In situ measurements of vertically resolved particle size
distributions based on a tethered balloon system were carried out for the first time in the highland city of Lhasa over the Tibetan Plateau in summer 2020, using portable optical counters for the size range of 0.124–32
Lhasa, the provincial capital of Tibet, lies almost in the heart of the sparsely populated Tibetan Plateau, which is the highest plateau in the world with an average altitude of approximately 4320 m (Zhang et al., 2021). As a result of its unique topography, the Tibetan Plateau plays a vital role in the East Asian Summer Monsoon and therefore in the regional and global climate (He et al., 2019; Chiang et al., 2020). As the most urbanized and populated highland city in Tibet, Lhasa is quite suitable for exploring the impact of anthropogenic activities on atmospheric components over this remote region.
In the past decade, the population in Lhasa has increased from about 0.55 million in 2010 to nearly 0.87 million in 2020, according to the newly published data from the Seventh National Population Census (
Previous studies on aerosols in Lhasa were quite limited. One particular focus was the chemical composition of single particle (D. Z. Zhang et al., 2001; Duo et al., 2015), bulk aerosol (Cong et al., 2011; Gong et al., 2011; Liu
et al., 2013; Chen et al., 2018), and size-segregated aerosols (X. Y. Zhang et al., 2001; Wan et al., 2016; L. L. Cui et al., 2018; Wei et al., 2019a, b) near the ground. Results from these studies revealed that vehicular exhaust, religious activities that involve incense burning and biomass burning, and the suspension of mineral dust were major sources. It was also pointed out from the results of the backward trajectory analysis that local emissions dominated during the monsoon season (Wei et al., 2019b). Aerosol optical properties such as multiwavelength aerosol absorption near the ground (Zhu et al., 2017) and aerosol optical depth (Bai et al., 2000; Zhu et al., 2019) have also been investigated. However, the particle size distribution, as a
critical microphysical property, has rarely been studied. Using a portable optical particle counter, Y. Y. Cui et al. (2018) measured the particle size distributions (14 size bins) within the range of 0.14–3
In this study, the vertical structure and temporal variability in aerosol profiles were explored, based on in situ measurements of particle size distributions within 1 km above the ground at a suburban site in Lhasa, using optical counters attached to a tethered balloon. Source apportionment based on measured particle size distributions was further performed to identify possible sources for aerosols in different layers.
The city of Lhasa lies in an east–west-oriented valley along the Lhasa River, surrounded by mountains up to an elevation of about 5500 m (Fig. 1a), as shown by Shuttle Radar Topography Mission (SRTM) data v4 from Jarvis et al. (2008). The field site is on the Najin Campus of Tibet University (29.64
The topographic map of the Lhasa River valley
Measurements were conducted in an open space, about 90 m away from the nearest building, in order to ensure the safety of launching the tethered balloon system. The 60 m
A lightweight optical counter, the Portable Optical Particle Spectrometer (POPS, Handix Scientific), was attached to the tethered balloon for in situ measurements of vertical profiles of atmospheric aerosols. Details about the principle on which the instrument operates were given in Gao et al. (2016). The POPS was calibrated by establishing a relationship between the scattering signal and the particle size before the campaign. Both polystyrene latex sphere (PSL) with known sizes and ammonium sulfate particles with sizes selected by a differential mobility analyzer were employed. Though the experimental responses of the two calibration materials generally agreed well with the simulated theoretical responses, both theoretical response curves were found to be highly oscillatory above the particle size of 600 nm (Gao et al., 2016). Considering that the refractive index of ammonium sulfate is closer to ambient dry aerosols than PSL (Shingler et al., 2016), a combined calibration curve from the experimental response of ammonium sulfate particles for diameters smaller than 600 nm, and the smoothed theoretical response of ammonium sulfate particles for diameters larger than 600 nm, was used to obtain 42 logarithmically equal bins over the size range of 0.124–2.55
A portable aerosol spectrometer (Model 11-C, GRIMM Aerosol Technik Ainring GmbH & Co. KG), equipped with the homemade silica-gel-filled diffusion dryer, was also attached to the tethered balloon for 24 launches to concurrently measure the PNSDs for dry particles within the size range of 0.25–32
Vertical profiles of meteorological parameters, including pressure (MS5540B, Intersema Sensoric SA), temperature (
Profiles of aerosol and meteorological parameters were all processed into 10 m averaged data for the subsequent analysis. A mixed layer (ML) for daytime profiles or a nocturnal boundary layer (NBL) for nighttime profiles, mentioned together as the planetary boundary layer (PBL) in what follows, could be identified for 72 profiles in the dataset of 112 profiles in total. For the remaining 40 profiles, the flight path was entirely within the daytime ML. The height of the PBL, denoted as
A bilinear factor-based receptor model, with positive matrix factorization (PMF), was used for the source apportionment of particle mass in this study based upon observed PMSDs (Paatero and Tapper, 1994; Paatero, 1997). By minimizing an objective function
Under the impact of the East Asian Summer Monsoon, Lhasa normally experiences a
rainy season from May to September (Ding, 2007; Ran et al., 2014). However, in August 2020 when the campaign took place, two periods with distinctly different conditions of water vapor were found, as can be clearly seen from
The time series of
Vertical profiles of meteorological parameters observed at the site were categorized into five time periods of the day, as given in Table S1, and were presented along
The evolution of the PBL was an important influencing factor in shaping the vertical structure of atmospheric aerosols, as has been pointed out by some previous studies (Ferrero et al., 2010; Ran et al., 2016; Zhang et al., 2017). During the campaign, heights of the shallow NBL were largely below 200 m around sunrise/sunset and at night (Fig. S5). In Period I,
Normalized by
Average vertical profiles of
Table 1 listed particle parameters in the PBL, the RL, and the FT. For the PBL category, averages within the ML and 10 m averages near the ground for the NBL were adopted. Inside the well-mixed layer, the surface level and the ML-averaged level were close to each other. Averaged
Particle parameters within the PBL, in the RL, and in the FT for Periods I and II, given as average value
It was found that PM
Profiles in Period II were mainly collected in the early morning and at night, when elevated aerosol layers were often encountered, though it was hard to tell whether those were only residual layers that could easily form above the NBL after sunset and remain until the PBL had evolved to a sufficient height on the next day or if there could also be contributions from
transported plume. Here, we generally denoted such elevated layers as residual layers (RLs). In total, the RL was identified for 29 profiles in Period II, whereas no profiles were identified in Period I. A layer above the PBL and beneath the FT was observed for four profiles collected in the morning
in Period I, with
Undoubtedly, air quality over the Tibetan Plateau has been increasingly influenced by anthropogenic activities and emissions during the processes of urbanization and economic growth in the past several decades, especially in the relatively densely populated cities like Lhasa. However, the level of
Particle mass size distributions (PMSDs) measured by the POPS were categorized into two periods and averaged for three layers (Fig. 4). In general, size-resolved particle mass concentrations were the highest at all diameters within the PBL, while being much lower in the FT. Nevertheless, a similarity was found among all PMSDs. Plainly, a distinct mode below 0.3
Average particle mass size distributions (
The vertical distribution of PMSDs within the size range of
0.124–32
A further examination was made of average PMSDs within 50 m above the ground for each category, considering that in the NBL or in an early morning ML measurements near the ground would help to better elucidate the impact religious activities brought to PMSDs. A marked mode, peaking around 0.6
Average PMSDs within 50 m above the ground for different cases, with the same markers and text as in Fig. 5.
In total, three source factors were identified from the PMF analysis of the PMSDs combined from the POPS and GRIMM 11-C measurements in Period II (Fig. 7a). The first factor (factor 1) revealed a potential source that predominantly contributed to particles smaller than 0.3
On average, particles in the PBL and the RL shared a similar makeup of potential sources (Fig. 7b), suggesting that particles in the two layers were probably of the same origin. The contribution by suspended dust particles took up a portion of more than 50 %, while the contribution by religious burning was comparable with the contribution by combustion-related or secondary formation sources with a fraction of roughly 20 %–25 %. In particular, the results for the PBL were
separated into one part on holiday mornings and the other under other conditions, namely non-holidays and holidays
In this study, vertical profiles of particle size distributions within 1 km were measured by a POPS and a GRIMM 11-C attached to a tethered balloon in summer 2020 at a suburban site in Lhasa. The variability in the vertical structure and temporal features of parameters, such as aerosol number and mass concentrations, the effective diameter, and particle mass size distributions were examined. Possible sources for aerosols in different layers were investigated by the PMF analysis.
The vertical distribution of aerosol properties was found to be largely governed by the diurnal variation in the boundary layer. Generally speaking, aerosols uniformly distributed in the daytime PBL and sharply declined in the NBL. Usually, the average particle number and mass concentrations in the FT were less than 25 % of the amount in the PBL. For the humid Period I under
the influence the East Asian Summer Monsoon, a lower level of aerosols and also
fewer large particles in the PBL were found than that in the relatively dry Period II, possibly due to frequent rainfall and associated efficient removal by wind. More emissions from religious activities in Period II might also contribute to the differences between the two periods. In contrast, both the total amount and the size distribution of particle mass in the FT
for the two periods were comparable. Residual layers with elevated aerosol were often encountered in Period II, with particle mass concentrations averaging around 55 % of that in the PBL. The PMSDs in the RL shared a similar pattern to that in the PBL, suggesting they were probably of the same origin. The source apportionment analysis revealed that the factor associated with suspended dust in the coarse mode contributed more than
50 % in the PBL and the RL. However, a distinct peak over 0.5–0.7
The data in this study can be publicly accessed via
The supplement related to this article is available online at:
JB and LR proposed the study. LR, ZD, and ZB designed and conducted the field campaign. YL participated in the field campaign. LR and ZD processed the data. JL and YW performed the positive matrix factorization modeling. DZ provided the surface meteorological data. LR visualized the data and wrote the paper. ZD and YW participated in several discussions and provided valuable suggestions. All authors reviewed the paper carefully.
The contact author has declared that neither they nor their co-authors have any competing interests.
Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
We are grateful to Yong Wang, Hanze Yu, and Qi Li, for their assistance in launching the tethered balloon. We also thank Tibet University, for providing the location for the campaign and all the support.
This research has been funded by the Second Tibetan Plateau Scientific Expedition and Research Program (grant no. 2019QZKK0604) and the National Natural Science Foundation of China (grant nos. 91837311 and 42061134012).
This paper was edited by Rolf Müller and reviewed by two anonymous referees.