key: cord-0924510-lea3vj4a
authors: Wang, Qinglu; Wang, Lili; Tao, Minghui; Chen, Nan; Lei, Yali; Sun, Yang; Xin, Jinyuan; Li, Tingting; Zhou, Jingxiang; Liu, Jingda; Ji, Dongsheng; Wang, Yuesi
title: Exploring the Variation of Black and Brown Carbon During COVID-19 lockdown in Megacity Wuhan and its Surrounding Cities, China
date: 2021-06-04
journal: Sci Total Environ
DOI: 10.1016/j.scitotenv.2021.148226
sha: 82b93b061ef9ad8b50f3f0d37494b632cdb63a3e
doc_id: 924510
cord_uid: lea3vj4a

Absorbing carbonaceous aerosols, i.e. black and brown carbon (BC and BrC), affected heavily on climate change, regional air quality and human health. The nationwide lockdown measures in 2020 were performed to against the COVID-19 outbreak, which could provide an important opportunity to understand their variations on light absorption, concentrations, sources and formation mechanism of carbonaceous aerosols. The BC concentration in Wuhan megacity (WH) was 1.9 μg m-3 during lockdown, which was 24% lower than those in the medium-sized cities and 26% higher than those in small city; in addition, 39% and 16-23% reductions occurred compared with the same periods in 2019 in WH and other cities, respectively. Fossil fuels from vehicles and industries were the major contributors to BC; and compared with other periods, minimum contribution (64-86%) mainly from fossil fuel to BC occurred during the lockdown in all cities. Secondary BrC (BrCsec) played a major role in the BrC light absorption, accounting for 65-77% in WH during different periods. BrCsec was promoted under high humidity, and decreased through the photobleaching of chromophores under higher Ox. Generally, the lockdown measures reduced the BC concentrations significantly; however, the variation of BrCsec was slight.

A new type of coronavirus disease has spread rapidly around the globe since late December 2019 (Zhu et al., 2020) , and become a major threat to public health. China, the first country hit seriously by the COVID-19 pandemic, has released a number of measures (e.g. lockdown city, shut down commercial activities, restrict travel and stay at home and so on) to control the spread of this disease from late January 2020 Wang et al., 2020a) . These nationwide restrictions have led to the lowest human activities, resulting in the lowest anthropogenic pollution emissions. The Chinese government has released the Air Pollution Prevention and Control Action Plan in 2013 to reduce air pollution on a national scale (Wang et al., 2020b) ; however, these restrictions during the COVID-19 breakout offered an unprecedented opportunity to assess the effectiveness of the reduction in anthropogenic emissions on air quality improvement, which will be helpful for optimize future emission control strategies.

Carbonaceous aerosols have profound effects on climate, air quality and human health through their light-absorbing components, e.g. black carbon (BC) and brown carbon (BrC) (Arola et al., 2011; Bond et al., 2013) . BC has been considered as secondary global warming contributor due to the most efficient light absorption in the full wavelength range (Bond et al., 2013; Moosmueller et al., 2009; Wang et al., 2019a) ; in addition, BC plays an important role in regional or urban heavy haze events through depressing the development of planetary boundary layer (PBL) (Ding et al., 2016; Ma et al., 2020) . Previous study reported that 1.0 μg m -3 BC increase had

The light absorption coefficients of the carbonaceous aerosols were measured by a multi-wavelength aethalometer (Model AE-31, Magee Scientific, USA) with seven-band (λ= 370, 470, 520, 590, 660, 880 and 950nm) in six cities. The flow rate and temporal resolution were set 5 L min -1 and 1 h, respectively. A detailed description of the dual-spot theory of the model AE31 aethalometer can be found in Drinovec et al. (2015) . More details about the calculation of BC mass concentration and aerosol absorption coefficient are given in Zheng et al. (2019) . The data were obtained in six cities in Hubei Province during the COVID-19 outbreak in 2020 and the same periods in 2019.

It is assumed that the total light absorption can be primarily due to light absorption by BC from fossil fuel combustion (BC ff ) and biomass burning (BC bb 

where the AAE ff and AAE bb are set 1.0 and 2.0, respectively (Sandradewi et al., 2008) .

b abs (370) and b abs (880) are the absorption coefficients at 370 and 880 nm wavelengths, respectively.

Assuming that the total b abs (λ) was apportioned between BC and BrC absorption (b abs BC(λ) and b abs BrC(λ), respectively) in the range of near ultraviolet and visible (i.e., λ=370,470,520,590 and 660 nm). The calculation was followed Lack and

where absorption b abs (λ) is the measured absorption at the short wavelength λ. BC absorption at λ (b abs BC(λ)) can be obtained from b abs (880) However, BC absorption varies weakly with wavelength in near-UV to infrared range showing an AAE around 1.0. The AAEBC value was set 1.0, which is consistent with previous studies (Wang et al., 2019c; Xie et al., 2019; Zhang et al., 2021b) .

A minimum R-squared (MRS) method was developed to separate light absorption by BrCsec (b abs BrCsec(λ)) versus primary BrC. The BC tracer method for calculating SOC was applied in this approach (Srivastava et al., 2018) . Aerosol light absorption is due to carbonaceous particles from both primary and secondary sources. Thus, b abs BrCsec(λ) can be calculated as the following equation:

where b abs (λ) is the measured light absorption coefficient at a given wavelength (e.g., λ= 370, 470, 520, 590 and 660 nm). (b abs (λ)/BC) pri is the ratio of the primary particle's light absorption to the BC mass concentration from combustion sources.

[BC] is the mass concentration of BC at 880 nm. The key step for the analyses is to find a value for b abs (λ)/BC that is the representative of the primary combustion sources which affected the sampling site, but finding that value is challenging because the ratio varies among sources. More details about MRS are given in Wang et al. (2019b) and Wu and Yu (2016) .

The average light absorption coefficient of BC (b abs BC) and BrC (b abs BrC) with their contributions to total light absorption at different wavelengths were calculated based on equations (5) and (6) (880) (Yuan et al., 2016) and Hangzhou (1.12-1.21) (Xu et al., 2020) ; however, it was lower than that measured in Chongqing (2.0) and Luohe (1.37) where household cooking, heating with coal and wildfires were the major source in winter (Chen et al., 2020c; Zheng et al., 2019) . (Zhang et al., 2018) , and close to that (1.7 μg m -3 )

during the 2016 G20 summit in Hangzhou . However, the lower BC (1.06 μg m -3 ) was measured in Hangzhou urban areas during COVID-19 lockdown, and the BC concentration reduced by 47% from the pre-lockdown to lockdown (Xu et J o u r n a l P r e -p r o o f al., 2020), which was higher about 2 times than this study because of pollution events occurred frequently during lockdown in Hubei Province .

In addition, the average BC concentration during P2 in 2020 in other five cities (e.g. HS, HG, XN, XG and EZ) decreased by approximately 0.6-1.2, 0.2-0.9 and 0.5-0.7 μg m -3 compared with that in P1, P3 in 2020 and P2 in 2019, respectively, and the corresponding decreasing percentages were 24-33%, 0.5-27% and 15-39%, respectively. The BC variation trend in different periods was decreasing at first (from P1 to P2), then increasing (from P2 to P3) and reducing in the end (from P3 to P4), and this was different from the BC variation patterns in WH. It might be that industry and transportation recovered faster than that in WH. An interesting phenomenon was observed that the BC levels in medium-sized cities (e.g. HS, HG, XN and XG) were higher, followed the WH megacity and the EZ small city during different periods (Table 1) 

The relative contributions between fossil fuel and biomass burning to BC were apportioned using the aethalometer model. which was consistent with previous results that the contribution of BC from fossil fuel was above 85% in WH (Zhang et al., 2021a; Zheng et al., 2019) . As shown in Table 2 J o u r n a l P r e -p r o o f

Diurnal variation of BC concentration and the %BC ff during different observed periods were shown in Figure 4 . The BC concentration during P1 and P3 in 2020 and different periods in 2019 exhibited a bimodal diurnal distribution with one peak between 8:00 and 10:00 (2.7-3.8 μg m -3 at 10:00 LT) and the other peak at 17:00-21:00 (3.0-4.0 μg m -3 at 21:00 LT). The morning BC peak value was lower than the evening peak. Meanwhile, the peak values of %BC ff reached 90-95% during different periods, except P2. The morning peak could be attributed to vehicle emissions during morning rush hour, and the evening peak could be influenced by later afternoon rush hour and the emission of heavy-duty diesel trucks, in addition, the BC was easier accumulated when the mixing height decreased in the evening.

However, the bimodal distribution of BC during P2 in 2020 disappeared in WH due to strict traffic control (Figure 4b1) , and the %BC ff kept lower level (ranging from 81 to 85%) (Figure 4b2 ). We compared the BC peak values during the lockdown in 2020 and 2019, and then roughly estimated that the decreased BC peak value of 1.2 μg m -3 (38%) in WH urban areas was attributed to the traffic reduction. During all periods, the concentration of BC at ground level decreased from 10:00 to 15:00 because of the increasing mixing height which facilitated the diffusion of air pollutants. Therefore, the diurnal variation of BC was mainly influenced by the intensity of local source and meteorological conditions (e.g. mixing height).

J o u r n a l P r e -p r o o f

The production process of BrCsec is affected by various factor such as precursors and environmental condition. The generation mechanism mainly include the oxidation of aromatic hydrocarbon precursors to generate nitroaromatic BrC, and the reaction of Previous studies reported that BrCsec would appear to be an important contributor to light absorption of BrC especially at 370 nm (Wang et al., 2019b; Zhang et al., 2020) . In this study, b abs BrCsec was further quantified using the MRS method. As shown in Table 1 (Zhao et al., 2015) . Strong negative correlation between b abs BrCsec (370) and Ox in P2 in 2020 (R 2 =0.70, Figure S3b ) and P3 (R 2 =0.24, Figure S4c ) were measured. Increasing b abs BrCsec(370)/∆CO trends after 20:00 LT for extensively long times at night can be explained by the occurrences of active aqueous formation of BrCsec. Whereas, the peak of b abs BrCsec (370) occurred in the periods of 10:00-11:00, and this can be explained by the increased solar radiation after sunrise and more gaseous precursors from emission sources (e.g. traffic and industry), which enhanced the photochemical reaction that led to the formation of BrCsec . In other words, the b abs BrC (370) conditions or after being oxidized by OH radicals, BrCsec generated by the reaction of glyoxal or methylglyoxal with ammonium sulfate will undergo rapid "photobleaching" phenomenon, and the light absorption will be significantly reduced (Wong et al., 2017; Zhao et al., 2015) .

In this study, in order to analyze the impact of strict city lockdown on air quality during COVID-19, the variations on light absorption, concentrations, sources and formation mechanism of BC and BrC were investigated with different periods, based on real-time measurements with the 7-wavelength Aethalometers in megacity Wuhan and its surrounding cites. The BC was the dominant light absorption contributor to the total light absorption of aerosol, while the BrC also was no-neglect in short wavelengths. The BC in the megacity WH was 2.5 and 1.9 μg m -3 during pre-lockdown and lockdown, respectively, which were lower by 25% and 28% than those in the medium-sized cities (e.g. HS, XN, XG and HG) and higher by 20 and 26%

in small city of EZ. The BC concentrations declined by 16-39% and 24-34% during lockdown in all cities compared to the same period in 2019 and pre-lockdown in 2020, respectively; and then they increased gradually during P3, except the WH megacity.

Our study indicated that the significant reduction of anthropogenic emission contributed to the decrease in the BC concentration. We found 0.6 and 0.6-1.2 μg m -3 BC decline in WH and other cities, respectively, due to a reduction in vehicle emissions. Compared with other periods, the minimum %BCff (64-86%) mainly from fossil fuel to BC occurred during the lockdown in all cities, and the values of %BC ff J o u r n a l P r e -p r o o f increased (≥ 73%) in other periods, which was attributed to vehicle emission reduction in the urban areas. Notably, the BC level in XG and HG was 2.3 and 2.7 μg m -3 during strict vehicle control, and %BC ff was range from 84 to 85%, indicating that the industrial was another BC source. Therefore, the new energy vehicles or the vehicle restriction policy should be used widely in the megacity and the coal fuel should be reduced in medium-sized cities (e.g. XG and HG).

The values of b abs BrCsec (370) and C BrCsec (370) during the lockdown in 2020 and 2019 indicated that the variation of BrCsec was more related to meteorological factors than emission reduction. The diurnal patterns of b abs BrCsec (370) and RH were similar, but were opposite to that of Ox, indicated that liquid phase reaction promoted the formation of BrCsec at night; however, the BrCsec chromophores were photobleached in the afternoon by the photo-oxidation process. Thus, we suggest that the effect of RH should be considered in atmospheric models in order to get accurate representation of secondary aerosol formation in aqueous droplets.

Generally, the lockdown measures reduced the BC concentrations significantly;

however, the variation of BrCsec was slight. We only investigated light characterization of BrC rather than the source of BrC. In the future, we will utilized the aerosol mass spectrometers (  Compared with other periods, the minimum contribution (64-86%) mainly from fossil fuel to BC occurred during lockdown.

 Secondary BrC accounts for 75% to the total BrC at 370 nm during the lockdown.

 Secondary BrC absorption was promoted (decreased) under higher humidity (Ox).

 The decline of primary pollutants is more obvious than secondary due to lockdown.

J o u r n a l P r e -p r o o f

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Author statement

Writing-original Draft, Visualization. Lili Wang: Conceptualization, Supervision, Resources and editing. Nan Chen: Data supporting, Investigation. Minghui Tao: Data supporting, Investigation

Model simulation. Yang Sun: Review and Editing. Jinyuan Xin: Review and Editing. Tingting Li: Review and Editing. Jingxiang Zhou: Sample collection, Visualization. Jingda Liu: Formal analysis, Data Curation. Dongsheng Ji: Review and Editing

This work was partially supported by the grant of National Key R&D Plan