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 and 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 mechanistic description for boreal forest environments which considers the seasonal variation in INP concentrations. For this, we use the ambient air temperature measured close to the ground at 4.2 m height as a proxy for the season, which appears to affect the source strength of biogenic emissions and, thus, the INP abundance over the boreal forest. 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.

The diurnal and seasonal variability of ice nucleating particles at the High Altitude Station Jungfraujoch (3580 m a.s.l.), Switzerland

“…We found a seasonal cycle, highest in spring with a median of 3.1 INP std L −1 , followed by summer (median: 1.6 INP std L −1 ), and lowest in fall and winter (median: 0.5 std L −1 and 0.7 INP std L −1 , respectively), all in absence of SDEs. This is consistent with the seasonality observed in other studies at the same site (Conen et al, 2015;Lacher et al, 2018) and at different sites (Wex et al, 2019;Tobo et al, 2020;Schneider et al, 2020). Here, INP concentrations show a larger seasonal dependency than the total particle concentrations.…”

Measurement report: Introduction to the HyICE-2018 campaign for measurements of ice nucleating particles in the Hyytiälä boreal forest

“…In this paper, we summarize the general results for the duration of the intensive campaign and focus on a few instances where defined efforts were made to inter-compare different INP measurement techniques and instrumentation. Longer term and more detailed studies that have emerged from specific instruments and/or activities have been or will be published independently, with many contributions aimed at the Copernicus special issue, "Ice nucleation in the boreal atmosphere" (Paramonov et al, 2020;Schneider et al, 2021).…”

“…For INSEKT, filters were collected beginning March 2 and continuing through the end of the intensive campaign in mid-June 2018. Further INSEKT filters were collected more intermittently at the site for more than one more year (until May 2019), and that data has yielded detailed information related to the seasonality of observed INPs (Schneider et al, 2021). Ambient aerosol particles were collected onto 0.2 µm filters (Whatman nuclepore track-etched polycarbonate membranes, 47 mm), which were pre-cleaned with a 10% H 2 O 2 solution and subsequently rinsed with deionized water that was passed through a 0.1µm syringe filter (Whatman).…”

“…To overcome our lack of knowledge, a measurement campaign was organized in 2018 at the Station for Measuring Ecosystem-Atmosphere Relations (SMEAR) II, at the Hyytiälä Forestry Research Station in Juupajoki, Finland. The intensive field campaign, called HyICE-2018, began in February and extended until June, with longer term INP monitoring efforts continuing for more than one year beyond the intensive measurement period (Schneider et al, 2021).…”

“…For the INSEKT data, the error bars represent the statistic as well as the systematic error of the INSEKT assay. More details related to the calculation of these error bars is given inSchneider et al (2021). For the µL-NIPI data, the error bars were calculated using the Poisson Monte Carlo procedure as described inHarrison et al (2016)…”