A comprehensive global investigation on the impact of reduction (changes) in aerosol emissions due to Coronavirus disease-2019 (COVID-19) lockdowns on aerosol single scattering albedo (SSA) utilizing satellite observations and model simulations is conducted for the first time. The absolute change in Ozone Monitoring Instrument (OMI) retrieved, and two highly-spatially resolved models (Modern-Era Retrospective Analysis for Research and Applications-2 (MERRA-2) and Copernicus Atmosphere Monitoring Service (CAMS)) simulated SSA is <4% (<0.04-0.05) globally during COVID (2020) compared to normal (2015-2019) period. Change in SSA during COVID is not significantly different from long-term and year-to-year variability in SSA. A small change in SSA indicates that significant reduction in anthropogenic aerosol emissions during COVID-19 induced lockdowns has a negligible effect in changing the net contribution of aerosol scattering and/or absorption to total aerosol extinction. The changes in species-wise aerosol optical depth (AOD) are examined in detail to explain the observed changes in SSA. Model simulations show that total AOD decreased during COVID-19 lockdowns, consistent with satellite observations. The respective contributions of sulfate and black carbon (BC) to total AOD increased, which resulted in a negligible change in SSA during the spring and summer seasons of COVID over South Asia. Europe and North America experience a small increase in SSA (<2%) during the summer season of COVID due to a decrease in BC contribution. The change in SSA (2%) is the same for a small change in BC AOD contribution (3%), and for a significant change in sulfate AOD contribution (20%) to total AOD. Since, BC SSA is 5-times lower (higher absorption) than that of sulfate SSA, the change in SSA remains the same. For a significant change in SSA to occur, the BC AOD contribution needs to be changed significantly (4-5 times) compared to other aerosol species. A sensitivity analysis reveals that change in aerosol radiative forcing during COVID is primarily dependent on change in AOD rather than SSA. These quantitative findings can be useful to devise more suitable future global and regional mitigation strategies aimed at regulating aerosol emissions to reduce environmental impacts, air pollution, and public health risks.

Objective In coal mining regions, extensive coal dust is generated during mining, transportation, and storage, coupled with substantial black carbon produced resulting from incomplete coal combustion in the industry chain. Over time, these materials form absorbable substances, evolving into core- shell aerosols with inorganic salt shells. These aerosols, including sulfate, nitrate, and water, exert significant climate impacts through direct and indirect radiation effects. The environmental and radiative forcing effects are substantial. Absorbing aerosol demonstrates strong solar radiation absorption across the ultraviolet to infrared spectrum. However, past studies primarily focus on their optical properties in visible and infrared bands, overlooking ultraviolet band absorption. Current research often assumes a lognormal particle size distribution for absorbing aerosols, neglecting variations in distribution and optical properties resulting from diverse emission scenarios. Therefore, a thorough analysis of absorbing aerosol optical properties at local scales is crucial. Quantitative assessments of particle size distribution, mixing state, and spatio-temporal variations are vital for elucidating the intricate interactions with boundary layer development, radiative forcing changes, and air pollution. Methods In our study conducted in the coal mining area of Changzhi City, Shanxi Province, various datasets are collected, including surface black carbon concentration, particle size distribution, and columnar aerosol optical depth (AOD). The investigation commenced with the utilization of the variance maximization method to categorize AOD data into distinct pollution events. Subsequent analysis involved evaluating the particle size distribution corresponding to different pollution degrees through probability density functions. The uncertainty of particle size for the desorption aerosol core and shell is then determined by integrating black carbon mass concentration data and particle size distribution information. These uncertainties are then used as input parameters to run the Mie scattering model based on the core- shell structure. This process results in the inversion of the multi- band optical characteristic parameters of absorbing aerosol in the coal mining area. The computations are carried out under both the assumption of a uniform distribution and a non- uniform distribution, representing different mixing degrees of aerosols. To complete the picture, the uncertainty interval for the single scattering albedo (SSA) of absorbing aerosol was constrained through the application of absorption & Aring;ngstr & ouml;m exponent (AAE) theory. This comprehensive approach provides a nuanced understanding of the complex dynamics of absorbing aerosol in the specific context of coal mining environments. Results and Discussions In the coal mining area, absorbing aerosols are influenced by emission sources, manifesting a particle size distribution divergent from the lognormal model. Under various pollution conditions, robust peaks are discernible in smaller particle size ranges (0.28 -0.3 mu m), with weaker peaks present around 0.58 -0.65 mu m. The relative proportion between the two peaks fluctuates in tandem with the pollution severity (Fig. 3). Using the Mie scattering model, the optical characteristics of absorbing aerosol are inverted based on AOD information, black carbon mass concentration, and particle number concentration. Results indicate that under the assumption of a uniform distribution (Fig. 4), the average size of the core particles at 0.28, 0.58, and 0.7 mu m is relatively low, leading to corresponding patterns in SSA with changes in core particle size. Additionally, the average core particle size shows no significant variation with changes in wavelength in different size ranges. SSA decreases with increasing wavelength, with greater fluctuations in the smaller particle size range (0.25-0.58 mu m) and more stable changes in the larger particle size range (0.58-1.6 mu m). Under this assumption, the AAE theory is found to be inapplicable. In the case of a non- uniform distribution (Fig. 5), SSA values exhibit a slow, followed by a gradual and then rapid increase in the shortwave region, while in the longwave region, SSA first rapidly increases and then gradually levels off. For shorter wavelengths (500 nm and above), AAE theory proves effective for absorbing aerosol with smaller particle sizes. For longer wavelengths (675 nm and above), AAE theory is applicable to absorbing aerosol with moderate particle sizes. However, for larger particles such as coal dust, AAE theory is not suitable. It is noteworthy that, under both assumptions, the inversion results of SSA values in the longwave spectrum (such as 870 and 936 nm) are relatively lower compared to the shortwave spectrum (such as 440 and 500 nm). This discrepancy will lead to an underestimation of emission quantities. Conclusions We conduct on- site observations in the coal mining area of Changzhi City, Shanxi Province, aiming to capture the variation characteristics of AOD, particle concentration, and black carbon mass concentration. Utilizing the Mie scattering model based on the core- shell hypothesis, we simulate the SSA of absorbing aerosol under two different mixing states. Additionally, we calculate the optical variations of absorbing aerosol constrained by the AAE. The research findings reveal the following: 1) The particle size distribution of absorbing aerosol in the coal mining area deviates from the assumptions made in previous studies, which typically assumed single or double- peaked distributions. Influenced by emission sources, the characteristics vary under different pollution conditions. Smaller particles predominantly originate from the incomplete combustion of coal in local power plants and coking factories, producing black carbon. Larger particles stem from the aging processes of black carbon in the atmospheric environment and coal dust generated during coal transportation. 2) Comparison of the SSA variations under different mixing states simulated by the two hypotheses indicates that particle size, mixing state, and spectral range significantly impact the SSA of absorbing. In contrast to previous studies using the infrared spectrum, the present investigation reveals higher SSA values in the ultraviolet and visible light spectrum, suggesting a potential underestimation of black carbon emissions. 3) The AAE theory is applicable only to certain particle size ranges in different spectral bands. For large- sized absorbing aerosol in the coal mining area, using the AAE theory to estimate SSA introduces uncertainty, and applying the AAE assumption across all particle size ranges leads to an underestimation of emissions. These findings underscore that the distribution characteristics of SSA in absorbing aerosol do not strictly adhere to the power- law relationship of the AAE index but are collectively determined by particle size distribution, mixing state, and spectral range.

Aerosol optical properties, including absorption and scattering coefficients (B-abs, and B-scat), extinction coefficient (B-ext), single scattering albedo (SSA), and so forth, are critical metrics to estimate the radiative balance of the atmosphere. However, their ground measurements are sparsely distributed in the world, where Central Asia is void in these measurements. We had been performing the measurements of AOPs and BC with a photoacoustic extinctiometer (PAX) in Jimunai, a border town of China neighboring Kazakhstan, Central Asia, from Aug 2016 to Apr 2019. This three-year study first reported statistically significant trends of B-abs, B-scat, B-ext, SSA, and derived concentrations of BC (Mann-Kendall trend test, p-value 0.05) in the Central-Asian area. B-abs and B-scat show increasing trends and SSA was decreasing determined by the greater increasing pace of B-abs than B-scat. Seasonal and diurnal variations of the AOPs were associated with climate shift and residents' commute activity, respectively. The difference in the magnitudes and trends of AOPs between the measurements and satellites' observations advise that more care should be invested when choosing remote-sensing data to represent the AOPs at a specific site. The increasing trend of derived BC concentrations is reflected in the deposition record of BC in a snowpit of the nearby Muz Taw glacier. We suppose that the dramatically increasing BC particles emitted from Jimunai are significant factors triggering the melting of the adjacent mountain glaciers. The outflow of dust from the neighboring Gurbantiinggiit Desert could occasionally invade into Jimunai and deteriorate the local air quality, as evidenced by a probable dust event captured by the PAX on Feb 15, 2018. Finally, we outlook the future perspectives of measurements in Jimunai as a long-standing station.

Knowledge of aerosol radiative effects in the Tibetan Plateau (TP) is limited due to the lack of reliable aerosol optical properties, especially the single scattering albedo (SSA). We firstly reported in situ measurement of SSA in Lhasa using a cavity enhanced albedometer (CEA) at lambda = 532 nm from 22nd May to 11th June 2021. Unexpected strong aerosol absorbing ability was observed with an average SSA of 0.69. Based on spectral absorptions measured by Aethalometer (AE33), black carbon (BC) was found to be the dominated absorbing species, accounting for about 83% at lambda = 370 nm, followed by primary and secondary brown carbon (BrCpri and BrCsec). The average direct aerosol radiative forcing at the top of atmosphere (DARFTOA) was 2.83 W/m2, indicating aerosol warming effect on the Earth-atmosphere system. Even though aerosol loading is low, aerosol heating effect plays a significant role on TP warming due to strong absorbing ability. The Tibetan Plateau (TP) has experienced rapid warming over the past decades, but the key factors affecting TP climate change haven't yet been clearly understood. Aerosol single scattering albedo (SSA) is a key optical parameter determining aerosol warming or cooling effect; however, reliable SSA measurement is scarce in TP. This study firstly reported in situ measurement of SSA in Lhasa and explored the direct radiative effect of aerosol on TP warming. Strong aerosol absorption, mainly contributed by black carbon (BC), was observed with an average SSA value of 0.69 in this city. Besides Lhasa, other sites over TP were also reported with low SSA (<= 0.77) from surface measurement. The strong aerosol absorption could cause heating effect on the Earth-atmosphere system. To relieve TP warming, reasonable pollutant emission control strategies should be taken urgently to weaken aerosol absorbing ability. Unexpected low aerosol single scattering albedo was observed in Lhasa via in situ measurement of multiple optical parameters simultaneously Black carbon was the dominant contributor (similar to 83%) to aerosol absorption at 370 nm, followed by primary and secondary brown carbon The strong absorption in Lhasa exerted positive direct aerosol radiative forcing (warming effect) at the top of atmosphere

According to the monitoring data of the optical and microphysical characteristics of smoke aerosol at AERONET stations during forest fires in the summer of 2019 in Alaska, the anomalous selective absorption of smoke aerosol has been detected in the visible and near-infrared spectral range from 440 to 1020 nm. With anomalous selective absorption, the imaginary part of the refractive index of smoke aerosol reached 0.315 at a wavelength of 1020 nm. A power-law approximation of the spectral dependence of the imaginary part of the refractive index with an exponent from 0.26 to 2.35 is proposed. It is shown that, for anomalous selective absorption, power-law approximations of the spectral dependences of the aerosol optical extinction and absorption depths are applicable with an angstrom ngstrom exponent from 0.96 to 1.65 for the aerosol optical extinction depth and from 0.97 to -0.89 for the aerosol optical absorption depth, which reached 0.72. Single scattering albedo varied from 0.62 to 0.96. In the size distribution of smoke aerosol particles with anomalous selective absorption, the fine fraction of particles of condensation origin dominated. The similarity of the fraction of particles distinguished by anomalous selective absorption with the fraction of tar balls (TBs) detected by electron microscopy in smoke aerosol, which, apparently, arise during the condensation of terpenes and their oxygen-containing derivatives, is noted.

Brown carbon (BrC), known as light-absorbing organic aerosol in the near-ultraviolet (UV) and short visible region, plays a significant role in the global and regional climate change. A detailed understanding of the spectral optical properties of BrC is beneficial for reducing the uncertainty in radiative forcing calculation. In this work, the spectral properties of primary BrC were investigated by using a four-wavelength broadband cavity-enhanced albedometer with central wavelengths at 365, 405, 532 and 660 nm. The BrC samples were generated by the pyrolysis of three types of wood. During the pyrolysis process, the measured average single scattering albedo (SSA) at 365 nm was about 0.66 to 0.86, where the average absorption angstrom ngstrom exponent (AAE) was between 5.8 and 7.8, and the average extinction angstrom ngstrom exponent (EAE) was within 2.1 to 3.5. The full spectral measurement of SSA (300-700 nm) was realized by an optical retrieval method and the retrieved SSA spectrum was directly applied to evaluate aerosol direct radiative forcing (DRF) efficiency. The DRF efficiency over ground of various primary BrC emissions increased from 5.3 % to 68 % as compared to the non-absorbing organic aerosol assumption. A decrease of about 35 % in SSA would cause the DRF efficiency over ground to change from cooling effect to warming effect (from -0.33 W/m2 to +0.15 W/m2) in the near-UV band (365-405 nm). The DRF efficiency over ground of strongly absorptive primary BrC (lower SSA) contributed 66 % more than weakly absorptive primary BrC (higher SSA). These findings proved the importance of broadband spectral properties of BrC, which are substantial for radiative forcing evaluation of BrC and should be considered in global climate models.

Aerosols influence the development of Atmospheric Boundary Layer (ABL) ensuing aggravated air pollution in megacities. ABL Height (ABLH) and Ventilation coefficient (VC) are key aspects in the pollution study that determines the vertical extent of dispersion of pollutants. To address the intricate, multi-sectoral problem of air pollution, a cogent and considerable approach of evalu-ating the influence of aerosol on ABLH is vital. The data from 2019 -2021 over Delhi, presents quantitative evaluation of radiation effect of aerosol on ABL. The study demonstrates qualita-tively that high aerosol loading lowers the ABLH, consecutively causes further increase in aerosol and PM2.5. Lowering of ABLH subsequently lowers the VC. The influence of aerosol on ABL and radiative forcing (RF) is critiqued with correlation coefficient (R) as-0.17 and -0.37 respec-tively. The cooling effect of the surface is enhanced as Aerosol Optical Depth (AOD) rises, further impeding the development of ABL. Further, through the interdependence between Single Scat-tering Albedo (SSA) and ABLH, it is speculated that the absorbing aerosols (AA) tend to increase the stability, ultimately leading to lower ABLH. Assessing the link between ABL and air pollution is vital not only for preventing and combating aerosol emissions in Delhi, but also for accurate air quality prediction and numerical weather prediction.

East Africa (EA) suffers from the inadequate characterization of atmospheric aerosols, with far-reaching consequences of its inability to quantify precisely the impacts of these particles on regional climate. The current study aimed at character-izing absorption and radiative properties of aerosols using the long-term (2001-2018) AErosol RObotic NETwork (AERONET) and Modern-Era Retrospective analysis for Research and Applications (MERRA-2) data over three environ-mentally specific sites in EA. The annual mean absorption aerosol optical depth (AAOD440 nm), absorption Angstrom Ex-ponent (AAE440-870 nm), total effective radius (REff), and total volume concentration (mu m3/mu m2) revealed significant spatial heterogeneity over the domain. The study domain exhibited a significant contribution of fine-mode aerosols com-pared to the coarse-mode particles. The monthly variation in SSA440 nm over EA explains the strength in absorption aero-sols that range from moderate to strong absorbing aerosols. The aerosols exhibited significant variability over the study domain, with the dominance of absorbing fine-mode aerosols over Mbita accounting for similar to 40 to similar to 50 %, while weakly absorbing coarse-mode particles accounted for similar to 8.2 % over Malindi. The study conclusively determined that Mbita was dominated by AAOD mainly from biomass burning in most of the months, whereas Malindi was coated with black carbon. The direct aerosol radiative forcing (DARF) retrieved from both the AERONET and MERRA-2 models showed strong cooling at the top of the atmosphere (TOA; -6 to -27 Wm-2) and the bottom of the atmosphere (BOA, -7 to -66 Wm-2). However, significant warming was noticed within the atmosphere (ATM; +14 to +76 Wm-2), an indica-tion of the role of aerosols in regional climate change. The study contributed to understanding aerosol absorption and ra-diative characteristics over EA and can form the basis of other related studies over the domain and beyond.

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.

According to the particle size and absorptivity as determined by the fine mode fraction and the single-scattering albedo (SSA) retrievals from AErosol RObotic NETwork (AERONET) sites around the world, aerosols are classified into four key categories: coarse and absorptive aerosol (Type I), mixed aerosol (Type II), fine and absorptive aerosol (Type III), fine and non-absorptive aerosol (Type IV). Seasonal variations of aerosol types with their corresponding direct radiative forcing efficiency (RFE) are observed on different continents. The RFE at the surface (RFEsfc) and top of the atmosphere (RFEtoa) reach their maximum (minimum) values over Asia and North America (Europe, Oceania, and South America) from June to August. The effects of solar zenith angle (SZA), surface albedo (SA), and SSA on RFEsfc and RFEtoa are investigated. The absolute values of RFE at TOA of all types of aerosols are largest at cos(SZA) =0.3 to 0.4. The increased SA reduces the absolute value of RFE both at SFC and TOA for all types of aerosols, and when SA reaches a specific threshold, depending on the type of aerosol, the RFEtoa turns positive. RFEtoa increases while RFEsfc decreases with decreasing SSA. The RFEsfc of the four categories of aerosol varies slightly in the same SZA, SSA and SA conditions, while RFEtoa is aerosol type dependent. It is found that larger particles reflect more solar energy into space per optical depth, resulting in an enhanced cooling effect under similar SZA, SSA, and SA conditions.