Contrasting Vertical Circulation between Severe and Light Air Pollution inside a Deep Basin: Results from the Collaborative Experiment of 3D Boundary-Layer Meteorology and Pollution at the Sichuan Basin (BLMP-SCB)

Suping Zhao 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, and Pingliang Land Surface Process and Severe Weather Research Station, Pingliang, China;

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Jianjun He State Key Laboratory of Severe Weather and Key Laboratory of Atmospheric Chemistry of CMA, Chinese Academy of Meteorological Sciences, Beijing, China;

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Longxiang Dong 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, and Pingliang Land Surface Process and Severe Weather Research Station, Pingliang, China;

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Shaofeng Qi 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, and University of Chinese Academy of Sciences, Beijing, China;

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Daiying Yin University of Chinese Academy of Sciences, Beijing, and Key Laboratory of Desert and Desertification, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou, China

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Jinbei Chen 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, and Pingliang Land Surface Process and Severe Weather Research Station, Pingliang, China;

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Ye Yu 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, and Pingliang Land Surface Process and Severe Weather Research Station, Pingliang, China;

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Abstract

The study of air pollution in a valley is a classic research subject. Compared with flat terrain, the formation and development of haze pollution are more complicated and unique within a deep basin. How a basin or valley plays a role in the horizontal and vertical distribution of air pollutants is poorly understood in highly industrialized deep basins in China due to scarce field observations. We conducted a collaborative experiment of three-dimensional (3D) boundary layer meteorology and pollution at the western Sichuan Basin (SCB) close to the Tibetan Plateau (TP). Generally, the concentrations of PM1 (particulate matter smaller than 1.0 μm), NO, and NO2 largely decline with elevation, while O3 shows a slight increasing trend inside the SCB. Three different types of pollutant profiles and the formation mechanisms are described below. The high PM1 near the surface layer corresponds to the vertical clockwise circulation, i.e., a wind shift with increasing altitude in a clockwise direction. The air pollutants at the central and eastern SCB can be transported to the eastern foothills of the TP by southeasterly winds and then are trapped within the western SCB by a strong surface temperature inversion. The pollutants over the eastern TP also can be dispersed to above the SCB by westerly winds. More aerosol particles are concentrated at about 2.0 km MSL by jointly ascending and descending motion below and above the layer over the valley. The relative uniform PM1 in the vertical direction correlates to the counterclockwise circulation. The trapped pollutants in the western SCB can be transported to the eastern region by westerly winds and then are dispersed to the upper air by unstable stratification.

© 2023 American Meteorological Society. This published article is licensed under the terms of the default AMS reuse license. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding authors: Ye Yu, yyu@lzb.ac.cn; Jianjun He, hejianjun@cma.gov.cn

Abstract

The study of air pollution in a valley is a classic research subject. Compared with flat terrain, the formation and development of haze pollution are more complicated and unique within a deep basin. How a basin or valley plays a role in the horizontal and vertical distribution of air pollutants is poorly understood in highly industrialized deep basins in China due to scarce field observations. We conducted a collaborative experiment of three-dimensional (3D) boundary layer meteorology and pollution at the western Sichuan Basin (SCB) close to the Tibetan Plateau (TP). Generally, the concentrations of PM1 (particulate matter smaller than 1.0 μm), NO, and NO2 largely decline with elevation, while O3 shows a slight increasing trend inside the SCB. Three different types of pollutant profiles and the formation mechanisms are described below. The high PM1 near the surface layer corresponds to the vertical clockwise circulation, i.e., a wind shift with increasing altitude in a clockwise direction. The air pollutants at the central and eastern SCB can be transported to the eastern foothills of the TP by southeasterly winds and then are trapped within the western SCB by a strong surface temperature inversion. The pollutants over the eastern TP also can be dispersed to above the SCB by westerly winds. More aerosol particles are concentrated at about 2.0 km MSL by jointly ascending and descending motion below and above the layer over the valley. The relative uniform PM1 in the vertical direction correlates to the counterclockwise circulation. The trapped pollutants in the western SCB can be transported to the eastern region by westerly winds and then are dispersed to the upper air by unstable stratification.

© 2023 American Meteorological Society. This published article is licensed under the terms of the default AMS reuse license. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding authors: Ye Yu, yyu@lzb.ac.cn; Jianjun He, hejianjun@cma.gov.cn

The high-loading absorption aerosol in the near-surface layer promotes planetary boundary layer (PBL) development, which is the stove effect (Barbaro et al. 2014). However, the absorption aerosol aloft traps solar radiation to strongly heat up the upper air to suppress PBL development, i.e., the dome effect (Ding et al. 2016). The disparate impacts largely depend on the vertical location of absorption aerosol layers relative to the residual layer (Z. Q. Li et al. 2017; Ma et al. 2022). The absorbing aerosols lying below or above the morning residual layer (MRL) promote PBL development by heating the MRL layer. They conversely suppress PBL development by strengthening the temperature inversion layer after sunrise. As one of the main light-absorbing aerosols, a large-eddy simulation indicated that the heating rate of black carbon (BC) at various altitudes is between 0.01 and 0.016 K h−1 (μm−3)−1 of BC for a haze episode in Beijing (Slater et al. 2022). The heating rate increases with altitude but decreases around the PBL top. The fractional composition of 10% BC within the PBL increases maximum PBL height by 0.4%, but the same loading of BC above the PBL can suppress the height by 6.5% (Slater et al. 2022). The impact of BC within the PBL on PBL height is small (0.4% increase), compared to a 4%–6% increase suggested by Z. L. Wang et al. (2018) and a 10% increase by Ma et al. (2020). The much smaller increase in PBL height is mainly related to the stronger temperature inversion, low PBL height, and high PM2.5 concentrations in Beijing, and BC heating rate is still not enough to fully weaken the strong temperature inversion. Brown carbon (BrC), a fraction of organic carbon, can absorb sunlight in the lower visible to ultraviolet (UV) wavelength range (Alexander et al. 2008). Radiative transfer calculations suggest that BrC accounts for about 24% of combined BC and BrC warming effect at the tropopause, and roughly two-thirds of the estimated BrC forcing occurs above 5 km by aircraft measurements (Zhang et al. 2017). Huang et al. (2020) associated long-range transport with aerosol–PBL feedback and found an amplified transboundary transport of haze from the North China Plain to the Yangtze River Delta by aerosol–PBL interaction in China. Therefore, the vertical profiles of aerosol chemical compositions and optical properties are critical to reveal the aerosol–PBL meteorology interactions (C. F. Zhao et al. 2020).

In recent years, many in situ observations in aerosol vertical distribution within PBL in China are conducted by means of tethered balloon (Guan et al. 2022), unmanned aerial vehicle (Wu et al. 2021), cable car (Duan et al. 2021), meteorological tower (Sun et al. 2015), and ground stations at rapidly increasing altitudes from basin to mountain (Yin et al. 2020; Zhao et al. 2019). As an important environmental condition for air pollution studies, basin and mountain meteorology are widely studied in urban and agricultural basins and valleys across the globe, such as in the Salt Lake Valley (Baasandorj et al. 2017; Green et al. 2015; Whiteman et al. 2014) and San Joaquin Valley, United States (Brown et al. 2006; Chen et al. 2018; Pusede et al. 2014); Po Valley, Italy (Bressi et al. 2016; Gilardoni et al. 2014); Indo-Gangetic Plain, India (Tiwari and Kulshrestha 2019); and more. The wintertime episodes of high aerosol concentrations occur frequently in urban basins and valleys worldwide. These pollution episodes often arise due to the development of persistent cold-air pools that limit lateral and vertical mixing and modify chemistry because of sheltering by surrounding topography (Lareau et al. 2013). Furthermore, fog events are frequent in the basin and valley regions in response to stagnant air, and thus particle loss by fog scavenging may be important when the fog penetrates to the surface (Gilardoni et al. 2014). The unique basin topography (e.g., slope, how enclosed the basin is, and the size of basin) is found to play a key role in modulating the rate of pollutant buildup and vertical profiles of temperature and moisture (Hallar et al. 2021). However, most previous experiments and studies mainly focused on Europe and North America. Fewer studies were conducted in heavily polluted basins in China. In addition, the coupling between interconnected chemical and meteorological processes within basins remains an insufficiently studied research area, especially for rapid basin urban expansion in western China.

The Sichuan Basin (SCB), enclosed by mountains with elevations ranging from 1 to 4 km, is one of the most populous and polluted regions in China, with the two megacities of Chengdu and Chongqing in the western and eastern basin, respectively. The unique basin topography, Tibetan Plateau (TP) higher than 4 km to the west, provides a good platform to study basin meteorology and pollution in the large urban agglomeration. The SCB is exposed to the air stagnation conditions for approximately half of the year due to the closed terrain impacts (X. Wang et al. 2018), and thus frequently triggers long-lasting regional haze pollution (Qiao et al. 2019; Zhao et al. 2018a,b). The air pollutants inside the SCB can be transported to eastern TP where altitude is lower than 3 km (Yin et al. 2020), which may be more and more significant due to stronger “heat pump” induced by elevation-dependent warming (C. F. Zhao et al. 2020; S. Y. Zhao et al. 2020). The air pollutants even can lift into the Asian monsoon anticyclone by convective processes (Lee et al. 2021). The dry low-trough synoptic patterns occur frequently inside SCB (Ning et al. 2019). In the case of this pattern, westerly wind prevails over the SCB and the aloft atmosphere is warmer than the TP at the same height, which induces the cold air over the TP moving eastward to the SCB. Under the synergistic effects of the cold air eastward movement and TP, the strong stable layer above the PBL due to strong descending motion suppresses secondary circulation and plays a key role in the formation of heavy air pollution during the wintertime within the basin (Ning et al. 2018, 2019).

The previous in situ observations of aerosols were mainly inside the basin (Yang et al. 2020; Zhang et al. 2019; Zhao et al. 2021), especially for the two megacities (Chen et al. 2014; Peng et al. 2020; H. Wang et al. 2018), while it is less known about the vertical distributions of aerosols and meteorology within PBL above the SCB. The three-dimensional (3D) in situ observation of air pollutants and meteorological variables from the basin to mountain or upper PBL (about 2–3 km AGL) is essential for revealing the formation mechanisms of heavy air pollution within the SCB. The aim of this paper is to reveal the impact of 3D PBL meteorology on air pollution at the unique terrain from deep basin to the plateau.

Design of the field experiment

We conducted the first collaborative experiment of 3D Boundary-Layer Meteorology and Pollution at the western edge of the Sichuan Basin (BLMP-SCB), where the terrain is most complex inside the SCB. There are two sites (Chengdu and Sanbacun) in the western SCB (Fig. 1). The air pollution is more severe and the formation mechanism is more complicated at the western SCB as compared to other regions of the basin (Ning et al. 2018). That phenomenon is mainly related to poor diffusion conditions due to TP blocking effects. Therefore, our study mainly concentrates on the region from the western SCB to eastern TP (Fig. 1) to better understand the impact of PBL meteorology on air pollution by the field campaign. Daytime [0730–1830 local time (LT); local time = UTC + 8] and nighttime (1930–0630 LT) PM1 samples were collected offline for the following determination of carbon components at the six sites with the elevation extending from about 500 m at Chengdu to 3,500 m at Hongyuan from 28 December 2018 to 26 January 2019, and thus the result of this study is limited to the winter season only. Offline here means that the chemical components cannot be directly obtained by the instrument, and the concentrations of carbonaceous aerosols and the optical properties are determined with a thermal/optical carbon analyzer in the laboratory.

Fig. 1.
Fig. 1.

Locations of six sampling sites and experimental setup of 3D pollution and meteorology within the PBL at the transition region from the western Sichuan Basin (SCB) to eastern Tibetan Plateau (TP).

Citation: Bulletin of the American Meteorological Society 104, 2; 10.1175/BAMS-D-22-0150.1

The mass concentrations of air pollutants (PM1, carbonaceous aerosols, NO, NO2, O3) and meteorological variables (temperature, relative humidity) are mea­sured in situ online respectively by a lightweight low-cost multipollutant sensor package, microAeth MA-200 (AethLabs, United States), and a radiosonde carried by a tethered balloon (Fig. 1). Online instruments can directly measure the concentrations of air pollutants and meteorological variables with high temporal resolution. The multipollutant (PM1, NO, NO2, and O3) sensor package, developed by Pang et al. (2021), is substantiated to be a reliable and accurate device for aerial measurements of air pollutants within the PBL. The performance of the sensors was compared with on-ground reference instruments and commercial portable instruments from Thermal Scientific and Vaisala in aerial observations (Pang et al. 2021). All gas and PM sensors showed good correlations with the reference instruments, with R2 varying from 0.81 to 0.93 and slope from 0.89 to 1.35. The instrument of MA-200 makes real-time five-wavelength (375, 470, 528, 625, and 880 nm) optical analyses by measuring the rate of decrease in transmitted light through the sample filter, due to continuous particle deposition on the filter. Measurement at 880 nm is interpreted as the concentration of BC. Measurement at 375 nm is interpreted as ultraviolet particulate matter (UVPM) indicative of woodsmoke, tobacco, and or biomass burning. The MA-200 may produce negative values in lower concentrations and at a high time resolution, which can contribute up to 30% of uncertainty for filter-based optical attenuation technique (Hagler et al. 2011). The optical noise-reduction averaging (ONA) program was used to postprocess the negative values from our real-time measurements. The algorithm conducts variable time averaging of carbon component data produced from MA-200 in order to reduce noise in the data. The ONA algorithm leads to significant noise reductions and much more reasonable temporal variations in concentrations of carbonaceous aerosols (Cheng and Lin 2013; Park et al. 2010). The horizontal and vertical wind profiles were continuously obtained by Doppler wind lidar at the western SCB. Detailed information on the instruments used during the campaign can be found in appendixes AC. The absorption Ångström exponent (AAE) is calculated by MA-200 measurements, which is described in detail in appendix E. Local Beijing time (UTC + 8) is used throughout the paper.

Vertical profiles of air pollutants and meteorological variables

The displacements of the tethered balloon were within 1.5 km (Fig. ES1 in the online supplemental material), and thus the measurements can reflect the vertical distributions of air pollutants and meteorological variables within the PBL during the campaign. The vertical profiles of air pollutants changed significantly among the same hour of days (see the profiles in each subplot of Fig. 2), probably responding to PBL meteorology changes. The diurnal variations of vertical profiles of the air pollutants may be jointly affected by primary sources and meteorology. For example, the PM1 vertical distributions were more uniform in the afternoon than that in the morning and evening due to strong afternoon mixing and high PBL. However, morning and evening PM1, NO, and NO2 had the higher concentrations at the near-surface layer due to a strong temperature inversion and deep weak wind layer (Fig. 3) corresponding to the unique bowl-shaped basin. Compared with PM1 and NO, NO2 reduction was more rapid with the increased altitude, which is consistent with the results of Pang et al. (2021). Unlike PM1, NO, and NO2, O3 slightly increased as the increased height, especially during daytime. Daytime strong convection can transport formed O3 from the surface layer to upper air. In addition, photolysis of stratospheric O2 by the short-wavelength UV radiation supplies atomic oxygen and facilitates formation of the O3 layer (Wang et al. 2017), and the formed stratospheric O3 can be downward transported to near ground level (Vingarzan 2004). In addition, the slightly increased O3 with the height may be partly attributed to O3 reduction near the surface by deposition (Wang et al. 2017).

Fig. 2.
Fig. 2.

Changes in vertical profiles of air pollutants (PM1, NO, NO2, and O3) during the campaign and the corresponding mean profiles for each 3 h of the day (the bold black lines).

Citation: Bulletin of the American Meteorological Society 104, 2; 10.1175/BAMS-D-22-0150.1

Fig. 3.
Fig. 3.

Changes in vertical profiles of meteorological variables (temperature, potential temperature, RH, wind speed, and vertical velocity) during the campaign and the corresponding mean profiles for each 3 h of the day (the bold black lines).

Citation: Bulletin of the American Meteorological Society 104, 2; 10.1175/BAMS-D-22-0150.1

PBL meteorology is considered to be the key factor modulating the formation and development of haze pollution episodes (Li et al. 2021), which may be more significant inside the deep SCB in response to strong temperature inversion and weak winds (Feng et al. 2022). However, previous studies were mainly based on reanalysis data or numerical simulation, and there is a lack of the intensive sounding observation within the SCB (Liu et al. 2021). The reanalysis data are grid cell averages, and thus PBL structure cannot be obtained accurately, especially for complex terrain covering a wide range of altitudes. Potential temperature does not depend on air pressure or elevation, and thus it is a more useful variable for PBL behavior as compared to temperature, especially for analysis of temperature inversion. In general, the diurnal variation of potential temperature profiles is similar to that of temperature profiles (Fig. 3). However, the potential temperature is much lower than the temperature at upper PBL while it is slightly higher than temperature at lower PBL, which causes the greater gradient in potential temperature from near the surface to upper PBL. In combination with potential temperature and RH, there is a shallow temperature inversion at 0200 LT due to surface longwave radiative cooling (Fig. 3), and then the inversion gradually strengthens and deepens until 0800 LT when thickness of inversion layer is about 300 m. Thereafter, the surface inversion layer is destroyed by solar shortwave radiative heating, and then the temperature inversion restarts to be established at 2000 LT on most of days during the campaign and gradually strengthens at 2300 LT due to more rapid temperature reduction at the surface than atmosphere. More interestingly, the capping inversion sometimes occurs in altitudes ranging from 2.0 to 2.5 km AGL within the western basin, which may be induced by the dry low-trough synoptic pattern (Ning et al. 2018). In the case of the pattern, the air layer is descending motion (known as foehn, Fig. 3) due to cold air over the TP moving eastward to the SCB along with prevailing westerly wind and terrain impacts, and the downward air masses warms up and triggers temperature inversion on the leeward slope of the towering TP (Ning et al. 2019). At the capping inversion layer, PM1 mass concentrations had high values (Fig. 2). Besides warm lid impacts, the high PM1 may be affected by long-range transported pollutants from South Asia, one of the most polluted regions, which will be analyzed deeply in the following sections.

Compared with temperature and RH, wind speed varied less in the vertical direction (Fig. 3), which probably reflects the barrier effect of the TP large topography. The mean horizontal wind speed is smaller than 3 m s−1 when the altitude is lower than 3 km, and it is even lower than 2 m s−1 at the altitude lower than 2 km. For some profiles, the increases in wind speed are more significant in the afternoon, which may be closely related to PBL development. For the vertical velocity (w), the mean value hovers around 0 at the altitudes lower than 2 km, but the largely negative w sometimes occurs in the higher altitude, which may be induced by the strong descending motion (known as foehn) on the leeward slope of the TP (Ning et al. 2018).

In general, the carbonaceous aerosol concentrations decline as the increased altitude, which is similar to PM1 profiles (Fig. ES2). The BC and UVPM concentrations near the surface level can reach 20 μg m−3 at 0800 LT, which may be induced by both strong temperature inversion in the morning (Fig. 3) and open local coal and biomass burning for cooking and heating at rural areas. At the same experimental site, the recent study of Zhao et al. (2021) also found that light absorption coefficient at 405 nm of brown carbon (BrC), a fraction of organic carbon, is about 5 times higher than that at urban site. Compared with secondary formation, the strong light absorption more depends on primary organic carbon (OC) from biomass and coal burning (Zhao et al. 2021). Unlike BC and UVPM profiles, the averaged AAE values are between 1.0 and 2.0 at an altitude lower than 1.5 km, and then increases to approaching or exceeding 2.0 at the higher altitude. AAE value is generally close to 1.0 for fossil fuel combustion (FF), while that is the larger for biomass burning (BB) (Day et al. 2006). It is inferred that the carbonaceous aerosols are mainly from both FF and BB at altitudes lower than 1.5 km, while BB is dominant at the higher altitudes within the western SCB, and the causes will be analyzed in detail in the following sections.

The concentration of UVPM decreases more slowly with increasing altitude than does BC, and the vertical profiles of UVPM minus BC (UVPM − BC) are contradicted to UVPM and BC while known high concentrations are near sources (Fig. 4). The previous studies also found a similar phenomenon by aircraft measurements (Liu et al. 2014, 2015; Zeng et al. 2020), and they attributed it to deep convective storms preferentially lofting BrC and in-cloud heterogeneous BrC production (Zhang et al. 2017). However, the deep convection in winter is relatively less within the SCB and the observation height is much lower due to the limitations of the tethered balloon, and thus the two causes may not be true for this study. The slower UVPM decrease with altitude than BC, the contradicted UVPM − BC profiles to UVPM and BC, is mainly related to long-range and regional UVPM transports from South Asia and Tibetan Plateau (Figs. 8 and 9) due to intensive biomass burning in the regions (Zhao et al. 2022). In addition, the transported UVPM is easily trapped above the SCB in response to stable atmospheric stratification inside the basin. More interestingly, at altitudes lower than 1.5 km, the mean concentrations between UVPM and BC are comparable during nighttime and in the morning and evening (stable stratification), while the differences between the two carbon components (UVPM − BC) are much larger at noon and in the afternoon (unstable stratification) and can even reach 2.0 μg m−3 at 1400 LT. The relatively high afternoon UVPM concentrations at the surface level may be mainly related to strong downward UVPM transport from the upper air (also see Fig. 3) corresponding to declining UVPM concentrations at altitudes higher than 1.5 km. Therefore, the contrasting UVPM − BC to UVPM and BC profiles are co-influenced by long-range transported UVPM from South Asia and TP, terrain, and atmospheric stratification conditions above the SCB.

Fig. 4.
Fig. 4.

Comparisons between mean BC and UVPM profiles and the profiles of UVPM minus BC (UVPM − BC) for each 3 h of the day during the campaign.

Citation: Bulletin of the American Meteorological Society 104, 2; 10.1175/BAMS-D-22-0150.1

Impact of meteorology on aerosol vertical profiles.

To better understand the structure of PM1 vertical profiles and reveal the causes inside the SCB, all the PM1 vertical profiles by means of the tethered balloon during the campaign were classified as three typical clusters with unique characteristics: cluster 1 has one PM1 peak at the altitude lower than 1.0 km, cluster 2 has a high concentration of PM1 mass at about 2.0 km above mean sea level (MSL), and cluster 3 has a uniform vertical distribution of PM1 concentrations. Before classifying the profiles, the PM1 mass concentrations for each profile were standardized with the following equation:
PM1,std=PM1μσ,
where μ and σ are mean value and standard deviation of PM1 mass concentration for each profile.

As can be seen from Fig. 5, for cluster 1, PM1 concentrations near the surface layer exceed 120 μg m−3 and then rapidly decline as the altitude increases, and decrease to 20 μg m−3 at an altitude of 2.65 km. Cluster 1 mainly occurs in the nighttime (0200, 0500, 0800, 2000, and 2300 LT) and has a strong temperature inversion and high relative humidity near the surface. The winds are southeasterly at the layer lower than 1.0 km, and then shift to southwesterly at the altitude of 1.0 km and wind speed increases significantly as the height increases. The wind direction varies little, while wind speed significantly increases with the altitude at an altitude higher than 1.0 km, and thus strong wind speed shears induce the stronger and deeper descending motion at the layer. In addition, the heavy PM1 pollution near the surface layer at western SCB is related to regional transport from central and eastern SCB by southeasterly winds, which is further enhanced by poor dispersion conditions, such as strong ground temperature inversion (Fig. 5c). Liao et al. (2017) also found that air masses from southeast might be the most important potential source regions of surface fine particulate matter at western SCB due to huge barrier effects of Tibetan Plateau. Wind shear turned out to be the important factor in terms of PM10 vertical profile modification in a complex terrain due to the presence of a large valley (Sekuła et al. 2021).

Fig. 5.
Fig. 5.

(a) Three clusters of typical PM1 profiles and (b) diurnal variations of occurred frequency, and the corresponding mean vertical profiles of (c) temperature and VTKE{VTKE=[(1/2)(u2¯+υ2¯+w2¯)]1/2}, (d) relative humidity, and (e)–(g) wind vectors in the x–y plane and vertical velocity (w) for three different types of PM1 profiles during the campaign. The colored circles in (e)–(g) represent the vertical circulations induced by wind shear.

Citation: Bulletin of the American Meteorological Society 104, 2; 10.1175/BAMS-D-22-0150.1

In contrast to cluster 1, the mean PM1 mass concentration of cluster 2 varies little at altitudes lower than 1.0 km, and it rapidly increases to 120 μg m−3 until an altitude of 2.0 km, and then PM1 sharply decreases as the elevation increases (Fig. 5a). Cluster 2 mainly appears at 0200, 0500, and 1700 LT, which accounts for three-quarters of all PM1 profiles of the cluster (Fig. 5b). The air temperature not only declines with increasing altitudes, but also decreases more significantly from the surface to an altitude of 2.0 km as compared to the other two clusters (Fig. 5c). Similarly, the relative humidity (RH) increase with altitude is the most significant at this layer among the three clusters (Fig. 5d). Therefore, the temperature and RH profiles of cluster 2 indicate that the atmospheric stratification at the layer between the surface and the altitude of 2.0 km is more unstable among the three clusters. The southwestern winds and positive vertical velocities (w) also represent strong upward motion at the altitudes lower than 2.0 km (Fig. 5e). However, at the layer of altitudes between 2.0 and 2.4 km, the lapse rate of the temperature decreases and RH declines as with increasing elevation, and the wind shifts from southwesterly to northeasterly or southeasterly with a strong downward motion at the layer. Therefore, the high PM1 concentration at the altitude of 2.0 km is mainly because the particulate matter is trapped within the layer by both strong upward and downward motion at the lower and upper layers.

Compared with the other two clusters, the mean PM1 profile of cluster 3 is between that of clusters 1 and 2 (Fig. 5a). PM1 mass concentrations decline significantly from 90 μg m−3 near the surface to below 60 μg m−3 at an altitude of 1.5 km, and then keep a constant value until an altitude of 2.0 km, and gradually reduce to 30 μg m−3 as the elevation increases. Cluster 3 mainly occurs at 0800, 1100, 1700, 2000, and 2300 LT, with about a quarter of PM1 profiles appearing at 1100 LT. The average RH change in the vertical directions for the cluster is negligible with gradual reduction at the layer between the altitudes of 1.5 and 2.0 km. From wind vectors and vertical velocity perspectives (Fig. 5e), the winds are southwesterly with upward motion at an altitude lower than 1.5 km, and shift to northeasterly and increase in speed with subsidence until an altitude of 3.0 km, and thereafter shift to southwesterly again. In brief, the clockwise and counterclockwise vertical circulations for clusters 1 and 2, and a clockwise cycle superimposed above a counterclockwise vertical circulation for cluster 3, are found within the western SCB (Fig. 5e). The circulation configuration at the vertical directions may be important for modulation of aerosol vertical profiles above the basin terrain. Sekuła et al. (2021) also found that wind shear largely impacts on PM10 vertical structure in a complex terrain with a sightseeing balloon.

For each cluster, we also checked the diurnal variations of wind speed, temperature, and relative humidity at the six sites from western SCB to eastern TP during the field campaign (Fig. 6). The wind speed shows a peak in the afternoon along with strongly developed PBL, especially at Hongyuan and Wenchuan. The high winds blow from the southwest at the two sites (Fig. ES3), which are consistent with wind lidar measurements (Fig. 5). The easterly or southeasterly winds occur in the daytime at Wenchuan and Lixian, closer to the SCB as compared to Maerkang and Hongyuan (Fig. 1), which may be due to the upward motion of valley air by radiative heating during day. As diurnal cycle of mountain meteorology showed in Fig. 6, the temperature significantly declines with the increasing altitudes from western SCB to eastern TP in the nighttime and early morning, while that is more comparable among the sites in the afternoon (1400 and 1700 LT) due to the stronger radiative heating over TP. The diurnal variations of wind speed and temperature are similar among the three clusters. For all hours of the day, relative humidity at Hongyuan and Maerkang are significantly lower than that at the other sites with the lower altitudes for the three clusters. However, at the basin sites (Chengdu and Sanbacun), relative humidity for cluster 1 is much higher than that for clusters 2 and 3, especially in the afternoon, which is related to weak turbulent mixing inside the deep SCB for cluster 1. In addition, we calculated profiles of turbulent intensity (VTKE, Lan et al. 2018) for each cluster to see the relationships between mixing state and clusters (Fig. 5c). The turbulent mixing at the altitudes lower than 0.9 km for cluster 1 is the weakest among the clusters, which corresponds well to high PM1 concentrations below the layer for cluster 1 (Fig. 5a). At altitudes higher than 2.0 km, high PM1 concentrations of cluster corresponding to strong turbulent mixing may be related to the radiative heating of absorbing aerosols.

Fig. 6.
Fig. 6.

Diurnal variations of wind speed, temperature, and relative humidity for each cluster (columns 1, 2, and 3 correspond to clusters 1, 2, and 3, respectively) at the six sites from the western SCB (Chengdu, Sanbacun) to eastern TP (Wenchuan, Lixian, Maerkang, and Hongyuan) during the campaign.

Citation: Bulletin of the American Meteorological Society 104, 2; 10.1175/BAMS-D-22-0150.1

We selected one case (1 January 2019) to better understand the effect of basic meteorological variables (temperature, RH, wind, and vertical velocity) on aerosol vertical distributions within the planetary boundary layer inside the SCB (Fig. 7). PM1 vertical profiles are similar from 0200 to 0800 LT with the highest concentrations between 0.7 and 0.8 km (Fig. 7a), which belong to the typical cluster 1 (also see Fig. 5). The air pollutants are easily trapped near the surface by the deep temperature inversion due to surface radiative cooling and thus PM1 concentration largely declines as the increasing altitude. The clockwise circulations at the vertical direction with southeasterly winds at the altitudes lower than 1.0 km and southwesterly winds at the higher elevation during 0200–0800 LT are consistent with cluster 1. Thereafter, the vertical location of the largest PM1 value for each profile becomes increasingly high from an altitude of 1.3 km at 1100 LT to that of 2.4 km at 1700 LT (Fig. 7a) due to more and more unstable atmospheric stratification (Figs. 7b,c) and strong upward motion (Fig. 7d). The winds are westerly in the whole layer with altitudes lower than 2.0 km, while winds shift to easterly at higher altitudes, and thus forms a counterclockwise vertical circulation belonging to cluster 2. The shallow temperature inversion layer begins to be established at 2000 LT, and the winds shift again to southeasterly with subsidence near the surface, which causes significant PM1 reduction from the surface to 1.5 km MSL. The PM1 mass peaks between 1.5 and 2.0 km MSL at 2000 and 2300 LT may be induced by wind shear at the layer (Fig. 7d). Synoptic patterns and thermodynamic structures in the lower troposphere are similar among the three clusters (Fig. ES4, the used reanalysis data are described in detail in appendix D), and thus the wind lidar observation is essential to studying atmospheric dispersion and its impacts on air pollution.

Fig. 7.
Fig. 7.

Diurnal variations in vertical profiles of (a) PM1 mass concentrations, (b) temperature, (c) relative humidity, and (d) wind vectors in the x–y plane on 1 Jan 2019. The colors of the wind vectors in (d) represent vertical velocity (w).

Citation: Bulletin of the American Meteorological Society 104, 2; 10.1175/BAMS-D-22-0150.1

Impact of regional and long-range transports on aerosol vertical profiles

PM1 pollution rise is largely dependent on cluster and height (Fig. 8). For C1 with the highest PM1 mass near the surface layer (Fig. 5), the locations of severe PM1 pollution toward the experiment site vary from the southeast and west at 750 m MSL to the northeast and southwest at 1,350 m MSL and then to the southwest at 1,950 m MSL. For C2 with the highest PM1 at ∼2.0 km MSL, the high surface PM1 mass corresponds to southwesterly winds at 750 m MSL and southwesterly and northeasterly winds at higher altitudes. The PM1 origination relative to the site at 750 and 1,350 m MSL for C3 is similar to that for C2, while at 1,950 m MSL for C3, it is mainly northeast. The gridded back trajectory concentration indicated that the high PM1 mass concentrations at 750 and 1,350 m MSL potentially originate from South Asia, while those at 1,950 m MSL mainly come from central and eastern SCB (Fig. 9, detailed information can be found in appendix F). Similar figures for BC and UVPM also were seen, and the results were consistent with PM1 mass concentrations (the figures are omitted).

Fig. 8.
Fig. 8.

PM1 pollution rose at 750 m (H1), 1,350 m (H2), and 1,950 m MSL (H3) for the obtained clusters 1, 2, and 3 in Fig. 5 marked as C1, C2, and C3, respectively. Mean PM1 mass concentrations are also given in the corresponding subplots.

Citation: Bulletin of the American Meteorological Society 104, 2; 10.1175/BAMS-D-22-0150.1

Fig. 9.
Fig. 9.

Gridded back trajectory concentrations showing mean PM1 concentrations using the CWT approach at 750 m (H1), 1,350 m (H2), and 1,950 m MSL (H3) for the obtained clusters 1, 2, and 3 in Fig. 5 marked as C1, C2, and C3, respectively.

Citation: Bulletin of the American Meteorological Society 104, 2; 10.1175/BAMS-D-22-0150.1

At the same geographical location, the carbonaceous aerosols increase synchronously between online and offline measurements at Sanbacun, especially during daytime with coefficients of determination (R2) larger than 0.40 (Figs. 10a1,a2). The low correlation between online measured BC and offline obtained elemental carbon (EC) may be partly attributed to the errors induced by the different measurement methods. The offline collected samples by the quartz filter membrane are stored in the refrigerator for the subsequent carbon component analysis by DRI-2015 in the laboratory, which may result in much higher concentrations of carbon components by optical measurements due to mixing with other chemical composition and hygroscopic growth. In addition, UVPM, interpreted as the optical measurement at 375 nm, may be comparable to BrC (a small fraction of OC), which may cause the low correlation between online measured UVPM and offline measured OC. The OC and EC tested by DRI-2015 is 1–25 times higher than UVPM and BC measured by MA-200 at Wenchuan, Lixian, and Maerkang. Besides the abovementioned causes, the low correlation may be mainly due to different measurement location with the same altitude. The ratio of UVPM above the SCB to OC over the TP with the same altitude as UVPM measurement decreases with the altitude, which may be mainly related to more OC sources in the wintertime TP. Additionally, decreasingly fewer TP aerosols can be transported to above the SCB with increasing altitude due to the farther distance between the plateau sites and the locations above the SCB (see Fig. 1). As the elevation increases, the relationships between offline measured EC at the plateau sites and online measured BC at the locations with the same altitude above the SCB become weaker and weaker, which also is because the fewer BC aerosols inside the SCB are dispersed to much higher altitudes in response to stable atmospheric stratification.

Fig. 10.
Fig. 10.

Relationships between BC (UVPM) online measured by the MA-200 by means of tethered balloon and EC (OC) offline tested by DRI-2015 by means of orographic lifting at (left) daytime and (right) nighttime during the campaign at (a) Sanbacun, (b) Wenchuan, (c) Lixian, and (d) Maerkang. The selected observation heights of MA-200 by tethered balloon are similar to the altitudes of the four sites to better compare carbon components at identical elevations. The error bars represent the standard deviation. The coefficients of determination (R2) larger than 0.25 passed the significance level of 0.01.

Citation: Bulletin of the American Meteorological Society 104, 2; 10.1175/BAMS-D-22-0150.1

More interestingly, OC concentrations at Wenchuan, Lixian, and Maerkang increase synchronously as UVPM concentrations above the SCB at the same heights as the plateau sites when UVPM is lower than 15 μg m−3 (Phase I), and thereafter the relation is opposite (Phase II). The difference between Phases I and II represents regional transport, PBL meteorology, and terrain impacts. For Phase I, the synchronous increases are more significant during daytime (the left column of Fig. 10), indicating that the origin of UVPM above the SCB and OC at the eastern TP may be identical and originates from basin biomass burning. The carbonaceous aerosols inside the SCB are dispersed to upper air by converging motion at basin during day. They also are transported to eastern TP by upward motion of valley air by radiative heating during daytime. However, for Phase II, the negative correlation mainly occurs in the nighttime, which is mainly related to that the organic carbon from biomass burning at the eastern TP may be transported to above the SCB by westerly winds (also see Fig. ES4). As can be seen from Fig. 5a, PM1 reduction above 1.5 km MSL starts to get slower than that at the lower elevations, which may be related to regional transport from the eastern TP, which also can confirm the above inference. Furthermore, the downward motion of mountain air by radiative cooling during nighttime brings biomass burning plumes at the eastern TP to the SCB, and then the aerosols are uplifted to the upper PBL. Briefly, the vertical profiles of air pollutants within the basin terrain are affected not only by PBL meteorology, but also by regional and long-range transports. The vertical distributions of air pollutants and the causes over the basin are more unique and complicated as compared to those over the flat terrain.

In brief, the heavy PM pollution near the surface layer corresponds to the vertical clockwise circulation, a wind shift with increasing altitude in a clockwise direction, and the transported aerosol particles from central and eastern SCB by southeasterly winds are trapped at the eastern foothills of large TP by strong surface temperature inversion (Fig. 11). The upper westerly winds can partly transport primary PM from biomass and coal burning over eastern TP to above the SCB, and then the aerosol particles are downward transported due to subsidence motion with the lower temperature. The light PM pollution correlates to the vertical counterclockwise circulation, a wind shift with increasing altitude in a counterclockwise direction. The air pollutants at the western SCB are dispersed to eastern areas by westerly winds and then are transported to upper air by the unstable atmospheric stratification.

Fig. 11.
Fig. 11.

Schematic diagram of PBL meteorology for (a) heavy and (b) light PM pollution within the Sichuan Basin. Air temperature is abbreviated as “Temp.” in each panel.

Citation: Bulletin of the American Meteorological Society 104, 2; 10.1175/BAMS-D-22-0150.1

Summary and conclusions

The concentrations of PM1, NO, and NO2 decline significantly as the elevation increases, especially for NO and NO2. However, O3 shows a slight increasing trend with altitudes. BC and UVPM concentrations can reach 20 μg m−3 near the surface layer in the morning and evening due to strong temperature inversion and more primary emissions within the SCB. The carbonaceous aerosols are mainly from fossil fuel (FF) and biomass burning (BB) at altitudes lower than 1.5 km, while BB is dominant at the higher elevations. UVPM decreases more slowly with the increasing elevation than does BC, and the afternoon higher concentrations below 1.5 km MSL may be mainly related to the strong downward transport from upper air. Besides primary sources, the vertical distributions in air pollutants are jointly affected by boundary layer thermal and dynamic effects.

There are three typical PM1 profiles (one peak below 1.0 km MSL, one peak at about 2.0 km MSL, and relatively uniform distributions at the vertical directions) in the western SCB. The high PM1 concentration near the surface layer corresponds to the vertical clockwise circulation, and the southeasterly winds can transport the air pollutants from the central and eastern SCB to the front areas of the TP, and the pollutants are trapped at the western SCB by strong temperature inversions and the barrier effect of large topography. The air pollutants from human activities over the eastern TP also can be dispersed to above the basin by the upper westerly winds. The high concentration of PM1 mass at about 2.0 km MSL is jointly modulated by both upward and downward motion at the lower and higher layers. The uniform vertical distribution of PM1 concentrations correlates to the vertical counterclockwise circulation. The trapped pollutants at the eastern foothills of the TP are transported to the eastern region and then are uplifted to the upper air by unstable stratification. The impact of aerosols on meteorology will be conducted by comparison of the field observations with the results from controlled tests of a numerical model in a future further study.

Acknowledgments.

This work was supported by the National Natural Science Foundation of China (42075185; 41605103), Excellent Member of Youth Innovation Promotion Association, ­Chinese Academy of Sciences (Y2021111), and Gansu Science and Technology Program key projects (20JR10RA037). Technical support was provided by Technology Service Center, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences.

Data availability statement.

The used data in this study can be obtained by contacting the corresponding author (zhaosp@lzb.ac.cn).

Appendix A: PM1 offline collection and carbon component analyses

The transition region from the deep SCB to high TP with large altitude difference is an ideal location for revealing aerosol–meteorology interactions over complex terrain (Yin et al. 2020). The six sampling sites, i.e., two within the western basin (Chengdu, Sanbacun) and four over the eastern TP (Wenchuan, Lixian, Maerkang, Hongyuan), were distributed along the steep slope from the western SCB to eastern TP extending the elevation from 500 m at Chengdu to 3,500 m at Hongyuan (Fig. 1). Compared with a coarser fraction of particulate matter (PM), the strong environment and health effects mainly focus on the submicrometer range, and thus PM1 samples were collected at daytime (0730–1830 LT) and nighttime (1930–0630 LT) during the campaign from 28 December 2018 to 26 January 2019. We used a medium-volume aerosol sampler (LY–2034, Laoying Instrument Co., Ltd., China) to collect PM1 samples at a flow rate of 100 L min−1. The quartz filter membrane (diameter: 91 mm, Whatman, United Kingdom) was used to collect PM1 samples to more accurately obtain the concentrations of chemical components. The collected samples were stored frozen in the refrigerator until further chemical composition analysis (Kawamura et al. 2010). The organic and elemental carbon (OC and EC) were measured by a thermal/optical carbon analyzer (DRI-2015, Desert Research Institute, United States) at seven wave bands, i.e., 405, 445, 532, 635, 780, 808, and 980 nm. The carbon components within PM1 were determined using the thermal/optical reflectance (TOR) method (Chow et al. 2007).

Appendix B: Tethered-balloon observations

At Sanbacun (30°54′59″N, 103°40′38″E), in the eastern foothills of Tibetan Plateau, we conducted the first in situ observations of vertical profiles of key PM (PM1 mass concentrations, and the carbonaceous components) and gaseous pollutants (NO, NO2, O3) and temperature and relative humidity (RH) within the planetary boundary layer (PBL) by means of a tethered balloon. The pollutants and meteorological variables were observed every 3 h (0200, 0500, 0800, 1100, 1400, 1700, 2000, and 2300 LT) during the campaign at Sanbacun, a rural site in the western SCB. Therefore, eight vertical profiles can be obtained every day, which is helpful in understanding diurnal variations of the vertical structures of key pollutants and meteorological parameters in the foothills of the TP. The jet-shaped balloon filled with 10 m3 of helium acted as a carrier with a maximum payload of 8.0 kg. The balloon ascends or descends at a steady speed of 0.5 m s−1 controlled by an electric winch from the ground to a specific height, and it is drawn in when wind speed is higher than 5.0 m s−1 at the upper layer to ensure the safety of the carried instruments.

A lightweight low-cost multipollutant sensor package, developed by Pang et al. (2021), is particularly portable to aerial measurements, and thus the system was deployed to observe the vertical profiles of air pollutants on a tethered balloon at our experiment (Fig. 1). The sensor system consists of electrochemical sensors measuring NO, NO2, O3, an optical counting (OPC) PM sensor for PM1 with time resolution of 10 s. All sensor signals were collected by a data acquirement logger, and the data were saved in a microcomputer. All sensors and the accessories were integrated and installed in a thermal insulation foam package, in which the temperature was stable at 25°C to avoid the temperature effect. Furthermore, a silicone desiccant tube was connected to the inlet of air sample to minimize the RH impact. The sensor performances were verified by comparing with on-ground reference instruments (Pang et al. 2021), it was substantiated to be a reliable and accurate device for aerial measurements of air pollutants within PBL.

The microAeth MA-200 (AethLabs, United States) is a highly sensitive, portable, and ­miniature five-wavelength (375, 470, 528, 625, and 880 nm) instrument designed for measuring the mass concentration of light-absorbing carbonaceous particles. The instrument has an 880-nm optical channel that is primarily interpreted as black carbon (BC). The instrument also measures ultraviolet particulate matter (UVPM) and makes measurements at three other wavelengths that can be used to calculate the Ångström exponent for source apportionment or other investigations into the optical properties of light-absorbing particles in the atmosphere. The microAeth device was carried by the tethered balloon to observe the vertical profiles of carbonaceous aerosols (Fig. 1). The microAeth draws an air sample at a flow rate of 100 mL min−1 through a 3-mm diameter portion of the filter media. Optical transmission through the “sensing” spot is illuminated by stabilized 880 nm (IR), 625 nm (red), 528 nm (green), 470 nm (blue), and 375 nm (UV) LED light sources and measured by a detector. The light attenuation (ATN) due to absorbance of particles collected on the spot is measured relative to an adjacent “reference” portion of the filter where no particles are accumulated. The 5-s time base was set to match the other observed data during the campaign.

A portable IMET-3050 403-MHz GPS upper-air sounding system was deployed to ­monitor the vertical profiles of temperature and RH by utilizing the IMET-1-AB radiosonde on the ­tethered balloon (Z. Li et al. 2017). Unlike the traditional usage, the radiosonde carried by the tethered balloon can be recycled during the campaign, which saved a significant cost. The time resolution of temperature and humidity data varied from 1 to 3 s in the ­vertical ­direction for each profile. The accuracies of temperature (T) and RH measurements were given by the manufacturer as ±0.3°C and ±5%. The observed T and RH profiles data were processed using IC-PCR1500/2500 software. IMET-1-AB radiosonde has been widely used and validated (Haman et al. 2012). There is a very slight difference in performance between IMET-1 and other radiosondes such as the Vaisala RS92 (Trapp et al. 2016).

Appendix C: Wind profile observations

The vertical profiles of horizontal winds (speed and direction) and vertical velocity were observed by a Windcube 200s Doppler wind lidar (Leosphere, France) during the campaign. The principle of the wind lidar is the Doppler effect, which is closely related to aerosol particles. The lidar emits a fixed pulse signal into the atmosphere, and the frequency of the pulse signal changes when the electromagnetic waves encounter moving particles. The radial wind speed and direction can be retrieved precisely by analyzing the frequency shift of the backscattered signal. The wind lidar was positioned about 20 m to the northeast of the tethered balloon. The Windcube 200s is designed to include a total of four scanning modes for different applications: plan position indicator (PPI), range height indicator (RHI), line-of-sight (LOS), and Doppler beam swinging (DBS) scan modes (Lundquist et al. 2017). We used the DBS mode to obtain horizontal and vertical wind profiles for studying aerosol–meteorology interactions within the SCB. In DBS mode, the lidar scanning head points a beam along four lines spaced 90° apart at a fixed half-opening angle of 15° and one vertical line to obtain the wind field at different heights.

The cycle time for each DBS mode is about 20 s, and thus the time resolutions of the horizontal and vertical winds of the wind lidar are 0.2 and 0.05 Hz, respectively. A vertical resolution of 50 m was used in this study. In view of the signal-to-noise ratio (SNR) threshold, the wind data with SNR lower than −26 dB were discarded from the raw data. The wind data are averaged each hour for studying aerosol–PBL meteorology feedbacks by combining with the other measurements on the tethered balloon. The wind lidar data were validated by comparing with Beijing 325-m meteorological tower data at the Institute of Atmospheric Physics, Chinese Academy of Sciences, and there is no significant difference for horizontal wind speed and direction between the lidar and sonic wind anemometer (Dai et al. 2020).

Appendix D: Reanalysis data

The air quality in the SCB is significantly affected by the 700-hPa weather systems moving eastward from the TP (Ning et al. 2018), and thus the 700-hPa geopotential height ERA-Interim daily data (0.75° spatial resolution) during the campaign were used for analyzing synoptic pattern impacts. The geopotential height reanalysis data are available for 0000, 0600, 1200, and 1800 UTC. In addition, the u and υ components of wind, the vertical velocity, and the temperature of the ERA-Interim daily dataset at the pressure level from 950 to 500 hPa were also obtained to investigate the physical mechanism of the synoptic pattern impacts.

Appendix E: Calculation of absorption Ångström exponent (AAE)

According to the aethalometer model suggested by Sandradewi et al. (2008), the relationships between BC concentrations and light absorption coefficients can be expressed as follows:
babs,λ(BC)=BCλ×MACλ,
where babs,λ is the light absorption coefficient at a wavelength of λ (unit: M m−1), and BCλ and MACλ are the BC concentration and mass absorption cross section for the wavelength, respectively. For the MA-200 instrument , MAC values at wavelengths of 375, 470, 528, 625, and 880 nm are 24.069, 19.070, 17.028, 14.091, and 10.120 m2 g−1, respectively. Referring to the Lambert–Beer law, the light absorption coefficients of aerosol particles also depend on the absorption Ångström exponent (AAE), which can be expressed as
babs,λ=K×λAAE,
where K is a constant. AAE can represent size, chemical compositions, and mixed state of aerosol particles. Therefore, based on Eq. (E2), we calculated AAE values for the wavelengths of 470 and 880 nm:
AAE=ln[babs(470)babs(880)]ln(470880).

The AAE value is generally close to 1 for fossil fuel combustion, while it is the larger for biomass burning (Day et al. 2006). Therefore, AAE can be used to roughly identify BC sources.

Appendix F: Identification of potential source regions for typical pollution profiles

On a regional scale, we used the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model, developed by the NOAA Air Resources Laboratory (ARL), to determine the origin of air masses and understand the potential source regions influencing SCB air quality during the campaign. The model calculation method is based on the Lagrangian approach, which uses a moving frame of reference for the advection and diffusion calculations as the trajectories or air parcels move from their initial location. For each cluster of pollution profile, the 96-h backward trajectories arriving at 100, 700, and 1,300 m above ground level (AGL) and initialized at the hours of day corresponding to tethered balloon launch were calculated with 0.25° spatial resolution Global Data Assimilation System (GDAS) data from National Centers for Environmental Prediction (NCEP).

Based on the calculated backward trajectories, we also used the concentration-weighted trajectory (CWT) method, developed by Hsu et al. (2003), to determine the potential source regions of the specific air pollutants, such as PM1, BC, and UVPM. For that method, each grid cell is assigned a weighted concentration by averaging pollutant concentrations that have associated trajectories crossing the grid cell, which is introduced in detail in the study of Yin et al. (2020). In addition, combining PM1, BC, and UVPM concentrations obtained by the MA-200 on the tethered balloon with wind data measured by wind lidar within the PBL, the pollution rose was calculated at 100, 700, and 1,300 m AGL by the Openair package of Rplot software. For each cluster of the pollution profile, the regional potential source regions at different heights can be better determined by combining the CWT method with the pollution rose.

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