor (E-abs) of BC is difficult to quantify due to an inadequate representation of its mixing structure and the interaction with radiation. Here, by tracking the evolution of the fresh BC particles in the ambient, we found a transitional stage of the particle E-abs with the non-BC-to-BC mass ratio (R-BC) at similar to 2, below which there were insufficient coating materials to encapsulate the BC core and the absorption enhancement was not significant (similar to 14%). When the R-BC >similar to 2, obvious absorption enhancement occurred as the BC cores were fully covered. Secondary inorganic species played the most critical role in the coating materials to enhance the lensing effect. We suppose the particle-resolved core-shell Mie model can be applied in the E-abs prediction for most cases.

The radiative forcing of soot is dependent on the morphology, mixing state and structure. Cloud processing has been predicted to affect their mixing properties but little is known about the resulting light absorption properties. We collected ambient particles in the pre-cloud period, the cloud residues and interstitials in the in-cloud period at Mt. Tianjing (southern China). The morphology parameters of soot aggregates with varying mixing materials [sulfate (S) and organics (OM)] and mixing structures were investigated by a transmission electron microscope, and their absorption cross were calculated based on discrete dipole approximation. We found that the number contribution of soot-S decreased from 45% in the pre-cloud period to 32% in the in-cloud period, and that of soot-OM increased from 44% to 60%. Moreover, the number proportion of soot-OM with fully embedded structure increased remarkably in the in-cloud period (29%), compared with that in the pre-cloud period (3%). In addition, the soot-S aggregates became denser after in-cloud aqueous process. However, for soot-OM aggregates, the morphology remained relatively constant. The distinctly different change of soot-S and soot-OM in morphology highlights the chemically resolved reconstruction of soot morphology. Theoretical calculation further shows that the changes of soot particles in the mixing state and morphological characteristics by the cloud process resulted in the light absorption enhancement increase from 1.57 to 2.01. This study highlights that the evolution of microphysical properties upon cloud processing should also be considered in climate models to more accurately evaluate the impacts of soot particles.

To establish the direct climatic and environmental effect of anthropogenic aerosols in East Asia in winter under external, internal, and partial internal mixing (EM, IM and PIM) states, a well-developed regional climate-chemical model RegCCMS is used by carrying out sensitive numerical simulations. Different aerosol mixing states yield different aerosol optical and radiative properties. The regional averaged EM aerosol single scattering albedo is approximately 1.4 times that of IM. The average aerosol effective radiative forcing in the atmosphere ranges from -0.35 to +1.40 W/m(2) with increasing internal mixed aerosols. Due to the absorption of black carbon aerosol, lower air temperatures are increased, which likely weakens the EAWM circulations and makes the atmospheric boundary more stable. Consequently, substantial accumulations of aerosols further appear in most regions of China. This type of interaction will be intensified when more aerosols are internally mixed. Overall, the aerosol mixing states may be important for regional air pollution and climate change assessments. The different aerosol mixing states in East Asia in winter will result in a variation from 0.04 to 0.11 K for the averaged lower air temperature anomaly and from approximately 0.45 to 2.98 mu g/m(3) for the aerosol loading anomaly, respectively, due to the different mixing aerosols.

The significant uncertainty associated with black carbon (BC) radiative forcing estimation is mainly due to discrepancies related to its mixing state. The in situ measurement-based understanding of absorption properties is limited to only a few locations worldwide, primarily as a result of the unavailability of sophisticated instrumentations for absorption enhancement (Eabs) measurements resulting from mixing with non-BC chemicals. Therefore, we have proposed an alternative approach for a more robust in situ measurement of absorption enhancement using a thermal-optical carbon analyzer. In the present study, the absorption spectra during different stages of thermal-optical carbon analysis were used to estimate the absorption coefficients of mixed and pure BC aerosols. Moreover, we have also explored the possibility of apportioning light absorption by the BC core and absorbing organics (brown carbon). The present method was applied on a few ground-based aerosol samples collected at two distinct Indo-Gangetic Plain (IGP) sampling stations. Eabs at 808 nm was observed to be approximately 1.2 at both of the sampling sites. Interestingly, the absorbing brown carbon chromophores showed a wide range of absorption in the ultraviolet to near-infrared wavelengths with minimum absorption at 635 nm. Thus, the present study suggests that the absorption of organics in near-infrared wavelengths cannot be neglected.

Atmospheric black carbon (BC) has a large yet highly uncertain contribution to global warming. When mixed with non-BC/coating material during atmospheric aging, the BC light absorption can be enhanced through the lensing effect. Laboratory and modeling studies have consistently found strong BC absorption enhancement, while the results in ambient measurements are conflicting, with some reporting weak absorption enhancement even for particles with large bulk coating amounts. Here, from our direct field observations, we report both large and minor absorption enhancement factors for different BC-containing particle populations with large bulk non-BC-to-BC mass ratios. By gaining insights into the measured coating material distribution across each particle population, we find that the level of absorption enhancement is strongly dependent on the particle-resolved mixing state. Our study shows that the greater mixing-state heterogeneity results in the larger difference between observed and predicted absorption enhancement. We demonstrate that by considering the variability in coating material thickness in the optical model, the previously observed model measurement discrepancy of absorption enhancement can be reconciled. The observations and improved optical models reported here highlight the importance of mixing-state heterogeneity on BC's radiative forcing, which should be better resolved in large-scale models to increase confidence when estimating the aerosol radiation effect.

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.