The effect of black carbon on aerosol-boundary layer feedback: Potential implications for Beijing haze episodes

Beijing suffers from poor air quality particularly during wintertime haze episodes when concentrations of PM2.5 can peak at > 400 ug/m. Black carbon (BC), an aerosol which strongly absorbs solar radiation can make up to 10 % of PM2.5 in Beijing. Black carbon is of interest due to its climatic and health impacts. Black carbon has also been found to impact planetary boundary layer (PBL) meteorology. Through interacting with radiation and altering the thermal profile of the lower atmosphere, BC can 5 either suppress or enhance PBL development to various degrees depending on the properties and altitude of the BC layer. Previous research assessing the impact of BC on PBL meteorology has been investigated through the use of regional models which are limited both by resolution and the chosen boundary layer schemes. In this work, we apply a high resolution coupled large eddy simulation-aerosol-radiation model (UCLALES-SALSA) to quantify the impact of black carbon at different altitudes on PBL dynamics using conditions from a specific haze episode which occurred from 1st-4th Dec 2016 in Beijing. 10 Results presented in this paper quantify the heating rate of BC at various altitudes to be between 0.01 and 0.016 K/h per μg/m of BC, increasing with altitude but decreasing across the PBL. Through utilising a high resolution model which explicitly calculates turbulent dynamics, this paper showcases the impact of BC on PBL dynamics both within and above the PBL. These results show that BC within the PBL increases maximum PBL height by 0.4 % but that the same loading of BC above the PBL can suppress PBL height by 6.5 %. Furthermore, when BC is present throughout the column the impact of BC suppressing PBL 15 development is further maximised, with BC causing a 17 % decrease in maximum PBL height compared to only scattering aerosols. Combining these results in this paper, we present a mechanism through which BC may play a prominent role in the intensity and longevity of Beijing’s pollution episodes.

development without the parameterisation of boundary layer processes which is necessary in regional models such as WRF-CHEM. This gives them a significant advantage in understanding and quantifying perturbations to the PBL. LES models have been used to examine the effect of absorbing aerosol layers on the development of stratocumulus and cumulus clouds. Herbert 80 et al. (2020) examined the effect of an absorbing layer on stratocumulus clouds and thus the PBL development and rates of entrainment, finding a significant reduction in the entrainment rate the closer the absorbing layer was to cloud top. Related to dissipation of radiation fog, Maalick et al. (2016) found that in a polluted conditions BC has a warming effect close to the fog top, and BC enhances fog dissipation due to absorption of solar radiation. However, if the increase in BC concentration is accompanied with an increase in CCN and thus fog droplet concentration, the CCN effect increasing fog lifetime is much 85 stronger than BC effect that shortens fog lifetime.
In this work, we use the coupled LES-aerosol-radiation model (UCLALES-SALSA), which has previously been set up and tested in Beijing, to examine the impact of BC on aerosol-PBL interactions and the implication on Beijing haze episodes (Slater et al., 2020(Slater et al., , 2021. The high resolution of LES models and their ability to calculate turbulent fluxes and perturbations thereof, allows for isolation and quantification of the different factors impacting the 'dome effect' of BC. We use meteorological 90 conditions from 2 days in the middle of a haze episode (2nd and 3rd Dec 2016), where a strong temperature inversion and shallow PBL already exist due to the convergence of cold northerly air masses with southerly warm air masses (Wang et al., 2019). Specifically, this work investigates the 'dome effect' of BC, through isolating the impact of BC both above and within the PBL and the impact on PBL dynamics. This paper is set out as follows. Section 2 describes model set up, including experimental setup for the different sensitivities examined, while section 3 details the results for the corresponding sensitivity 95 studies. Section 4 briefly discusses the overall results and their implications in more detail.

Model Description
The model used in this study is UCLALES-SALSA, which is a large eddy simulation model (UCLALES) fully coupled to the sectional aerosol model (SALSA). UCLALES-SALSA has been used to examine the impact of aerosols on stratocumulus 100 clouds, radiation fog, cloud seeding and to examine the aerosol-PBL feedback in Beijing (Tonttila et al., 2017;Slater et al., 2020;Tonttila et al., 2021;Slater et al., 2021). UCLALES is based on the Smagorinsky-Lilly subgrid model, with leapfrog time stepping used for advection of momentum variables and forward time stepping for advection of scalar variables, based on fourth order differential equations. Boundary conditions are doubly periodic in the horizontal and fixed in the vertical. The surface scheme is based on a coupled soil moisture and surface temperature scheme by Ács et al. (1991), which explicitly 105 calculates surface temperature and, sensible and latent heat fluxes. The surface scheme used in this case has been adapted and tested for the urban environment of Beijing and the set up is as detailed in Slater et al. (2020).
SALSA is a sectional aerosol model which has been fully coupled to both UCLALES and the climate model ECHAM (Kokkola et al., 2008(Kokkola et al., , 2018. SALSA bins aerosol particles according to size and has two sets of parallel size bins which allow Processes including deposition of aerosols, semi-volatile condensation, nucleation and emissions are switched off but aerosol coagulation and water condensation on aerosol particles are turned on. For these simulations, organic carbon, sulphate, nitrate, black carbon and ammonium are included. We use the same size distribution for all simulations (Table 1) and composition is varied slightly to examine the impact of fractional composition changes of BC (Table 2).
To calculate aerosol-radiation interactions, SALSA uses a four stream radiative transfer scheme based on the work by Fu 115 and Liou (1993). This scheme is fully coupled to UCLALES to allow feed back on turbulent dynamics and is a four stream method integrating over 6 SW bands and 12 LW bands. To account for the impact of size on aerosol-radiation interactions we use set refractive indices, and use look up tables for the aerosol-extinction cross section, asymmetry parameter and singlescattering albedo, which are calculated as a function of the size parameter (α = Dp λ ). In this work we set all imaginary parts of the refractive indices in the SW to zero apart from BC which is set to values according to Bond and Bergstrom (2006). This 120 allows us to consider BC as the only absorbing aerosol. SALSA treats internal mixing for optical properties simply, through volume averaging of the complex refractive index of each component in each particle. Optical properties of the entire particle are calculated from the average refractive index of the particle according to volume as detailed in Jacobson (2005). Therefore, the potential of scattering aerosols to enhance absorption of BC through the 'lensing effect' is not considered here.

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The work presented here is divided into three sections and specific set up for each sensitivity is detailed in the appropriate section. We perform simulations for 2nd and 3rd Dec 2016 in Beijing, varying the altitude of the aerosol layers and fractional composition. Initial meteorological conditions were taken from radiosonde profiles. Aerosol composition and size parameters were calculated based on ground based measurements taken at 8am on 3rd Dec 2016 during the APHH winter field campaign (Shi et. al 2019). The initial aerosol vertical profiles were estimated based on the gradient of boundary layer profiles. The  Table 2. For all results, PBL top or maximum PBL height is taken as the height at which there is a maximum gradient in potential temperature (θ).

Parameter
Mode 1  are 3 different altitudes for an aerosol layer and each layer either has a composition of 10 % BC or no BC. In these cases, the aerosols are only present within the specified layer, with no aerosols present initially above or below the layer. Section 2.4 outlines the setup for case 2 simulations where we include a surface aerosol layer only for simulations of 02 Dec and 03 Dec. This case compares the impact of BC heating within the PBL on boundary layer meteorology for the different initial 140 meteorological conditions. Section 2.3 describes the setup for case 3 simulations which vary the fraction of BC in vertical layers within a full column aerosol profile.

Case 1 -Vertically varied aerosol loading
To isolate the impact of BC and the altitude of the BC layer on the aerosol-PBL feedback, we performed sensitivity studies with meteorological conditions taken from measurements on the morning of 3rd Dec 2016. In the simulations presented in 145 this section, we varied the altitude of the aerosol layer as well as the fractional composition of the aerosols, to either have BC or no BC within the layer (Table 2). We included aerosol layers with identical mass mixing ratios within the PBL (0-350 m), at and above PBL top (500-950 m) and high aloft (700-1150 m) as shown in figure 2 (red, blue and cyan lines respectively).
Maximum PBL height (top) was considered to be 510 m based on simulations performed on this day without aerosols. For all simulations in this section, total aerosol loading was kept constant, while the composition was varied as detailed in Table 2

Case 2-Varying initial conditions
To examine how sensitive BC heating at the surface is to the initial meteorological conditions, particularly the strength of 160 temperature inversions both in the morning and throughout the day, we included an aerosol layer with BC at the surface for simulations on 2nd Dec and compared the results to simulations performed on 3rd Dec. We then compared each simulation to a base case, which was simulations not including aerosol-radiation interactions. The difference in initial meteorological conditions are outlined in figure 3. We can see that on the 2nd Dec, relative humidity is lower, while surface wind speeds are higher and the total temperature inversion throughout the profile is weaker than on 3rd Dec. Furthermore, initial conditions 165 show that at 9am there is a shallow layer with a sharp inversion at the surface in the 2nd Dec initial profile. However, the free tropospheric lapse rates for 3rd Dec is more stable, which may also impact PBL growth throughout the day. The aim of this case study was to examine the influence of these initial conditions on BC heating within the PBL and the associated impact on PBL dynamics.
For these simulations, the aerosol profiles for each day were kept the same so the only variable was the different initial For further analysis, we calculates SW heating rate by BC as the change in SW radiative flux (↓SW -↑SW) divided by specific heat capacity of air (C p ) multiplied by density of air (ρ) as in equation 1.

SW HeatingRate
We calculate that the heating rate of BC varied between 0.1-0.2 K/h which will lead to a maximum heating of the PBL throughout the day in wintertime Beijing of 1.6 -2 K. If the temperature inversion, during the day was small (1-3 K), this additional heating by BC within the PBL and at PBL top of 1.6 -2 K could break the inversion at PBL top and increase PBL height. However, under a strong temperature inversion as on 03 Dec, this heating within the PBL was not strong enough to 205 reduce the temperature inversion fully and so there was a very small increase in PBL height. Consequently, BC heating within the PBL in this case only resulted in a very small increase in PBL height (Table 3). Furthermore, the low albedo (0.2) and high heat capacity of the underlying surface, typical of an urban environment, mean that BC at the surface will have a lower impact than studies which have examined the effect of polluted environments over high albedo surfaces, for example, clouds or rural environments (Wang et al., 2018).   develops and the aerosol layer subsides. This results in a strong heating at the top of and above the PBL, causing a decrease in 6.7 % in PBL height compared to no aerosols. This is a decrease of 4.2 % in PBL height for the high aerosol layer (700-1150 m) compared to the low aerosol layer (500-950 m). Figure 6 shows the potential temperature lapse rate throughout the day for each of the aerosol layers. This shows the inversion for including the aerosol layer at 500 m is much stronger than for the 700 m aerosol layer. This causes the larger suppression of PBL development observed under these conditions.  (blue) and BC above 700m (cyan)

Case 2-Varied Initial Conditions
In these simulations we examined the sensitivity of BC surface heating on turbulent dynamics to different meteorological conditions. Specifically, we assessed whether BC at the surface can cause heating to a large enough extent to overcome the temperature inversion and enhance PBL development, under different initial conditions. On the 2nd Dec, simulations with no aerosols show a temperature inversion for the 400 m above PBL top of ∼ 4 K at 14:00 LST, compared to ∼ 7 K on 3rd 230 Dec ( Figure 7). Consequently, including BC at the surface shows a larger enhancement in turbulence for 2nd Dec, when the initial temperature inversion is lower. In this case, BC at the surface causes a 5% increase in PBL height, increasing TKE and minimising the decrease in sensible heat flux. This is despite the change in SW downwelling and upwelling radiation being similar for both days (Figure 8).
Due to the variation in initial meteorological conditions such as humidity, temperature and wind (Figure 3)  BC heating within the PBL under these conditions will be unlikely to promote haze dissipation due to the strength of the temperature inversion. Figure 9. Potential Temperature Lapse Rate at 12pm, 2pm and 4pm for simulations with surface aerosols (blue and green) and no aerosols (red and cyan) on 2nd Dec (cyan and green) and 3rd Dec (red and blue) Figure 9 shows the potential temperature lapse rate at 12pm, 2pm and 4pm for simulations with no aerosols and with BC at 245 the surface for 2nd Dec and 3rd Dec. This shows BC at the surface reduces the inversion at PBL top in both cases. Furthermore, at 2pm the PBL top is higher on 2nd Dec for simulations including BC at the surface. Here, compared to 3rd Dec, the heating within the PBL and at PBL top appears to be almost strong enough to break the temperature inversion at PBL top and enhance PBL development. As can be seen in figure 8 (

Case 3-Vertically varied BC
255 Section 3.1 shows the aerosol radiative forcing and perturbations due to BC are higher than the scattering effect of other aerosols. However, this case only identifies the effect of either aerosol concentrations within or above the PBL, where they can exist both within and above the PBL for several reasons. Here, we examine the idea of fully reducing BC at the surface as a proxy to decreasing BC emissions locally, where other species are still present. So for example targeting sources of BC, such as biomass burning without tackling other sources of inorganic aerosols or volatile gases. BC aloft is considered to be brought into 260 Beijing through regional transport or entrainment from a polluted residual layer. A study by Ferrero et al. (2014) suggested that the impact of local BC emissions will heat the PBL and lead to pollutant dissipation through promoting atmospheric buoyant turbulence. Results from section 3.1 show the reasonably low impact of BC at the surface in enhancing PBL development, compared to the suppression caused by the BC layer at PBL top. Furthermore, if BC from the surface gets mixed into the residual layer it will negatively impact turbulent mixing the next day. This section looks at including BC and other aerosols 265 both within and above the PBL and changing the relative BC concentration in the column.
In this section, we include aerosols throughout the column and varied the fractional aerosol composition to have BC and no BC throughout the profile and BC above 500m and 1000 m (Figure 9). This was done as a proxy to examine the impact of tackling local emissions of BC but allowing regional emissions of BC. Our results show that including BC both within and above the PBL causes a large reduction in PBL height (17 %) compared to no BC (Table 4). In section 3.1 and 3.2 simulations 270 with BC have a slightly higher PBL height compared to those without (Table 3). Therefore, the decrease in PBL height for these simulations (BC within and above the PBL) indicates that the potential enhancement in turbulence by BC within the PBL (as seen in sections 3.1 and 3.2) is eclipsed by the effect of BC above the PBL which acts strongly to prevent PBL development through the day. This is likely due to the low level of SWR available for BC heating at the surface in the full column BC simulations, due to absorption by BC at higher altitudes.    Table 4 shows that including BC has a significant impact on reducing SW downwelling and upwelling radiation, which consequently feeds back and reduces surface temperature and sensible heat flux. Simulations including BC across the entire column, show the largest decrease in downwelling and upwelling SW radiation, due to the overall larger columnar concentration of BC. Including BC at the surface (Full column BC) leads to higher air temperature at 10 m compared to simulations with BC aloft (BC 500 m and BC 1000 m) only, but lower air temperature at 10 m than not including BC at the surface (No BC). This is likely due to BC throughout the column absorbing radiation, which leads to heating but also reduces the amount of SW radiation reaching the air at the surface, consequently reducing surfac air temperature. This work shows that any increase in PBL height due to BC at the surface is outweighed compared to the stronger impact of BC above and at PBL top (BC 500 m), which results in the largest decrease in PBL height for simulation with BC across the column. Figure 10. SW heating rate per unit mass of BC at 10am, 12pm and 2pm on 3rd Dec for simulations with BC throughout the column Figure 10 shows the BC heating rate per unit mass of BC, taken as the heating rate for column aerosol with BC -without 285 BC. This shows the heating rate per unit mass of BC increases with height as suggested by Wang et al. (2018). Firstly, we see a strong heating effect increasing up to the bottom of the PBL with a decrease in heating rate across the PBL, and a further constant increase above the PBL. The larger heating rate of BC at higher altitudes is thought to be due to the higher incident radiation available for BC absorption. Consequently, the heating caused by BC in the atmosphere and the effect on PBL development will be dependent on the altitude of the BC layer as well as the total BC within the aerosol column. This may 290 be important when examining the impact of BC within the PBL as if BC also exists aloft, as there will be less SW radiation reaching the surface due to BC at higher altitudes and consequently as shown here, BC heating in the lower layers will be smaller.
The results here show that BC causes heating in the atmosphere, and that absorption of solar radiation by BC has the largest 295 impact on the temperature profile of the PBL compared to the effect of scattering aerosols. Specifically, BC can cause surface cooling through reducing SW radiation reaching the surface. In this study, BC causes heating in the aerosol layer at a rate of around 0.01-0.016 K h −1 /µgm −3 of BC (Figure 10) , which is similar to that proposed by Ding et. al (2016) and Wang et. al (2018). For the concentrations used in this study, this works out at an overall heating of around 0. are included throughout the column, we observe an enhanced effect on PBL suppression, with a decrease of 16 % compared to not including BC and only slightly less effect than having BC throughout the entire column (Section 3.3). When there are aerosols throughout the column, BC at the surface will receive less SW downwelling radiation compared to BC aloft due to the interactions of the aerosols above it preventing SW downwelling reaching lower levels. Consequently, there will be more SW radiation available for the BC aloft to absorb and heat the atmosphere. Figure 10 shows that the heating rate per unit mass of from 01 -04 Dec 2016 and the meteorological and synoptic condtions are detailed in the paper by Wang et al. (2019). In their work, they suggest the strong temperature inversion on the 3rd Dec is due to both the impact of synoptic conditions and the aerosol-PBL feedback from the previous day causing surface cooling. Overall, we find that surface BC causes warming and 330 enhances turbulence. This increases PBL height by 0.26 % on 3rd Dec due to the strong initial temperature inversion (7.0 K in the lowest 500m at 10am) but increases PBL height by 5 % on 2nd Dec due to the weaker temperature inversion (4.2 K in the lowest 500 m at 10am). However in these conditions, the heating rate is still not enough to fully weaken the strong temperature inversion (Figure 9). this polluted absorbing layer will heat the layer above the PBL, thus changing the temperature profile of the PBL to reduce buoyancy. This reduces PBL height and enhances the aerosol-PBL feedback to increase surface PM 2.5 and intensify pollution episodes ( Figure 11). Our results show that BC above the PBL has more impact than BC below and consequently if BC is present throughout the column, the effect of suppressing turbulent motion by BC is greater than the enhancement effect.
In performing simulations including BC throughout the column (at multiple layers) this work can directly show that the 350 impact of BC heating within the PBL is negated by the stronger impact of BC aloft, which absorbs a significant proportion of SW solar radiation, meaning less absorption of SW radiation by BC within the PBL or at lower altitudes. This work therefore adds on to the studies by Ding et al. (2016), which only shows the effect of BC at and above PBL top, and Wang et al. (2018) which examines the impact of BC layers at different altitudes separately rather than the effect of multiple BC layers. Our work also shows the importance of initial conditions on the BC surface heating effect as outlined for aerosols in general by Slater 355 et al. (2020) and Slater et al. (2021). This is important as these conditions change over the course of the haze episode, with PBL height found to decrease by as much as 50 % due to synoptic influences alone (Wang et al., 2019;Slater et al., 2021).
In the work by Wang et al. (2018) only one set of meteorological conditions are examined, which limits the applicability of the results to periods with similar conditions. While our work shows that conditions on 02 Dec lead to a PBL enhancement of 5 %, compared to 0.4 % on 03 Dec. Combining all the results presented in this paper as well as other research by Wang Beijing (Figure 11). Although this mechanism has not been fully tested in this work due to computational cost, we hypothesise that locally emitted BC which heats the PBL could promote PBL development (section 3.2), resulting in the BC becoming well mixed through the PBL. When the PBL collapses overnight, the BC will remain in the residual layer overnight and exist above the PBL the next day. This would then suppress PBL development as shown in section 3.1 and 3.3 of this work and in 365 the work by Ding et al. (2016). However, if synoptic conditions on the next day changed to weaken the temperature inversion and the PBL developed, as observed during this haze episode by Wang et al. (2019), the BC aloft could become entrained into the PBL to heat the surface layer and help promote buoyant turbulence and the dissipation of pollutants ( Figure 11). This mechanism could have strong influences for policy and we would therefore recommend that further research be performed to directly investigate the mechanism and its potential to influence the severity and longevity of haze episodes.

Conclusions
From this work, we suggest: a) The impact of BC aloft on PBL suppression is dependent on the altitude of the aerosol layer in relation to PBL height, b) BC surface heating impact on PBL development is dependent on the strength of the initial