Abstract
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 μm. The vertical structure of 112 aerosol profiles was found to be largely shaped by the evolution of the boundary layer (BL), with a nearly uniform distribution of aerosols within the daytime mixing layer and a sharp decline with the height in the shallow nocturnal boundary layer. During the campaign, the average mass concentration of particulate matters smaller than 2.5 μm in aerodynamic diameter (PM2.5) within the BL was around 3 μg m−3, almost four times of the amount in the free troposphere (FT), which was rarely affected by surface anthropogenic emissions. Though there was a lower level of particle mass in the residual layer (RL) than in the BL, a similarity in particle mass size distributions (PMSDs) suggested that particles in the RL might be of the same origin as particles in the BL. This was also in consistence with the source apportionment analysis based on the PMSDs. Three distinct modes were observed in the PMSDs for the BL and the RL. One mode was exclusively coarse particles up to roughly 15 μm and peaked around 5 μm. More than 50 % of total particle mass was often contributed by coarse mode particles in this area, which was thought to be associated with local dust resuspension. The mode peaking over 0.5~0.7 μm was representative of biomass burning on religious holidays and was found to be most pronounced on holiday mornings. The contribution from the religious burning factor rose from about 25 % on non-holidays to nearly 50 % on holiday mornings. The mode dominated by particles smaller than 0.3 μm was thought to be associated with combustion related emissions and/or secondary aerosol formation. In the FT coarse mode particles only accounted for less than 10 % of the total mass and particles larger than 5 μm were negligible. The predominant submicron particles in the FT might be related to secondary aerosol formation and the aging of existed particles. To give a full picture of aerosol physical and chemical properties and better understand the origin and impacts of aerosols in this area, intensive field campaigns involving measurements of vertically resolved aerosol chemical compositions in different seasons would be much encouraged in the future.
Highlights
Lhasa, the provincial capital of Tibet, lies almost in the heart of the sparsely populated Tibetan Plateau, which is the highest 35 plateau in the world with an average altitude of approximately 4320 m (Zhang et al, 2021)
Unlike the diurnal variation of T being a peak in the late afternoon and a valley in the early morning without much differences between the two periods, average relative humidity (RH) reached a maximum in the early morning of about 79%±11% for period I and 64%±6% for Period II, as well as a minimum in the late afternoon of about 43%±11% for period I and 28%±11% for Period II
325 A further examination was taken on average Particle mass size distributions (PMSD) within 50 m above the ground for each category, considering that in the nocturnal boundary layer (NBL) or in an early morning mixed layer (ML) measurements near the ground would better help elucidate the impact religious activities brought to PMSDs
Summary
The provincial capital of Tibet, lies almost in the heart of the sparsely populated Tibetan Plateau, which is the highest 35 plateau in the world with an average altitude of approximately 4320 m (Zhang et al, 2021). One particular focus was the chemical composition of single particle 55 (Zhang et al, 2001a; 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 (Zhang et al, 2001b; Wan et al, 2016; Cui et al, 2018a; Wei et al, 2019a, 2019b) near the ground Results from these studies revealed that vehicular exhaust, religious activities involved incense burning and biomass burning, and the suspension of mineral dust were major sources. 70 In this study, the vertical structure and temporal variability of 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. Emissions from residential, vehicular and religious sources are expected to be much weaker in the surrounding area of the site than in the downtown
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