Measurement report: Vertical profiling of particle size distributions over Lhasa, Tibet – tethered balloon-based in situ measurements and source apportionment

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 planetary boundary layer (PBL), 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, mass concentrations of particulate matter smaller than 2.5 µm in aerodynamic diameter (PM2.5) within the PBL ranged from 0.5 to 12.0 µg m−3, with an average and standard deviation of 3.4 ± 2.3 µg m−3, almost 4 times 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 PBL, a similarity in particle mass size distributions (PMSDs) suggested that particles in the RL might be of the same origin as the particles in the PBL. This was also consistent with the source apportionment analysis based on the PMSDs. In total, three distinct modes were observed in the PMSDs for the PBL and the RL. One mode was exclusively coarse particles up to roughly 15 µm, peaking 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 existing 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 highly encouraged in the future.