Radiative absorption enhancements by black carbon controlled by particle-to-particle heterogeneity in composition.

Black carbon (BC) absorbs solar radiation, leading to a strong but uncertain warming effect on climate. A key challenge in modeling and quantifying BC's radiative effect on climate is predicting enhancements in light absorption that result from internal mixing between BC and other aerosol components. Modeling and laboratory studies show that BC, when mixed with other aerosol components, absorbs more strongly than pure, uncoated BC; however, some ambient observations suggest more variable and weaker absorption enhancement. We show that the lower-than-expected enhancements in ambient measurements result from a combination of two factors. First, the often used spherical, concentric core-shell approximation generally overestimates the absorption by BC. Second, and more importantly, inadequate consideration of heterogeneity in particle-to-particle composition engenders substantial overestimation in absorption by the total particle population, with greater heterogeneity associated with larger model-measurement differences. We show that accounting for these two effects-variability in per-particle composition and deviations from the core-shell approximation-reconciles absorption enhancement predictions with laboratory and field observations and resolves the apparent discrepancy. Furthermore, our consistent model framework provides a path forward for improving predictions of BC's radiative effect on climate.

Aerosol-induced direct radiative forcing effects on terrestrial ecosystem carbon fluxes over China.

Atmospheric aerosols can change vegetation photosynthesis through the effects of aerosols on radiation, which will affect the peak carbon dioxide emissions and carbon neutrality at global scales. In this study, we quantify the aerosol-induced direct radiation forcing (ADRF) in China from 2001 to 2014 based on the radiation flux simulation used by the Fu-Liou radiation transfer model under with-aerosols and no-aerosols scenarios. Using the radiation simulation results, we modify the atmospheric forcing datasets to drive Community Land Model 4.5 (CLM4.5) to gain the changes in carbon fluxes in China caused by ADRF. The results show that these two models are accurate in estimating radiation (R2 = 0.78-0.88) and carbon fluxes (R2 = 0.73-0.75) in China. High levels of ADRFs were captured in China, especially with increasing diffuse fraction, resulting in the diffusing fertilization effect occurring in most areas of China. The ADRF can increase cumulative gross primary productivity (GPP) and total ecosystem respiration (ER) by 3.20 gC m-2 and 5.13 gC m-2 per year, respectively. From 2001 to 2014, the diffusing fertilization effects experienced trends of increasing first and then decreasing. However, ADRFs in some regions of China show negative effects on carbon fluxes due to vulnerable vegetation functional types and high aerosol loading. The ADRF will also enable soil temperature decreases and volumetric soil water increases, which is closely related to changes in carbon fluxes. Meanwhile, due to changes in soil water and heat conditions, N2O and CH4 production will also be disturbed, and ADRF increases the global warming potential (GWP) for both greenhouse gases. This phenomenon indicated that atmospheric aerosol pollution control is far-reaching significance for peaking carbon dioxide emissions before 2030.

Aerosol optical properties and the mixing state of black carbon at a background mountainous site in Eastern China.

In-situ measurements of aerosol optical properties were conducted at Mt. Huang from September 23 to October 28, 2012. Low averages of 82.2, 10.9, and 14.1 Mm-1 for scattering coefficient (σsp, neph, 550), hemispheric backscattering coefficient (σhbsp, neph, 550), and absorption coefficient (σap, 550), respectively, were obtained. Atmospheric aging process resulted in the increase of σap, 550 but the decrease of the single scattering albedo (ω550) at constant aerosol concentration. However, the proportion of non-light-absorbing components (non-BCs) was getting higher during the aging process, resulting in the increase of aerosol diameter, which also contributed to relatively higher σsp, neph, 550 and ω550. Diurnal cycles of σsp, neph, 550 and σap, 550 with high values in the morning and low values in the afternoon were observed closely related to the development of the planetary boundary layer and the mountain-valley breeze. BC mixing state, represented by the volume fraction of externally mixed BC to total BC (r), was retrieved by using the modified Mie model. The results showed r reduced from about 70% to 50% when the externally mixed non-BCs were considered. The periodical change and different diurnal patterns of r were due to the atmospheric aging and different air sources under different synoptic systems. Local biomass burning emissions were also one of the influencing factors on r. Aerosol radiative forcing for different mixing state were evaluated by a "two-layer-single-wavelength" model, showing the cooling effect of aerosols weakened with BC mixing state changing from external to core-shell mixture.

Field evidence of brown carbon absorption enhancement linked to organic nitrogen formation in Indo-Gangetic Plain.

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 (MAEWS-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 MAEWS-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 MAEWS-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 âˆ¼ 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.

Effects of hygroscopicity on aerosol optical properties and direct radiative forcing in Beijing: Based on two-year observations.

The influence of relative humidity on aerosol properties and the direct radiative forcing of PM10 and PM1 were investigated in Beijing from January 2018 to December 2019. The annual mean scattering hygroscopic growth factor at RH = 80 % [f(80 %)] of PM10 and PM1 were 1.60 ± 0.24 and 1.58 ± 0.22, respectively. The variation of aerosol hygroscopic growth factors of PM10 and PM1 aerosols was similar, which is mainly due to the fact that aerosol scattering in Beijing is dominated by fine particles. The seasonal mean f(80 %) of PM10 from spring to winter were 1.66 ± 0.23, 1.71 ± 0.25, 1.51 ± 0.20, 1.49 ± 0.16, respectively, which were higher in spring and summer, and lower in autumn and winter. The diurnal variation of f(80 %) was relatively higher from 12:00 to 18:00, which could be related to the formation of secondary aerosols by photochemical reactions. f(80 %) shows a strong positive relationship with both the scattering Angström exponent (SAE) and the single scattering albedo (ω0) under dry conditions; therefore, the scattering hygroscopic growth factor could be estimated using these two parameters. The upscatter fraction (ß) and single scattering albedo, which are the key aerosol optical properties for the calculation of direct radiative forcing, are also RH-dependent. As RH increases, the upscatter fraction (backscatter fraction) decreases and ω0 increases. The aerosol radiative forcing at RH 80 % was 1.48 times as that in the dry state. The sensitivity experiment showed that the variation in the scattering coefficient with relative humidity had the greatest influence on radiation forcing, followed by ß and ω0. The seasonal variation of ΔF(80 %)/ΔF(dry) coincides with that of the aerosol hygroscopic growth factor. Our study suggests that understanding the influence of relative humidity on aerosol properties and direct radiative forcing is important for accurately estimating the radiative forcing of aerosols.

Spatiotemporal variations and relationships of aerosol-radiation-ecosystem productivity over China during 2001-2014.

Several air pollution episodes occurred in China in the past decade, and high levels of aerosols load also caused the changes of radiation, which could further influence the gross primary productivity (GPP) in the terrestrial ecosystem. This paper focuses on the spatiotemporal variations and relationship of aerosol-radiation-GPP in China during a heavy pollution period (2001-2014). For this purpose, the Fu-Liou radiation transfer mechanism model was used to estimate total radiation (TR) and diffuse radiation (DIFR) at the spatial resolution of 1° × 1° based on the satellite aerosol optical depth (AOD) and other auxiliary data. This model shows excellent performance with an R2 of 0.88 and 0.79 for TR and DIFR, respectively. A significant increasing trend (0.23 W m-2 year-1) in TR was found in China in this phase, and it was mainly attributed to DIFR. Furthermore, a scenario without aerosols (AOD = 0) was simulated as a comparison to quantify the aerosol radiative forcing, which indicated that aerosols play a catalytic role in DIFR, increasing it by approximately 19.55%. Despite all this, aerosols have weakened the brightening of China due to the negative forcing on direct radiation. Meanwhile, 0.65-4.20 kgC m-2 year-1 increase of GPP was also captured in seven regions of China during this phase.However, the significant negative response of GPP to aerosol was found in most ecosystems in the growing season of vegetation, and the highest correlation of -0.76 (p < .01) existed in the central China forest regions. It suggests although aerosol causes a diffuse fertilization effect, GPP is still lost due to high levels of aerosol load in most areas of China during growing season of vegetation. This paper aims to determine the relationship among the aerosol-radiation-ecosystem productivity in different regions of China, which could provide a reference for the divisional strategy formulation and classification management in different ecosystems.

Analysis of water vapor effects on aerosol properties and direct radiative forcing in China.

The effects of column water vapor (CWV) on aerosol optical properties, radiative effects and classification are studied by using aerosol and CWV data from eight Aerosol Robotic Network (AERONET) sites in China: Beijing, XiangHe, Shouxian, Taihu, Hong_Kong, Zhongshan_Univ, SACOL, and Mt_WLG, which represents 5 distinct aerosol climatologies in China. Contrast in correlations between aerosol optical depth (AOD) and CWV is found. High correlation coefficient (R) ranging from 0.63-0.94 is observed at Beijing and XiangHe (North China Plain), SACOL (Northwest China) and Mt_WLG (the Tibetan Plateau). R values at stations in the Middle-East China (Shouxian and Taihu) are within 0.32-0.45. AOD shows poor correlation to CWV in Southeast China (R at Hong_Kong and Zhongshan_Univ of 0.15 and 0.27). At most sites, the asymmetry (ASYM) of fine-mode aerosol increases with CWV with R larger than ~0.4. Aerosol direct radiative forcing efficiency (ADRFE) at the bottom of the atmosphere (BOA) is affected by CWV, with R >~0.5 over the north and Middle-East China sites. The statistic results show that an increase of CWV by 0.1 cm could result in enhancements of ADREF at the BOA by about 1.1-2.8 W m-2 at all the sites except Mt_WLG. The aerosol classification shows that the mix-small aerosol type is always dominated under the high CWV air. The clusters of back-trajectories with relative humidity (RH) from Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model indicate that the air mass with high RH is often from south and east directions. The influence of CWV on aerosol properties is mainly shown in the properties of fine mode aerosol, which needs to be considered in the study of aerosol radiative forcing and climate effects.

Characterization of vertical distribution and radiative forcing of ambient aerosol over the Yangtze River Delta during 2013-2015.

As the central part of eastern China, the Yangtze River Delta (YRD) region, with its rapid economic growth and industrial expansion, has experienced severe air quality issues. In this study, the monthly variation and interaction between aerosol direct radiative forcing (ADRF) and aerosol vertical structure during 2013-2015 over the YRD were investigated using ground-based observations from a Micro Pulse Lidar (MPL) and a CE-318 sun-photometer. Combining satellite products from MODIS and CALIPSO, and reanalysis wind fields, an integrated discussion of a biomass burning episode in Hangzhou during August 2015 was conducted by applying analysis of optical properties, planetary boundary layer (PBL), spatial-temporal and vertical distributions, backward trajectories, Potential Source Contribution Function (PSCF), and Concentration Weighted Trajectory (CWT). The results reveal that a shallower PBL coincides with higher scattering extinction at low altitude, resulting in less heating to the atmosphere and radiative forcing to the surface, which in turn further depresses the PBL. In months with a deeper PBL, the extinction coefficient decreases rapidly with altitude, showing stronger atmospheric heating effects and ADRF to the surface, facilitating the turbulence and vertical diffusion of aerosol particles, which further reduces the extinction and raises the PBL. Because of the hygroscopic growth facilitated by high relative humidity, June stands out for its high scattering extinction coefficient and relatively low PBL, and the reduced ADRF at the surface and the enhanced cooling effect on near-surface layer in turn depresses the PBL. Absorptive aerosols transported from biomass burning events located in Zhejiang, Jiangxi, and Taiwan provinces at 1.5 km, result in high ADRF efficiency for atmospheric heating. And the enhanced heating effect on near-surface layer caused by absorptive particles facilitates PBL development in August over the YRD.

Importance of aerosol non-sphericity in estimating aerosol radiative forcing in Indo-Gangetic Basin.

Aerosols are usually presumed spherical in shape while estimating the direct radiative forcing (DRF) using observations or in the models. In the Indo-Gangetic Basin (IGB), a regional aerosol hotspot where dust is a major aerosol species and has been observed to be non-spherical in shape, it is important to test the validity of this assumption. We address this issue using measured chemical composition at megacity Delhi, a representative site of the western IGB. Based on the observation, we choose three non-spherical shapes - spheroid, cylinder and chebyshev, and compute their optical properties. Non-spherical dust enhances aerosol extinction coefficient (ßext) and single scattering albedo (SSA) at visible wavelengths by >0.05km-1 and >0.04 respectively, while it decreases asymmetry parameter (g) by ~0.1. Accounting non-sphericity leads top-of-the-atmosphere (TOA) dust DRF to more cooling due to enhanced backscattering and increases surface dimming due to enhanced ßext. Outgoing shortwave flux at TOA increases by up to 3.3% for composite aerosols with non-spherical dust externally mixed with other spherical species. Our results show that while non-sphericity needs to be accounted for, choice of shape may not be important in estimating aerosol DRF in the IGB.