Vertical evolution of black carbon characteristics and heating rate during a haze event in Beijing winter

Science of the Total Environment 709 (2020) 136251

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Vertical evolution of black carbon characteristics and heating rate during a haze event in Beijing winter Delong Zhao a,c,e,f, Dantong Liu b,⁎, Chenjie Yu h, Ping Tian a,c,f,⁎⁎, Dawei Hu h, Wei Zhou a, Shuo Ding b, Kang Hu b, Zhaobin Sun g, Mengyu Huang a, Yu Huang a, Yan Yang a, Fei Wang a, Jiujiang Sheng a, Quan Liu a, Shaofei Kong d, Xinming Li g, Hui He a, Deping Ding a,c,f a

Beijing Weather Modification Office, China Department of Atmospheric Sciences, School of Earth Sciences, Zhejiang University, Hangzhou, Zhejiang, China c Beijing Key Laboratory of Cloud, Precipitation and Atmospheric Water Resources, Beijing, China d Department of Atmospheric Sciences, School of Environmental Studies, China University of Geosciences (Wuhan), Wuhan, China e Nanjing University, Nanjing, China f Field Experiment Base of Cloud and Precipitation Research in North China, China Meteorological Administration, Beijing, China g Institute of Urban Meteorology, China Meteorological Administration, Beijing, China h Centre for Atmospheric Sciences, School of Earth and Environmental Sciences, University of Manchester, Manchester M13 9PL, UK b

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Successive fights were conducted to capture the evolution of BC vertical profiles. • The coatings may introduce additional heating rate as high as 0.1 K/h during hazy day. • The positive vertical gradient of heating rate may enhance temperature inversion.

a r t i c l e

i n f o

Article history: Received 5 September 2019 Received in revised form 16 December 2019 Accepted 19 December 2019 Available online xxxx Editor: Jianmin Chen Keywords:

a b s t r a c t Black carbon aerosol plays an important role on absorbing shortwave solar radiation. The absorption of BC in urban environment with intensive anthropogenic emissions may modify the atmospheric thermodynamics by heating the planetary boundary layer (PBL), however the exact impacts are still largely uncertain due to lack of in-situ observations. Here we report the detailed in-situ characterization on vertical profiles of BC-related properties including the BC mass, size distribution and mixing state over Beijing by successive flights, during which a full process of haze initialization, development and ceasing were captured and processes of BC properties during this typical haze event was in detail investigated. We found the shallow PBL and the temperature inversion importantly enhanced the BC mass loading in the polluted day and these BC particles were significantly coated with

⁎ Corresponding author. ⁎⁎ Correspondence to: P. Tian, Beijing Weather Modification Office, China. E-mail addresses: [email protected] (D. Liu), [email protected] (P. Tian).

https://doi.org/10.1016/j.scitotenv.2019.136251 0048-9697/© 2018 Published by Elsevier B.V.

2 Black carbon Vertical profile

D. Zhao et al. / Science of the Total Environment 709 (2020) 136251

mass ratio of coating over refractory BC increasing from about 1 to 10, whereas when the capping was released the BC was dispersed throughout the column and the coating was reduced. The coatings may cause the enhancement of BC absorption by 95% and introduce additional heating rate as high as 0.1 K/h during hazy day. The absorbing power efficiency and heat rate of BC showed positive vertical gradient during peak pollution, which may enhance the temperature inversion at upper level of the PBL. These results provide impacts of BC mixing state on atmospheric heating, and emphasize the importance of including BC mixing state, especially under highly polluted environment, to model the aerosol-boundary layer interaction over urban environment with high BC emission. © 2018 Published by Elsevier B.V.

1. Introduction

1.1. Instrumentation and data analysis

Black carbon (BC) emitted from incomplete combustion is the most important light absorbing component in atmospheric aerosols. BC can affect solar radiation through direct radiative impacts, by heating the atmosphere (Ramanathan and Carmichael, 2008; Ramanathan et al., 2007). China is one of the BC hotspots with intensive anthropogenic emissions (Cao et al., 2006; Song et al., 2012). From 2012 to 2018, extremely heavy haze pollution events frequently occurred in northern China during wintertime (An et al., 2019; Renhe et al., 2014). The increased pollution and high frequency of hazy days were revealed to be associated with both emission and meteorological conditions (Wang et al., 2014; Zhang et al., 2016). In particular, the aerosol interaction with planetary boundary layer (PBL) led to feedback effect between the reduced height of PBL and increase of aerosol concentration (Huang et al., 2018; Quan et al., 2013). This doming effect of PBL, i.e. by reducing the solar radiation reaching the surface, could be enhanced if considerable presence of BC in the PBL (Ding et al., 2016). This is because the heating effect of BC throughout the PBL may enhance the inversion and increase the stability of the PBL depending on the location of the BC layer in the atmospheric column. However, most of the hypothesis is model-based and lack of direct observational support. In addition, besides the mass loading, the absorption efficiency of BC is importantly determined by its mixing state (Liu et al., 2017a). It is therefore important to understand the vertical profile of these information related to BC absorption and heating effect. Previous studies found different vertical structures of aerosol absorbing properties over Beijing by in-situ measurements via balloon platform (Ran et al., 2016), and showed diurnal variations of vertical profiles modulated by boundary layer development. Using aircraft platform, the particle extinction showed seasonal variation (Liu et al., 2009). Particularly, an elevated layer over Beijing was found above the PBL when being influenced by regional transport from the polluted southern area (Ding et al., 2019; Liu et al., 2018; Tian et al., 2019a). A recent study characterized the vertical profiles of BC mass loading and mixing state in both warm and cold seasons over Beijing and found the vertical structures were influenced by both local and synoptic meteorological conditions (Zhao et al., 2019). Due to the meteorological influence combined with the terrain effect, the pollution in Beijing could be built up in less than a day (Guo et al., 2014), however the evolution of aerosol vertical profiles and how the BC could play roles in terms of atmospheric heating during the pollution aggravation process has not been evaluated. Based on aircraft in-situ characterization of BC vertical profiles of mass loading and size-resolved mixing state, we investigated the evolution of BC physical properties in vertical profiles during a heavy pollution event, and clear processes of aerosol-PBL interactions were experienced during our measurements. The contribution of BC on modifying the solar radiation during this process is in detail explored, and the results provide potential guides on lessening pollution by regulating BC emissions.

The measurements in this study were performed on the king-air350 research aircraft platform with true air speed of approximately 250–300 km/h (Liu et al., 2018; Tian et al., 2019b); (Zhao et al., 2019). The observations were from November 25th to 27th 2018 at consistent time in the day 10:00–12:00, with on 25th an additional flight in the afternoon. The flight tracks are shown in Fig. 1. The isokinetic inlet (BMI, Brechtel Manufacturing Inc.) was implemented for aircraft sampling and the sample efficiency is 95% for particle size between 0.01 and 6 μm. The operation of most flights was carried out to avoid clouds where possible, and the results presented here have been screened to remove in-cloud data, as determined by measurements of relative humidity (N90%) and cloud liquid water content (N0.001 g/m3). AircraftIntegrated Meteorological Measurement System (AIMMS-20, Aventech, Canada) was used to in-situ measure meteorological parameters, including ambient air temperature, relative humidity (RH), wind speed and wind direction. All data time resolution is 1 s. (See Table 1.) The refractory BC of each individual particle was measured by a Single Particle Soot Photometer (SP2, DMT Inc.) (Baumgardner et al., 2004; Schwarz et al., 2008). The SP2 incandescence signal was calibrated for rBC mass using the monodispersed Aquadag® black carbon particle standard (Acheson Inc.). The mass of each mobility size of Aquadag is obtained through the empirical correlation obtained by Gysel et al. (2011), and then further corrected for ambient BC with a factor of

40.3

25am Nov 25pm Nov 26 Nov 27 Nov

Latitude(°)

40.2

Shahe

40.1

40.0

Beijing 39.9

39.8

PM2.5 MPL Wind Radar Aeronet 116.20

116.30 116.40 Longtitude(°)

Fig. 1. Flight tracks from November 25th–27th, 2018. The solid circle shows the location of ground station for PM2.5 measurement, the solid square for lidar measurement, the solid triangle for wind profile radar and the open circle for AERONET site.

D. Zhao et al. / Science of the Total Environment 709 (2020) 136251

3

Table 1 Summary of flight mean ± σ for key parameters in the PBL and FT. Date

Time

RHPBL (%) PM2.5 (μg m−3) PBLH (m) BCPBL (μg m−3) MMDPBL (μm) Mcoating/MrBC_PBL BCFT (μg m−3) MMDFT (μm) Mcoating/MrBC _FT

25th Nov 25th Nov 26th Nov 27th Nov

09:09–11:59 15:50–18:41 09:06–11:17 09:46–12:41

49 18 70 11

90 ± 21 70 ± 17 217 ± 74 48 ± 6

400 490 400 1600

1.34 ± 0.36 2 ± 0.26 5.2 ± 0.1 0.07 ± 0.06

0.21 0.21 0.22 0.18

0.75 (Laborde et al., 2012). The rBC mass outside of the detection limit is obtained by a lognormal fitting on the BC core size distribution (as shown in Fig. 2, (Zhao et al., 2019) and this fraction of missing rBC mass ranged from 5 to 10% and is accounted according to the extrapolation. The coated size of BC (Dp) is obtained by applying a Mie lookup table to match the SP2 measured scattering signal with core-shell model by assuming a coating refractive index (RI) of 1.50 + 0i, and this retrieval is insensitive to the composition-dependent coating RI (Liu et al., 2014; Taylor et al., 2015). The mass ratio of coating and rBC (MR) of each particle is obtained from the Dp/Dc analysis by assuming a rBC density of 1.8 g cm−3 (Bond and Bergstrom, 2006) and coating density of 1.5 g cm−3 (Cross et al., 2007). The coating mass over rBC mass for a given time window is calculated as: 0X Mcoating =MrBC

B i ¼B @X

Dp;i 3 Dc;i

3

1 C ρcoating −1C A ρ

ð1Þ

rBC

i

where Dp,i and Dc,i denotes the coated and uncoated BC diameter for each single particle. The mass absorption cross section of BC at λ = 550 nm (MAC550) is calculated for each single particle by assuming the refractive index of rBC core 1.95 + 0.79i (Bond and Bergstrom, 2006) and the coating refractive index 1.50 + 0i (Liu et al., 2015), using the Mie core-shell approach. The calculation method of MAC is based on the MR for each particle according to (Liu et al., 2017b), the absorption enhancement (Eabs) due to coating is considered to only occur when MR N 3, no Eabs when MR b 1.5 and in a transition state when MR = 1.5–3. The

± ± ± ±

0.003 0.002 0.002 0.02

1.68 ± 0.35 4.4 ± 0.36 7.8 ± 1.02 0.55 ± 0.16

0.09 ± 0.18 0.14 ± 0.27 0.4 ± 0.7 0.03 ± 0.01

0.17 0.18 0.18 0.16

± ± ± ±

0.01 0.02 0.01 0.01

0.28 ± 0.2 0.72 ± 0.75 0.85 ± 1.42 0.5 ± 0.15

MAC550 in bulk for a given time window is then calculated as the integrated absorption coefficient (MAC × mrBC) for all particles divided by the integrated particle masses. Note that this scenario generally separates the BC-containing particles based on the MR, however at the same MR there may be a range of mixing rules or microphysical structures (Wang et al., 2017) which may lead to uncertainties in modelling optical properties (He et al., 2015; Scarnato et al., 2013). This study only gives the best estimates according to the measured MR in single particle based on the hybrid model proposed by Liu et al., 2017b, but further investigation in different mixing rules may require additional measurements which warrants future study. The locations of other instruments are shown in Fig. 1. Ground PM2.5 was measured by Grimm instrument (Aerosol Technik., Germany) at northern Shahe. Micro-pulse lidar at 532 nm (MPL-4B, Sigmaspace Co., USA) located at the southeast of Beijing to monitor the temporal evolution of vertical profile of particle extinction. A Wind-Profile-Radar (Airda-3000, Airda Co., China) was located close to Shahe airport (40.1°N,116.3°E) to measure wind profiles (marked as solid triangle in Fig. 1). The AOD from AERONET network was from the CMA (China Meteorological Administration) site (marked as black hollow circle in Fig. 1). The daily gridded (1° × 1° resolution) Level 3 AOD (at 550 nm) data is obtained from the combined Dark Target and Deep Blue MODIS Terra satellites (Kharol et al., 2011). 1.2. Calculation of aerosol optical properties A wing-mounted Passive Cavity Aerosol Spectrometer Probe (PCASP-100×, DMT Inc., USA) was used to measure the particle size distribution at diameter = 0.12–2.5 μm, at a time resolution of 1 s. A wired heater on top of the inlet, and the dry sheath flow, assured the particles

Fig. 2. Time series of PM2.5 (grey column), AERONET AOD (black dot), RH (relative humidity), WD (wind direction), WS (wind speed) and extinction from ground Lidar. The black bars mark the time for each aircraft profile.

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D. Zhao et al. / Science of the Total Environment 709 (2020) 136251

measured by the PCASP were in a dry state, with RH b 40% (Strapp et al., 1992).The scattering coefficient (σsca) is obtained by integrating the number concentration (N(D)) and scattering cross section (Csca) measured by the PCASP. The absorption coefficient (σabs) is the integration of absorption cross section (Cabs) and BC number concentration N(Dc) for extrapolated all SP2 BC core size bins. The sum of both gives the extinction coefficient, as expressed in Eq. (2): Z σ ext ¼ σ sca þ σ abs ¼

2μm

NðDÞCsca ðDÞdD

D¼0:12μm Z 0:80μm

þ

Dc ¼0:08μm

NðDc ÞCabs ðDc ÞdDc

ð2Þ

This calculation is performed for every 200 m altitude bin using the mean PCASP and SP2 size distribution. The single scattering albedo (SSA=σsca/σext) and asymmetry parameter (g) are also obtained for each altitude bin. The aerosol optical depth - AOD(h) for each altitude bin (h) is obtained from the altitude-integrated σext(h), as expressed in Eq. (3): AODðhÞ ¼ σ ext ðhÞΔh

ð3Þ

The AOD, SSA and g as a function of altitude (Fig. 6) serve as inputs for the radiative transfer calculation given below. 1.3. Radiative transfer calculation The actinic flux spectrum at λ = 250–2550 nm was calculated using the Discrete Ordinates Radiative Transfer Code (DISORT), as implemented in the libRadtran software package (Emde et al., 2016). In this study, the aerosol optical depth (AOD), single scattering albedo (SSA) and asymmetry parameter (g) used are derived from the in-situ measured parameters based on the PCASP and SP2 measurements as above and calculated at each λ. The λ-dependent AOD (Ångström, 1929; King and Byrne, 1976) is applied, expressed as: AODðλÞ=AODð870Þ ¼ 0:17 þ 19  expð−0:0037  λÞ

ð4Þ

in order to convert the measured AOD at λ = 870 nm to the other wavelengths as the input of radiative transfer calculation. The other input parameters used in DISORT module is summarized in Table 2. The absorbing power of BC is calculated as absorption coefficient of BC (from BC mass × MAC) multiplied by the actinic flux, both of which are integrated over all λ (250–2550 nm) and BC core sizes Table 2 Parameters used as inputs for radiative transfer calculation. Parameter

Input value

Radiative transfer solver Gas absorption parameterization Wavelength range Atmosphere Aerosol

DISORT, 12-streams, delta-m method

Location Time Solar zenith angle Surface albedo

LOWTRAN/SBDART parameterization 250–2550 nm Standard Mid-latitude atmosphere 200 m altitude-averaged profiles AOD values were derived from in-situ PCASP measurement, also applying an exponential λ-dependent function SSA values are from in-situ PCASP and SP2 measurement Asymmetry factor (g) is derived from the in-situ PCASP measurement Henyey-Greenstein phase function 39.54°N, 116.23°E Local time 10:00–15:00 Effective solar zenith angle Using local time and aircraft location IGBP surface type 13 (Urban)

(50–800 nm), expressed in Eq. (5): Z Pabs ¼

800nm

Z

Dc ¼50nm

2550nm

λ¼250nm

σ abs;rBC ðλ; Dc ÞMrBC ðDc ÞFac ðλÞdλdDc

ð5Þ

where σabs, is the BC mass absorption cross section (in m2 g−1) depending on incident λ and BC core size (Dc), MrBC is the rBC mass concentration at each Dc (in μg m−3), and the actinic flux (Fac, in mW m−2) is calculated from the radiative transfer module as above. Integrating over all wavelengths and Dc range gives the BC absorption power (Pabs) in unit volume of air (in mW m−3). The absorbing efficiency (Peff, in mW/μg rBC) is calculated as the Pabs normalized by rBC mass loading. The heating rate of BC is then calculated as the Pabs divided by the heating capacity of air. 1.4. Meteorology This study reported a heavy pollution event occurring during 25–27th Nov. 2018 and captured this full process by continuous fights on each day. Fig. 2 showed the temporal evolution of surface PM2.5 concentration during this period. Nov. 25th showed PM2.5 at 64 μg/m3 (termed as pollution start), increased by a factor of 5 to 304 μg/m3 in one day on Nov. 26th (pollution development), then sharply decreased to 50 μg/m3 on Nov. 27th (pollution cease). The column-integrated AERONET AOD increased from 0.24 to 1.39 during this pollution event. The wind profile radar showed that the height of wind shear decreased from 1 km to 100–200 m during this pollution process, with lower wind speed in the boundary layer. This is consistent with the aerosol extinction vertical profiles measured by the lidar, with the majority of pollutants accumulated in a shallow boundary layer during pollution, and was released into a larger atmospheric column when pollution ceased. Fig. 3 showed the synoptic wind and pressure field at the surface and 850hpa (in the PBL) during this pollution event. At 8 am on Nov. 25th, Beijing area was in front of a high pressure centered in the southwest of Beijing, with dominant northwesterly wind. As the high-pressure system moved eastward and merged with the western Pacific high pressure in the afternoon and formed a larger scale of high-pressure system. At 14:00, the wind shifted to southwesterly at 850 hPa level. The convergence area of low pressure in the northern region continued to move eastward, and the conditions for air pollution dispersion began to deteriorate. The 24 h HYSPLIT back trajectories (Fig. 3c) initializing at aircraft location on Nov. 25th indicated the air mass was mainly northwesterly in the FT. In the PBL, there was a branch of air mass from southwest and the pollution started to be transported to Beijing from polluted southwestern regions. On Nov. 26th, Beijing was influenced by this low-pressure convergence region, and the low-pressure trough was over the northeast Beijing. The atmosphere had a typical “sandwich” dynamic structure, leading to strong southwesterly air mass throughout the column, allowing pollutants accumulated through regional transport from the polluted southwest region. Backtrajectory analysis showed consistent southwesterly air mass on Nov. 26th. MODIS AOD showed horizontal evolution of aerosol pollutants over the course of this pollution event (Fig. 4). The accumulation of pollutants over Beijing was contributed by regional transport from the polluted southwest region via the northward movement of air mass, while the cease of pollution in Beijing was caused by prevailing northwesterly air mass and the pollution hotspot had been moved to the south. Fig. 5 showed vertical profiles of in-situ measured meteorological parameters. The PBL height (PBLH) is determined by considering combined factors(Zhao et al., 2019): a weak variation of potential temperature in vertical direction (dθ/dz. b 5 K/km), a temperature inversion above the PBL and an increased RH in the PBL. The PBLH was below 500 m through the pollution initialization (25th Nov) to the pollution full development (26th Nov), which indicates the shallow PBLH has

D. Zhao et al. / Science of the Total Environment 709 (2020) 136251

5

Fig. 3. Synoptic wind and pressure chart at surface level and 850hpa at 00:00 UTC from Nov. 25th–27th 2018. The HYSPLIT backward trajectories were initialized at aircraft location (Shahe, 40.1°N,116.3°E) in the PBL (upper panels) and FT (bottom panels).

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D. Zhao et al. / Science of the Total Environment 709 (2020) 136251

Fig. 4. AOD distribution from the MODIS Terra satellite at λ = 550 nm during the pollution event.

promoted the accumulation of pollutants. The RH in the PBL was below 40% in the first two days but increased to 60% at the most polluted day. The shallow PBLH lasting for three days facilitated the accumulation of pollutants and moisture in the PBL, along with the increased stability of surface layer (indicated by increased vertical gradient of θ) On Nov. 27th when pollution ceased, the PBLH dramatically increased to 1.6 km by 3-fold, with decreased RH. 1.5. Vertical profiles of all aerosols

Altitude/m

Altitude/m

Altitude/m

The particulate mass from 0.12–2.5 μm derived from the PCASP measurement is shown in Fig. 6a. The surface PM2.5 increased by a factor of 5 from 64 μg m−3 (pollution start) to 304 μg m−3 (peak pollution). The PBLH slightly increased during this pollution event and the pollutants

(a1) 20181125am

(b1)

(c1) 400m

(a2)

(b2)

(c2)

20181125pm

490m

(a3)

20181126

(b3)

(c3)

400m

(b4)

(a4) Altitude/m

became well mixed in the PBL, even showing a positive vertical gradient in the PBL during the peak pollution day. The increase of PM2.5 in the PBL was in line with the accumulation of moisture in the PBL, from 40% to 56% in the PBL from Nov. 25th to 26th. This suggests the aqueous reactions may have played an important role in promoting the aerosol formation during this pollution event. In the pollution ceasing day, the PM2.5 decreased by about two orders of magnitude to the background PM2.5 of 50 μg m−3. The particle size Deff was generally positively correlated with the pollution level, with the background Deff at 0.25 μm and reached as large as 0.4 μm on top of the PBL during peak pollution, which may result from the vertically increased RH. The AOD500 integrated over 100 m altitude (Fig. 6c) showed that over 60% of the AOD was contributed by the aerosol loadings in the PBL in the pollution day as most of the pollutants were accumulated in the PBL, whereas in

(c4)

20181127

1600m

T(°C)

RH(%)

Theta(K)

Fig. 5. Vertical profiles of aircraft in-situ measured temperature, RH and potential temperature during the project. The dash lines show the determined PBLH.

D. Zhao et al. / Science of the Total Environment 709 (2020) 136251

7

Fig. 6. Vertical profiles of PM2.5 concentration, Deff (effective diameter, calculated by the third divided by the second moment from number size distributon), AOD and g at λ = 500 nm measured or derived from the PCASP measurement. Black and grey markers denote the averages of the samples within the PBL and free troposphere, respectively.

the clean day the AOD was more dispersed in the column. Larger particle size has a more fraction of forward scattering thus larger asymmetry parameter (g): in polluted day g reached 0.64 while background showed g at 0.6. 1.6. Vertical profiles of BC physical properties The vertical profiles of rBC mass loading through lower troposphere during this pollution episode were shown in Fig. 7a. The rBC mass loading initialized at surface concentration 1.3 μg m−3 on Nov. 25th and increased to 5.2 μg m−3 in one day time by a factor of 4. For the pollution episode observed here, over 70% of the total BC mass was trapped in the PBL and in the FT the concentration was lower by a factor of 4. The atmospheric volume of the PBL thus importantly influenced the mass loading and the PBL processing on the pollutants. The rBC mass loading showed a decreasing trend with increasing altitude during pollution initialization but exhibited a slightly positive gradient with altitude when peak pollution. After pollution ceased, the BC was lowered by a factor of 15 and reached the background level throughout the column. The BC showed smaller core size at 0.20 μm (Fig. 7b) when less polluted and increased to 0.22 μm when high pollution. There was clear decreasing trend of core size at higher altitude, especially for the pollution ceasing day when BC core size rapidly decreased as soon as rBC mass loading dropped. The larger core size in the PBL reflected the possible coagulation process of BC. The core MMD on the surface was generally consistent with the ground measurements in Beijing during wintertime (Liu et al., 2019) and the smaller MMD in the FT (0.18 ± 0.01 μm) may represent the background characteristics of BC in this season. The mixing state of BC showed high variability in the PBL with Mcoating/ MrBC initialized from 2 and increased up to 10 in the peak pollution day. Note that the increase of rBC mass loading mostly covaried with BC coatings, suggesting that a higher chance for BC to be thickly coated during high pollution, because of the high concentration of gas precursors (for condensation) and pre-existing particles (for coagulation). During the peak pollution day, the Mcoating/MrBC showed a positive vertical gradient from the surface to the top of PBL, consistent with the increased particle Deff and PM2.5/rBC (Fig. 7), suggesting the likely enhanced secondary formation in the PBL (as RH increased in the PBL shown in Fig. 5). The lower Mcoating/MrBC (0.85 ± 1.5) in the FT as well as lower gas precursors and particles, in the opposite way suggested the importance of existing pollutants in the formation of BC coatings. The mass absorption cross section (MAC) of BC without considering the coating effect was solely determined by the core size, showing a slightly lower value ranging from 6 to 7 m2 g−1 in the PBL but increased to 7.5 m2 g−1 in the FT due to decreased core MMD (Fig. 7b). The MAC of uncoated BC showed moderate variation due to relative stable core MMD. However, by considering the coating effect, the absorption

efficiency of BC showed larger variability ranging from 7 to 12 m2 g−1. In line with the variation of coatings, the MAC for coated BC in the PBL showed an increase by a factor of 1.3 from pollution initialization to the peak pollution day (Fig. 7e) but decreased to a similar level comparable to uncoated BC MAC when pollution ceased. The enhancement of absorption (Eabs) due to coatings, calculated as MACcoated/MACuncoated is shown in Fig. 7f. The Eabs started from 1.4 and reached up to 1.9 in the peak pollution day. Consistent with the coatings, the Eabs showed positive vertical gradient in the PBL and reached maximum on top of the PBL. The FT and low pollution day showed Eabs 1.2 ± 0.04. Combined high rBC mass loading and higher MAC in the PBL, the most absorbing component of the aerosol ensemble was accumulated in the PBL for this pollution event. The PBL showed remarkably lower SSA than in the FT, which suggests a higher BC fraction in the polluted PBL; in other words, most BC had been trapped in the PBL and BC had not sufficiently reached upper level, thus showing a high and consistent SSA of 0.98 ± 0.01 in the FT. The SSA ranged from 0.91 to 0.99 in the PBL, and in the peak pollution day the SSA was slightly higher than other days which may be because of a higher production of secondary scattering materials than BC in the heavy pollution. To include the coating effect on BC absorption caused a reduction of SSA by 0.05, and enhanced the contrast between PBL and FT. This is because the scattering components were dominated by other aerosols and the coating enhancement on absorption of BC will decrease the SSA. 1.7. Heating rate of BC during the pollution event The total downward global irradiance is composed of direct irradiance and downward diffuse irradiance. The direct irradiance is mainly determined by the solar zenith angle, with the afternoon case showing a reduced direct solar radiation (Figs. 8a1 and a2). The presence of aerosol loadings, reflected by AOD, significantly reduced the direct solar radiation reaching the surface, which is the main reason that the global irradiance decreased during the pollution day. The presence of aerosol however increased the upward diffuse irradiance (Fig. 8b), which may to some extent balance the dimming effect. The reduced global irradiance caused by aerosol loading was as much as ~300 W m−2 on Nov. 26th. The downward irradiance, which is increased due to increased forward scattering of aerosol by 180 w m−2 during heavy pollution. The net irradiance caused by aerosols was as much as −480 w m−2 during the polluted days, which reflects the pronounced dimming effect during pollution, with the radiation received by the surface as low as null during the pollution event. Fig. 8c shows a clear evolution of increased aerosol dimming effect from −200 W m−2 when the pollution initialized (Nov. 25th) and increased up to −300 W m−2 on the surface during the peak pollution; when pollution ceased, the aerosol dimming effect

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D. Zhao et al. / Science of the Total Environment 709 (2020) 136251

(b)

(c)

Altitude (m)

(a)

3

BC mass (µg/m )

MMD(µm)

Mcoating/MrBC 3000

(e)

(d)

(f)

2500 Altitude(m)

2000 1500 1000 500 0 2

6

-1

7

Altitude/m

MAC550 (m g ) (uncoated) 3000

3000

2500

2500

2000

2000

1500

1500

1000

1000

500

500

0

0

0.88

0.92

0.96

SSA550_uncoated

1.00

8 9 10 2 -1 MAC550 (m g ) (coated)

0.88

0.92

11

0.96

12

1.0

1.2

1.4

1.6

1.8

2.0

Enhancement of absorption

1.00

SSA550_coated

Fig. 7. Vertical profiles of rBC mass loading (a), rBC core mass median diameter (MMD) (b), and bulk coating mixing ratio (c) for all flights. The black and grey markers denote the PBL and FT respectively. Mass absorption cross section (MAC550) for without (d) and with (e) considering the coating effect of BC; the enhancement of absorption (coated/uncoated) is shown in (f), calculated single scattering albedo (SSA550) without(g) and with(h) considering the coating effect.

reduced to about −100 W m−2, which may reflect the background aerosol effect. It is also notable that the dimming was reduced at higher altitude due to reduced column-integrated AOD, e.g. less than 100 w m−2 diming for altitude N 2 km (Fig. 8a). Assuming coated or uncoated BC has not impacted importantly on the overall dimming effect because the coating of BC has not added much on the total extinction. It shows the generally accumulated diming in the PBL but the upper level will receive more solar radiation. The resulting actinic flux which is the sum of global and upward irradiance is shown in Fig. 9a. Consistent with the diming effect, the actinic flux (Fac) received in the PBL was reduced by aerosol loadings however increased due to enhanced diffusion irradiance in the upper level. The absorbing power efficiency, which is determined by Fac multiplied by the absorbing efficiency of BC (MAC), is shown in Fig. 9b. Though the Fac in the PBL was reduced by 200 W m−2 in the polluted day, leading to a reduced Peff about 4 mW/μgBC in the polluted day if

not considering the coating effect on BC. The increased coating of BC significantly enhanced MAC and considerably enhanced the absorbing power efficiency by a factor of 2.5 up to 10 mW/μgBC. This is especially the case in the PBL, where most of the BC was accumulated and both enhanced BC mass loading and absorbing efficiency resulted in the remarkably enhanced heating rate during the pollution day. In the daytime, at the pollution initialization the heating rate of BC was about 0.03 K/h in the PBL with additional 0.01 K/h enhancement due to coatings; whereas in the polluted day, the adding of coatings introduced as high as 0.12 K/h in the polluted PBL. The heating rate of BC was higher than the observation in North America to about one order of magnitude, but generally consistent with the observation in Nanjing, China (Ding et al., 2016). The heating rate of absorbing component depends on both actinic flux deposited on the aerosols and the loadings of aerosols. Fac is more intensified in summer due to stronger solar radiation whereas in winter the BC emission is higher. The evolution of the

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heating effect of BC during this pollution event suggested that both enhanced BC absorbing efficiency and mass loading occurring in the polluted PBL, will exacerbate the heating effect of BC.

The heating rate of BC is determined by combined BC mass loading, absorbing efficiency and actinic flux, the resulting vertical structure of BC heating rate is thus determined by the three factors. Fig. 10 shows

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the evolution of each factor during this pollution episode. The PBLH maintained at a low level (b450 m) during the pollution accumulation processes from 25th–26th, with the mean BC mass loading in the PBL increased from 1.7 to 5.2 μg m−3. At the last day when pollution ceased, the PBLH was developed to 1500 m and BC mass was reduced to 0.07 μg m−3. The BC mass loading showed negative vertical gradient (Fig. 10b) and this gradient stared to approach to zero, even reaching a slightly positive gradient of 0.3 per m altitude in the peak pollution day. This suggested an eventually accumulated pollution in the PBL during the pollution development when the pollutants became more homogenously mixing in the PBL. The absorption efficiency of BC (as reflected by MAC) showed enhancement during the pollution process

if considering the coating effect, from 8.8 to11.6 m2 g−1, but deceased to 8 m2 g−1 when pollution creased (Fig. 10c). The increase of AOD caused dimming effect to the lower level but enhanced actinic flux at higher level, leading to significant positive vertical gradient of Fac under pollution, from 430 to 1100 mWm−2 per altitude during the pollution process (Fig. 10d). Contributed by enhancement of MAC and Fac vertical gradient, the absorbing power efficiency of BC showed increase of positive vertical gradient during the pollution as high as 6 mW/μg rBC per m altitude (Fig. 10e). The resulting BC heating rate (Fig. 10f) is a combination of BC mass loading and absorbing power efficiency, showing a notably positive vertical gradient in the peak pollution day. This stronger heating of BC on top of the PBL will potentially introduce

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convection. For example, a heating at lower level may promote the convective mixing while heating layer on top of the PBL will inhibit the PBL development by forming temperature inversion (Wang et al., 2018); and a heating layer below cloud may promote the convection while above the cloud will suppress the convection (Koch and Del Genio, 2010). This heating effect of BC is determined by BC mass, absorbing efficiency and actinic flux at different layers, and the vertical structures of three factors will vary according to emission and meteorology. This study reveals the positive vertical gradient of BC heating impacts during the heavy pollution day, which will lead to feedback impacts and further enhancing the doming effect of the PBL top. However this vertical structure of BC heating rate may only be limited to the environment with sufficient condensable gas pre-cursors or pre-existing particles to allow coating growth on BC particles during pollution, and if there is strong surface emissions prevailed the upward vertical mixing, the BC mass concentration may exhibit a negative vertical gradient hereby compensating the positive gradient of heating. Therefore the phenomenon observed in this study may only represent a typical pollutant event but measurements influenced by various mereological and emission conditions should provide a full understanding on the vertical structure of BC heating profile. Acknowledgments This research was supported by the National Key Research and Development Program of China (2016YFA0602001), the National Natural Science Foundation of China (41605108, 41875167, 41675038, 41675138, 41807313, and 41875044). Part of this work is supported by the National Center of Meteorology, Abu Dhabi, UAE under the UAE Research Program for Rain Enhancement Science. Declaration of competing interest There is no conflict of interest.

Fig. 10. Evolution of parameters averaged within the PBL during the pollution event. From bottom to top panel: PBLH, rBC mass loading, mass absorption cross section (MAC550), actinic flux, Power efficiency per unit mass of rBC and heating rate of BC in the PBL. All markers with error bars show the mean ± σ. The red dots using right y-axis show the vertical gradients of corresponding parameters in the PBL. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

enhanced temperature inversion over the PBL, further limiting the dispersion of pollutants trapped within the PBL. 2. Conclusion This study evaluates the vertical structures of BC physical properties during an entire pollution event over an urban megacity. The evolution of particle mass loading, size distribution and BC mixing state in the planetary boundary layer is investigated. During the pollution event, the BC was rapidly accumulated in two days from 1.7 to 5.2 μg m−3 on the surface and became more homogenously mixed in vertical direction within the PBL. The coatings on BC increased with BC mass loading and reached maximum during peak pollution day. For this event, the particle size and BC coatings showed positive vertical gradient from the surface to the top of PBL, which may result from enhanced secondary formation with increased altitude. Radiative transfer calculation shows increased diming effect at lower level thus enhancing its positive vertical gradient and allowing more actinic flux received at upper level. The factors including enhanced coatings from surface to the top of PBL and reduced actinic flux at lower level, contributed to the increased BC absorbing power efficiency with altitude in the PBL. The vertical gradient of BC heating rate or at which level the BC heating exhibited maxima, will importantly determine its impacts on modifying the thermodynamics hence the atmospheric stability and

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