Summary
Case study of the influence of atmospheric boundary-layer (ABL) development on new particle formation (NPF) during the morning transition.} Continuous in-situ measurements of vertical profiles of the ABL were measured near Melpitz, Germany by unmanned aerial systems to understand the potential connection between NPF and boundary-layer development in the context of turbulence, temperature and humidity fluctuations.
Airborne Measured New Particle Formation Event in the Atmospheric Boundary Layer
Andreas Platis1, Barbara Altst3, Birgit Wehner2, Norman Wildmann1, Astrid Lampert3, Markus Hermann2, Wolfram~Birmilli2, Jens Bange1
1Center for Applied Geoscience (ZAG), University Tübingen
2Leibniz-Institute for Tropospheric Research
3Institute of Flight Guidance TU Braunschweig
ALADINA (Application of Light-weight Aircraft for Detecting In situ Aerosol)
Here, a new UAV-borne measurement system ALADINA (Application of Light-weight Aircraft for Detecting In situ Aerosol) has been developed. It has a payload of approx. 2.8 kg and is equipped with instruments to measure basic meteorology (Fig. 2) and aerosol number concentrations in different size channels between 5 nm and 10 µm (Fig.3).
Aerosol instrumentation on ALADINA
Two Condensation Particle Counters (CPC, Type3007, TSI Inc., St. Paul, USA) were modified to measure number concentrations larger than 5 nm (CPC1, N5) and larger than 10 nm (CPC2, N10) with a time resolution of 1.5 s. The difference between both CPCs results in the number concentration between 5 and 10 nm N5-10.
Ground-based aerosol measurements in Melpitz
A Neutral cluster and Air Ion Spectrometer (NAIS) measures mobility distributions in the size range from 1 to 40 nm with a time resolution of 5 min. Measurements of meteorologocal, trace gas and other aerosol parameters are performed continuously at the measurement site Melpitz.
Results
On April 3, 2014 nine measurement flights were performed with ALADINA between 6:20 and 12:30 UTC. During these flights mainly vertical profiles up to 1000 m asl (above sea level) were flown to measure the vertical distrubution of ultrafine particle number concentrations (N5-10), potential temperature Θ, mixing ratio m, correlation (cor) between Θ and m, structure parameters Cq2 and CT2 and σT, σq.
Fig.1 shows a strong inversion at 200 m asl above a shallow convective boundary layer (SCBL) with constant N5, N10 below with high temperature and humidity fluctuation, expressed by large CT2 and Cq2. Above the inversion, a layer with up to 20000 cm-3 in N5 and N10 is present between 400 an 500 m but no significant values in N5-10. This changes at 9:58 UTC (Fig.2) where the height of the well-mixed layer reached 400 m asl and 25000 cm-3 particles were measured in N5-10 within a distinct layer between 400 and 500 m. At 10:04 UTC (Fig.3) the SCBL breaks through the inversion and thus the layer with increased N5-10 is mixed downwards.
This results in an increasing vertical thickness of this layer from 300 to 500 m asl. The following profile at 10:45 UTC (Fig.4) shows a nearly well-mixed layer below ~550 m asl and increased values in N5-10 of 10000 - 40000 cm-3 over all heights, also close to the ground. This corresponds to NAIS measurements (Fig.5), showing that the N5-10 measured on ground start to increase around 10:30 UTC.
Conclusion
High number concentrations of nucleation mode particles up to (6.0 ⋅ 104 cm-3 in the diameter range 5-10 nm) were observed in an inversion layer located about 450 m above ground level. Importantly, the inversion layer exhibited a spatial temperature structure parameter CT2 10 times higher and a spatial humidity structure parameter Cq2 5 times higher than in the remaining part of the vertical profile. We assume that the inversion layer is responsible for creating favorable thermodynamic conditions for a NPF event. In addition, this layer showed a strong anti-correlation of humidity and temperature fluctuations. Using estimates of the turbulent mixing and dissipation rates, we conclude that the downward transport of particles by convective eddies was also the cause of the sudden increase of nucleation mode particles in the surface observations. This work supports the hypothesis that many of the NPF events that are frequently observed near the ground may, in fact, originate at elevated altitude, with newly formed particles subsequently being mixed down to the ground.