The seasonal cycle of ice-nucleating particles linked to the abundance of biogenic aerosol in boreal forests

Abstract

Abstract. Ice-nucleating particles (INPs) trigger the formation of cloud ice crystals in the atmosphere. Therefore, they strongly influence cloud microphysical and optical properties, as well as precipitation and the life cycle of clouds. Improving weather forecasting and climate projection requires an appropriate formulation of atmospheric INP concentrations. This remains challenging, as the global INP distribution and variability depend on a variety of aerosol types and sources, and neither their short-term variability nor their long-term seasonal cycles are well covered by continuous measurements. Here, we provide the first year-long set of observations with a pronounced INP seasonal cycle in a boreal forest environment. Besides the observed seasonal cycle in INP concentrations with a minimum in wintertime and maxima in early and late summer, we also provide indications for a seasonal variation in the prevalent INP type. We show that the seasonal dependency of INP concentrations and prevalent INP types is most likely driven by the abundance of biogenic aerosol. As current parameterizations do not reproduce this variability, we suggest a new parameterization approach, which considers the seasonal variation of INP concentrations. For this, we use the ambient air temperature as a proxy for the season which affects the source strength of biogenic emissions and by that the INP abundance over the boreal forest areas. Furthermore, we provide new INP parameterizations based on the Ice Nucleation Active Surface Site (INAS) approach, which specifically describes the ice nucleation activity of boreal aerosols particles prevalent in different seasons. Our results characterize the boreal forest as an important but variable INP source and provide new perspectives to describe these new findings in atmospheric models.

Long-term deposition and condensation ice-nucleating particle measurements from four stations across the globe

“…The FRIDGE (Frankfurt Ice Deposition Freezing Experiment) instrument was originally introduced by Bundke et al (2008) and Klein et al (2010) but was fundamentally reevaluated and updated by Schrod et al (2016). Since this effort FRIDGE has participated in laboratory intercomparisons (Hiranuma et al, 2015;DeMott et al, 2018;Hiranuma et al, 2019) and field campaigns (Schrod et al, 2017;Thomson et al, 2018;Gute et al, 2019;Marinou et al, 2019).…”

“…However, due to several difficulties in quantifying and characterizing INPs in the atmosphere, there are still large knowledge gaps concerning geographic and vertical distributions, seasonal and/or interannual variations, chemical composition, and sources of INPs. Although there have been significant advances in identifying some of the INP species of global relevance (e.g., Atkinson et al, 2013;O'Sullivan et al, 2015;Wilson et al, 2015;Hiranuma et al, 2019), understanding microscopic freezing processes (e.g., Marcolli, 2014;Kiselev et al, 2017;David et al, 2020), parameterizing INP concentrations (e.g., DeMott et al, 2010DeMott et al, , 2015Niemand et al, 2012), intercomparing INP instrumentation in standardized procedures (Hiranuma et al, 2015;DeMott et al, 2018;Hiranuma et al, 2019), and designing and characterizing new measurement techniques (e.g., Garimella et al, 2016;Schrod et al, 2016;Möhler et al, 2020), we are still far from having a complete picture. A major shortcoming is the lack of global coverage and continuous longer-term observations of INPs.…”

“…Aerosol samples were collected using electrostatic precipitation on silicon substrates in a semiautomated routine. Samples were shipped to our laboratory in Frankfurt and were subsequently analyzed for ice nucleation activity using the FRIDGE isothermal vacuum diffusion chamber (Schrod et al, 2016(Schrod et al, , 2017. Each sample was analyzed at multiple combinations of temperature (−20, −25 and −30 • C) and relative humidity (95 %, 97 %, 99 % and 101 % with respect to water).…”

Referee Comment

“…Additional use of immersion freezing methods, as in that study, to investigate if that mode of ice nucleation also shows a lack of long-term trends at sites. Also, please note that the full publication on the noted results is now in press and under review in ACP (Schneider et al, 2020).…”

“…Page 2, Lines 30-32: I find this statement quickly becoming untrue, with many laboratories now involved in long-term measurements of immersion freezing (e.g., Schneider et al, 2020), some with agency support, and multiple online instruments are in development (or are already there) for automated or semi-automated operation.…”