Abstract
Abstract. This study describes the appearance of ultrafine boundary layer aerosol particles under classical “non-favourable” conditions at the research site of TROPOS (Leibniz Institute for Tropospheric Research). Airborne measurements of meteorological and aerosol properties of the atmospheric boundary layer (ABL) were repeatedly performed with the unmanned aerial system ALADINA (Application of Light-weight Aircraft for Detecting IN-situ Aerosol) during three seasons between October 2013 and July 2015. More than 100 measurement flights were conducted on 23 different days with a total flight duration of 53 h. In 26 % of the cases, maxima of ultrafine particles were observed close to the inversion layer at altitudes between 400 and 600 m and the particles were rapidly mixed vertically and mainly transported downwards during short time intervals of cloud gaps. This study focuses on two measurement days affected by low-level stratocumulus clouds, but different wind directions (NE, SW) and minimal concentrations (< 4.6 µg m−3) of SO2, as a common indicator for precursor gases at ground. Taken from vertical profiles, the onset of clouds led to a non-linearity of humidity that resulted in an increased turbulence at the local-scale and caused fast nucleation (e.g. Bigg, 1997; Wehner et al., 2010), but in relation to rapid dilution of surrounding air, seen in sporadic clusters of ground data, so that ultrafine particles disappeared in the verticality. The typical “banana shape” (Heintzenberg et al., 2007) of new particle formation (NPF) and growth was not seen at ground and thus these days might not have been classified as NPF event days by pure surface studies.
Highlights
Knowledge of atmospheric aerosols is still incomplete and contributes to the most significant uncertainties in climate model predictions (IPCC, 2007)
According to data of surface pressure systems that are publicly available by DWD, the research station was influenced by a low-pressure system and especially by the passage of a cold front that occluded around 18:00 UTC (−02:00 MEZ)
During six of these measurement days, new particle formation (NPF) events were observed under non-favourable conditions near the inversion layer and were mixed vertically induced by atmospheric boundary layer (ABL) dynamic processes
Summary
Knowledge of atmospheric aerosols is still incomplete and contributes to the most significant uncertainties in climate model predictions (IPCC, 2007). The small-scale vertical distribution of aerosols in the atmospheric boundary layer (ABL) needs a more profound understanding and has to be implemented in models (Boy et al, 2006). The measurements mentioned so far, except for the balloon, were conducted only in one fixed location so that the situation on a larger scale, and in particular a temporal development at different altitudes, is missing In this context, the use of airborne systems for atmospheric research is essential to deliver a detailed four-dimensional picture of the aerosol spatiotemporal distribution from the surface up to the free troposphere. Hamburger et al (2012) studied atmospheric aerosols with the research aircraft FAAM BAe–146 and DLR Falcon 20 and suggested nucleation events in the ABL caused by the presence of high-pressure systems and one event in the free troposphere around 8 km altitude due to the updraught during frontal passages.
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