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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. 

Abstract. Ice nucleating particles (INP) have been found to influence the amount, phase, and efficiency of precipitation from winter storms, including atmospheric rivers. Warm INP, those that initiate freezing at temperatures warmer than −10°C, are thought to be particularly impactful because they can create primary ice in mixed-phase clouds, enhancing precipitation efficiency. The dominant sources of warm INP during atmospheric rivers, the role of meteorology in modulating transport and injection of warm INP into atmospheric river clouds and the impact of warm INP on mixed-phase cloud properties are not well-understood. Time-resolved precipitation samples were collected during an atmospheric river in Northern California, USA during winter 2016. Precipitation was collected at two sites, one coastal and one inland, that are separated by less than 35km. The sites are sufficiently close that airmass sources during this storm were almost identical, but the inland site was exposed to terrestrial sources of warm INP while the coastal site was not. Warm INP were more numerous in precipitation at the inland site by an order of magnitude. Using FLEXPART dispersion modelling and radar-derived cloud vertical structure, we detected influence from terrestrial INP sources at the inland site, but did not find clear evidence of marine warm INP at either site. We episodically detected warm INP from long-range transported sources at both sites. By extending the FLEXPART modelling using a meteorological reanalysis, we demonstrate that long-range transported warm INP are observed only when the upper tropospheric jet provided transport to cloud tops. Using radar-derived hydrometeor classifications, we demonstrate that hydrometeors over the terrestrially-influenced inland site were more likely to be in the ice phase for cloud temperatures between 0°C and −10°C. We thus conclude that terrestrial and long-range transported aerosol were important sources of warm INP during this atmospheric river. Meteorological details such as transport mechanism and cloud structure were important in determining warm INP source strength and injection temperature, and ultimately the impact of warm INP on mixed phase cloud properties.