Best practices for precipitation sample storage for offline studies of ice nucleation in marine and coastal environments

Abstract

Abstract. Ice-nucleating particles (INPs) are efficiently removed from clouds through precipitation, a convenience of nature for the study of these very rare particles that influence multiple climate-relevant cloud properties including ice crystal concentrations, size distributions and phase-partitioning processes. INPs suspended in precipitation can be used to estimate in-cloud INP concentrations and to infer their original composition. Offline droplet assays are commonly used to measure INP concentrations in precipitation samples. Heat and filtration treatments are also used to probe INP composition and size ranges. Many previous studies report storing samples prior to INP analyses, but little is known about the effects of storage on INP concentration or their sensitivity to treatments. Here, through a study of 15 precipitation samples collected at a coastal location in La Jolla, CA, USA, we found INP concentration changes up to > 1 order of magnitude caused by storage to concentrations of INPs with warm to moderate freezing temperatures (−7 to −19 ∘C). We compared four conditions: (1) storage at room temperature (+21–23 ∘C), (2) storage at +4 ∘C, (3) storage at −20 ∘C and (4) flash-freezing samples with liquid nitrogen prior to storage at −20 ∘C. Results demonstrate that storage can lead to both enhancements and losses of greater than 1 order of magnitude, with non-heat-labile INPs being generally less sensitive to storage regime, but significant losses of INPs smaller than 0.45 µm in all tested storage protocols. Correlations between total storage time (1–166 d) and changes in INP concentrations were weak across sampling protocols, with the exception of INPs with freezing temperatures ≥ −9 ∘C in samples stored at room temperature. We provide the following recommendations for preservation of precipitation samples from coastal or marine environments intended for INP analysis: that samples be stored at −20 ∘C to minimize storage artifacts, that changes due to storage are likely an additional uncertainty in INP concentrations, and that filtration treatments be applied only to fresh samples. At the freezing temperature −11 ∘C, average INP concentration losses of 51 %, 74 %, 16 % and 41 % were observed for untreated samples stored using the room temperature, +4, −20 ∘C, and flash-frozen protocols, respectively. Finally, the estimated uncertainties associated with the four storage protocols are provided for untreated, heat-treated and filtered samples for INPs between −9 and −17 ∘C.

Ice-nucleating particles in precipitation samples from the Texas Panhandle

“…This process starts with the formation of cloud droplet followed by freezing due to an INP immersed in the supercooled droplet. In addition, past studies have identified other modes of heterogeneous nucleation, such as condensation freezing (Belosi and Santachiara, 2019), contact freezing (Hoffmann et al, 2013), and inside-out evaporation freezing (Durant and Shaw, 2005). These modes are relatively less relevant in mixed-phase clouds (MPCs) as discussed in the next section.…”

Towards parameterising atmospheric concentrations of ice-nucleating particles active at moderate supercooling

“…Water losses due to evaporation were compensated for by replenishing the circulating water after 10 and 20 min. To avoid storage effects (Beall et al, 2020), samples were analysed immediately after collection in a drop-freezing assay with 52 droplets of 100 µL each, as previously described (Stopelli et al, 2014), and cumulative [INP] was calculated (Vali, 1971). Sampling and analysis were designed in such a way that expected [INP −15 ] of each sample would be well within the detection limits, meaning that several but not all droplets in the assay would be frozen.…”

“…DeMott et al, 2010;Phillips et al, 2013) are based. Furthermore, an increase in atmospheric abundance of INPs active at moderate supercooling has been observed during precipitation (Bigg and Miles, 1964;Huffman et al, 2013;Hara et al, 2016;Conen et al, 2017). This might be explained by aerosolisation of INPs by rain itself, a mechanism similar to the generation of bioaerosol by raindrop impingement (Joung et al, 2017), which is probably dependent on various parameters like surface wetness or land cover.…”

Continuous online monitoring of ice-nucleating particles: development of the automated Horizontal Ice Nucleation Chamber (HINC-Auto)

“…A prime limitation for the absence of longterm monitoring data sets was that online real-time measurements of INP concentrations via INP counters required human operators, as no autonomous online INP counter were available. Bi et al (2019) presented the first autonomous online INP counter based on a continuous flow thermal gradient diffusion chamber (CFDC). A novel paper by Möhler et al (2020) introduced the Portable Ice Nucleation Experiment (PINE), an autonomous online INP counter that uses the adiabatic cooling during expansion to activate the INPs at the targeted supersaturation.…”