The recent large reduction in anthropogenic aerosol emissions across China has improved China's air quality but has potential consequences for climate forcing. This sharp reduction in anthropogenic emissions has occurred against a background influenced by changing regional biomass burning emissions over a similar period of time. Here, we use the UK Earth System Model (UKESM) to estimate aerosol instantaneous radiative forcing (IRF) due to changes in emissions of aerosols and precursors from biomass burning and anthropogenic sources (separately and in combination) over 2008-2016, with a focus on China and regions downwind. We also separately quantify the IRF due to changes in anthropogenic aerosol emissions inside China (CHN) and the Rest Of the World (ROW). Reductions in Chinese anthropogenic emissions of BC, SO2 and OC contributed -0.30 +/- 0.01, +1.00 +/- 0.04, and +0.05 +/- 0.01 W m-2, respectively to IRF over China, accounting for similar to 97% of the total local anthropogenic aerosol IRF. These emission changes contributed a remote regional IRF of 0.22 +/- 0.04 W m-2 over the North Pacific Ocean. The reduction in SO2 emissions from China contributed a global IRF of equal magnitude to that from SO2 emissions from ROW (similar to 0.08 W m-2). Changes in global biomass burning emissions contributed 0.03 W m-2 (equivalent to over 20% of the magnitude of anthropogenic aerosol IRF), enhancing the global anthropogenic aerosol IRF, whereas they partly offset the anthropogenic IRF over China. Meanwhile, biomass burning emissions dominated the total IRF (around 98%) over the Arctic.

Accurately reproducing the measured scattering matrix of black carbon (BC) through numerical simulations remains a challenge. Researchers have developed various morphological models of BC and computed their scattering matrices in attempts to replicate experimental measurements. However, prior simulation endeavors frequently encountered issues such as significant discrepancies with observational data, implausible particle shapes, or unsuitable computational parameters. In this study, we developed a fractal-based overlapping and necking model to represent the morphology of bare BC particles. We computed the scattering matrices for both individual particles and particle population using these models and compared the results with the previously reported measurements. Our findings revealed that the overlapping model reproduces the measured scattering matrix elements more accurately, whereas the necking model fails to achieve similar consistency. At a wavelength of 532 nm, the overlapping model yields F 22(pi)/F 11(pi) ranging from 0.80 to 0.99 for single particles and from 0.86 to 0.99 for particle population, both of which are much closer to the experimental observations than those of the necking model. In contrast, only a small subset of results from the necking model falls within the measured range. The overlapping model outperforms the necking model in reproducing the scattering matrix and should be preferred for representing bare BC particles. The established understanding provides useful guidance for retrieving microphysical parameters of BC from polarization features and for diminishing the uncertainties associated with its radiative forcing estimates.

Permafrost is both a product of climate change and an indicator of its progression. Rising air temperature has led to permafrost degradation, resulting in the melting of ground ice and the release of carbon to the atmosphere, creating a positive feedback loop. Extreme weather events, particularly extreme rainfall, have been increasing, yet the effects of extreme rainfall on permafrost remain unclear. Here, we use long-term observational data to investigate the effects of extreme rainfall on the hydrothermal properties of the active layer at two sites in China's upper Heihe River Basin, EBoA and PT5. Two methods are applied: hierarchical linear regression and a relative variation ratio. The results for both sites indicate that when rainfall exceeds 10 mm, soil temperatures increase. This suggests warming effects of extreme rainfall on the active layer that maybe attributable to reduced heat loss from decreased actual evapotranspiration, as well as increased thermal conductivity and heat transfer due to elevated soil moisture during extreme rainfall events. Future studies investigating the effects of extreme rainfall via irrigation experiments and physical modeling could benefit our results as a reference for the design of controlled experiments.

Stratospheric aerosol injection (SAI) introduces aerosols or their precursors into the stratosphere, reflecting sunlight and mitigating global warming. However, delivering these materials to the stratosphere at the required altitudes (18-25 km) poses practical challenges. Here, we evaluate a novel delivery method called solar-powered lofting (SPL), inspired by self-lofting during extreme wildfires. SPL coinjects a small amount of black carbon (BC) with SO2 at lower altitudes accessible to commercial aircraft (similar to 13 km), allowing the SO2 to self-loft into the stratosphere. Using the Community Earth System Model, we compare SPL simulations with traditional SAI simulation that injects equivalent SO2 mass at 20 km at the same locations, but without BC. SPL and SAI scenarios generate similar global aerosol optical depths and effective radiative forcing. BC induces an additional 1.5 K annual mean warming in the tropical stratosphere, raising stratospheric water vapor by 0.42 ppm. The coinjected BC accounts for 20% of the annual mean temperature and water vapor anomalies. Furthermore, the BC strengthens the polar vortex and enhances the Brewer-Dobson circulation. As a result of the changes in dynamics and chemistry, the coinjected BC results in a 5% increase in Antarctic ozone depletion in October. The SPL method at aircraft-accessible altitudes offers comparable cooling efficiency but requires careful evaluation of additional BC impacts.

The extensive utilization of agricultural machinery in China has made it a prominent contributor to particulate matter (PM). However, there still exist significant knowledge gaps in understanding optical characteristics and molecular composition of chromophores of brown carbon (BrC) in PM emitted from agricultural machinery. Therefore, BrC in PM from six typical agricultural machines in China were measured to investigate the light absorption, chromophore characteristics, and influencing factors. Results showed that the average emission factors of methanol-soluble organic carbon (MSOC) and water-soluble organic carbon (WSOC) were 0.96 and 0.21 g (kg fuel)-1, respectively, exhibiting clear decreasing trends with increasing engine power and improving emission standards. Despite the light absorption coefficient of methanol-extracted BrC (Abs365,M) being approximately 2.2 times higher than that of water (Abs365,W), mass absorption efficiency of water-extracted BrC (MAE365,W) exhibited significantly greater values than MAE365,M. Among the detected chromophores, nitro-aromatic compounds (NACs) exhibited the highest contribution to light absorption that was about 14.5 times more than to total light absorption compared to their mass contributions to MSOC (0.04%), followed by polycyclic aromatic hydrocarbons (PAHs) and oxygenated PAHs (OPAHs). Besides, the average integrated simple forcing efficiency values were estimated to be 1.5 W g-1 for MSOC and 3.7 W g-1 for WSOC, indicating significant radiative forcing absorption of agricultural machinery. The findings in this study not only provide fundamental data for climate impact estimation of but also propose effective strategies to mitigate BrC emissions, such as enhancing emission standards and promoting the adoption of high-power agricultural machinery.

Wet scavenging of black carbon (BC) is essential for evaluating their atmospheric lifetime and radiative forcing. However, it is crucial to differentiate atmospheric BC into char and soot subgroups, given their significant disparities in physicochemical properties and potential impacts. We first conducted a comparative study of char/soot in PM10 and rainwater, collected over a year in urban Guangzhou, China. The mean char/soot ratio in PM10 (similar to 2.5) is obviously higher than that in rainwater (similar to 0.8), corresponding to higher wet scavenging efficiency of soot. Through sequence rainwater sampling during individual rainfall events, we further distinguished the contributions of in-cloud and below-cloud scavenging, with in-cloud scavenging predominantly contributed to the distinct difference between char and soot. Such a distinct wet scavenging behavior of char and soot would have substantial implications for the atmospheric behavior of BC, which should be considered in future models for accurate evaluation of its lifetime and climate impact.

Given the considerable influence of surface soil freeze-thaw cycles on the surface energy balance, hydrological processes, and ecosystems, there is significant interest in exploring changes in surface soil freeze-thaw cycles in the context of climate warming. In this study, we investigated changes in the duration of seasonal surface soil freeze-thaw cycles across China and subregions divided by climate and ecosystem types (temperate and warm-temperate deserts of northwestern China, temperate grasslands of Inner Mongolia, temperate humid and subhumid zones of northeastern China, warm-temperate humid and subhumid zones of North China, and high-elevation and cold zones of the Tibetan Plateau) from 1981 to 2017 and examined their relationships with meteorological elements using both homogenized weather station data and gridded observations. The results showed that the freeze start date has been delayed by 8.6 days and that the freeze end date has advanced by 8.6 days, resulting in a shortened freeze duration by 17.2 days in China. This change was most pronounced in the high-elevation and cold zones of the Tibetan Plateau, with a shortened freeze duration by 25.2 days, and the weakest change was present in the temperate humid and subhumid zones of northeastern China. Nationwide, the decreasing trend of the freeze duration first increased but then decreased with increasing elevation, and it consistently decreased with increasing latitude. Changes in the freeze duration are significantly correlated with the following factors: air temperature in spring, autumn and winter, snow depth in spring, autumn and winter; and vegetation in autumn. Distinct regional differences exist in these relationships. These results provide a new understanding of surface freeze-thaw cycle changes and their causes in China.

Refractory black carbon (rBC) is a primary aerosol species, produced through incomplete combustion, that absorbs sunlight and contributes to positive radiative forcing. The overall climate effect of rBC depends on its spatial distribution and atmospheric lifetime, both of which are impacted by the efficiency with which rBC is transported or removed by convective systems. These processes are poorly constrained by observations. It is especially interesting to investigate rBC transport efficiency through the Asian Summer Monsoon (ASM) since this meteorological pattern delivers vast quantities of boundary layer air from Asia, where rBC emissions are high to the upper troposphere/lower stratosphere (UT/LS) where the lifetime of rBC is expected to be long. Here, we present in situ observations of rBC made during the Asian Summer Monsoon Chemistry and Climate Impact Project of summer, 2022. We use observed relationships between rBC and CO in ASM outflow to show that rBC is removed nearly completely (>98%) from uplifted air and that rBC concentrations in ASM outflow are statistically indistinguishable from the UT/LS background. We compare observed rBC and CO concentrations to those expected based on two chemical transport models and find that the models reproduce CO to within a factor of 2 at all altitudes whereas rBC is overpredicted by a factor of 20-100 at altitudes associated with ASM outflow. We find that the rBC particles in recently convected air have thinner coatings than those found in the UTLS background, suggesting transport of a small number of rBC particles that are negligible for concentration.

In this study, we used satellite observations to identify 10 typical dust-loading events over the Indian Himalayas. Next, the aerosol microphysical and optical properties during these identified dust storms are characterized using cotemporal in situ measurements over Mukteshwar, a representative site in Indian Himalayas. Relative to the background values, the mass of coarse particles (size range between 2.5 and 10 mu m) and the extinction coefficient were found to be enhanced by 400% (from 24 +/- 15 to 98 +/- 40 mu g/m3) and 175% (from 89 +/- 57 Mm-1 to 156 +/- 79 Mm-1), respectively, during these premonsoonal dust-loading events. Moreover, based on the air mass trajectory, these dust storms can be categorized into two categories: (a) mineral dust events (MDEs), which involve long-range transported dust plumes traversing through the lower troposphere to reach the Himalayas and (b) polluted dust events (PDEs), which involve short-range transported dust plumes originating from the arid western regions of the Indian subcontinent and traveling within the heavily polluted boundary layer of the Gangetic plains before reaching the Himalayas. Interestingly, compared to the background, the SSA and AAE decrease during PDEs but increase during MDEs. More importantly, we observe a twofold increase in black carbon concentrations and the aerosol absorption coefficient (relative to the background values) during the PDEs with negligible changes during MDEs. Consequently, the aerosol-induced snow albedo reduction (SAR) also doubles during MDEs and PDEs relative to background conditions. Thus, our findings provide robust observational evidence of substantial dust-induced snow and glacier melting over the Himalayas.

Atmospheric particulate matter (PM) as light-absorbing particles (LAPs) deposited to snow cover can result in early onset and rapid snow melting, challenging management of downstream water resources. We identified LAPs in 38 snow samples (water years 2013-2016) from the mountainous Upper Colorado River basin by comparing among laboratory-measured spectral reflectance, chemical, physical, and magnetic properties. Dust sample reflectance, averaged over the wavelength range of 0.35-2.50 mu m, varied by a factor of 1.9 (range, 0.2300-0.4444) and was suppressed mainly by three components: (a) carbonaceous matter measured as total organic carbon (1.6-22.5 wt. %) including inferred black carbon, natural organic matter, and carbon-based synthetic, black road-tire-wear particles, (b) dark rock and mineral particles, indicated by amounts of magnetite (0.11-0.37 wt. %) as their proxy, and (c) ferric oxide minerals identified by reflectance spectroscopy and magnetic properties. Fundamental compositional differences were associated with different iron oxide groups defined by dominant hematite, goethite, or magnetite. These differences in iron oxide mineralogy are attributed to temporally varying source-area contributions implying strong interannual changes in regional source behavior, dust-storm frequency, and (or) transport tracks. Observations of dust-storm activity in the western U.S. and particle-size averages for all samples (median, 25 mu m) indicated that regional dust from deserts dominated mineral-dust masses. Fugitive contaminants, nevertheless, contributed important amounts of LAPs from many types of anthropogenic sources.