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.
Highlights
Cloud processes are of particular importance for the evolution of weather and climate, as they regulate the global distribution of precipitation and influence Earth’s radiative budget (Hoose and Möhler, 2012; Murray et al, 2012)
The Ice-nucleating particles (INPs) concentrations range from about 10−4 to 10−2 std L−1 at the highest and from about 100 to 102 std L−1 at the lowest temperatures. These concentration values fall within the range of INP concentrations measured during previous globally distributed field studies, which are summarized in Kanji et al (2017)
This study provides a unique data set of continuously recorded INP concentrations for more than 1 year at the Station for Measuring Ecosystem–Atmosphere Relations (SMEARII) station located in the Finnish boreal forest
Summary
Cloud processes are of particular importance for the evolution of weather and climate, as they regulate the global distribution of precipitation and influence Earth’s radiative budget (Hoose and Möhler, 2012; Murray et al, 2012). Hartmann et al (2019) report INP concentrations from the past 500 years derived from ice core samples collected at two Arctic sites They do suggest indications that biological INPs contribute to Arctic INP populations throughout the past centuries and assume that it is likely that the strength of local biological particle sources is enhanced during a particular time of the year, which influences the INP variability. The short sampling times (low volumes; 1 h per day) and/or colder activation temperatures may serve to maximize sampling variability and mask any potential biological signal (Schrod et al 2020) To date, these studies present the first observations and indications of the seasonal variability of INP concentrations, addressing the need for more long-term INP observations. The focus of most of the previous studies, except for the Schrod et al (2020) study was especially at Arctic INPs
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