Brown carbon (BrC) is the ubiquitous part of the atmospheric organic carbon. It absorbs solar lights and greatly impacts the Earth's radiative balance. This study examines the spectral characteristics of BrC and its radiative effect in the Dhaka South (DS) site and Dhaka North (DN) site from July 2023 to January 2024 with a high-volume particulate matter sampler on quartz filters. Spectral characteristics such as absorption coefficient (babe,), mass absorption efficiency (MAE), absorption angstrom exponent (AAE), and refractive index (Kabs-x) were determined by using a UV -visible spectrophotometer, and fluorescence emission spectra were analyzed in different pH by the fluorescent spectrophotometer. The concentrations of BrC and black carbon (BC) were determined by an aethalometer. The mean concentrations of BrC and BC in Dhaka city were 18.63 +/- 3.84 mu g 111-3 and 17.93 +/- 3.82 pg M-3, respectively. The AAE values lie in the range of 3.20-4.01 (DN) and 3.27-4.53 (DS), and the radiative forcing efficiency of BrC was obtained at 4.43 +/- 1.02 W g-1 in DN and 3.93 +/- 0.74 W g-1 in DS, indicating the presence of highly light-absorbing BrC in these locations. Average MAE and Kabs_k values were 1.55 +/- 0.45 m2g1 and 0.044 + 0.013, respectively, in DS, alternatively 1.84 +/- 0.59 m2g1 and 0.052 +/- 0.016 in DN. The fluorescence excitation-emission spectra confirmed the presence of a polyconjugate cyclic ring with multifunctional groups in the structure of BrC. Light absorption properties and fluorescence emission spectra were varied with the change of pH. As the pH increased (2-8), the AAE value decreased and MAEB,c_365 increased due to protonation or deprotonation. This study highlights that the BrC has a significant impact on the air quality as well as the Earth's radiative balance, emphasizing its strong light-absorbing properties and variability with environmental factors.

Aviation emissions contribute to climate change and local air pollution, with important contributions from non-CO2 emissions. These exhibit diverse impacts on atmospheric chemistry and radiative forcing (RF), varying with location, altitude, and time. Assessments of local mitigation strategies with global emission metrics may overlook this variability, but detailed studies of aviation emissions in areas smaller than continents are scarce. Integrating the AviTeam emission model and OsloCTM3, we quantify CO2, NOx, BC, OC, and SOx emissions, tropospheric concentration changes, RF, region-specific metrics, and assess alternative fuels for Norwegian domestic aviation. Mitigation potentials fora fuel switch to LH2 differ by up to 3.1 x 108 kgCO2-equivalents (GWP20) when using region-specific compared to global metrics. These differences result from a lower, region- specific contribution of non-CO2 emissions, particularly related to NOx. This study underscores the importance of accounting for non-CO2 variability in regional assessments, whether through region-specific metrics or advanced atmospheric modelling techniques.

Light-absorbing organic carbon (i.e., brown carbon, BrC) significantly contributes to light absorption and radiative forcing in the atmospheric particles. However, the secondary formation of BrC and optical properties of secondary BrC are poorly understood. In this study, we analyzed and evaluated the light absorption and environmental effects of BrC and secondary BrC from July 1st to 31st, 2022 (summer) and January 20th to February 20th, 2023 (winter) in Chongqing. BrC and secondary BrC light absorption were estimated via a seven- wavelength aethalometer and the statistical approach. The average values of secondary BrC light absorption (Abs(BrC,sec,lambda)) accounted for 46.2-56.5% of Abs(BrC). Abs(BrC,370) and Abs(BrC,sec,370) were significantly higher during winter (26.2 +/- 13.2 and 9.1 +/- 5.2 Mm(-1) respectively) than that during summer (7.2 +/- 4.1 and 5.2 +/- 3.5 Mm(-1) respectively) (p < 0.001), suggesting secondary formation played an essential role in BrC. A diurnal cycle of Abs(BrC,sec,370) was explained by the photobleaching of light-absorbing chromophores under the oxidizing conditions in the daytime, and the formation of chromophores via aqueous reactions with NH(4)(+ )and NO(x )after sunset during winter. PSCF analysis showed that transport of anthropogenic emissions from the northeastern and southeastern areas of Chongqing was the important source of the secondary BrC in PP during winter. During winter, the average values of SFEBrC and SFEBrC,sec were 31.9 and 27.4 W g(-1) lower than that during summer (64.7 and 44.5 W g(-1)), respectively. In contrast, J[NO2] values of SFEBrC and SFEBrC,sec decreased by 23.3% and 8.7% during winter higher than that during summer (19.9% and 5.6%), indicating that BrC and secondary BrC cause substantial radiative effects and atmospheric photochemistry. Overall, this study is helpful in understanding the characterization and secondary formation of BrC and accurately evaluating the environmental effects of BrC in Chongqing.

Light absorbing particles (LAPs) present high absorbance and contribute to reducing the snow albedo when deposited on snow surfaces. This deposition can be caused by aerosols transported from natural or anthropogenic, either distant or nearby sources. In this study, snow was artificially contaminated with soil samples collected in the Central Andes (near El Yeso dam) to simulate the most common nearby source of Mineral Dust (MD) deposition onto snow surface. Andean soil samples previously conditioned were characterized through Single Particle Optical Sizing (SPOS), X-ray diffraction (XRD) analysis, and Scanning Electron Microscope (SEM) for the determination of optical properties. Spectral snow albedo was measured in situ with a spectroradiometric system. To evaluate the heterogeneity of the particle distribution over the snow surface, aerial photographs were taken with a drone to apply a visual color segmentation of the surface and to determine the equivalent MD concentration. Experimental snow albedo was compared with theoretical values obtained with the OptiPar radiative transfer model. Inputs for the model were: the MD refractive index (calculated from the mineralogical composition and morphology of MD) and particle size, cloudiness, snow density, surface roughness, snow grain size, and LAPs concentration (obtained from the snow samples collected during the experiments and analyzed in the laboratory). Small black carbon concentrations were found in natural snow and considered in the simulations. Spectral albedo measurements showed high albedo reductions in the UV and VIS range (300-800 nm), being less significant in the NIR range (800-1700 nm). A nonlinear behavior was observed in broadband albedo when increasing MD concentration. For lower values of MD concentration (lower than 1500 mg center dot kg (-1) ), a significant albedo reduction rate of 0.1 units per 1000 mg center dot kg (-1) was found, while at higher concentrations (> 3500 mg center dot kg (-1) ), such reduction tends to the minimum. Simulated values with OptiPar are in agreement with measured albedo, but some differences are observed, probably due to the refractive index considered, the snow surface roughness, and the non-uniform MD concentration in snow.

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.

Charge distribution measurements are required to understand the spatiotemporal distribution of the number concentrations of submicron atmospheric particles that affect radiative forcing and particle deposition in human airways. The number concentrations of non -charged and charged particles within the 0.3-0.5 pm diameter (D) range were measured at Keio University in Yokohama, Japan, from June 2022 to January 2023 by combining a parallel -pate particle separator and optical particle counters to investigate critical parameters controlling the charging state of submicron atmospheric particles. The measurement uncertainties in the average charge number per particle (pave) and the standard deviation (1 sigma), derived from the charge distribution of the submicron particles, were within 15%. The monthly median values of 1 sigma increased in summer and decreased in winter and correlated with the water vapor amount and wind speed. The 1 sigma values in summer and winter, derived from the seasonally averaged charge distributions of particles, were close to those from the theoretically calculated charge distribution of particles within 0.387-0.5 pm D range and with D = 0.3 pm, respectively, suggesting that the observed particle charge distributions approached the stationary charge distribution for the effective D. In summer, the frequent transport of water molecules and ions from the Pacific Ocean causes efficient collisions between multiple ions and submicron particles with a larger effective D, which may expand the charge distribution of particles. The polarity ratio, the concentration of positively charged particles relative to that of negatively charged particles, was almost unity, indicating the well-balanced charge polarity of the submicron atmospheric particles. The polarity ratio and pave changed significantly during lightning events, indicating that the atmospheric particle charge balance broke. Our findings show that the charge distribution of submicron atmospheric particles can be partly controlled by meteorological parameters (e.g., absolute humidity) and the microphysical properties of the particles.

Biomass burning (BB) greatly impacts the Maritime Continent through various mechanisms including agricultural burning, land clearing and natural response to drought. The dynamic characteristics of BB in terms of its spatiotemporal distribution, seasonality, transport mechanism, and aerosol properties have prompted numerous research efforts including field campaigns, in -situ measurements, remote sensing, and modelling. Although the differing perspectives of these studies have offered insights on understanding the regional BB issues, it is challenging to compare and resolve the wider picture because of the diversity of approaches. Human -induced global warming has certainly caused multiple observed changes in the regional meteorological characteristics. In this study, we review BB events in the Maritime Continent from 2012 to 2021, focusing on the meteorological influence and knowledge evolution in cloud -aerosol -radiation (CAR). Unlike other reviews, our review examines the occurrence of BB events using synergistic application of ground -based measurement, global reanalysis model and satellite product, which allows us to examine the anomalies for comparison with other studies and identify the unique features of the event. We identified four dominant modes of variability responsible for the occurrence of large-scale BB in the Maritime Continent: (1) El Nin similar to o Southern Oscillations (ENSO), (2) extreme positiveIndian Ocean Dipole (pIOD), (3) tropical cyclone (TC) activity, and (4) Madden -Julian Oscillations (MJO). We reconcile the past CAR studies and summarize their findings based on the four key CAR mechanisms: (1) instantanous radiative forcing from aerosol -radiation interactions, IRFari (2) and its subsequent adjustments, SAari, (3) instantanous radiative forcing from aerosol -cloud interactions, IRFaci, and (4) and its subsequent adjustments, SAaci. We urge future CAR studies in the Maritime Continent should focus on accurate characterization of the composition of biomass burning plume which is a mixture of peatland, agricultural burning and anthropogenic sources.

In the second of two companion papers, we quantify the potential ultimate benefits for public health and radiative forcing from the phase-out of coal use in China. With the baseline year of 2015, we estimate that the premature deaths and disability-adjusted life years (DALYs) in China will be reduced by 375,000 persons and 8.90 x 106 years (GEMM model), which is 41.7% and 54.5% of the total number of premature deaths and DALYs caused by PM2.5 from anthropogenic sources, effectively improving the health of the population. The provinces with the most significant decreases in fatalities and disabilities are concentrated in large coal-consuming provinces. Regarding age distribution, the phase-out of coal use will effectively extend the life expectancy of the elderly, especially in the age group of 65-89 years. Health benefits' growth rate upsurges notably once more than half of coal is phased out. Based on the Parallel Offline Radiative Transfer (PORT) model, the global annual average radiative forcing due to changes in short-lived climate pollutants (SLCPs), including sulfate, black carbon, organic carbon, and O3 associated with the phase-out of coal use, is 0.34 W m- 2,-0.048 W m- 2, 0.00064 W m- 2, and 0.00055 W m- 2, respectively. The complete phase-out of coal use in China also reduces the emission of long-lived climate pollutants (LLCPs) and causes negative radiative forcing. Among them, the global annual average radiative forcing of CO2, CH4, and N2O is-0.0073 W m- 2,-5.48 x 10- 5 W m- 2,-1.60 x 10- 5 W m- 2. If the lifetimes of pollutants are considered, the total global reduction in heat uptake is-2.29 x 107 J m- 2 over 100 years, dominated by the decrease of CO2, which will slow the rate of global warming and thus may reduce the probability of extreme weather events. This report will incentivize relevant policymakers to remove coal significantly to achieve carbon neutrality.

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.

Emissions from road traffic are one of the most important sources of soot aerosols and they can affect the surfaces in the area. In the case of snow surfaces, this effect may lead to changes in the radiative forcing and snow melt, also influenced by particle transport such as particle diffusion and advection. An experimental campaign in The Andes, Chile, was carried out measuring the radiance and irradiance of the snow perpendicularly to a road with high traffic load, together with the meteorological conditions (to identify the diffusion and advection phenomena), the aerosol size distribution, the concentration of particles (PM1, PM2.5 and PM10), and the concentration of black carbon (BC) in both, the atmosphere and the snowpack. The aim of the study was to quantify the contribution of four factors affecting the albedo (black carbon -BC- concentration in snow, grain size, cloudiness, and roughness) by comparing the measured albedo of the contaminated snow surface with that of the same surface prior to contamination, considered as a reference. Results showed a trade-off between diffusion and advection. Close to the road, diffusion was predominant, leading to an increase in BC concentration and a reduction in snow albedo. On the contrary, far from the road, where winds are channeled along the centre of the valley, advection of particles became dominant, leading to another increase in particle concentration and another reduction in snow albedo. Among the factors contributing to reduce the snow albedo, BC concentration dominates at all distances from the road, although the effect of the grain size becomes as significant as that of BC in the centre of the valley, with the effects of surface roughness and cloudiness remaining minor. This information can be used in snow models to get a better knowledge of the effect of particle deposition on albedo reductions.