Long-Term Coastal Ice-Nucleating Particle Measurements from Mace Head Research Station

Low-level marine clouds reflect incoming short-wave solar radiation which would otherwise be readily absorbed by the dark ocean surface. Predicting primary ice formation by Ice Nucleating Particles (INPs) for warm (> -30°C) clouds in the marine environment is important for accurately predicting the albedo effect of clouds. Primary ice formation followed by secondary ice processes changes the phase of the cloud, the cloud radiative scattering and absorption effects, the cloud lifetime, and precipitation. INP concentrations can be extremely low and still cause a cloud to transition from liquid to ice phase.

In terms of coastal INP abundance at mixed-phase cloud temperatures, marine organics are the most important contributing factor. Dust, transported (or re-suspended or settling cosmic dust), may even play a role in marine regions at colder temperatures. However, there is a dearth of information on what occurs in winter versus summer months in the marine environment, which leaves it hard to determine whether the marine organics showing good warm temperature INP activation (< -15°C) are linked to marine productivity (which would show higher particulate organic carbon) or are more closely related to the dissolved organic carbon (DOC) which persists in the marine environment throughout the year. This could also be clarified by size segregation of the INP sampling, although that has yet to be tested for marine samples. McCluskey et al. (2018) suggest that the most prominent INPs from SSA (which may not be source specific) are heat stable but susceptible to peroxide digestion treatment resulting in a reduction in INP activation behaviour. This indicates that a large fraction of this INP type was some form of marine organic matter (OM), not made up of proteins or other biological material that could be less INP active from a deformity of exposure to 95°C, which may likely be DOC. Simultaneously, a smaller fraction of marine INP were augmented in increased INP activation behaviour above -22°C, these particles were heat labile, and likely more like POC. However, these types were less frequent and even less active than the terrestrial OM types. Depending on the temperature of the mixed phase cloud, POC in small quantities could be significant enough to initiate the Wegener-Bergeron-Findeisen precipitation process.

To assess primary ice formation in mixed-phase oceanic clouds as a function of the season, on-going aerosol filter based INP measurements were taken at Mace Head research station (MHD) as a combined projection between the National University of Ireland Galway and the Karlsruhe Institute of Technology (KIT). The project, called Long-term Ice Nucleating measurements At Mace head (LINAM), is a long-term campaign that lasted from November 2017- February 2019. INP samples were collected at MHD on 0.2 µm pore width NucleporeTM track-etched polycarbonate membrane filters (Whatman). Analysis of the samples is performed at KIT using INSEKT (a detailed methodology can be found in Schiebel (2017)). The NucleporeTM filters are pre-treated and cleaned in the same process described above following the description in CSU-IS procedure in the supplement of Hiranuma et al. (2015) at KIT and shipped in batch to MHD with two filters per sterile aluminium envelopes. Two samples are collected per week at MHD atmospheric research station at 10 m height, using PM10 Leckel head inlets (Sven Leckel, Berlin, Germany), with ~ 1m of stainless steel 1” diameter tubing before reaching the NucleporeTM filter holders, which are custom manufactured by KIT. The pump draws 15 L min-1 of sample through each filter membrane often starting with a pressure of ~700 hPa behind the membrane filter, and dropping to ~300 hPa at the end of a week of sampling. Ambient pressure, start/end time/pressure are recorded for all the samples. Three sample types are collected at MHD, clean sector, all sector, and polluted sector. Polluted sector is always run in parallel for ~24-72 hours depending on the loading conditions. Clean sector samples are run for a minimum of 3 days. All sector samples are run for ~7 days at a time. Samples are prepared and collected from the filter holders in a clean hood at MHD. Collected samples are frozen and shipped to Germany for analysis with the INSEKT instruments in batches with collected field blanks.

Preliminary results show temperature dependent time-trends of INP concentration for clean sector (A) and all sector (B) samples collected at MHD. Parallel measurements of all sector and clean sector samples can also be compared to identify marine origin INP concentrations over temperature ranges. The data will be analysed with collocated measurements of aerosol size and chemistry to better understand the conditions which favour anomalous INP concentration, as well as to further our understanding of both marine and coastal INP seasonality.

References:

Hiranuma, N., and Coauthors, 2015: A comprehensive laboratory study on the immersion freezing behavior of illite NX particles: a comparison of 17 ice nucleation measurement techniques. Atmos. Chem. Phys., 15, 2489-2518.

McCluskey, C. S., and Coauthors, 2018: Marine and Terrestrial Organic Ice-Nucleating Particles in Pristine Marine to Continentally Influenced Northeast Atlantic Air Masses. Journal of Geophysical Research: Atmospheres, 123, 6196-6212.

Schiebel, T., 2017: Ice Nucleation Activity of Soil Dust Aerosols, PhD thesis, Karlsruher Institut für Technologie (KIT).