Reduced light absorption of black carbon (BC) and its influence on BC-boundary-layer interactions during “APEC Blue”

Light absorption and radiative forcing of black carbon (BC) is influenced by both BC itself and its interactions with other aerosol chemical compositions. Although the changes in BC concentrations in response to emission reduction measures have been well documented, the influence of emission reductions on the light absorption properties of BC and its 25 influence on BC-boundary-layer interactions has been less explored. In this study, we used the online coupled WRF-Chem model to examine how emission control measures during APEC affect the mixing state/light absorption of BC, and the associated implications for BC-PBL interactions. We found that both the mass concentration of BC and the BC coating materials declined during the APEC week, which reduced the light absorption and light absorption enhancement (Eab) of BC. The reduced absorption aerosol optical depth (AAOD) during APEC were caused by both the declines in mass 30 concentration of BC itself (52.0%), and the lensing effect of BC (48.0%). The reductions in coating materials (39.4%) dominated the influence of lensing effect, and the reduced light absorption capability (Eab) contributed 3.2% to the total reductions in AAOD. Reduced light absorption of BC due to emission control during APEC enhanced planetary boundary layer height (PBLH) by 8.2 m. Different responses of PM2.5 and O3 were found to the changes in light absorption of BC. Reduced light absorption of BC due to emission reductions decreased near surface PM2.5 concentrations but enhanced near 35 surface O3 concentrations in the North China Plain. These results suggest that current measures to control SO2, NOx, etc. would be efficient to reduce the absorption enhancement of BC, and to inhibit the feedback of BC on boundary layer. Yet https://doi.org/10.5194/acp-2021-170 Preprint. Discussion started: 17 March 2021 c © Author(s) 2021. CC BY 4.0 License.

enhanced ground O3 might be a side effect of current emission control strategies. How to control emissions to offset this side effect of current emission control measures on O3 should be an area of further focus.

Introduction
Black carbon (BC) in the atmosphere is produced both naturally and by human activities, attributable to the incomplete combustion of hydrocarbons (Bond et al., 2013;Ramanathan and Carmichael, 2008). In addition to contributing considerably to particulate matter and degraded air quality, it is the dominant absorber of visible solar radiation, playing a unique and pivotal role in the Earth's climate system (Bond et al., 2013;Menon et al., 2002;Ramanathan and Carmichael, 45 2008;Yang et al., 2019). The absorption of BC occurs not only in the atmosphere, but when it is deposited over snow or ice, it triggers positive feedbacks and exert a positive radiative forcing (Flanner et la., 2007;Grieshop et al., 2009). The direct radiative forcing of atmospheric black carbon was estimated to be 0.4 W m -2 (0.05-0.8 W m -2 ) (IPCC, 2014), and BC has been targeted in emission control policies to mitigate both air pollution and global warming (Grieshop et al., 2009).
Before the 1950s, intense emissions of BC were concentrated in North America and Western Europe. In recent 50 decades, South and East Asia have emerged to become major source regions (Ramanathan and Carmichael, 2008). BC emitted from China is responsible for a quarter of the total global emissions (Bond et al., 2004). Chemical transport model simulations suggest that the residential sector is the leading source for mass concentration of BC in China, followed by the industrial sector (Li et al., 2016). Mean BC direct radiative forcing in China is ~1.22 W m -2 , more than three times the global mean forcing (Li et al., 2016), with two-thirds to three fourths of which contributed by local emissions of BC in China, and 55 the rest by emissions in other countries (Li et al., 2016;Yang et al., 2017).
Specific policies to address BC emissions have not been implemented in China, yet multiple measures targeting PM2.5 reduction have resulted in declines in BC (Gao et al., 2018b;Yamineva and Liu, 2019). A number of observational studies have revealed the declining trend of BC concentrations in China in recent years (Ji et al., 2018(Ji et al., , 2019a(Ji et al., , 2019bQin et al., 2019). From 2013 to 2018, the annual mean BC concentrations in Beijing declined from 4.0 µg m -3 to 2.6 µg m -3 (Ji et 60 al., 2019b). Associated changes in BC radiative forcing can be expected from declines in mass concentration of BC in China, while the radiative forcing of BC is influenced also by the changes in other aerosol components.
BC absorption is closely connected with the aging process, which is defined as the interaction between BC and other aerosol chemical compositions (Jacobson, 2001). After being emitted from combustion processes, BC particles can coagulate and grow by condensation, during which both self-coagulation and hetero-coagulation happen (Jacobson, 2001). 65 Although BC is mixed internally with other components, the system is impossible to be well-mixed due to the irregular shape of BC (Jacobson, 2000). A core-shell morphology is commonly established, with BC as the core and the coating materials (organics, sulfate, etc.) as the shell (Jacobson, 2001;Zhang et al., 2018). Numerous efforts have been made to explore the influence of aerosol components on internally mixed BC absorption (Cappa et al., 2012;Chen et al., 2021;Bond https://doi.org/10.5194/acp-2021-170 Preprint. Fuller et al., 1999;Jacobson, 2001;Liu et al., 2017;Peng et al., 2016). It was proposed that the coating 70 components (shell) could act as a lens to focus more photons onto the core to enhance the light absorption of BC (Fuller et al., 1999). Bond et al. (2006) estimated that this lensing effect would increase the light absorption of BC by 50-100%. Jacobson (2001) reported a global average BC absorption enhancement factor of 2, whereas other values, from negligible (Cappa et al., 2012) to as high as 2.4 (Peng et al., 2016) were also found previously. This lensing effect has been recognized also as an important factor affecting radiative forcing of BC (Jacobson, 2001). 75 Over the past several years, the State Council of China has issued a comprehensive Air Pollution Prevention and Control Action Plan (APPCAP), covering major emission sectors (Zhang et al., 2019a). Long-term observations of aerosol chemical composition indicate that both concentrations of BC and other coating components have declined rapidly (Gao et al., 2020b;Ji et al., 2019b;Zhou et al., 2019). Although the changes in BC concentrations in response to emission reduction measures have been documented (Ji et al., 2019b;Gao et al., 2020b), the influence of emission reductions on the aging 80 processes and light absorption of BC has been less explored (Zhang et al., 2019b). Zhang et al. (2018) observed that the declines in absorption of BC was mainly dominated by decreases in BC mass concentration (86%), and the weakening of BC light absorption capability also played a role (14%). However, this finding was formulated based on surface observations, little is known about the changes at upper layers. Given the importance of BC absorption in the upper boundary layer as to buildup of pollution (Ding et al., 2016), the impact of emission reductions on the light absorption of BC, and its implications 85 for the development of boundary layer and pollution episodes need further investigations. On November 5-11, 2014, Beijing, China hosted the Asia-Pacific Economic Cooperation (APEC) meeting, during which Beijing and surrounding regions cooperated to implement short-term emission control measures to ensure good air quality. This event offers a great opportunity to study physical and chemical responses of atmospheric composition to emission reductions.
In this study, we address the following questions using the APEC event as a study case: (1) how did emission 90 reductions affect the aging processes and light absorption of BC during APEC; (2) what were the relative contributions of reduced mass concentrations of BC, aging processes of BC, and reshaped mixing state of BC to the changes in light absorption of BC during APEC; and (3) how did these processes affect BC-PBL interactions and formation of air pollution?
In Sect. 2, we describe the WRF-Chem model configurations and observational datasets used in this study. Results are presented in Sect. 3, and conclusions/discussions are provided in Sect. 4. 95 2 Methods and data 2.1 WRF-Chem model configuration WRF-Chem model (Grell et al., 2005) version 3.8.1 was adopted in this study to simulate emission, chemical transformation and deposition of aerosols, as well as their interactions with radiation. We demonstrated in previous studies 100 https://doi.org/10.5194/acp-2021-170 Preprint. Discussion started: 17 March 2021 c Author(s) 2021. CC BY 4.0 License. (Gao et al., 2016a(Gao et al., , 2016b(Gao et al., , 2020b(Gao et al., , 2020c) that the spatio-temporal variations of air pollutants over China could be reproduced well by WRF-Chem. WRF-Chem enables multiple options for gas phase chemistry and aerosol modules (Grell et al., 2005). We employed the Carbon Bond Mechanism version Z (CBMZ) gas phase chemistry (Zaveri and Peters, 1999) coupled with the Model for Simulating Aerosol Interactions and Chemistry (MOSAIC) (Zaveri et al., 2008) aerosol module in this study. MOSAIC treats size resolved aerosol species, and we used 8 bins version in this study, corresponding to the 105 particle diameter ranges of 0.039-0.078, 0.078-0.156, 0.156-0.312, 0.312-0.625, 0.625-1.25, 1.25-2.5, 2.5-5.0, 5.0-10.0 µm, respectively. Secondary organic aerosol (SOA) formation in MOSAIC was simulated with volatility basis set (VBS) (Shrivastava et al., 2011). We configured two nested domains with horizontal resolutions of 81km and 27km, and 31 vertical layers up to a pressure level of 50hPa. The configured domains cover most areas of East Asia and focus on the North China region (same as Figure 1 in Gao et al., 2017). Other chosen options for key physical parameterizations follow Gao et al. 110 (2016b). Meteorological initial and boundary conditions were provided by the NCEP 1°×1° degree final reanalysis dataset (FNL), and chemical initial and boundary conditions were obtained from the MOZART global chemistry simulations (Emmons et al., 2010). To allow the effects of aerosol on meteorological conditions in the model, we did not apply observational nudging or reanalysis nudging.
Anthropogenic emissions of particles and gases in China in the model were taken from the Multi-resolution 115 Emission Inventory for China (MEIC) for year 2014 developed by Tsinghua University . Anthropogenic emissions for areas outside China were obtained from the MIX Asian emission inventory developed for MICS-Asia and HTAP, which combines five emission inventories for Asia (Li et al., 2017). Both MEIC and MIX datasets provide monthly emissions of air pollutants at 0.25°×0.25° grids, which were interpolated to WRF-Chem modeling domains in this study. We adopted the MEGAN model version 2.04 to estimate biogenic emissions of gases and particles online (Guenther et al., 120 2006). The Global Fire Emissions Database version 4 (GFEDv4) (Giglio et al., 2013) were used as open fire emissions.
We simulated the period from October 16 to November 13, and discarded the first seven days as spin-up to avoid the influences of initial conditions. To explore the influences of coordinated emission control measures on BC absorption, we conducted multiple sets of simulations, as described in Table 1. For the NOCTL experiments, simulations were conducted with no perturbations in emissions. For the CTL experiments, emissions of SO2, NOx, PM10, PM2.5, VOCs, and 125 other species in Beijing were reduced by 39.2%, 49.6%, 66.6%, 61.6%, 33.6%, and 50%, respectively, over November 3-11 period. Emissions in Inner Mongolia, Shanxi, Hebei, Tianjin, and Shandong were reduced by 35%. These perturbation factors were taken from the BMEPB reports (Gao et al., 2017). The locations of these provinces are marked in Figure 1 in Gao et al. (2017).
The influences of BC absorption under different assumptions, including external/core-shell mixing and with/without 130 , can be derived with equations (1-5) below. The description of each simulation is documented in Table 1.
The influences of emission reductions during APEC on changes in light absorption of BC and associated changes in meteorological/pollution conditions under external/core-shell mixing assumptions (∆ − and ∆ − ) can be inferred with equations (6-7) below. We use equation (8) to derive the impact of changed BC aging processes by comparing 140 the differences between core-shell simulation and external mixing simulation. The influences of reduced coating due to emission control measures during APEC are calculated with equation (9). We use equation (10) to derive the influences of changes in light absorption enhancement ( ) of BC.

Calculation of aerosol optical properties in WRF-Chem 150
WRF-Chem uses Mie theory to calculate layer aerosol optical depth (AOD), single scattering albedo (SSA), and asymmetry factor (g). First, the size parameter and spectral refractive index are used to calculate the Mie extinction efficiency . Then, the extinction coefficient (λ) is provided by the integral of with consideration of the geometric size of the particle (π 2 ) and the particle number size distribution n(r) (equation (11)). (λ) is a equation of wavelength λ.
Similarly, absorption coefficient (λ) and scattering coefficient (λ) can be obtained with Mie absorption efficiency 155 and Mie scattering efficiency . The value of SSA can be calculated with equation (12) using (λ) and (λ).
The calculated optical properties vary with the assumption of mixing state of aerosols. For external mixing, each particle is assumed to be a single chemical species. There are several models proposed for internal mixing, and the 160 commonly used ones include the volume averaging model and core-shell model. In the volume averaging model, all species are assumed to be well mixed, while the core-shell model assumes that BC is coated by a well-mixed shell of other species (Jacobson, 2001). The volume-weighted refractive index is obtained with the equation below: In equation (13), denotes the volume of species and represents the refractive index of species . The official 165 version of WRF-Chem does not calculate optical properties of aerosols with external mixing assumption. To assess the influence of aging process on the light absorption of BC, estimated light absorption of BC with external mixing assumption is required. We modified the optical calculation module in WRF-Chem so that it does not mix BC with other chemical species in the calculation of optical properties, which can be used as optical properties of BC with external mixing assumption. 170

Observations
Both observations of meteorological variables and air pollutants were used to evaluate the performance of model over the APEC study period in Gao et al. (2017) and in this study. The meteorological measurements were retrieved from the National Centers for Environmental Information website (https://gis.ncdc.noaa.gov/maps/ncei#app=cdo), which includes 175 near surface temperature, relative humidity (RH), wind speed, and wind direction. The hourly surface concentrations of PM2.5 and daily PM2.5 chemical compositions were measured at the Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences (CAS) site (Liu et al., 2017;Yang et al., 2020). We obtained also AAOD (absorption aerosol optical depth) from the AERONET network (Dubovik and King, 2000;Holben et al., 1998) to evaluate model performance. Data from more than 500 sites across the world are provided online at the AERONET website (http://aeronet.gsfc.nasa.gov). 180

Model Evaluation
Model evaluation was conducted with surface observations of meteorological variables, PM2.5, PM2.5 chemical components, and AAOD. Data at two meteorological sites in urban Beijing were averaged, and were compared against the 185 model values for the domain grid cell containing the monitoring site. Figure 1 indicates that the daily mean temperature and relative humidity (RH) are captured well by the model. Observed strong wind conditions are slightly underestimated, which is a common issue due to inaccurate land use inputs or other problems in the model (Gao et al., 2018a). Our previous investigation (Gao et al., 2017) suggested that temperature and RH were lower and northerly winds became more frequent from before APEC to during APEC periods, contributing to pleasant air quality. to -4.0 µg m -3 . The performance of WRF-Chem in simulating wintertime PM2.5 chemical compositions was explored extensively in our previous investigations (Gao et al., 2016b(Gao et al., , 2018a. Similarly, measured high concentrations of inorganic 195 aerosols (sulfate, nitrate and ammonium) are underestimated, which could be partly due to missing sulfate formation pathways (Cheng et al. 2016). We used the updated version with heterogeneous sulfate formation (Gao et al., 2016a) to reduce the underestimation of sulfate in this study. Simulated BC concentration shows high degree of consistency with observations, while OC is slightly underestimated due to large uncertainties in current status of SOA modeling (Figure 2f).
In general, the temporal variations and magnitudes of air pollutants are well represented in our model. Figure 1(d) compares 200 simulated AAOD with external mixing assumption and core-shell model against AERONET inferred AAOD during the APEC study period. AAOD simulated with external mixing assumption exhibits much lower values than observation. With the core-shell model, this underestimation is largely reduced. However, AAOD is still underestimated by the model, which might be caused by missing sources of absorbing particles in the model. Currently, the absorption of organics is not treated in the WRF-Chem model, which is likely to underestimate the light-absorbing capability of carbonaceous aerosols in the 205 atmosphere (Andreae and Gelencser, 2006). Uncertainties in the aerosol size distribution in emissions may also contribute to this mismatch between the model and observations (Matsui, 2006).

Reductions in the concentrations of BC/coating pollutants and changes in BC aging degree
Previously, the reductions of air pollutants were estimated by comparing concentrations of air pollutants during the 210 APEC period with those during other periods. Given the differences in meteorological conditions, such a comparison is not able to indicate the influence of emission control measures. As displayed in Figure 2(a), the concentrations during October 24-25 can be two times of those during October 26-27, although no emission reduction measures were implemented.
Previously, we concluded that the meteorological conditions during the APEC week were generally favorable for good air quality compared to it during the week before the APEC week (Gao et al., 2017). Thus, we perturbated emissions in this 215 study to examine how it would affect concentrations of air pollutants, including both BC and its coating pollutants. As displayed in Figure 3(a-b), mean concentrations of SO2 and NO2 in urban Beijing declined by 38.7% and 36.3%, respectively, in response to short-term emission control measures. Based on observations, Zhang et al. (2018) reported that SO2 concentrations decreased by 35% (67%) and NO2 concentrations decreased by 34% (45%) compared with that before The changes in BC aging process is determined by both the decrease in BC and primary/secondary pollutants condensed on BC. We used the ratio of the sum of pollutants (primary as well as secondary) to black carbon concentrations 225 ( ) to track the changes of BC aging degree: As shown in Figure 3(c), the impacts of emission reductions during APEC on behave differently at different sizes. For ultrafine particles, emission reductions generally lower the aging degree of BC. This is consistent with the observational evidence that smaller BC cores show larger reductions in aging degree as a result of emission control measures 230 during APEC (Zhang et al., 2018). As most secondary aerosols are in smaller sizes, the effect of emission reduction on BC aging is more significant for smaller particles. Zhang et al. (2018) reported only the changes in sizes below 0.2 µm, our modeling results suggest, however, that the aging degree of BC might be enhanced under emission reductions for relatively larger particles (Figure 3c). The impact of emission reductions on behaves differently near the surface and at higher layers ( Figure 3d). The aging degree is lowered in the CTL case near the surface, mainly due to reductions in coating 235 materials. However, at layers higher than 200 meters, the aging degree of BC increases with emission reductions. In-situ near surface measurements indicate also that was reduced during APEC, and the reduction was most likely caused by lower photochemical production (Zhang et al., 2018).

Changes in AAOD and the light-absorption enhancement ( ) of BC during APEC 240
values describe the aging degree of BC, while the exploration of how emission reductions affect light absorption of BC requires a sophisticated calculation of optical properties of BC. Mie theory is commonly used to calculate the light absorption enhancement of BC (E ) from lensing effect with a core-shell model. Zhang et al. (2018) estimated E by dividing the light-absorption cross section of the whole BC-containing particle by that of BC core at a certain wavelength.
Here we follow the method in Curci et al. (2019), and calculate E as the ratio of BC AAOD estimated assuming core-shell 245 internal mixing to that calculated with external mixing assumption:  (Peng et al., 2016). Liu et al. (2017) pointed that the enhancement factors depend on the particles' mass ratio of non-black carbon to black carbon. Our  Due to emission reductions (differences between CTL and NOCTL scenarios), mean daytime BC AAOD decrease by 0.0075 during the APEC week, as a result of declines in mass concentration of BC (52.0%, Table 2). However, the lensing effect of BC induces a further decline of 0.0069 (48.0%, Table 2). The influence of lensing effect is dominated by the reductions in coating materials (39.4%, equation (9), Table 2). The BC absorption enhancement (E ) factor decreased by 0.003 due to reductions in emissions (Figure 4b). We further quantified that the reduced light absorption capability (E ) resulting from 265 emission reductions during APEC contributed 3.2% to the total reductions in AAOD (equation (10), Table 2).

Influences on boundary layer process and air pollution
The vertical distribution of BC absorption plays an important role in modulating the temperature gradient and changing boundary layer meteorology (Ding et al., 2016). We conducted a series of numerical experiments to understand the 270 influences of reshaped BC absorption due to emission reductions during APEC on boundary layer process and the formation of air pollution. Figure 4c illustrates the vertical profiles of BC absorption induced changes in equivalent potential temperature (EPT), which is commonly used to indicate the stability of air in the atmosphere (Obremski et al., 1989). When EPT decreases with height, the atmosphere is unstable and vertical motion/convection is likely to occur. In all experiments, BC absorption induces a positive impact on EPT in the air above ground acting to enhance the stability of the atmosphere 275 ( Figure 4c). The maximum enhancement occurs at layers close to 1-2km (Figure 4c). At ~2.6km, the maximum ratio of changes with core-shell model to those with external mixing reach above 2.5, indicating the important effects of mixing state of BC in the upper boundary layer (Figure 4c).
In urban Beijing, BC absorption induced mean changes of daytime planetary boundary layer height (PBLH) during the APEC week are -11.6 and -24.0 m for external mixing and core-shell model, respectively (Figure 4d). Under a relatively 280 clean condition (CTL scenarios), these values change to -8.8 and -15.6 m for external mixing and core-shell model for NOCTL emissions (Figure 4d). Due to emission reductions, the impacts of BC absorption on PBL inhibition decrease by 8.2 m (reduced emissions enhance PBLH by 8.2 m). The influences of reduced mass concentration of BC itself account for 35% of the total changes, while the lensing effect of BC explain the rest (65%, Table 2). The decreased coating due to emission reductions dominate the lensing effect of BC (47.4%, Table 2).
The corresponding changes in daytime mean near surface concentrations of O3 and PM2.5 in Beijing are displayed in Figure   4e and Figure 4f, respectively. The inhibited development of PBL due to BC absorption results in higher abundance of PM2.5 within the PBL (Ding et al., 2016;Gao et al., 2016b). Previously, we quantified that the co-benefits of reduced aerosol feedbacks could explain ~11% of the total decreases in PM2.5 in Beijing during APEC. Here we focus on light absorption of BC, and find that the lensing effect of BC decreases PM2.5 concentration by 0.8 µg m -3 on average (Figure 4f). On average, 290 declines in BC mass concentration itself account for 64.3% of the total impact of reduced light absorption of BC on PM2.5, while 35.7% is attributed to the lensing effect of BC. However, inhibited PBL development does not necessarily lead to enhanced levels of near surface O3, as the formation of O3 is also affected by changes in aerosols and photolysis reactions above the ground. As displayed in Figure  The responses of O3 to reduced light absorption of BC during APEC are in the opposite direction (Gao et al., 2018c), compared to those for PM2.5. Strong absorption of BC tends to enhance photolysis above the aerosol layer, but to reduce photolysis near the ground. Figure 7d, 7g illustrate the changes in O31D and NO2 photolysis rates with emission reductions 310 inferred from an external mixing assumption. With emission control implemented, photolysis rates near the ground are enhanced due to lower light absorption of BC, while the photolysis rates above the aerosol layer are reduced. Similar patterns but with larger values are found using the core-shell model (Figure 7e, 7h). The responses of O3 are generally in line with the responses of O31D and NO2 photolysis rates (Figure 7a, 7b).

Summary and Discussions
In this study, we used the online coupled WRF-Chem model to understand how emission control measures during the APEC event would affect the mixing state/light absorption of BC, and the implications for BC-PBL interactions.
Multiple observations, including surface observations of meteorological variables, PM2.5, PM2.5 chemical composition, and AAOD were used to evaluate model performance. A series of numerical experiments were conducted to address three 320 questions: (1) how did emission reductions affect the aging processes and light absorption of BC during APEC; (2)  This study with perturbations of emissions during APEC offer important implications on the potential effects of 340 China's Clean Air Act. As discussed in our previous investigation (Gao et al., 2017), emission control measures have the cobenefits of reducing aerosol feedbacks to accelerate the cleaning of air, which accounts for ~11% of the decreased PM2.5 concentrations during APEC. In this study, we further clarified that the ongoing measures to control SO2, NOx, etc. would be efficient to reduce the absorption capability of BC to inhibit the feedback of BC on the boundary layer. Our results also show that near ground O3 responds differently from the changes in PM2.5, which might be a side effect of current emission control 345 strategies. Ma et al. (2021) reported that aerosol radiative effect could explain 23% of the total change in surface summertime O3 in China. How to control emissions to offset this side effect of current emission control measures on O3 should be an area of further focus. In addition to the influences on air quality and weather, a sudden reduction in aerosol emissions may potentially affect climate (Ren et al., 2020;Yang et al., 2020), which warrants further investigation.
Although careful validation was conducted in this study, uncertainties still remain in the current study. We 350 concluded that the core-shell model captures the variation of AAOD better than external mixing assumption. However, the core-shell model is an ideal scenario that assumes all non-BC materials are internally mixed and coated on BC. Zhang et al. (2016) observed that BC particles are heavily coated and are in a near-spherical shape in the North China Plain. The usage of core-shell model seems to be reasonable in this study, whereas the assumption that all non-BC materials are coated on BC might not be true in real atmosphere. The observed ratio of coatings to PM1 was ~25-70% in summer in Beijing (Xu et al., 355 2019), and the observed ratio of coatings to PM2.5 was ~10-40% in winter in Beijing . Thus, the assumption of BC coating in the model might have overestimated in this study, leading to uncertainties in the results. In the near future, we would examine how different assumptions of BC coating would affect the light absorption properties of BC. Additionally, the simulated feedbacks of BC absorption on boundary layer processes are not well constrained. We used multiple coupled models to examine how these processes are represented, and we calculated ensemble mean to obtain the 360 best current understanding (Gao et al., 2018a(Gao et al., , 2020a. In the future, further efforts are needed also to constrain the uncertainties of these processes in the model.  No perturbations in emissions; calculating optical properties using core-shell assumption.