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High uncertainty in optical properties of black carbon (BC) involving heterogeneous chemistry has recently attracted increasing attention in the field of atmospheric climatology. To fill the gap in BC optical knowledge so as to estimate more accurate climate effects and serve the response to global warming, it is beneficial to conduct site-level studies on BC light absorption enhancement (E abs) characteristics. Real-time surface gas and particulate pollutant observations during the summer and winter over Wuhan were utilized for the analysis of E abs simulated by minimum R squared (MRS), considering two distinct atmospheric conditions (2015 and 2017). In general, differences in aerosol emissions led to E abs differential behaviors. The summer average of E abs (1.92 ± 0.55) in 2015 was higher than the winter average (1.27 ± 0.42), while the average (1.11 ± 0.20) in 2017 summer was lower than that (1.67 ± 0.69) in winter. E abs and R BC (representing the mass ratio of non-refractory constituents to elemental carbon) constraints suggest that E abs increased with the increase in R BC under the ambient condition enriched by secondary inorganic aerosol (SIA), with a maximum growth rate of 70.6% in 2015 summer. However, E abs demonstrated a negative trend against R BC in 2017 winter due to the more complicated mixing state. The result arose from the opposite impact of hygroscopic SIA and absorbing OC/irregular distributed coatings on amplifying the light absorbency of BC. Furthermore, sensitivity analysis revealed a robust positive correlation (R > 0.9) between aerosol chemical compositions (including sulfate, nitrate, ammonium and secondary organic carbon), which could be significantly perturbed by only a small fraction of absorbing materials or restructuring BC through gaps filling. The above findings not only deepen the understanding of BC, but also provide useful information for the scientific decision-making in government to mitigate particulate pollution and obtain more precise BC radiative forcing. [Display omitted] • In 2015, E abs was high in summer and low in winter. • Compared to 2015, the seasonal pattern of E abs reversed in 2017. • E abs demonstrated a negative trend against R BC in 2017 winter. • The positive correlation between E abs and SIA was affected by BC mixing-state diversity. • Aqueous phase heterogeneous reactions dominated the generation of SIA. [ABSTRACT FROM AUTHOR]

• We studied BCRF in ultraviolet, visible, and near-infrared spectra under haze. • ARF and BCRF increase with the development of haze. • Radiative forcing efficiency is weakened by increased forward scattering. The optical properties and radiative forcing of atmospheric aerosol (ARF) and black carbon (BC) aerosol (BCRF) in ultraviolet (UV), visible (VIS), near-infrared (NIR), and shortwave (SW) spectra were investigated under haze conditions based on the observations of the Aethalometer and sun-sky radiometer and simulations from libRadtran. The results show that the BC concentrations increased greatly from 2.73 μg/m3 under clear-air conditions to 7.95 μg/m3 under severe haze conditions, while BC aerosol optical depth (AOD) increased from 0.025 to 0.092. A high correlation (R2=0.62) was found between BC AOD and absorbing aerosol optical depth (AAOD) derived from the sun-sky radiometer. The BCRF in SW (BCRF SW) varied from -10.20 W/m2 under clear-air conditions to -25.40 W/m2 under severe hazy conditions. However, its fraction in ARF (ARF SW) decreased from 19% to 17% simultaneously, which is mainly related to the decrease of the ratio of BC AOD to AOD. The fraction of ARF in VIS in ARF SW decreased from 56.3% under clear-air conditions to 50.5% under severe haze conditions, while the fraction of BCRF in VIS in BCRF SW was much larger, and increased from 72.9% to 73.8%. The BCRF efficiency (BCRFE) was much larger than ARF efficiency (ARFE), and both of them decreased with the development of haze. The ARFE in SW decreased from -173.84 W/m2 under clear-air conditions to -112.75 W/m2 under severe haze conditions while BCRFE SW varied from -482.50 W/m2 to -321.88 W/m2. The decrease of ARFE and BCRFE is related to the increase of aerosol loading and asymmetry factor (ASY). The ASY increased and the forward scattering was enhanced with the development of haze due to the hygroscopic growth of aerosol particles, which reduced the extinction efficiency of aerosols including BC on solar radiation and the cooling effect on the surface. [ABSTRACT FROM AUTHOR]

[Display omitted] • China is 'turning green' and the LAI increased significantly from 2001 to 2020. • The responses of different vegetation types to climate change were studied. • The order of driving force for LAI is: precipitation > temperature > AOD > PAR. • The influence of aerosols on LAI cannot be ignored. • LAI of deciduous broadleaf forests is more sensitive to climate change. Vegetation in China has experienced significant improvements under the combined effects of human activities and climate change. However, when exploring the effects of climate change on vegetation dynamics, existing studies mainly focus on two meteorological factors (temperature and precipitation) and rarely consider aerosol optical depth (AOD) and photosynthetically active radiation (PAR). Therefore, in this study, we further explored the potential dependence of leaf area index (LAI) on temperature (Tem), precipitation (Pre), AOD, and PAR, based on the analysis of the spatiotemporal variations of LAI in different vegetation types in China. Results showed that the LAI increased significantly from 2001 to 2020, with an overall growth rate of 0.0202/yr; the response of LAI for different vegetation types to each influencing factor varied greatly, and the deciduous broadleaf forests were more sensitive to climate change. Quantitative analysis based on the GeoDetector method showed that the most important driving force of LAI change is Pre (0.596); while Tem (0.394) and AOD (0.325) showed similar effects; by contrast, PAR (0.279) had the lowest driving force on LAI. Among them, the change of Pre is the dominant factor affecting LAI for most vegetation types. From the perspective of regional research, we found that the increase of LAI in the South was greater than that in North China, while human activities (urbanization) were unfavorable to the increase of LAI, such as the Yangtze River Delta where the LAI decreased significantly. This study contributes to a better understanding of the response of different vegetation types to climate and atmospheric changes in China over the past 20 years and provides a scientific reference for developing and implementing ecological restoration measures and promoting sustainable development. Overall, China is 'turning green', and air quality is improving. [ABSTRACT FROM AUTHOR]

The changes in aerosol optical and microphysical properties, and sub-band shortwave direct radiative forcing (DARF) in Beijing and Wuhan were compared at different haze levels in winters. The occurrence of haze is found to be governed by the wind circulation and boundary layer in Beijing where the ground wind speed and the height of boundary layer decreased significantly with the development of haze. Compared with the boundary layer and wind, the relative humidity has a stronger impact on haze in Wuhan. Especially, hygroscopic growth of aerosol particles is observed in Wuhan. The increase of fine-mode non-absorbing particles is the main characteristic of aerosol change during haze periods. With the development of haze, the larger increase in DARF at the top of atmosphere is found in Beijing, while the change in DARF at the atmosphere is more obvious in Wuhan. Efficiency of the DARF shows that the change of DARF depends highly on the single scattering albedo in Beijing due to the obvious enhancement of particle scattering, while it depends much more on particle radius over Wuhan. The increase in particle size can also change DARF proportion of each sub-band in shortwave that the DARF proportion in ultraviolet and visible decreased. However, the DARF proportion in near-infrared increased during haze, as the light in shorter wavelength is more sensitive to the change of the fine particle radius. • Aerosol properties in Beijing were compared with Wuhan at different haze levels. • Meteorological parameters are more significant in driving haze in Beijing than Wuhan. • Decrease in direct radiative forcing efficiency was observed with haze developing. [ABSTRACT FROM AUTHOR]