Report
Teaser, summary, work performed and final results
Periodic Reporting for period 1 - ATM-METFIN (Quantifying atmospheric ice nucleating particle (INP) concentrations via advances in measurement techniques and field studies of ice nucleation)
Teaser
Atmospheric aerosol particles play an important role in the global climate by way of influencing Earth’s hydrological cycle, energy and radiation balance. Due to their importance, aerosol-cloud interactions have been a subject of intense research over the last several...
Summary
Atmospheric aerosol particles play an important role in the global climate by way of influencing Earth’s hydrological cycle, energy and radiation balance. Due to their importance, aerosol-cloud interactions have been a subject of intense research over the last several decades. Despite that, exact quantification of aerosol effects on the changing cloud properties and the ability to predict future climate based on expected changes to global aerosol burden have been challenging. Ubiquitous in the atmosphere, atmospheric aerosol particles can participate in the formation of liquid and ice clouds by acting as cloud condensation nuclei (CCN) and ice nucleating particles (INPs), respectively. Both have received a lot of attention in recent years, with a multitude of studies attempting to quantify the importance of aerosols in the aerosol-cloud-climate system. The objectives of the project included the quantification of ice nucleating particles in condensation/immersion freezing modes under mixed-phase cloud conditions, the comparison to previously published data from other locations around the globe, the investigation of INP physical and chemical properties and the identification of the already known INP species in the boreal environment. The main results presented several important questions and conclusions for future ambient INP studies. First and foremost, it was not possible to identify physical and chemical properties of ambient INPs despite the complexity of instrumental setup and a multitude of additional instrumentations deployed during the campaign. Unfortunately, the current instrumental setup coupled with the rarity of ambient INPs rendered any speculations about INP identity inconclusive. If the future INP field studies aim to explicitly identify the physical and chemical properties of INPs, a single-particle analysis of ice crystal residuals must be carried out. The second important question that needs to be addressed as a result of this work is the necessity for future INP measurements. Future ambient INP studies need to explicitly focus on the physical and chemical properties of the INPs and whether they are different from the rest of the aerosol population. Additionally, future studies should aim to assess whether the explicit physics and chemistry of INPs can be and need to be parameterised in order to understand aerosol-cloud interactions on the global scale.
Work performed
Ice nucleating particle (INP) measurements were performed in the boreal environment of Southern Finland at the Station for Measuring Ecosystem-Atmosphere Relations SMEAR II in the winter-spring of 2018. Measurements with the Portable Ice Nucleation Chamber (PINC) were conducted at 242 K and 105% relative humidity with respect to water. The median INP number concentration [INP] during a six-week measurement period was found to be 13 L-1. [INP] varied by three orders of magnitude and showed a general increase from mid-February until early April. No local or regional sources of INPs in the boreal environment of Southern Finland could be identified. Rather, it is believed that the INPs at SMEAR II are a result of mixing during long-range transport. Despite high variability, the measured [INP] values fall within the range expected for INP number concentrations measured elsewhere at similar thermodynamic conditions. [INP] did not correlate with any of the examined relevant parameters during the entire field campaign, indicating that no one single parameter can be used to predict the INP number concentration at the measurement location during the examined time period. The absence of correlation across the entire field campaign also suggests that a variety of particles are acting as INPs at different times, although it was indirectly determined that, on average, ambient INPs are most likely in the size range of 0.1–0.5 μm in diameter. On shorter time scales, several particle species correlated well with [INP] implying their potential role as INPs. Depending on the meteorological conditions, signatures of black carbon BC, supermicron biological particles and sub-0.1 μm particles have been found in the INP signal. However, an increase in the concentration of any of these particle species may not necessarily lead to the increase in [INP], reasons for which remain unknown.
A laboratory study of the ice nucleation efficiency of several types of mineral and soil dust was also finished during the project duration. The study showed that it is not possible to predict the ice nucleation activity of surface-collected soil dust based on the presence and amount of certain minerals or any particular class of compounds, such as soluble or proteinaceous/organic. Instead, at temperatures of 238–243 K the ice nucleation activity of the untreated, surface-collected soil dust in the condensation freezing mode can be roughly approximated by one of the existing surrogates for atmospheric mineral dust, such as illite NX.
The project resulted in one peer-reviewed, Open Access publication, the one included in the Publications list. The publication related to the field campaign is still in its preparatory phase. It will, however, be also published with an Open Access publisher within the next couple of months.
The work carried out during the project duration has been presented in several conferences and workshops. Main results were presented during the International Conference on Nucleation and Atmospheric Aerosols 2017, International Aerosol Conference 2018 and European Aerosol Conference 2019. The project was also presented during several visits to the University of Helsinki and Gothenburg University, as well as internally within the ETH group.
Final results
The measurements performed during the project run were the first-of-their-kind, state-of-the-art measurements and the first ones conducted in the boreal environment. Advances in technical expertise and collaboration with several international groups allowed to probe very low INP number concentrations, as well as to intercompare the relevant measurement techniques. The main research conclusions all point to the necessity to understand ice nucleation on the microphysical, molecular level. The performed measurements corroborate previously published results and require an imminent assessment of the necessity of similar future measurements. Studies of the behaviour of the INPs in ambient atmosphere and their interaction with water vapour are crucial in improving our understanding of the aerosol-cloud interactions.