Current research predominantly emphasizes elemental carbon (EC) light absorption, while ignoring its relationship with aerosol hygroscopic scattering. In this study, concentrations and optical properties of aerosol components were measured during a full-year monitoring campaign at an urban site in Suzhou. Results from a multiple linear regression model suggested that secondary organic carbon was a primary contributor to high mass absorption efficiency of EC in summer. Through the estimation of aerosol scattering coefficients under both dry and ambient atmospheric conditions, it was found that hygroscopic growth accounted for more than 35.0 % of the total aerosol scattering coefficient. Hygroscopic growth of nitrate and sulfate enhanced their annual mean scattering contributions by 42.1 % and 45.2 %, respectively. A negative correlation between EC concentration and the hygroscopic growth factor (f(RH)) was observed under varying relative humidity (RH) conditions. Associated with the decrease in f(RH), reductions in PM2.5 scattering coefficients (14.0 f 2.2, 29.4 f 5.2, and 24.5 f 8.2 Mm-1) were linked to EC concentration increases of 0.37 f 0.1, 0.40 f 0.1, and 0.21 f 0.1 mu g/m3 under low, medium, and high RH conditions, respectively. An increase in EC concentration by 0.19-0.37 mu g/m3 elevated the PM2.5 absorption coefficient by 2.66-5.41 Mm-1, and reduced the scattering coefficient by 10.53-17.91 Mm-1. Collectively, increased EC concentrations reduced aerosol single scattering albedo (SSA), particularly under high RH conditions. This study reveals that EC not only reduces aerosol extinction coefficients but also shifts aerosol radiative forcing in a positive direction by suppressing hygroscopic scattering.

Under environment with various water contents, the variations in the mixing state and particle size of coated black carbon (BC) aerosols cause changes in optical and radiative effects. In this study, fractal models for thinly, partially, and thickly coated BC under six relative humidities (RHs 1/4 0-95%) are constructed and optically simulated at 1064 and 532 nm. Differential scattering cross-sections are selected to retrieve the mixing state (Dp/Dc) of BC to investigate the possible retrieval errors caused by the nonspherical morphology when using the single-particle soot photometer (SP2). Furthermore, the radiative forcing of BC aerosols at different RHs are analyzed. Results showed that the retrieval errors (REs) of Dp/Dc are negative for coated particles with BC volume fraction smaller than 0.10, indicating that the mixing states of coated fractal BC are underestimated during the hygroscopic growth. The partiallycoated BC has the best retrieval accuracy of the mixing state, followed by the closed-cell and coatedaggregate model, judging from averaged REs. Radiative forcing enhancements for partially-coated aerosols with different BC volume fractions exponentially increase to opposite values, resulting in a warming or cooling effect. This study helps understand the uncertainties in Dp/Dcof BC aerosols retrieved by SP2 and their radiative forcing at different RHs. (c) 2025 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights are reserved, including those for text and data mining, AI training, and similar technologies.

In this study, the sensitivity of the optical properties of carbonaceous aerosols, especially humic-like substances (HULIS), are investigated based on a one-year measurement of ambient fine atmospheric particulate matter (PM2.5) at a Global Atmospheric Watch (GAW) station in South Korea. The extinction, absorption coefficient, and radiative forcing (RF) are calculated from the analysis data of water soluble (WSOC) and insoluble (WISOC) organic aerosols, elemental carbon (EC), and HULIS. The sensitivity of the optical properties on the variations of refractive index, hygroscopicity, and light absorption properties of HULIS as well as the polydispersity of organic aerosols are studied. The results showed that the seasonal absorption coefficient of HULIS varied from 0.09 to 11.64 Mm(-1) and EC varied from 0.11 to 3.04 Mm(-1) if the geometric mean diameter varied from 0.1 to 1.0 mu m and the geometric standard deviation varied from 1.1 to 2.0, with the imaginary refractive index (IRI) of HULIS varying from 0.006 to 0.3. Subsequently, this study shows that the RF of HULIS was larger than other constituents, which suggested that HULIS contributed significantly to radiative forcing.

This investigation focuses on the characterisation of the aerosol particle hygroscopicity. Aerosol particle optical properties were measured at Granada, Spain, during winter and spring seasons in 2013. Measured optical properties included particle light-absorption coefficient (sigma(ap)) and particle light-scattering coefficient (sigma(sp)) at dry conditions and at relative humidity (RH) of 85 +/- 10%. The scattering enhancement factor, f(RH = 85%), had a mean value of 1.5 +/- 0.2 and 1.6 +/- 0.3 for winter and spring campaigns, respectively. Cases of high scattering enhancement were more frequent during the spring campaign with 27% of the f(RH = 85%) values above 1.8, while during the winter campaign only 8% of the data were above 1.8. A Saharan dust event (SDE), which occurred during the spring campaign, was characterised by a predominance of large particles with low hygroscopicity. For the day when the SDE was more intense, a mean daily value of f(RH = 85%) = 1.3 +/- 0.2 was calculated. f(RH = 85%) diurnal cycle showed two minima during the morning and afternoon traffic rush hours due to the increase in non-hygroscopic particles such as black carbon and road dust. This was confirmed by small values of the single-scattering albedo and the scattering Angstrom exponent. A significant correlation between f(RH = 85%) and the fraction of particulate organic matter and sulphate was obtained. Finally, the impact of ambient RH in the aerosol radiative forcing was found to be very small due to the low ambient RH. For high RH values, the hygroscopic effect should be taken into account since the aerosol forcing efficiency changed from -13W/m(2) at dry conditions to -17W/m(2) at RH = 85%.