Flow-induced errors in airborne in situ measurements of aerosols and clouds
Abstract. Aerosols and clouds affect atmospheric radiative processes and climate in many complex ways and still pose the largest uncertainty in current estimates of the Earth’s changing energy budget. Airborne in situ sensors such as the Cloud, Aerosol, and Precipitation Spectrometer (CAPS) or other optical spectrometers and optical array probes provide detailed information about the horizontal and vertical distribution of aerosol and cloud properties. However, flow distortions occurring at the location where these instruments are mounted on the outside of an aircraft may directly produce artifacts in detected particle number concentration and also cause droplet deformation and/or breakup during the measurement process.
Several studies have investigated flow-induced errors assuming that air is incompressible. However, for fast-flying aircraft, the impact of air compressibility is no longer negligible. In this study, we combine airborne data with numerical simulations to investigate the flow around wing-mounted instruments and the induced errors for different realistic flight conditions. A correction scheme for deriving particle number concentrations from in situ aerosol and cloud probes is proposed, and a new formula is provided for deriving the droplet volume from images taken by optical array probes.
Shape distortions of liquid droplets can either be caused by errors in the speed with which the images are recorded or by aerodynamic forces acting at the droplet surface caused by changes of the airflow when it approaches the instrument. These forces can lead to the dynamic breakup of droplets causing artifacts in particle number concentration and size. An estimation of the critical breakup diameter as a function of flight conditions is provided.
Experimental data show that the flow speed at the instrument location is smaller than the ambient flow speed. Our simulations confirm the observed difference and reveal a size-dependent impact on particle speed and concentration. This leads, on average, to a 25 % overestimation of the number concentration of particles with diameters larger than 10 µm diameter and causes distorted images of droplets and ice crystals if the flow values recorded at the instrument are used. With the proposed corrections, errors of particle number concentration and droplet volume, as well as image distortions, are significantly reduced by up to 1 order of magnitude.
Although the presented correction scheme is derived for the DLR Falcon research aircraft (Saharan Aerosol Longrange Transport and Aerosol-Cloud-Interaction Experiment (SALTRACE) campaign) and validated for the DLR Falcon (Absorbing aerosol layers in a changing climate: aging, lifetime and dynamics mission conducted in 2017 (A-LIFE) campaign) and the NASA DC-8 (Atmospheric Tomography Mission (ATom) campaigns), the general conclusions hold for any fast-flying research aircraft.
Introduction
Aerosol–cloud–radiation interactions are one of the largest uncertainties in current climate predictions (Stocker et al., 2014). The size distribution of cloud and aerosol particles is a crucial parameter for aerosol–radiation and aerosol–cloud interaction (Albrecht, 1989; Rosenfeld and Lensky, 1998; Pruppacher and Klett, 2010). For example, an increase of the fraction of coarse particles can modify the direct radiative forcing of desert dust from cooling to warming (Kok et al., 2017) and also increase the reservoir of ice-nucleating particles (e.g., DeMott et al., 2010).
Airborne in situ measurements are fundamental to extend our knowledge of cloud and aerosol distributions, especially in the coarse mode. Instruments typically used by the aerosol and cloud community, for measuring coarse particles, are open-path or passive-inlet1 optical particle counters (OPCs) and optical array probes (OAPs). OPCs and OAPs measure particle flux as they record, within a time interval, the number of particles passing through a specific region named sampling area. The flux is later converted into a concentration using the airflow speed. Therefore, errors in the flow speed are directly affecting the calculated particle and cloud hydrometeor concentrations. For example, a too-low flow speed leads to a higher calculated particle concentration. Since the aircraft itself can influence the surrounding air and the flow measurements (Kalogiros and Wang, 2002), airborne measurements are challenging. Flow distortion caused by the fuselage and wings not only impacts the flow velocity but also modifies air temperature, pressure, and density as compared to free stream conditions, thereby further affecting the aerosol and cloud measurements. For example, a higher air density leads to a higher number concentration of aerosol particles if the particles are sufficiently small to be able to follow the airflow. Furthermore, large droplets may be deformed or may even break up during high-speed sampling due to aerodynamic forces acting on the droplet surface, as studied by Szakall et al. (2009); Vargas and Feo (2010).
Whereas droplet deformation does not change the detected number concentrations, breakup results in enhanced droplet number concentrations (Weber et al., 1998). These shattering artifacts may originate not only from aerodynamic forces but also from impaction breakup of cloud droplets and ice particles in and around the aerosol inlet (Korolev and Isaac, 2005; Craig et al., 2013). In contrast to these effects, droplets may appear as deformed on the OAP images, but they are not deformed in reality. This is the case if the camera does not use the correct particle velocity for taking the images.