China is an important emitter of light-absorbing carbonaceous aerosols (LACs), including black carbon (BC) and brown carbon (BrC). Currently, there are large uncertainties in model-estimated direct radiative forcing (DRF) of LACs, partially due to the poor understanding of the emissions and optical properties of LACs. In this study, we estimated the DRF of LACs over China during the implementation of the Air Pollution Prevention and Control Action Plan (APPCAP) using the global chemical transport model (GEOS-Chem) coupled with the Rapid Radiative Transfer Model of GCMs (RRTMG). We updated the refractive index of BC, includedbiomass burning (BB) sources, biofuel (BF) and coal combustion (CC) sources in the residential sector as BrC emission sources and the optical properties were updated, which were not fully considered in the previous model studies. Our results showed that model could reasonably capture the spatial and temporal variations of LACs in China with the correlation coefficients between model simulated and Aerosol Robotic Network (AERONET) observed daily absorption aerosol optical depth (AAOD) of LACs at 440 nm above 0.63 and the corresponding values of the normalized mean bias within +/- 30%. The simulated annual mean LACs AAOD at 440 nm in China was 0.016 (0.021) in 2017 (2014) and BrC contributed about 20% (21%). The estimated annual mean clear-sky LACs DRF at the top of the atmosphere in China was 1.02 W m(-2) in 2017 and 1.38 W m(-2) in 2014, and the contribution of BrC was about 10% and 11%, respectively, which was dominated by the BF sources (46% in 2017 and 44% in 2014) and the BB sources (38% in 2017 and 43% in 2014), with CC sources being low (16% in 2017 and 13% in 2014). The annual mean AAOD and DRF of LACs in China decreased by 0.005 and 0.36 W m(-2) from 2014 to 2017, which were largely attributed to the reductions of anthropogenic emissions during the implementation of APPCAP. Our results would improve the understanding of the light absorption capacity and climate effects of LACs in China.

Atmospheric brown carbon (BrC), a short-lived climate forcer, absorbs solar radiation and is a substantial contributor to the warming of the Earth ' s atmosphere. BrC composition, its absorption properties, and their evolution are poorly represented in climate models, especially during atmospheric aqueous events such as fog and clouds. These aqueous events, especially fog, are quite prevalent during wintertime in Indo-Gangetic Plain (IGP) and involve several stages (e.g., activation, formation, and dissipation, etc.), resulting in a large variation of relative humidity (RH) in the atmosphere. The huge RH variability allowed us to examine the evolution of water-soluble brown carbon (WS-BrC) diurnally and as a function of aerosol liquid water content (ALWC) and RH in this study. We explored links between the evolution of WS-BrC mass absorption efficiency at 365 nm (MAE WS- BrC-365 ) and chemical characteristics, viz., low-volatility organics and water-soluble organic nitrogen (WSON) to water-soluble organic carbon (WSOC) ratio (org-N/C), in the field (at Kanpur in central IGP) for the first time worldwide. We observed that WSON formation governed enhancement in MAE WS-BrC-365 diurnally (except during the afternoon) in the IGP. During the afternoon, the WS-BrC photochemical bleaching dwarfed the absorption enhancement caused by WSON formation. Further, both MAE WS-BrC-365 and org-N/C ratio increased with a decrease in ALWC and RH in this study, signifying that evaporation of fog droplets or bulk aerosol particles accelerated the formation of nitrogen-containing organic chromophores, resulting in the enhancement of WS-BrC absorptivity. The direct radiative forcing of WS-BrC relative to that of elemental carbon (EC) was -19 % during wintertime in Kanpur, and - 40 % of this contribution was in the UV -region. These findings highlight the importance of further examining the links between the evolution of BrC absorption behavior and chemical composition in the field and incorporating it in the BrC framework of climate models to constrain the predictions.

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.

Light-absorbing aerosols (LAA) impact the atmosphere by heating it. Their effect in the Arctic was investigated during two summer Arctic oceanographic campaigns (2018 and 2019) around the Svalbard Archipelago in order to unravel the differences between the Arctic background and the local anthropic settlements. Therefore, the LAA heating rate (HR) was experimentally determined. Both the chemical composition and high-resolution measurements highlighted substantial differences between the Arctic Ocean background (average eBC concentration of 11.7 +/- 0.1 ng/m3) and the human settlements, among which the most impacting appeared to be Tromso and Isfjorden (mean eBC of 99.4 +/- 3.1 ng/m3). Consequently, the HR in Isfjorden (8.2 x 10-3 +/- 0.3 x 10-3 K/day) was one order of magnitude higher than in the pristine background conditions (0.8 x 10-3 +/- 0.9 x 10-5 K/day). Therefore, we conclude that the direct climate impact of local LAA sources on the Arctic atmosphere is not negligible and may rise in the future due to ice retreat and enhanced marine traffic.

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.

Aerosol direct radiative forcing (ADRF) has substantial impacts on regional and global climate changes, whereas it remains one of the largest uncertainties among various climate forcing factors. The 40-year detailed clear-sky ADRFs over China from 1981 to 2020 are systematically studied based on the MERRA-2 satellite reanalysis data, which hopefully benefit for the evaluation of the performances of aerosol-climate models. The clear-sky short-wave ADRFs show diverse spatial distributions with strong top of atmosphere (TOA) and surface cooling and atmospheric heating over the Taklamakan desert, eastern China, and southern China. A high clear-sky longwave surface ADRF reaching-6.5 W/m2 over dust source regions is found, and continuous increase of ADRFs over the Taklamakan desert during the past four decades may indicate that dust episodes therein are becoming severe. The mean clear-sky shortwave surface, TOA and atmospheric ADRFs, and longwave surface ADRF over China during 1981-2020 are found to be-12.3,-5.1, 7.2, and 1.0 W/m2, respectively. The seasonality of clear-sky ADRFs shows strongest mean values in spring, moderate forcings in summer and fall, and weakest levels in winter. The monthly average clear-sky surface and TOA ADRFs over China vary by approximately twofold, ranging the weakest values of-7.8 and-3.9 W/m2 in December to the strongest-17.6 and-7.1 W/m2 in April, respectively. Distinctive seasonal and monthly patterns of clear-sky ADRFs are generally seen among the Beijing -Tianjin-Hebei region, Yangtze River Delta, Pearl River Delta and Tarim Basin in China, while the monthly ADRF patterns over a typical area of these four regions are similar during four different decades. The clear-sky ADRFs over China are highly correlated with aerosol optical thickness, and dust has strong influences on clear-sky shortwave aerosol direct radiative forcing among column aerosol compositions. Our study indicates general underestimations of clear-sky atmospheric ADRF over China by aerosol-climate models and stronger impacts of aerosol scattering than absorption on the TOA radiation budgets in China.

Studies on optical properties of aerosols can reduce the uncertainty for modelling direct radiative forcing (DRF) and improve the accuracy for discussing aerosols effects on the Tibetan Plateau (TP) climate. This study investigated the spatiotemporal variation of aerosol optical and microphysical properties over TP based on OMI and MERRA2, and assessed the influence of aerosol optical properties on DRF at NamCo station (30 degrees 46.440N, 90 degrees 59.310E, 4730 m) in the central TP from 2006 to 2017 based on a long measurement of AERONET and the modelling of SBDART model. The results show that aerosol optical depth (AOD) exhibits obvious seasonal variation over TP, with higher AOD500nm (>0.75) during spring and summer, and lower value (<0.25) in autumn and winter. The aerosol concentrations show a fluctuated rising from 1980 to 2000, significant increasing from 2000 to 2010 and slight declining trend after 2013. Based on sensitivity experiments, it is found that AOD and single scattering albedo (SSA) have more important impact on the DRF compared with a values and ASY. When AOD440nm increases by 60%, DRF at the TOA and ATM is increased by 57.2% and 60.2%, respectively. When SSA440nm increases by 20%, DRF at the TOA and ATM decreases by 121% and 96.7%, respectively. (c) 2022 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

The Aerosol Optical Depth (AOD) and aerosol-induced radiative forcing trends inferred for the period 1995-2019 over the Arabian Peninsula region (APR) are extensively studied using the state-of-the-art Modern-Era Retrospective analysis for Research and Applications, version 2 (MERRA-2) reanalysis data. We examine the long-term AOD change for five major aerosol species: Dust (DU), Sea Salt (SS), Sulfate (SU), Black Carbon (BC), and Organic Carbon (OC) over the APR. The MERRA-2 AOD comparisons with surface measurements show that it is capable to reproduce the AOD features over APR. The total AOD over the region shows a high value in JJA with the combined effect of DU and SU being major contributors. The total AOD over APR shows an increasing trend at a rate of similar to 0.05/decade. Along with an incline in DU AOD , the anthropogenic signature on total AOD also hikes contributed mainly by the SU and OC. The increase in AOD also results in a surge in aerosol-induced atmospheric forcing (ATM) with a trend of 0.13 Wm(-2) year and 0.15 Wm(-2) year during MAM and JJA respectively. Overall, the study gives a comprehensive picture of the capability of the MERRA-2 in long-term aerosol monitoring over APR, primarily situated in the dust-belt region.

The spatiotemporal characteristics of aerosol direct radiative forcing (RF) and the relative contributions from aerosol species, as well as the impacts of cloud coverage and relative humidity on aerosol direct RF were quantified in East Asia using a regional climate model. Generally, the total aerosol produces net RFs of -12.78 W m- 2 at surface, 1.72 W m- 2 at TOA (top-of-atmosphere), and atmospheric heating of 14.50 W m- 2. It was found that dust, black carbon, and sulfate made dominant contributions to the total RF at surface and TOA, and all aerosol species induced atmospheric heating, whereas more than 96% of which was induced by dust and black carbon. The remarkably seasonally decreasing tendency of the total and the absorbing aerosol RFs was found from spring to winter at surface. Moreover, dust contributes relatively larger to the positive TOA RF and to the atmospheric heating in spring and summer, which were weakened and smaller than black carbon in other seasons. Sensitivity studies further demonstrated cloud strengthens the dust and black carbon direct RF and weakens the other species direct RF at TOA, while induces weak direct RFs of all aerosol species at surface. Particularly, cloud induced larger reduction in dust longwave RF than shortwave leads to remarkable enhanced net surface direct RF of dust, especially in JJA. The aerosol swelling effect induced by relative humidity strengthens aerosol direct RF at both TOA and surface. The percentage changes in aerosol RF and its seasonal amplitude by cloud are considered larger at TOA than surface, however, the effects of relative humidity distribute relatively uniform vertically. Meteorological factors impact on scattering aerosols direct RF is assumed larger than absorbing aerosols. The impacts of cloud on aerosol direct RF are compared to the relative humidity and are supposed to be more important at TOA and surface.

An extreme biomass burning event occurred in the Amazonian rainforest from July through September 2019 due to the extensive wildfires used to clear the land, which allowed for more significant forest burning than previously occurred. In this study, we reclustered the clear-sky ambient aerosols to adapt the black carbon (BC) aerosol retrieval algorithm to Amazonia. This not only isolated the volumetric fraction of BC (f(bc)) from moderate-resolution imaging spectroradiometer (MODIS) aerosol data, but also facilitated the use of aerosol mixing and scattering models to estimate the absorption properties of smoke plumes. The retrieved MODIS aerosol dataset provided a space perspective on characterizing the aerosol changes and trends of the 2019 pollution event. A very high aerosol optical depth (AOD) was found to affect the source areas continuously, with higher and thus stronger aerosol absorption. These pollutants also affected the atmosphere downwind due to the transport of air masses. In addition, properties of aerosols emitted from the 2019 Amazonian wildfire events visualized a significant year-to-year enhancement, with the averaged AOD at 550 nm increased by 150%. A 200% increase in the aerosol-absorption optical depth (AAOD) at 550 nm was recognized due to the low single-scattering albedo (SSA) caused by the explosive BC emissions during the pollution peak. Further simulations of aerosol radiative forcing (ARF) showed that the biomass-burning aerosols emitted during the extreme Amazonian wildfires event in 2019 forced a significant change in the radiative balance, which not only produced greater heating of the atmospheric column through strong absorption of BC, but also reduced the radiation reaching the top-of-atmosphere (TOA) and surface level. The negative radiative forcing at the TOA and surface level, as well as the positive radiative forcing in the atmosphere, were elevated by similar to 30% across the whole of South America compared to 2018. These radiative effects of the absorbing aerosol could have the ability to accelerate the deterioration cycle of drought and fire over the Amazonian rainforest.