Reduced volatility of aerosols from surface emissions to the top of the planetary boundary layer

Quan Liu,Ping Tian,Guiqian Tang,Deping Ding,Shuo Ding,Dantong Liu,Hui He,Kai Bi,Mengyu Huang,Kang Hu,Qian Gao,Shaofei Kong,Yangzhou Wu,Yunfei Wu,Fei Wang,Wenkang Gao,Delong Zhao,Siyuan Li,Chenjie Yu

doi:10.5194/acp-21-14749-2021

Abstract

Abstract. Aerosols from surface emission can be transported upwards through convective mixing in the planetary boundary layer (PBL), which subsequently interact with clouds, serving as important sources to nucleate droplets or ice particles. However, the evolution of aerosol composition during this vertical transport has yet to be explicitly understood. In this study, simultaneous measurements of detailed aerosol compositions were conducted at two sites, namely urban Beijing (50 m above sea level – a.s.l.) and Haituo mountain (1344 m a.s.l.) during wintertime, representing the anthropogenically polluted surface environment and the top of the PBL, respectively. The pollutants from surface emissions were observed to reach the mountain site on daily basis through daytime PBL convective mixing. From the surface to the top of PBL, we found efficient transport or formation of lower-volatility species (black carbon, sulfate, and low-volatile organic aerosol, OA); however, a notable reduction in semivolatile substances, such as the fractions of nitrate and semivolatile OA reduced by 74 % and 76 %, respectively, during the upward transport. This implies that the mass loss of these semivolatile species was driven by the evaporation process, which repartitioned the condensed semivolatile substances to the gas phase when aerosols were transported and exposed to a cleaner environment. In combination with the oxidation processes, these led to an enhanced oxidation state of OA at the top of the PBL compared to surface environment, with an increase of oxygen to carbon atomic ratio by 0.2. Such a reduction in aerosol volatility during vertical transport may be important in modifying its viscosity, nucleation activity, and atmospheric lifetime.

Highlights

Substances in the atmosphere, present as aerosol and gas phases, are subject to phase transformation during their lifetime (Pankow, 1994, 1987)
The maximum local influence was consistent with the most developed PBL height (PBLH) (Fig. 1c). This suggested the strongest influence of surface emissions to the mountain through midday convective mixing (CM; termed, hereafter, as the CM period)
The increase in the oxidation state could be caused by the evaporation process losing the less oxidized and more volatile species, and the evaporated gases could be further oxidized to partition to a more oxidized phase

Summary

Introduction

Substances in the atmosphere, present as aerosol and gas phases, are subject to phase transformation during their lifetime (Pankow, 1994, 1987). The condensation process leads to gas molecular partitioning in the condensed phase, while the evaporation process occurs when aerosols were diluted in an environment with lower concentration (Donahue et al, 2006). Hereby, their physiochemical properties could be modified, such as the condensation results in enlarging aerosol size (Riipinen et al, 2011, 2012) or the production of new particles (Zhang et al, 2004; Kulmala et al, 2013); the evaporation led to a loss of particulate mass (May et al, 2015; Cubison et al, 2011). The repartitioned gases from aerosols during dilution could experience chemical evolution and further contribute to the modification of aerosol properties (Zhang et al, 2007; Robinson et al, 2007)

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Conclusion