Long-term trends of black carbon and particle number concentration in the lower free troposphere in Central Europe

Jia Sun,Kay Weinhold,Laurent Poulain,Ralf Sohmer,Martine Collaud Coen,Cedric Couret,Benjamin Tobias Brem,Leizel Madueño,Ye Yuan,Martin Gysel-Beer,Gabriele Frank,Alfred Wiedensohler,Wolfram Birmili,Thomas Tuch,Markus Hermann,Ludwig Ries,Nan Ma

doi:10.1186/s12302-021-00488-w

Abstract

BackgroundThe implementation of emission mitigation policies in Europe over the last two decades has generally improved the air quality, which resulted in lower aerosol particle mass, particle number, and black carbon mass concentration. However, little is known whether the decreasing particle concentrations at a lower-altitude level can be observed in the free troposphere (FT), an important layer of the atmosphere, where aerosol particles have a longer lifetime and may affect climate dynamics. In this study, we used data from two high-Alpine observatories, Zugspitze-Schneefernerhaus (ZSF) and Jungfraujoch (JFJ), to assess the long-term trends on size-resolved particle number concentrations (PNCs) and equivalent black carbon (eBC) mass concentration separated for undisturbed lower FT conditions and under the influence of air from the planetary boundary layer (PBL) from 2009 to 2018.ResultsThe FT and PBL-influenced conditions were segregated for both sites. We found that the FT conditions in cold months were more prevalent than in warm months, while the measured aerosol parameters showed different seasonal patterns for the FT and PBL-influenced conditions. The pollutants in the PBL-influenced condition have a higher chance to be transported to high-altitudes due to the mountainous topography, leading to a higher concentration and more distinct seasonal variation, and vice versa. The long-term trends of the measured aerosol parameters were evaluated and the decreased aerosol concentrations were observed for both FT and PBL-influenced conditions. The observed decreasing trends in eBC concentration in the PBL-influenced condition are well consistent with the reported trends in total BC emission in Germany and Switzerland. The decreased concentrations in the FT condition suggest that the background aerosol concentration in the lower FT over Central Europe has correspondingly decreased. The change of back trajectories in the FT condition at ZSF and JFJ was further evaluated to investigate the other possible drivers for the decreasing trends.ConclusionsThe background aerosol concentration in the lower FT over Central Europe has significantly decreased during 2009–2018. The implementation of emission mitigation policies is the most decisive factor and the decrease of the regional airmass occurrence over Central Europe also has contributed to the decreasing trends.

Highlights

The implementation of emission mitigation policies in Europe over the last two decades has gener‐ ally improved the air quality, which resulted in lower aerosol particle mass, particle number, and black carbon mass concentration
Diurnal cycle and seasonal differences in equivalent black carbon (eBC) and particle number concentration (PNC) Thermally driven vertical transport is one of the most decisive factors contributing to aerosol concentrations at high Alpine sites [35, 47, 52]
Transport from the planetary boundary layer (PBL) is mostly responsible for elevated Black carbon (BC) mass concentrations [53], while on-site emissions related to tourism such as smoke, cooking, etc., only have a minor effect [33]

Summary

Introduction

The implementation of emission mitigation policies in Europe over the last two decades has gener‐ ally improved the air quality, which resulted in lower aerosol particle mass, particle number, and black carbon mass concentration. Little is known whether the decreasing particle concentrations at a lower-altitude level can be observed in the free troposphere (FT), an important layer of the atmosphere, where aerosol particles have a longer lifetime and may affect climate dynamics. In terms of climatic impacts, aerosol particles are more important in the free troposphere (FT) than in the planet boundary layer (PBL), since BC above clouds have much higher direct radiative efficiency than BC close to the surface [5,6,7]. Aerosol particles participate in the formation of clouds by acting as condensation nuclei, which depends on their size and composition [8]. This leads to in- or semi-direct effects of aerosols on climate [9, 10]. The aerosol measurements in the FT can represent the background aerosol concentration over a large spatial scale [11, 12]

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