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Abstract. Ice-nucleating particles (INPs) represent a rare subset of aerosol particlesthat initiate cloud droplet freezing at temperatures above the homogenousfreezing point of water (−38 ∘C). Considering that the oceancovers 71 % of the Earth's surface and represents a large potential sourceof INPs, it is imperative that the identities, properties and relativeemissions of ocean INPs become better understood. However, the specificunderlying drivers of marine INP emissions remain largely unknown due tolimited observations and the challenges associated with isolating rare INPs. Bygenerating isolated nascent sea spray aerosol (SSA) over a range ofbiological conditions, mesocosm studies have shown that marine microbes cancontribute to INPs. Here, we identify 14 (30 %) cultivable halotolerantice-nucleating microbes and fungi among 47 total isolates recovered fromprecipitation and aerosol samples collected in coastal air in southernCalifornia. Ice-nucleating (IN) isolates collected in coastal air were nucleated ice fromextremely warm to moderate freezing temperatures (−2.3 to −18 ∘C). While some Gammaproteobacteria and fungi are known to nucleate ice attemperatures as high as −2 ∘C, Brevibacterium sp. is the first Actinobacteriafound to be capable of ice nucleation at a relatively high freezingtemperature (−2.3 ∘C). Air mass trajectory analysis demonstratesthat marine aerosol sources were dominant during all sampling periods, andphylogenetic analysis indicates that at least 2 of the 14 IN isolates areclosely related to marine taxa. Moreover, results from cell-washingexperiments demonstrate that most IN isolates maintained freezing activityin the absence of nutrients and cell growth media. This study supportsprevious studies that implicated microbes as a potential source of marineINPs, and it additionally demonstrates links between precipitation, marineaerosol and IN microbes. 

Abstract. Ice-nucleating particles (INPs) are efficiently removed fromclouds through precipitation, a convenience of nature for the study of thesevery rare particles that influence multiple climate-relevant cloudproperties including ice crystal concentrations, size distributions andphase-partitioning processes. INPs suspended in precipitation can be used toestimate in-cloud INP concentrations and to infer their originalcomposition. Offline droplet assays are commonly used to measure INPconcentrations in precipitation samples. Heat and filtration treatmentsare also used to probe INP composition and size ranges. Many previousstudies report storing samples prior to INP analyses, but little is knownabout the effects of storage on INP concentration or their sensitivity totreatments. Here, through a study of 15 precipitation samples collected at acoastal location in La Jolla, CA, USA, we found INP concentration changes upto > 1 order of magnitude caused by storage to concentrations ofINPs with warm to moderate freezing temperatures (−7 to−19 ∘C). We compared four conditions: (1) storage at roomtemperature (+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 bothenhancements and losses of greater than 1 order of magnitude, withnon-heat-labile INPs being generally less sensitive to storage regime, butsignificant losses of INPs smaller than 0.45 µm in all tested storageprotocols. Correlations between total storage time (1–166 d) and changesin INP concentrations were weak across sampling protocols, with theexception of INPs with freezing temperatures ≥ −9 ∘C in samples stored at room temperature. We provide thefollowing recommendations for preservation of precipitation samples fromcoastal or marine environments intended for INP analysis: that samples bestored at −20 ∘C to minimize storage artifacts, thatchanges due to storage are likely an additional uncertainty in INPconcentrations, and that filtration treatments be applied only to freshsamples. At the freezing temperature −11 ∘C, average INPconcentration losses of 51 %, 74 %, 16 % and 41 % were observed foruntreated samples stored using the room temperature, +4, −20 ∘C, and flash-frozen protocols, respectively.Finally, the estimated uncertainties associated with the four storage protocolsare provided for untreated, heat-treated and filtered samples for INPsbetween −9 and −17 ∘C.