Aerosols are liquid and solid particles suspended in the atmosphere and have a broad size range; they can cool the Earth by scattering radiation back to space or warm the Earth by absorbing radiation directly. Since the industrial revolution, the loading of aerosols in the Earth's atmosphere has increased significantly, yielding modifications to the Earth's energy budget and further affecting the climate state. Aerosol direct radiative forcing (ADRF), defined as the difference in radiation with and without total or specific aerosols, is an important concept used to describe the direct impact of aerosols on radiation. Accurate quantification of ADRF is the premise for understanding and predicting the Earth's climate state. To improve the estimation and evaluation of ADRF, numerous researchers have dedicated their efforts to developing a series of observations and models in recent decades. However, due to the limited availability of wide spatial and high-precision observations of aerosol optical characteristics, as well as an insufficient model description of aerosol properties and physical and chemical processes, the ADRF uncertainty is still high. This paper first reviews the spatio-temporal distribution of aerosol optical depth (AOD), single scattering albedo (SSA) and corresponding ADRF by using observations and models. The aerosol optical properties and ADRF show distinct discrepancies among various regions due to the impact of anthropogenic emissions and meteorological and climate conditions. In regions with rapid economic development, such as India, AOD demonstrates a long-term increasing trend with higher average values. However, regions influenced by environmental protection policies, such as North America and Europe, show a long-term decreasing trend in AOD, accompanied by lower average values. Based on site observations, most of Europe, North America, Africa, and Asia exhibit a significant long-term increasing trend in SSA. However, in seasons with biomass burning or dust outbreaks, specific regions, such as southern and southwestern China in late autumn and early spring, and northern and northwestern China in spring, exhibit a reduction in SSA. In the future, with the global and regional emissions of aerosols and precursors declining, ADRF is expected to weaken, highlighting the warming effect of greenhouse gases. However, the ADRF trend is closely linked to the present development level and trajectory of each region. Second, we systematically summarize the impacts of the influential factors on the ADRF, considering the AOD, SSA, surface albedo (SA), solar zenith angle (SZA), asymmetry factor (ASY), relative altitude between aerosols and clouds, and relative altitude between different types of aerosols. Subsequently, we proceed to review the sensitivities of ADRF to different influential factors, as well as the contributions of these factors to the overall uncertainty of ADRF, which indicate that ADRF is more sensitive to AOD and SSA while SSA emerges as the most significant source of uncertainty in ADRF due to the larger errors associated with its measurement. It should be noted that the uncertainty caused by SA and ASY cannot be ignored in polluted regions. Finally, from the perspective of observations and models, a brief outlook on improving the accuracy of ADRF evaluation is provided. In the future, advanced observation technologies, such as multi-angle, hyperspectral, polarized remote sensing observations, and precise in-situ measurements, should be developed to obtain more accurate information about the aerosols and environment. Furthermore, we need to properly combine various observations and models, including Earth system models, to improve the simulation of aerosols and their precursors. With improved understanding of aerosol-radiation interactions and refining techniques in observations and model simulations, the evaluation of ADRF will be more accurate.

Estimates of the effective radiative forcing from aerosol-radiation interaction (ERFari) of anthropogenic Black Carbon (BC) have been disputable and require better constraints. Here we find a substantial decline in atmospheric absorption of -5.79Wm(-2)decade(-1) over eastern central China (ECC) responding to recent anthropogenic BC emission reductions. By combining the observational finding with advances from Coupled Model Intercomparison Project phase6 (CMIP6), we identify an emergent constraint on the ERFari of anthropogenic BC. We show that across CMIP6 models the simulated trends correlate well with simulated annual mean shortwave atmospheric absorption by anthropogenic BC over China. Making use of this emergent relationship allows us to constrain the aerosol absorption optical depth of anthropogenic BC and further provide a constrained range of 2.4-3.0 Wm(-2) for its top-of-atmosphere ERFari over China, higher than existing estimates. Our work supports a strong warming effect of BC over China, and highlights the need to improve BC simulations over source regions.

Black carbon (BC) is a major light-absorbing component in the atmosphere and plays an important role in aerosol radiative forcing. In this study, the combination of monitoring data and the WRF-Chem model was used to study the source apportionment of BC in China during January 2017. Meanwhile, the aerosol-radiation interaction (ARI) effect of BC was also simulated. We found that the average BC/PM(2.5)ratios were 4.8%, 4.2%, and 3.8% in Shijiazhuang, Tangshan, and Beijing, respectively. The source apportionment suggested that traffic emissions played a dominant role in the BC concentration over Beijing. The traffic, residential, industrial, and power contributions accounted for 41%, 32%, 25%, and 2% of total concentration, respectively. The BC concentration in Beijing was also affected by regional transport. During January, the contributions of monthly regional transport to BC and PM(2.5)concentrations in Beijing were 41% and 49%, respectively. BC emissions decreased downward shortwave radiation (SWDOWN) at the surface, leading to a decrease in temperature. As a result, the planetary boundary layer height (PBLH) development was suppressed and the relative humidity increased. The stable meteorological conditions suppressed the dispersion of air pollutants and increased BC concentrations. Traffic emissions decreased the monthly SWDOWN by approximately 2.2 W/m(2), decreased 2 m temperature (T2) by approximately 0.1 degrees C, increased 2 m relative humidity (RH2) by approximately 0.5%, and decreased PBLH by approximately 4.4 m.

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.

A partial internal mixing (PIM) treatment of black carbon (BC), organic carbon (OC), and sulphate was examined, and the core-shell model was used to represent the internally mixed aerosols with BC as the core and sulphate or OC as the shell. The influences of PIM treatment on the effective radiative forcing due to aerosol-radiative interaction (ERFari) and global temperature were examined and compared to those of external mixing (EM) treatment using an aerosol-climate online coupled model of BCC_AGCM2.0_CUACE/Aero. Radiative forcing due to aerosol-radiation interaction (RFari) of the anthropogenic aerosols since the preindustrial era was -0.34 W m(-2) for EM and -0.23 W m(-2) for PIM, respectively. The global annual mean ERFari of anthropogenic aerosols since the preindustrial era was -0.42 W m(-2) for EM and -0.34 W m(-2) for PIM, respectively. The change in global annual mean surface temperature increased accordingly from -0.18K in the EM case to -0.125K in the PIM case. Well geographic consistence between the change in low-level cloud amount and the change in temperature can be found. The atmospheric temperature in the troposphere was markedly less reduced in the PIM case than in the EM case. The RFari/ERFari for 50% and 100% were -0.11/-0.07 and 0.13/0.14 W m(-2), respectively. RFari, ERFari, and surface temperature changed approximately linearly with the internal mixing proportion.

Arctic observations show large decreases in the concentrations of sulfate and black carbon (BC) aerosols since the early 1980s. These near-term climate-forcing pollutants perturb the radiative balance of the atmosphere and may have played an important role in recent Arctic warming. We use the GEOS-Chem global chemical transport model to construct a 3-D representation of Arctic aerosols that is generally consistent with observations and their trends from 1980 to 2010. Observations at Arctic surface sites show significant decreases in sulfate and BC mass concentrations of 2-3% per year. We find that anthropogenic aerosols yield a negative forcing over the Arctic, with an average 2005-2010 Arctic shortwave radiative forcing (RF) of -0.190.05Wm(-2) at the top of atmosphere (TOA). Anthropogenic sulfate in our study yields more strongly negative forcings over the Arctic troposphere in spring (-1.170.10Wm(-2)) than previously reported. From 1980 to 2010, TOA negative RF by Arctic aerosol declined, from -0.670.06Wm(-2) to -0.190.05Wm(-2), yielding a net TOA RF of +0.48 +/- 0.06Wm(-2). The net positive RF is due almost entirely to decreases in anthropogenic sulfate loading over the Arctic. We estimate that 1980-2010 trends in aerosol-radiation interactions over the Arctic and Northern Hemisphere midlatitudes have contributed a net warming at the Arctic surface of +0.27 +/- 0.04K, roughly one quarter of the observed warming. Our study does not consider BC emissions from gas flaring nor the regional climate response to aerosol-cloud interactions or BC deposition on snow.