Abstract
Abstract. The incomplete understanding of aerosol–cloud interactions introduces large uncertainties when simulating the cloud radiative forcing in climate models. The physical and optical properties of a cloud, as well as the evolution of precipitation, are strong functions of the cloud hydrometeor phase. Aerosol particles support the phase transition of water in the atmosphere from a meta-stable to a thermodynamically preferred stable phase. In the troposphere, the transition of liquid droplets to ice crystals in clouds, via ice-nucleating particles (INPs) which make up only a tiny fraction of all tropospheric aerosol, is of particular relevance. For accurate cloud modeling in climate projections, the parameterization of cloud processes and information such as the concentrations of atmospheric INPs are needed. Presently, only few continuous real-time INP counters are available and the data acquisition often still requires a human operator. To address this restriction, we developed HINC-Auto, a fully automated online INP counter, by adapting an existing custom-built instrument, the Horizontal Ice Nucleation Chamber. HINC-Auto was able to autonomously sample INPs in the immersion mode at a temperature of 243 K and a water saturation ratio of 1.04 for 97 % of the time for 90 consecutive days. Here, we present the technical setup used to acquire automation, discuss improvements to the experimental precision and sampling time, and validate the instrument performance. In the future, the chamber will allow a detailed temporal analysis (including seasonal and annual variability) of ambient INP concentrations observing repeated meteorological phenomena compared to previous episodic events detected during campaigns. In addition, by deploying multiple chambers at different locations, a spatiotemporal variability of INPs at any sampling site used for monitoring INP analysis can be achieved for temperatures ≤ 243 K.
Highlights
The interaction between aerosols and clouds contributes to the global energy budget by indirectly influencing the radiative forcing of the climate system
The condensation particle counter (CPC) used for validation experiments has a counting uncertainty of ± 10 % which yields in a relative uncertainty in the reported activated fraction (AF) of ± 14 %
To maximize the sampling time, the duration of the re-wetting procedure was reduced by reducing the warming and cooling time of the chamber by using a lower thermal mass for the chamber walls
Summary
The interaction between aerosols and clouds contributes to the global energy budget by indirectly influencing the radiative forcing of the climate system. It is essential to understand the mechanism of ice formation within mixed-phase clouds. One such mechanism is immersion freezing, the formation of ice crystals on ice-nucleating particles (INPs) immersed in liquid droplets (Vali et al, 2015), which has been studied extensively in laboratory studies (see, e.g., Zuberi et al, 2002; DeMott et al, 2003b; Marcolli et al, 2007; Lüönd et al, 2010; Niemand et al, 2012; Murray et al, 2012; Atkinson et al, 2013; Hiranuma et al, 2015). A large number of field studies based on intensive observation periods to quantify the concentration, properties, identity and sources of immersion-mode INPs have been reported
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