Abstract
Abstract. Ice particle activation and evolution have important atmospheric implications for cloud formation, initiation of precipitation and radiative interactions. The initial formation of atmospheric ice by heterogeneous ice nucleation requires the presence of a nucleating seed, an ice-nucleating particle (INP), to facilitate its first emergence. Unfortunately, only a few long-term measurements of INPs exist, and as a result, knowledge about geographic and seasonal variations of INP concentrations is sparse. Here we present data from nearly 2 years of INP measurements from four stations in different regions of the world: the Amazon (Brazil), the Caribbean (Martinique), central Europe (Germany) and the Arctic (Svalbard). The sites feature diverse geographical climates and ecosystems that are associated with dissimilar transport patterns, aerosol characteristics and levels of anthropogenic impact (ranging from near pristine to mostly rural). Interestingly, observed INP concentrations, which represent measurements in the deposition and condensation freezing modes, do not differ greatly from site to site but usually fall well within the same order of magnitude. Moreover, short-term variability overwhelms all long-term trends and/or seasonality in the INP concentration at all locations. An analysis of the frequency distributions of INP concentrations suggests that INPs tend to be well mixed and reflective of large-scale air mass movements. No universal physical or chemical parameter could be identified to be a causal link driving INP climatology, highlighting the complex nature of the ice nucleation process. Amazonian INP concentrations were mostly unaffected by the biomass burning season, even though aerosol concentrations increase by a factor of 10 from the wet to dry season. Caribbean INPs were positively correlated to parameters related to transported mineral dust, which is known to increase during the Northern Hemisphere summer. A wind sector analysis revealed the absence of an anthropogenic impact on average INP concentrations at the site in central Europe. Likewise, no Arctic haze influence was observed on INPs at the Arctic site, where low concentrations were generally measured. We consider the collected data to be a unique resource for the community that illustrates some of the challenges and knowledge gaps of the field in general, while specifically highlighting the need for more long-term observations of INPs worldwide.
Highlights
Ice-nucleating particles (INPs) are a crucial element in cloud formation and precipitation processes (DeMott et al, 2010; Lohmann, 2015)
When transforming the INP concentrations to log space, we find that the two operational modes are well correlated (R = 0.81, N = 44), with the immersion freezing INPs being on average a factor of 10 higher than deposition and condensation INPs
The INP concentrations from May 2015 to January 2017 at the four stations at −20, −25 and −30 ◦C are presented in Figs. 2, 3 and 4
Summary
Ice-nucleating particles (INPs) are a crucial element in cloud formation and precipitation processes (DeMott et al, 2010; Lohmann, 2015). In a supercooled and supersaturated cloud regime INPs may activate to ice crystals, which will grow and possibly form secondary ice by splintering or other multiplication processes. Once grown to their critical size, crystals may initiate precipitation. This is especially important for mixed-phase clouds that consist of both supercooled water droplets and ice crystals. INPs influence local and global radiation budgets and related aerosol–cloud interactions by affecting the phase of clouds (Lohmann, 2015)
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