HOVERCAT: a novel aerial system for evaluation of aerosol-cloud interactions Atmospheric Measurement Techniques

Aerosols have a profound impact on cloud microphysics through their ability to serve as ice nucleating particles (INPs). As a result, cloud radiative properties and precipitation processes can be modulated by such aerosolcloud interactions. However, one of the largest uncertainties associated with atmospheric processes is the indirect effect of aerosols on clouds. The need for more advanced observations of INPs in the atmospheric vertical profile is apparent, yet most ice nucleation measurements are conducted on the ground or during infrequent and intensive airborne field campaigns. Here, we describe a novel measurement platform that is less expensive and smaller (< 5 kg) when compared to traditional aircraft and tethered balloon platforms and that can be used for evaluating two modes of ice nucleation (i.e., immersion and deposition). HOVERCAT (Honing On VERtical Cloud and Aerosol properTies) flew during a pilot study in Colorado, USA, up to 2.6 km above mean sea level (1.1 km above ground level) and consists of an aerosol module that includes an optical particle counter for size distributions (0.38-17 mu m in diameter) and a new sampler that collects up to 10 filter samples for offline ice nucleation and aerosol analyses on a launched balloon platform. During the May 2017 test flight, total particle concentrations were highest closest to the ground (up to 50 cm(-3) at < 50m above ground level) and up to 2 in 102 particles were ice nucleation active in the immersion mode (at 23 degrees C). The warmest temperature immersion and deposition mode INPs (observed up to -6 and -40.4 degrees C, respectively) were observed closest to the ground, but overall INP concentrations did not exhibit an inverse correlation with increasing altitude. HOVERCAT is a prototype that can be further modified for other airborne platforms, including tethered balloon and unmanned aircraft systems. The versatility of HOVERCAT affords future opportunities to profile the atmospheric column for more comprehensive evaluations of aerosol-cloud interactions. Based on our test flight experiences, we provide a set of recommendations for future deployments of similar measurement systems and platforms.

气溶胶可作为冰核粒子（ice nucleating particles, INPs），对云微物理过程产生深远影响。由此，气溶胶-云相互作用可调控云的辐射特性与降水过程。然而，大气过程中最大的不确定性来源之一便是气溶胶对云的间接效应。当前，学界亟需针对大气垂直廓线内冰核粒子的高精度观测，但现有冰核测量多在地面开展，或依托频次较低但强度较高的机载野外试验。
本文介绍一款新型测量平台：相较于传统飞机及系留气球平台，该平台成本更低、体积更小（重量小于5千克），可用于评估两种冰核作用模式——浸润模式与沉积模式。HOVERCAT（Honing On VERtical Cloud and Aerosol properTies）已于美国科罗拉多州开展试点试飞，最大爬升高度可达海拔2.6千米（地面以上1.1千米）。该平台搭载的气溶胶模块包含用于测量粒径分布（直径0.38~17μm）的光学粒子计数器，以及一款新型采样器：该采样器可在升空气球平台上采集至多10份滤膜样品，用于后续离线冰核与气溶胶分析。
2017年5月的试飞数据显示，粒子总浓度在贴近地面处最高（地面以上50米范围内可达50 cm⁻³）；在23℃条件下，每10²颗粒子中最多有2颗在浸润模式下表现出冰核活性。贴近地面处观测到了温度最高的浸润模式与沉积模式冰核粒子（分别可达-6℃与-40.4℃），但整体冰核粒子浓度并未随海拔升高呈现负相关关系。
HOVERCAT属于原型平台，可针对系留气球、无人飞行器系统（unmanned aircraft systems）等其他机载平台进行适配改造。该平台的多功能性为后续开展大气柱廓线监测提供了契机，以更全面地评估气溶胶-云相互作用。基于本次试飞经验，本文还为同类测量系统与平台的后续部署提出了一系列建议。