Size-resolved carbonaceous aerosols at near surface level and the hilltop in a typical valley city, China

Atmospheric Pollution Research 11 (2020) 129–140 HOSTED BY

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Size-resolved carbonaceous aerosols at near surface level and the hilltop in a typical valley city, China

T

Suping Zhaoa,b,c,∗, Ye Yua, Zhiheng Dub, Daiying Yind, Jiancai Yange, Longxiang Donga, Ping Lia a

Key Laboratory of Land Surface Process and Climate Change in Cold and Arid Regions, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou, 730000, China b State Key Laboratory of Cryospheric Sciences, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou, 730000, China c Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention (LAP3), Shanghai, 200433, China d Key Laboratory of Desert and Desertification, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou, 730000, China e Lanzhou Central Weather Station, Lanzhou, 730000, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Carbonaceous aerosols Vertical distributions Urban valley size distributions Lanzhou

The lack of light–absorbing aerosols vertical distributions data largely limited to revealing the formation mechanism of severe haze pollution in Chinese cities. Based on the synchronous measurements of size–resolved carbonaceous aerosols and meteorological data at near surface level and hilltop (about 620 m above the valley) in Lanzhou of northwest China, this study compared organic and elemental carbon (OC, EC) size distributions at the two altitudes and revealed the key influencing factors in a typical urban valley, China. The winter OC size distributions were typically bimodal with two comparable peaks in the accumulation and coarse modes, while those in summer were unimodal with the highest value in the size bin of 4.7–5.8 μm. The size-resolved OC and EC at near the surface were significantly higher than those at the hilltop. The difference (concentrations and size distributions) of OC and EC between the surface and hilltop in summer was much smaller than that in winter due to stronger vertical mixing and larger summer SOC contributions at the hilltop. The winds paralleling with running urban valley were conducive to dispersing the air pollutants from near the surface to the upper air. The roles of horizontal and vertical dispersions to carbonaceous aerosols were comparable at near the surface, while horizontal dispersion was more important at the hilltop. Furthermore, the vertical dispersion was a main factor controlling size–resolved carbonaceous aerosols under highly polluted conditions in a typical urban valley. This study will provide the basis for regulation of severe haze pollution over complex terrain.

1. Introduction As a typical valley city, Lanzhou is located at the intersections of Tibetan Plateau, Mongolian Plateau and Loess Plateau. Air pollution at urban Lanzhou was exacerbated by weak winds and strong temperature inversion with the exception of lots of primary emissions (Chu et al., 2008; Zhang and Li, 2011). A lot of observations and studies have been conducted to reveal the formation mechanisms of daytime inversion and heavy air pollution over the specific mountainous region (An et al., 2008; Chen et al., 1993; Hu and Zhang, 1999; Shen et al., 1982; Zhang and Hu, 1992; Zhang, 2001; Zhao et al., 2015, 2017). The clearly positive feedback between boundary layer meteorology and light-absorbing aerosols were found to be a key factor in formation and development of strong daytime inversion over urban Lanzhou. In the

winter of recent decades, the large spatio-temporal scale haze pollution episodes occurred frequently at the Chinese urban agglomerations such as North China Plain, Yangtze River Delta, Pearl River Delta and Sichuan Basin (Cai et al., 2017; Guo et al., 2014; Huang et al., 2014, 2018a,2018b,2018c; Li et al., 2015a,2015b; Shi et al., 2018; Wang et al., 2018a,2018b,2018c,2018d; Zhao et al., 2016, 2018a; 2018b; Zhong et al., 2018). The formation causes were revealed by various aspects, including increased primary emissions, more secondary aerosol formation and unfavorable meteorological conditions. The main causes were attributed to reduced surface winter northerlies winds and increased air-stagnation events under global warming (Cai et al., 2017; Huang et al., 2018a,2018b,2018c; Wang et al., 2018a,2018b,2018c,2018d), more primary emissions accompanied with urban growth (Wu et al., 2018; Zhao et al., 2017), and highly

Peer review under responsibility of Turkish National Committee for Air Pollution Research and Control. ∗ Corresponding author. Key Laboratory of Land Surface Process & Climate Change in Cold & Arid Regions, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou, 730000, Gansu, China. E-mail address: [email protected] (S. Zhao). https://doi.org/10.1016/j.apr.2019.09.022 Received 21 May 2019; Received in revised form 20 September 2019; Accepted 29 September 2019 Available online 03 October 2019 1309-1042/ © 2019 Turkish National Committee for Air Pollution Research and Control. Production and hosting by Elsevier B.V. All rights reserved.

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Fig. 1. Locations of Lanzhou city and the surface and hilltop sites.

light-absorbing component, the observations on carbonaceous aerosols mainly focused on near the surface and the specific size bins (Arhami et al., 2018; Bian et al., 2018; Chang et al., 2017; Chen et al., 2014; Ji et al., 2018; Li et al., 2016; Querol et al., 2013). The vertical distributions of carbonaceous aerosols were observed in few studies and found largely different types of profiles due to turbulent mixing impacts (Bisht et al., 2016; Chilinski et al., 2016; Ding et al., 2017; Li et al., 2015a,2015b, 2018; Markowicz et al., 2017; Safai et al., 2012; Shi et al., 2012; Tripathi et al., 2007; Wang et al., 2018a,2018b,2018c,2018d; Zawadzka et al., 2017; Zhao et al., 2019a, 2019b). However, the vertically size-resolved carbonaceous aerosols was still unclear in the valley megacities. Size-segregated aerosol chemical characterization was highly informative and largely impacted on radiative, health and environmental effects of aerosols (Cuccia et al., 2013). The lack of the information was not conducive to revealing the causes of severe haze pollution in the Chinese cities. As the most important light-absorbing aerosols, the optical properties and radiative effects of carbonaceous aerosols largely depended on black carbon (BC) size distributions and vertical profiles (Wang et al.,

secondary aerosol formation (Guo et al., 2014; Huang et al., 2014). The human and ecological health and regional and even global climate were affected largely by severe air pollution in Chinese cities (Tie et al., 2016; Xie et al., 2016; Zhong et al., 2018). The air pollution might be further enhanced by the positive feedbacks between light-absorbing aerosols and boundary layer meteorology in China (Ding et al., 2016; Huang et al., 2018a,2018b,2018c; Petäjä et al., 2016; Wang et al., 2018a,2018b,2018c,2018d). High concentration of particulate matter (PM) enhanced the stability of an urban boundary layer, which in turn decreased boundary layer height and consequently further increased PM concentrations. In upper planetary boundary layer (PBL), the maximum temperature increased by ~ 0.7 °C on average and a mean temperature decreased by ~ −2.2 °C at near surface under polluted condition due to aerosol impacts (Huang et al., 2018a,2018b,2018c). Furthermore, the effect of light-absorbing aerosols on PBL largely depended on the altitude of aerosol layer, and the aerosols near the capping inversion was more essential to suppressing the PBL height and to weakening the turbulent mixing (Wang et al., 2018a,2018b,2018c,2018d). However, as the most important 130

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Fig. 2. Comparison of averagely size-resolved OC (a, b) and EC (c, d) concentrations between near the surface and the hilltop for winter (a, c) and summer (b, d).

Table 1 Concentrations of carbonaceous aerosols in each size bin and the corresponding meteorological parameters for winter and summer at near the surface and the hilltop. Items

Winter Surface /μg m−3 Winter Hilltop /μg m−3 Summer Surface /μg m−3 Summer, Hilltop /μg m−3

a

OC EC SOC OC EC SOC OC EC SOC OC EC SOC

0.4–0.7 /μm

0.7–1.1 /μm

1.1–2.1 /μm

2.1–3.3 /μm

3.3–4.7 /μm

4.7–5.8 /μm

5.8–9.0 /μm

> 9.0 /μm

Meteorological parametera (T, RH and WS)

2.0 0.9 0.5 1.3 0.8 0.3 1.1 0.4 0.4 0.8 0.2 0.2

3.3 0.6 0.4 1.9 0.8 0.2 1.3 0.4 0.3 0.8 0.1 0.2

3.2 0.7 1.0 2.2 0.5 0.7 1.0 0.2 0.1 0.8 0.1 0.3

1.6 ± 0.7 0.4 ± 0.2 0.2 ± 0.1 1.0 ± 0.3 0.1 ± 0.1 0.3 ± 0.3 0.9 ± 0.2 0.1 ± 0.1 0.3 ± 0.2 0.7 ± 0.1 0.02 ± 0.01 0.4 ± 0.2

1.4 ± 0.6 0.5 ± 0.3 0.2 ± 0.1 1.0 ± 0.5 0.2 ± 0.1 0.2 ± 0.2 1.1 ± 0.2 0.2 ± 0.1 0.4 ± 0.2 0.9 ± 0.3 0.06 ± 0.04 0.3 ± 0.2

1.1 ± 0.5 0.4 ± 0.2 0.3 ± 0.2 0.7 ± 0.3 0.2 ± 0.2 0.2 ± 0.1 1.0 ± 0.1 0.1 ± .1 0.2 ± 0.1 0.8 ± 0.2 0.04 ± 0.02 0.4 ± 0.3

1.0 0.5 0.2 0.9 0.3 0.2 1.1 0.3 0.3 0.7 0.1 0.2

1.3 0.7 0.3 1.1 0.5 0.2 1.2 0.3 0.3 0.9 0.1 0.2

T: (4.2 ± 1.6) oC RH: (48.1 ± 8.5) % WS: (1.6 ± 0.2) m s−1 T: (−7.3 ± 3.1) oC RH: (60.2 ± 18.1) % WS: (2.4 ± 0.6) m s−1 T: (23.5 ± 1.5) oC RH: (51.4 ± 10.0) % WS: (1.6 ± 0.1) m s−1 T: – RH: – WS:–

± ± ± ± ± ± ± ± ± ± ± ±

0.5 0.3 0.4 0.2 0.2 0.2 0.1 0.1 0.2 0.2 0.1 0.3

± ± ± ± ± ± ± ± ± ± ± ±

0.6 0.1 0.4 0.5 0.3 0.1 0.8 0.4 0.2 0.2 0.1 0.2

± ± ± ± ± ± ± ± ± ± ± ±

1.1 0.1 0.9 0.6 0.2 0.5 0.2 0.1 0.1 0.3 0.1 0.2

± ± ± ± ± ± ± ± ± ± ± ±

0.3 0.3 0.2 0.5 0.2 0.2 0.3 0.1 0.1 0.2 0.1 0.2

± ± ± ± ± ± ± ± ± ± ± ±

0.6 0.4 0.2 1.2 0.6 0.2 0.3 0.2 0.2 0.2 0.1 0.2

T, RH and WS are abbreviations of temperature, relative humidity and wind speed.

will help to understand vertical profiles of carbonaceous aerosols size distributions and to provide the basis for regulation of severe haze pollution over complex terrain.

2018a,2018b,2018c,2018d). However, in the previous studies, BC within a specific size bin was mainly observed at near surface (Li et al., 2014; Wang et al., 2010; Xie and Xu, 2017; Xu et al., 2014), which largely limited the understanding of haze pollution over complex terrain. Therefore, size-segregated samples was collected synchronously at near the surface and the hilltop in Lanzhou, and the collected filters were analyzed as regards the content of OC and EC, and these species levels have been extensively studied in the aerosol particulate matter in different environments (Daellenbach et al., 2016; Fermo et al., 2006; Vassura et al., 2014). The objective of this study is to compare carbonaceous aerosols size distribution between the two sites and to reveal the key controlling factors using synchronously measured carbonaceous aerosols and meteorological variables at the two altitudes. The study

2. Data and methods 2.1. Sampling sites The size-resolved PM samples were collected at near surface and the hilltop to better analyze the difference of size-resolved carbonaceous aerosols between below and above the capping inversion at an urban valley. The instruments in the near surface were mounted on the rooftop of the research building of Northwest Institute of Eco131

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Fig. 3. Variations of OC (the upper panel) and EC (the lower panel) size distributions near the surface and the hilltop for winter and summer. In the horizontal axes, WS (WH) represents the samples near the surface (hilltop) in winter, and SS (SH) represents the samples near the surface (hilltop) in summer.

precision electronic balance (BT125D, Sartorius, Germany), and the mass concentrations were calculated by the weight differences between before and after sampling. After sampling, the filters were stored in the refrigerator at −18 °C at the lab until chemical composition was analyzed. All of the OC/EC data reported in this study were corrected by the field blanks. Generally, cutoff size of fine and coarse particles was 2.5 μm, and submicron fraction was the particles smaller than 1.0 μm (Seinfeld and Pandis, 2012). However, in view of the limitations of used Anderson sampler, submicron (fine) particles were defined as the particles in the size bin of 0.4–1.1 μm (0.4–2.1 μm), while accumulation or coarse mode was defined as the particles smaller or larger than 1.1 μm in this study.

Environment and Resources (NIEER, 36° 2′ 59.46″ N, 103° 51′ 28.63″ E, altitude of 1520 m), Chinese Academy of Sciences. The site could represent typical urban air pollution with sampling height of ~34 m above the surface. The instruments at the hilltop were mounted at a countryside with the sampling height of ~620 m above the valley. The locations of surface and hilltop sites were showed in Fig. 1. The horizontal distance between the two sites was about 4 km (also see from distance scale in Fig. 1), and they can be used as the comparison study of vertical distribution of aerosols. The PM samples in the two sites were collected simultaneously with two 9-stage cascade impactor samplers (Andersen, Model 20–800, USA) at a flow rate of 28.3 L min−1 with 50% cutoff points as 0.4, 0.7, 1.1, 2.1, 3.3, 4.7, 5.8 and 9.0 μm in summer (sampling months: August) and winter (sampling months: from December to January next year). The sampling flow rates of the two Andersen samplers were calibrated by a flow meter before collecting each sample. The different air pressure and temperature at varying altitudes can affect the sample volume in the surface and hilltop sites, and thus the sample volume was calibrated with the following equation:

V=

V ′ × 273 P ⋅ T 101.3

2.2. OC/EC and meteorology measurements OC and EC concentrations for the samples were determined using the thermal/optical carbon analyzer (DRI Model, 2001A, Desert Research Institute, USA) with the thermal/optical reflectance (TOR) method, following the Interagency Monitoring of Protected Visual Environments (IMPROVE) protocol (Chow et al., 2007). The working principles of the instrument could be found at the study of Zhao et al. (2019b). Primary and secondary organic carbon (POC, SOC) could be estimated by the EC-tracer method (Turpin and Lim, 2001), and the relevant equations referred to Zhao et al. (2019b). However, the calculated SOC and POC were only an approximation with uncertainties. The high vertical resolution temperature data were measured using a ground-based multi-channel microwave radiometer (MWP967KV, Xi'an, China). Additionally, the 10-min basic meteorological data (temperature, RH and wind speed and direction) were obtained with an automatic meteorological station at the surface and hilltop sites. The measurement principle of the sensors and the corresponding precisions were introduced in detail in the study of Zhao et al. (2019b). The

(1)

where, V and V’ are volumes after and before calibration, and P and T are air pressure and temperature. Each set of the size-segregated samples was collected continuously for 72 h and then replaced by new filters. In total, 180 samples (20 sets of samples with each set having 9 filters) were collected on 81 mm quartz-fiber filters (Whatman, Clifton, UK) during this campaign. The filters were preconditioned by heating at 800 °C for 5 h to minimize the potential effect of organic carbon during carbon analysis. The filters were conditioned at the glassware dryer at the temperature of 20 °C and relative humidity of 30% at the lab before and after sampling. The filter membranes were weighed before and after sampling with a high132

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Fig. 4. Relationships of size-resolved (a, c) OC and (b, d) EC for (a, b) winter and (c, d) summer between near the surface and the hilltop.

(Wan et al., 2015). The SOC concentrations in 0.7–1.1 μm were comparable between winter and summer, but the winter mean OC in that size bin were 3.3 ± 0.6 (1.9 ± 0.5) μg m−3 at the near surface (hilltop), which were twice as high as summer OC. Furthermore, OC aerosols larger than 3.3 μm was comparable between winter and summer (Table 1). The higher accumulation mode OC and EC in winter than summer was largely related to more primary emissions in winter such as coal combustion or biomass burning. The coarse OC may come from agglomerated small OC particles and condensation of VOCs on dust particles in winter, while coarse OC in summer may be affected by secondary formation at higher temperature. Generally, size-resolved OC and EC at near the surface were much higher than the hilltop due to more surface sources such as motor vehicles, which may also be related to the fact that surface wind speed was about 50% lower than the hilltop in winter (Table 1). Additionally, winter submicron EC differences between near the surface and the higher hilltop were significantly smaller than summer mainly due to more coal or biomass utilization for domestic heating at the hilltop countryside. Wang et al. (2017) found that the carbonaceous species in fine particles maybe originated from coal and biomass burning, which can support our results (Piazzalunga et al., 2011; Vassura et al., 2014). However, more interestingly, summer OC difference between near the surface and the hilltop was much smaller than winter, which was not only due to stronger vertical mixing but also due to larger summer SOC contributions at the hilltop (see also Table 1). The air pollutants at near the surface can be vertically transported to the upper air by strongly vertical convection in summer, which may be more obvious for the

maintenance and regular checking for the instruments could also be found at Zhao et al. (2019b). The summer meteorological data at the hilltop were absent during the campaign, and thus this paper only discussed the impacts of meteorology on winter size-resolved carbonaceous aerosols. Beijing Time (BT) (=UTC+8) was used throughout the paper.

3. Results and discussion 3.1. Size distribution of OC and EC fractions The size distributions of carbonaceous aerosols have been not reported in urban Lanzhou even if it has been extensively studied for other sites (Cuccia et al., 2013). In this study, the averagely winter and summer size-resolved OC and EC concentrations were first compared between near the surface and the hilltop (Fig. 2). For the two sites, the winter OC size distributions were typically bimodal with two comparable peaks in the specific bin of accumulation (0.65–1.1 μm) and coarse modes (4.7–5.8 μm), and similar peaks in the fine and coarse modes were reported at Chengdu (Cheng et al., 2015), while those in summer were unimodal with the highest OC value in the size bin of 4.7–5.8 μm. The high winter OC in 0.65–1.1 μm was potentially due to the direct emission of organic particles, the growth process through condensation and coagulation, as well as the gas to particle reactions of volatile organic compounds (Cheng et al., 2015). The peak at coarse mode was mainly induced by dust particles, providing surfaces for the uptake of gas precursors and serving as a carrier for carbonaceous components 133

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Fig. 5. Relationships between size-resolved OC and EC near the surface and at the hilltop for winter. The relationships were fitted using unary linear regression and coefficients of determination (R2) also were given in the subplots. The coefficients of determination with superscript asterisk passed significance level of 0.05.

lots of motor vehicles near the surface (Wan et al., 2015). The winter EC in accumulation mode at the hilltop was sometime higher than that at near the surface due to the effect of more coal or biomass combustion for domestic heating at the hilltop with lower temperature (Table 1, Wang et al., 2017). There are 1 town and 6 villages with about 10 thousand population at the hilltop. Although population is less at the hilltop compared to urban areas, unlike urban areas with central heating, more bulk coal or biomass were burned by traditional stoves for winter domestic cooking and heating at every hilltop household. The bulk coal accounting for below 10% of fuels contributed to about 50% of air pollution (http://dy.163.com/v2/article/detail/ E3880D1D053855WR.html).

urban valley with weakly horizontal and vertical dispersions. Reduced vertical mixing due to temperature inversion and weak winds enhanced air stagnation and easily formed haze pollution in valleys (Chilinski et al., 2016; Guinot et al., 2006). The recent studies also demonstrated that vertical convection as indicated by mixing layer height, temperature inversion, and local emissions were three major factors affecting the structures of vertical profiles (Wang et al., 2018a,2018b,2018c,2018d; Zhang et al., 2009), which was consistent with our results. Only mean OC and EC size distributions were analyzed in the above paragraph. To better compare size-segregated OC and EC concentrations between near the surface and hilltop, the variations of OC and EC size distributions at the two sites for winter and summer were shown in Fig. 3. The figure was drew by the software of Sigmaplot. The shapes of OC and EC size distributions were similar with two comparable peaks in accumulation and coarse modes in winter and a peak in coarse mode in summer both at the near surface and the hilltop, indicating that the sources affecting OC and EC may be similar for each site. However, the size-resolved OC and EC variations had large discrepancy between the near surface and the hilltop and especially for EC due to different sources for the two sites and more favorable diffusion conditions at the hilltop. For example, the higher OC and EC in coarse mode at the near surface than the hilltop during 26–30 December 2017 was mainly induced by dust events (http://gansu.gansudaily.com.cn/system/2018/ 01/11/016888328.shtml), and dust particles were easily accumulated at near the surface and were difficult to be diffused to the upper air due to its large size and heavy weights. The abundant dust particles provided surface for chemical reactions among the species emitted from

3.2. Factors affecting OC and EC size distributions 3.2.1. Emission sources impacts The fossil fuel, motor vehicles and open biomass burning were identified as the main sources of OC and EC in urban or rural area (Bian et al., 2018; Chen et al., 2014; Ji et al., 2018; Shi et al., 2012; Vassura et al., 2014). However, contributions of the sources to carbonaceous aerosols varied greatly among the different microenvironments and depended significantly on the sampling heights above the surface (Wang et al., 2018a,2018b,2018c,2018d). Therefore, to better understand the difference of the sources affecting OC and EC size distributions between the near surface and the hilltop, the relationships of sizeresolved OC or EC between the two sites were first given in Fig. 4. In general, the size-resolved OC and EC at near the surface were much higher than those at the hilltop due to more sources such as lots of 134

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Fig. 6. Relationships between size-resolved OC and EC near the surface and at the hilltop for summer. The relationships were fitted using unary linear regression and coefficients of determination (R2) also were given in the subplots. The coefficients of determination with superscript asterisk passed significance level of 0.05.

winter size-resolved OC was comparable between the near surface and hilltop sites during the low OC periods. However, winter fine OC (0.65–1.1 μm and 1.1–2.1 μm) at near the surface was about twice as high as that at the hilltop during the high OC periods (see Fig. 4a), which was mainly related to the fact that the primary pollutants were mainly accumulated at near the surface and SOC was easily formed by gas-to-particle conversion of volatile organic compounds during highly polluted periods (Kroll and Seinfeld, 2008). The relationship between OC and EC can provide some valuable information for identification of the sources of carbonaceous aerosols (Turpin and Huntzicker, 1995). The strong correlation demonstrated similar source origins and transport process. Overall, the size–resolved OC were positively related to the corresponding EC at the near surface and hilltop (Figs. 5 and 6), inferring that they may had common sources to some degree. The strong correlations between OC and EC also were reported in a remote site (Wan et al., 2015), a rural site (Guo, 2016) and a megacity in China (Hou et al., 2011). However, the positive correlations were stronger for the particles larger than 3.3 μm than finer fractions and especially for the near surface site during wintertime, suggesting that the sources of carbonaceous components for the particles larger than 3.3 μm may be more similar and mainly from primary sources, which was consistent with the results of Wan et al. (2015). The weak solar radiation during dust events was not conducive to formation of secondary organic carbon by photochemical reactions. In addition, the correlations between OC and EC in the size ranges of 0.43–0.65 μm and 1.1–2.1 μm at the hilltop during wintertime were weaker than summertime with coefficients of determination lower than 0.50, which suggested complex mixture of source contributions (Guo, 2016). The

Fig. 7. The mean ratios of size-resolved OC to EC near the surface and at the hilltop for winter and summer.

motor vehicles near the surface. Interestingly, the winter accumulation mode EC at the hilltop were even higher than that at near the surface, which was induced by the primary pollutants from coal and biomass combustion at the countryside. Some studies also indicated that incompletely combusted biomass and fossil fuel were the primary source for EC (Seinfeld and Pandis, 2012), while OC had another origins with the exception of the same primary source such as atmospheric gas-toparticle conversion of volatile organic compounds and biogenic emissions (Calvo et al., 2013; Kroll and Seinfeld, 2008). Additionally, the 135

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Fig. 8. Wind speed frequencies and OC and EC size distributions for each sample (a) near the surface and (b) at the hilltop in winter. S1–S5 represent the samples near the surface (hilltop) in winter.

surface and hilltop sites for wintertime and summertime, the mean size–resolved OC/EC ratios were shown for each case in Fig. 7. The ratios varied greatly among the different size bins with a main peak in the size range of 2.1–3.3 μm. Generally, the ratios in summer at the hilltop were much higher than those in winter at the near surface. There was much more vegetation around the hilltop, which can emit larger volatile organic compounds, producing more secondary organic carbon by photochemical reactions with strong solar radiation. Moreover, there may be less fossil fuel and biomass combustion and more primary biogenic particles at the hilltop. They were the possible reasons of higher OC/EC ratios in summer at the hilltop. Winter and summer OC/EC ratios were similar near the surface for accumulation mode. Lower winter OC/EC was only observed for coarse mode near the surface supporting local EC emissions from biomass or coal burning. However, higher summer OC/ EC ratios were observed over the entire size range at the hilltop, which also may be related to large SOA contribution or lower EC for all size bins in summer (Fig. 2). Differences between summer and winter OC were not large compared to differences between summer and winter EC

above analyses indicated that winter carbonaceous aerosols were jointly affected by the various sources. The carbonaceous aerosols were mainly from primary emissions during dust events and dust particles can provide a carrier for primarily emitted OC and EC aerosols. As an important diagnostic index, the ratio of OC to EC has been frequently used to identify the type and source strength of carbonaceous aerosols although there are some limitations for this method (Turpin and Lim, 2001; Zhang et al., 2013). The ratios are largely influenced by sources and secondary organic aerosol (SOA) formation. The OC/EC ratios exceeding 2.0, 6.6 and 12 indicated that the effect of secondary organic aerosol, biomass burning and long-range transport, respectively (Chow et al., 1996; Saarikoski et al., 2008). Cao et al. (2005) found the ratios of 12.0 for coal combustion. SOC in Lanzhou varied less in different seasons and mean percentage of SOC in total carbonaceous aerosols was about 17% (Zhang and Kang, 2019). Therefore, the contribution of SOC to OC/EC ratios was varied little in different seasons. To better identify the sources of carbonaceous aerosols in each size range and understand the differences between the near 136

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Fig. 9. Relationships between (a, b) OC or (c, d) EC in different size bins (a, c) near the surface and (b, d) at the hilltop and T200m–0m in the winter.

Local wind directions can reflect primary pollutants transport from nearby sources to the sampling site to some degree (Zhao et al., 2015). The relationships between wind direction frequencies and OC and EC size distributions at the surface and hilltop sites were shown in Fig. 8. The more obviously dominant wind direction at near the surface maybe affected aerosol size distributions. The largest size–resolved OC differences among the five samples were in the size ranges of 0.65–1.1 μm and 1.1–2.1 μm. The OC in 1.1–2.1 μm for S5 was the highest among the samples, followed by S1, S2, S3 and S4 at near the surface, which was consistent with frequencies of northeasterly winds and more than 50% of winds was from the northeast during S1. In addition, OC and EC in all size bins for S4 was the lowest among the five samples at near the surface with high frequency of easterly and southeasterly winds. That may be induced by almost paralleled winds with running urban valley (Fig. 1) and thus the pollutants can be effectively dispersed to outside of the valley by means of less recirculated winds (Venegas and Mazzeo, 1999). At the hilltop, the coarse mode OC and EC for S1 were significantly higher than the other samples with higher frequency of southwesterly winds, which may be due to residential coal or biomass combustion at the countryside in that direction. The above analyses indicated that winds paralleling with running urban valley were conducive to dispersing the pollutants from near the surface to the upper air and the residential sources from the southwest had an important effect on carbonaceous aerosol size distributions at the hilltop. The ambient temperature maybe largely impacts on particle concentrations in different environment (Hussein et al., 2006; Zhao et al., 2015). Generally, the size-resolved OC and EC at near the surface (at the hilltop) reduced (increased significantly) with the increase of air temperature in winter (see Fig. S3 of the supplementary materials).

(Fig. 2). This was the reason for the lower OC/EC ratio in winter due to high EC from biomass burning. 3.2.2. Meteorological condition impacts The particle size distributions were significantly modulated by meteorological conditions such as wind speed and direction, temperature and relative humidity (Zhao et al., 2015). We first analyzed the wind roses at the near surface and hilltop sites during the campaign (see Fig. S1 of the supplementary materials). The wind speed was lower and direction was more monotonous with dominantly northeast winds at near the surface than that at the hilltop, which was mainly due to terrain blocking and the running urban valley from the east to the west (also see Fig. 1). Generally, compared with near the surface, the wind speed was stronger and direction was more multidirectional, the temperature was lower, and the relative humidity was higher at the hilltop (Table 1). The meteorological conditions discrepancy maybe affected size-resolved OC (EC) differences between the two sites, which will be analyzed more deeply in the following sections. The decreases of size–resolved OC and EC with the increased wind speed were significant at near the surface and especially for the carbonaceous components in the two size ranges of 0.65–1.1 μm and 1.1–2.1 μm (see Fig. S2 of the supplementary materials), which was consistent with the results from the previous studies. For example, Hussein et al. (2006) and Zhao et al. (2015) found that local wind conditions were the most important factor controlling fine particles concentrations. More interestingly, OC and EC in fine particles reduced as the increases of wind speed, but those in coarse particles varied less and even increased as wind speed at the hilltop, which was mainly induced by the dust events during the campaign. 137

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Fig. 10. Normalized ratios of size-resolved OC or EC to horizontal wind speed vs. to T0m–200m (a) near the surface and (b) at the hilltop in winter. The red circle showed Sample 1 (during 26–30 December 2017).

(0.43–0.65 μm, 0.65–1.1 μm and 1.1–2.1 μm), which may be due to more coal or biomass burning for heating under low temperature conditions at near the surface. The above section indicated that surface carbonaceous aerosols were more easily influenced by atmospheric stratification. To better evaluate horizontal or vertical dispersion was more important factor to modulate size–resolved carbonaceous aerosols at different altitudes, we further compared normalized ratios of size–resolved OC (EC) to wind speed with normalized ratios of size–resolved OC (EC) to T0m–200m at near the surface and the hilltop in the winter (Fig. 10). The normalized ratios (NR) were calculated using the following equation (Zhao et al., 2019a):

Some studies also found that particle concentration was inversely proportional with ambient temperature (Hussein et al., 2004; Zhao et al., 2015). The OC variations as increased temperature were more obvious than EC, which was mainly due to the impact of more primary emissions on winter EC. As temperature rose at near the surface, winter size–resolved OC and EC decreased due to mixing layer development under high temperature conditions (Hussein et al., 2006). However, high temperature at the hilltop may be induced by absorbing solar radiation by carbonaceous aerosols. Wang et al., 2018a,2018b,2018c,2018d found that the upper-level black carbon, especially that near the capping inversion, was more essential in suppressing the mixing layer height and weakening the turbulent mixing by its radiative effects. Therefore, high hilltop temperature corresponding to more stable atmosphere was not conducive to diffusing carbonaceous aerosols to the upper air. To better understand the impacts of atmospheric stratification on carbonaceous aerosols, the variations of size–resolved OC and EC as T200m–0m (temperature difference between 200 m above the surface and near the surface) at near the surface and the hilltop were given in Fig. 9. The T200m–0m indexes were calculated using the temperature profiles based on on–line measurement with ground-based multi-channel microwave radiometer. The larger T200m–0m corresponds to more stagnant air at near the surface level. The size–resolved OC and EC increased significantly as T200m–0m increased at near the surface, while OC and EC variations as T200m–0m were not obvious at the hilltop, suggesting that the surface carbonaceous aerosols were more easily affected by atmospheric stratification. Furthermore, the effects of atmospheric stratification on aerosols were more significant for fine fractions

NR =

C−μ σ

(2)

where, C is the ratio of size–resolved OC (EC) to horizontal wind speed or to T0m–200m,‾μ and σ are mean value and standard deviation of the ratios, respectively. The scales of horizontal and vertical axes in Fig. 10 were set to be equal to more intuitively see the importance of horizontal or vertical dispersion. The dots locating above (below) 1:1 line (the dashed lines) indicated that horizontal (vertical) dispersion was more effective to affect size–resolved carbonaceous aerosols. As it can be seen from Fig. 10, the roles of horizontal and vertical dispersions to carbonaceous aerosols were comparable at near the surface, while horizontal dispersion was more important for size–resolved carbonaceous aerosols at the hilltop. Interestingly, the ratios of size–resolved carbonaceous aerosols to wind speed were similar among the samples, but the ratios 138

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of size–resolved carbonaceous aerosols to T0m–200m for Sample 1 (during 26–30 December 2017) were significantly higher than those for the other samples (the red circle in Fig. 10), indicating that vertical dispersion was a main factor controlling size–resolved carbonaceous aerosols under highly polluted conditions in a typical urban valley.

based measurements of black carbon and particulate profiles within the lower troposphere during the foggy period in Delhi, India. Sci. Total Environ. 573, 894–905. Cai, W., Li, K., Liao, H., Wang, H., Wu, L., 2017. Weather conditions conducive to Beijing severe haze more frequent under climate change. Nat. Clim. Chang. 7, 257–262. Calvo, A.I., Alves, C., Castro, A., Pont, V., Vicente, A.M., Fraile, R., 2013. Research on aerosol sources and chemical composition: Past, current and emerging issues. Atmos. Res. 120–121, 1–28. Cao, J.J., Wu, F., Chow, J.C., Lee, S.C., Li, Y., Chen, S.W., An, Z.S., Fung, K.K., Watson, J.G., Zhu, C.S., Liu, S.X., 2005. Characterization and source apportionment of atmospheric organic and elemental carbon during fall and winter of 2003 in Xi'an, China. Atmos. Chem. Phys. 5, 3127–3137. Chang, Y., Deng, C., Cao, F., Cao, C., Zou, Z., Liu, S., Lee, X., Li, J., Zhang, G., Zhang, Y., 2017. Assessment of carbonaceous aerosols in Shanghai, China – Part 1: long-term evolution, seasonal variations, and meteorological effects. Atmos. Chem. Phys. 17, 9945–9964. Chen, C., Huang, J., Cheng, L., 1993. The Research on the Atmospheric Boundary Layer and Regularity of Atmospheric Diffusion over Complex Terrain. China Meteorological Press, Beijing, pp. 213 (in Chinese). Chen, Y., Xie, S., Luo, B., Zhai, C., 2014. Characteristics and origins of carbonaceous aerosol in the Sichuan Basin, China. Atmos. Environ. 94, 215–223. Cheng, C., Wang, G., Meng, J., Wang, Q., Cao, J., et al., 2015. Size-resolved airborne particulate oxalic and related secondary organic aerosol species in the urban atmosphere of Chengdu, China. Atmos. Res. 161–162, 134–142. Chilinski, M.T., Markowicz, K.M., Markowicz, J., 2016. Observation of vertical variability of black carbon concentration in lower troposphere on campaigns in Poland. Atmos. Environ. 137, 155–170. Chow, J.C., Watson, J.G., Lu, Z., Lowenthal, D.H., Frazier, C.A., Solomon, P.A., Thuillier, R.H., Magliano, K., 1996. Descriptive analysis of PM2.5 and PM10 at regionally representative locations during SJVAQS/AUSPEX. Atmos. Environ. 30 (12), 2079–2112. Chow, J.C., Watson, J.G., Chen, L.-W.A., Chang, M.O., Robinson, N.F., Trimble, D., Kohl, S., 2007. The IMPROVE_A temperature protocol for thermal/optical carbon analysis: maintaining consistency with a long-term database. J. Air Waste Manage. Assoc. 57, 1014–1023. Chu, P.C., Chen, Y., Lu, S., Li, Z., Lu, Y., 2008. Particulate air pollution in Lanzhou China. Environ. Int. 34 (5), 698–713. Cuccia, E., Massabò, D., Ariola, V., Bove, M.C., Fermo, P., Piazzalunga, A., Prati, P., 2013. Size-resolved comprehensive characterization of airborne particulate matter. Atmos. Environ. 67, 14–26. Daellenbach, K.R., Bozzetti, C., Křepelová, A., Canonaco, F., Wolf, R., Zotter, P., Fermo, P., Crippa, M., Slowik, J.G., Sosedova, Y., Zhang, Y., Huang, R.-J., Poulain, L., Szidat, S., Baltensperger, U., El Haddad, I., Prévôt, A.S.H., 2016. Characterization and source apportionment of organic aerosol using offline aerosol mass spectrometry. Atmospheric Measurement TechnAtmos. Meas. Tech.iques 9 (1), 23–39. Ding, A.J., Huang, X., Nie, W., Sun, J.N., Kerminen, V.-M., Petäjä, T., Su, H., Cheng, Y.F., Yang, X.-Q., Wang, M.H., Chi, X.G., Wang, J.P., Virkkula, A., Guo, W.D., Yuan, J., Wang, S.Y., Zhang, R.J., Wu, Y.F., Song, Y., Zhu, T., Zilitinkevich, S., Kulmala, M., Fu, C.B., 2016. Enhanced haze pollution by black carbon in megacities in China. Geophys. Res. Lett. 43, 2873–2879. https://doi.org/10.1002/2016GL067745. Ding, X.X., Kong, L.D., Du, C.T., Zhanzakova, A., Fu, H.B., Tang, X.F., Wang, L., Yang, X., Chen, J.M., Cheng, T.T., 2017. Characteristics of size-resolved atmospheric inorganic and carbonaceous aerosols in urban Shanghai. Atmos. Environ. 167, 625–641. Fermo, P., Piazzalunga, A., Vecchi, R., Valli, G., Ceriani, M., 2006. A TGA/FT-IR study for measuring OC and EC in aerosol samples. Atmos. Chem. Phys. 6 (1), 255–266. Guinot, B., Roger, J.-C., Cachier, H., Wang, P., Bai, J., Yu, T., 2006. Impact of vertical atmospheric structure on Beijing aerosol distribution. Atmos. Environ. 40, 5167–5180. Guo, S., Hu, M., Zamora, M.L., Peng, J., Shang, D., Zheng, J., Du, Z., Wu, Z., Shao, M., Zeng, L., Molina, M.J., Zhang, R., 2014. Elucidating severe urban haze formation in China. Proc. Natl. Acad. Sci. U.S.A. 111 (49), 17373–17378. Guo, Y., 2016. Size distribution characteristics of carbonaceous aerosol in a rural location in northwestern China. Air Qual., Atmosphere and Health 9, 193–200. Hou, B., Zhuang, G., Zhang, R., Liu, T., Guo, Z., Chen, Y., 2011. The implication of carbonaceous aerosol to the formation of haze: revealed from the characteristics and sources of OC/EC over a mega-city in China. J. Hazard Mater. 190 (1–3), 529–536. Hu, Y., Zhang, Q., 1999. Atmosphere pollution mechanism along with prevention cure countermeasure of the Lanzhou hollow basin. China Environ. Sci. 19 (2), 119–122 (in Chinese). Huang, R.J., Zhang, Y., Bozzetti, C., Ho, K.F., Cao, J.J., Han, Y., Daellenbach, K.R., Slowik, J.G., Platt, S.M., Canonaco, F., Zotter, P., Wolf, R., Pieber, S.M., Bruns, E.A., Crippa, M., Ciarelli, G., Piazzalunga, A., Schwikowski, M., Abbaszade, G., SchnelleKreis, J., Zimmermann, R., An, Z., Szidat, S., Baltensperger, U., Haddad, I.E., Prévôt, A.S.H., 2014. High secondary aerosol contribution to particulate pollution during haze events in China. Nature 514, 218–222. Huang, X., Wang, Z., Ding, A., 2018a. Impact of aerosol-PBL interaction on haze pollution: Multiyear observational evidences in North China. Geophys. Res. Lett. 45. https://doi.org/10.1029/2018GL079239. Huang, Y., Deng, T., Li, Z., Wang, N., Yin, C., Wang, S., Fan, S., 2018b. Numerical simulations for the sources apportionment and control strategies of PM2.5 over Pearl River Delta, China, part I: Inventory and PM2.5 sources apportionment. Sci. Total Environ. 634, 1631–1644. Huang, Q., Cai, X., Wang, J., Song, Y., Zhu, T., 2018c. Climatological study of the Boundary-layer air Stagnation Index for China and its relationship with air pollution. Atmos. Chem. Phys. 18, 7573–7593. Hussein, T., Puustinen, A., Aalto, P.P., Makela, J.M., Hameri, K., Kulmala, M., 2004. Urban aerosol number size distributions. Atmos. Chem. Phys. 4, 391–411.

4. Conclusions The vertical distributions of size–resolved carbonaceous aerosols were less studied in Chinese cities, which was not conducive to revealing the formation mechanism of haze pollution. In this study, the OC and EC size distributions at the two altitudes were analyzed and the key influencing factors were revealed with the help of boundary–layer profiles and basic meteorological parameter at the near surface and the hilltop. Some main conclusions were obtained as follows. The winter OC size distributions were typically bimodal with two comparable peaks in the specific bins of accumulation (0.65–1.1 μm) and coarse modes (4.7–5.8 μm), while those in summer were unimodal with the highest value in the size bin of 4.7–5.8 μm. Generally, sizeresolved OC and EC at near the surface were significantly higher than at the hilltop. The winter surface fine OC was about twice as high as that at the hilltop during the high OC periods, while for low OC values, the winter size-resolved OC was comparable between the two altitudes, The more coal or biomass utilization for winter domestic heating at the hilltop countryside make winter submicron EC differences between the two sites much smaller than in summer. Interestingly, summer OC and EC differences between the surface and hilltop sites were much smaller maybe due to stronger vertical mixing and larger summer SOC contributions at the hilltop. Overall, the size–resolved OC and EC may have common sources to some degree. Secondarily formed aerosols had some contributions to summer organic carbon. The residential sources from the southwest such as coal combustion or biomass burning largely impacted on carbonaceous aerosol size distributions at the hilltop. The winds paralleling with running urban valley were conducive to diffusing the air pollutants from near the surface to the upper air. The roles of horizontal and vertical dispersions to carbonaceous aerosols were comparable at near the surface, while horizontal dispersion was more important on size–resolved carbonaceous aerosols at the hilltop. Furthermore, the vertical dispersion was a main factor controlling size–resolved carbonaceous aerosols under highly polluted conditions in a typical urban valley. Acknowledgement The study is supported by Youth Innovation Promotion Association, CAS (2017462), Natural Science Foundation of China (41605103), CAS “Light of West China” Program and the Excellent Post-Doctoral Program (2016LH0020). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apr.2019.09.022. References An, X., Hu, Y., Lu, S., Zuo, H., 2008. Numerical simulation of heating effect of mountain peak. Plateau Meteorol. 27 (2), 286–292 (in Chinese). Arhami, M., Shahne, M.Z., Hosseini, V., Haghighat, N.R., Lai, A.M., Schauer, J.J., 2018. Seasonal trends in the composition and sources of PM2.5 and carbonaceous aerosol in Tehran, Iran. Environ. Pollut. 239, 69–81. Bian, Q., Alharbi, B., Shareef, M.M., Husain, T., Pasha, M.J., Atwood, S.A., Kreidenweis, S.M., 2018. Sources of PM2.5 carbonaceous aerosol in Riyadh, Saudi Arabia. Atmos. Chem. Phys. 18, 3969–3985. Bisht, D.S., Tiwari, S., Dumka, U.C., Srivastava, A.K., Safai, P.D., Ghude, S.D., Chate, D.M., Rao, P.S.P., Ali, K., Prabhakaran, T., Panickar, A.S., Soni, V.K., Attri, S.D., Tunved, P., Chakrabarty, R.K., Hopke, P.K., 2016. Tethered balloon-born and ground-

139

Atmospheric Pollution Research 11 (2020) 129–140

S. Zhao, et al.

aerosols at a high-altitude site on the central Tibetan plateau (Nam Co station, 4730 m asl). Atmos. Res. 153, 155–164. Wang, X., Huang, J., Zhang, R., Chen, B., Bi, J., 2010. Surface measurements of aerosol properties over northwest China during ARM China 2008 deployment. J. Geophys. Res. 115, D00K27. https://doi.org/10.1029/2009JD013467. Wang, H., An, J., Zhu, B., Shen, L., Duan, Q., Shi, Y., 2017. Characteristics of carbonaceous aerosol in a typical industrial city—Nanjing in Yangtze River Delta, China: size distributions, seasonal variations, and sources. Atmosphere 8, 73. https://doi. org/10.3390/atmos8040073. Wang, J., Qu, W., Li, C., Zhao, C., Zhong, X., 2018a. Spatial distribution of wintertime air pollution in major cities over eastern China: relationship with the evolution of trough, ridge and synoptic system over East Asia. Atmos. Res. 212, 186–201. Wang, X., Dickinson, R.E., Su, L., Zhou, C., Wang, K., 2018b. PM2.5 pollution in China and how it has been exacerbated by terrain and meteorological conditions. Bull. Am. Meteorol. Soc. 99 (1), 105–120. Wang, Q., Sun, Y., Xu, W., Du, W., Zhou, L., Tang, G., Chen, C., Cheng, X., Zhao, X., Ji, D., Han, T., Wang, Z., Li, J., Wang, Z., 2018c. Vertically resolved characteristics of air pollution during two severe winter haze episodes in Urban Beijing, China. Atmos. Chem. Phys. 18, 2495–2509. Wang, Z., Huang, X., Ding, A., 2018d. Dome effect of black carbon and its key influencing factors: a one-dimensional modelling study. Atmos. Chem. Phys. 18, 2821–2834. Wu, Y., Wang, P., Yu, S., Wang, L., Li, P., Li, Z., Mehmood, K., Liu, W., Wu, J., Lichtfouse, E., Rosenfeld, D., Seinfeld, J.H., 2018. Residential emissions predicted as a major source of fine particulate matter in winter over the Yangtze River Delta, China. Environ. Chem. Lett. 16 (3), 1117–1127. Xie, R., Sabel, C.E., Lu, X., Zhu, W., Kan, H., Nielsen, C.P., Wang, H., 2016. Long-term trend and spatial pattern of PM2.5 induced premature mortality in China. Environ. Int. 97, 180–186. Xie, C., Xu, J., 2017. Study on the absorption characteristics of winter aerosol in Lanzhou: the distinction between black carbon and brown carbon and a characteristic analysis. J. Glaciol. Geocryol. 39 (6), 1249–1257. Xu, J., Zhang, Q., Chen, M., Ge, X., Ren, J., Qin, D., 2014. Chemical composition, sources, and processes of urban aerosols during summertime in northwest China: insights from high-resolution aerosol mass spectrometry. Atmos. Chem. Phys. 14, 12593–12611. Zawadzka, O., Posyniak, M., Nelken, K., Markuszewski, P., Chilinski, M.T., Czyzewska, D., Lisok, J., Markowicz, K.M., 2017. Study of the vertical variability of aerosol properties based on cable cars in-situ measurements. Atmospheric Pollut. Res. 8, 968–978. Zhang, Q., 2001. The influence of terrain and inversion layer on pollutant transfer over Lanzhou City. China Environ. Sci. 19 (3), 230–235 (in Chinese). Zhang, Q., Hu, Y., 1992. A research about turbulent transport and intensity over complex terrain of Lanzhou. Plateau Meteorol. 11 (2), 126–132 (in Chinese). Zhang, Q., Li, H., 2011. A study of the relationship between air pollutants and inversion in the ABL over the city of Lanzhou. Adv. Atmos. Sci. 28 (4), 879–886. Zhang, Q., Ma, X., Tie, X., Huang, M., Zhao, C., 2009. Vertical distributions of aerosols under different weather conditions: analysis of in-situ aircraft measurements in Beijing, China. Atmos. Environ. 43, 5526–5535. Zhang, R., Jing, J., Tao, J., Hsu, S.-C., Wang, G., Cao, J., Lee, C.S.L., Zhu, L., Chen, Z., Zhao, Y., Shen, Z., 2013. Chemical characterization and source apportionment of PM2.5 in Beijing: seasonal perspective. Atmos. Chem. Phys. 13, 7053–7074. Zhang, Y.L., Kang, S.C., 2019. Characteristics of carbonaceous aerosols analyzed using a multiwavelength thermal/optical carbon analyzer: a case study in Lanzhou City. Sci. China Earth Sci. 62 (2), 389–402. Zhao, S.P., Yu, Y., Yin, D.Y., He, J.J., 2015. Meteorological dependence of particle number concentrations in an urban area of complex Terrain, Northwestern China. Atmos. Res. 164–165, 304–317. Zhao, S.P., Yu, Y., Yin, D.Y., He, J.J., Liu, N., Qu, J.J., Xiao, J.H., 2016. Annual and diurnal variations of gaseous and particulate pollutants in 31 provincial capital cities based on in situ air quality monitoring data from China national environmental monitoring center. Environ. Int. 86, 92–106. Zhao, S.P., Yu, Y., Qin, D.H., Yin, D.Y., He, J.J., 2017. Assessment of long-term and largescale even-odd license plate controlled plan effects on urban air quality and its implication. Atmos. Environ. 170, 82–95. Zhao, S.P., Yu, Y., Yin, D.Y., Qin, D.H., He, J.J., Dong, L.X., 2018a. Spatial patterns and temporal variations of six criteria air pollutants during 2015 to 2017 in the city clusters of Sichuan Basin, China. Sci. Total Environ. 624, 540–557. Zhao, S.P., Yu, Y., Yin, D.Y., Qin, D.H., He, J.J., Li, J.L., Dong, L.X., 2018b. Two winter PM2.5 pollution types and the causes in the city clusters of Sichuan Basin, Western China. Sci. Total Environ. 636, 1228–1240. Zhao, S.P., Yu, Y., Qin, D.H., Yin, D.Y., Du, Z.H., Li, J.L., Dong, L.X., He, J.J., Li, P., 2019a. Measurements of submicron particles vertical profiles by means of topographic relief in a typical valley city, China. Atmos. Environ. 199, 102–113. Zhao, S.P., Yu, Y., Yin, D.Y., Qin, D.H., Yu, Z.S., Dong, L.X., Yang, J.C., Mao, Z.L., He, J.J., Li, P., 2019b. PM1 carbonaceous aerosols during winter in a typical valley city of western China: vertical profiles and the key influencing factors. Atmos. Environ. 202, 75–92. Zhong, J., Zhang, X., Wang, Y., Liu, C., Dong, Y., 2018. Heavy aerosol pollution episodes in winter Beijing enhanced by radiative cooling effects of aerosols. Atmos. Res. 209, 59–64.

Hussein, T., Karppinen, A., Kukkonen, J., Harkonen, J., Aalto, P.P., Hameri, K., Kerminen, V.M., Kulmala, M., 2006. Meteorological dependence of size-fractionated number concentrations of urban aerosol particles. Atmos. Environ. 40 (8), 1427–1440. Ji, D., Yan, Y., Wang, Z., He, J., Liu, B., Sun, Y., Gao, M., Li, Y., Cao, W., Cui, Y., Hu, B., Xin, J., Wang, L., Liu, Z., Tang, G., Wang, Y., 2018. Two-year continuous measurements of carbonaceous aerosols in urban Beijing, China: temporal variations, characteristics and source analyses. Chemosphere 200, 191–200. Kroll, J.H., Seinfeld, J.H., 2008. Chemistry of secondary organic aerosol: formation and evolution of low-volatility organics in the atmosphere. Atmos. Environ. 42, 3593–3624. Li, J., Fu, Q., Huo, J., Wang, D., Yang, W., Bian, Q., Duan, Y., Zhang, Y., Pan, J., Lin, Y., Huang, K., Bai, Z., Wang, S.-H., Fu, J.S., Louie, P.K.K., 2015a. Tethered balloon-based black carbon profiles within the lower troposphere of Shanghai in the 2013 East China smog. Atmos. Environ. 123, 327–338. Li, P., Yan, R., Yu, S., Wang, S., Liu, W., Bao, H., 2015b. Reinstate regional transport of PM2.5 as a major cause of severe haze in Beijing. Proc. Natl. Acad. Sci. U.S.A. 112 (21), E2739–E2740. Li, C., Chen, P., Kang, S., Yan, F., Hu, Z., Qu, B., Sillanpaa, M., 2016. Concentrations and light absorption characteristics of carbonaceous aerosol in PM2.5 and PM10 of Lhasa city, the Tibetan Plateau. Atmos. Environ. 127, 340–346. Li, X.B., Wang, D.S., Lu, Q.C., Peng, Z.R., Wang, Z.Y., 2018. Investigating vertical distribution patterns of lower tropospheric PM2.5 using unmanned aerial vehicle measurements. Atmos. Environ. 173, 62–71. Li, Y., Zhang, L., Cao, X., Yue, Y., Shi, J., 2014. Property of black carbon concentration over urban and suburban of Lanzhou. China Environ. Sci. 34 (6), 1397–1403. Markowicz, K.M., Ritter, C., Lisok, J., Makuch, P., Stachlewska, I.S., Cappelletti, D., Mazzola, M., Chilinski, M.T., 2017. Vertical variability of aerosol single-scattering albedo and equivalent black carbon concentration based on in-situ and remote sensing techniques during the iAREA campaigns in Ny-Ålesund. Atmos. Environ. 164, 431–447. Petäjä, T., Järvi, L., Kerminen, V.-M., Ding, A.J., Sun, J.N., Nie, W., Kujansuu, J., Virkkula, A., Yang, X.Q., Fu, C.B., Zilitinkevich, S., Kulmala, M., 2016. Enhanced air pollution via aerosol-boundary layer feedback in China. Sci. Rep. 6, 18998. https:// doi.org/10.1038/srep18998. Piazzalunga, A., Belis, C., Bernardoni, V., Cazzuli, O., Fermo, P., Valli, G., Vecchi, R., 2011. Estimates of wood burning contribution to PM by the macro-tracer method using tailored emission factors. Atmos. Environ. 45 (37), 6642–6649. Querol, X., Alastuey, A., Viana, M., Moreno, T., Reche, C., Minguillon, M.C., Ripoll, A., Pandolfi, M., Amato, F., Karanasiou, A., Perez, N., Pey, J., Cusack, M., Vazquez, R., Plana, F., Dall’Osto, M., de la Rosa, J., Sanchez dela Campa, A., Fernandez-Camacho, R., Rodrıguez, S., Pio, C., Alados-Arboledas, L., Titos, G., Artınano, B., Salvador, P., Garcıa Dos Santos, S., Fernandez Patier, R., 2013. Variability of carbonaceous aerosols in remote, rural, urban and industrial environments in Spain: implications for air quality policy. Atmos. Chem. Phys. 13, 6185–6206. Saarikoski, S., Timonen, S., Saarnio, K., Aurela, M., Järvi, L., Keronen, P., Kerminen, V.M., Hillamo, R., 2008. Sources of organic carbon in fine particulate matter in northern European urban air. Atmos. Chem. Phys. 8, 6281–6295. Safai, P.D., Raju, M.P., Maheshkumar, R.S., Kulkarni, J.R., Rao, P.S.P., Devara, P.C.S., 2012. Vertical profiles of black carbon aerosols over the urban locations in South India. Sci. Total Environ. 431, 323–331. Seinfeld, J.H., Pandis, S.N., 2012. Atmospheric Chemistry and Physics: from Air Pollution to Climate Change. John Wiley & Sons. Shen, Z., Wang, Y., Ji, G., 1982. Weakening of solar radiation crossing polluted atmosphere layer in Lanzhou hollow. Plateau Meteorol. 10 (1), 74–83 (in Chinese). Shi, C., Yuan, R., Wu, B., Meng, Y., Zhang, H., Zhang, H., Gong, Z., 2018. Meteorological conditions conducive to PM2.5 pollution in winter 2016/2017 in the Western Yangtze River Delta, China. Sci. Total Environ. 642, 1221–1232. Shi, G.L., Tian, Y.Z., Han, S.Q., Zhang, Y.F., Li, X., Feng, Y.C., Wu, J.H., Zhu, T., 2012. Vertical characteristics of carbonaceous species and their source contributions in a Chinese mega city. Atmos. Environ. 60, 358–365. Tie, X., Huang, R., Dai, W., Cao, J., Long, X., Su, X., Zhao, S., Wang, Q., Li, G., 2016. Effect of heavy haze and aerosol pollution on rice and wheat productions in China. Sci. Rep. 6, 29612. https://doi.org/10.1038/srep29612. Tripathi, S.N., Srivastava, A.K., Dey, S., Satheesh, S.K., Krishnamoorthy, K., 2007. The vertical profile of atmospheric heating rate of black carbon aerosols at Kanpur in Northern India. Atmos. Environ. 41, 6909–6915. Turpin, B.J., Huntzicker, J.J., 1995. Identification of secondary organic aerosol episodes and quantitation of primary and secondary organic aerosol concentrations during SCAQS. Atmos. Environ. 29, 3527–3544. Turpin, B.J., Lim, H.J., 2001. Species contributions to PM2.5 mass concentrations: Revisiting common assumptions for estimating organic mass. Aerosol Sci. Technol. 35, 602–610. Vassura, I., Venturini, E., Marchetti, S., Piazzalunga, A., Bernardi, E., Fermo, P., Passarini, F., 2014. Markers and influence of open biomass burning on atmospheric particulate size and composition during a major bonfire event. Atmos. Environ. 82, 218–225. Venegas, L.E., Mazzeo, N.A., 1999. Atmospheric stagnation, recirculation and ventilation potential of several sites in Argentina. Atmos. Res. 52, 43–57. Wan, X., Kang, S., Wang, Y., Xin, J., Liu, B., et al., 2015. Size distribution of carbonaceous

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