Background Free‐Tropospheric Ice Nucleating Particle Concentrations at Mixed‐Phase Cloud Conditions

Abstract

Clouds containing ice are vital for precipitation formation and are important in determining the Earth's radiative budget. However, primary formation of ice in clouds is not fully understood. In the presence of ice nucleating particles (INPs), the phase change to ice is promoted, but identification and quantification of INPs in a natural environment remains challenging because of their low numbers. In this paper, we quantify INP number concentrations in the free troposphere (FT) as measured at the High Altitude Research Station Jungfraujoch (JFJ), during the winter, spring, and summer of the years 2014–2017. INPs were measured at conditions relevant for mixed‐phase cloud formation at T = 241/242 K. To date, this is the longest timeline of semiregular measurements akin to online INP monitoring at this site and sampling conditions. We find that INP concentrations in the background FT are on average capped at 10/stdL (liter of air at standard conditions [T = 273 K and p = 1013 hPa]) with an interquartile range of 0.4–9.6/stdL, as compared to measurements during times when other air mass origins (e.g., Sahara or marine boundary layer) prevailed. Elevated concentrations were measured in the field campaigns of 2016, which might be due to enhanced influence from Saharan dust and marine boundary layer air arriving at the JFJ. The upper limit of INP concentrations in the background FT is supported by measurements performed at similar conditions, but at different locations in the FT, where we find INP concentrations to be below 13/stdL most of the time.

Impact of Air Mass Conditions and Aerosol Properties on Ice Nucleating Particle Concentrations at the High Altitude Research Station Jungfraujoch

“…This is in fair agreement with Herrmann, et al [60], measuring FT conditions more than 60% in winter, and less than 20% and 30% in spring and summer, respectively. For a detailed description of the occurrence of SDEs and marine events, as well as on FT conditions of each individual campaign, see Lacher, et al [58]. Values presented here may differ from Lacher et al [58] since here we only consider measurements taken with HINC and as such exclude measurements from winter 2014 (presented in Boose, et al [30]) which are included in Lacher et al [58].…”

“…The measurements were conducted during a total of 8 field campaigns in the years 2014-2017, out of which two were in spring (May/June), two in summer (July/August), and three in winter (January/February/March). Each field campaign represents 2 to 5 weeks sampling time (see Lacher, et al [58] for a detailed description).…”

“…The latter is most prominent in the warm season and results in a seasonal cycle of aerosol particle concentration and population [52,53,[59][60][61]. In addition, the station is regularly affected by Saharan dust events (SDE) [62] and air masses originating in the marine boundary layer [63], both of which are transported to the JFJ in the FT, and can lead to elevated INP concentrations [30,58,64]. The JFJ is part of the Global Atmospheric Watch (GAW) monitoring project as well as of the ACTRIS2 Infrastructure (European Research Infrastructure for the observation of Aerosol, Clouds, and Trace gases), the Swiss National Air Pollution Monitoring Network (NABEL), and the Swiss meteorological network (SwissMetNet).…”

“…The discrimination between FT and BLI is based on two criterions which have to be fulfilled simultaneously (see Lacher, et al [58] for detailed description): The trace gas ratio of NO y to CO is used to assess FT [60,61,[77][78][79] because both species are emitted from anthropogenic sources; while CO is relatively constant on the time scale of few days to weeks, NO y decreases with atmospheric age. As such, NO y /CO gives information on the last boundary layer contact.…”

“…In order to improve our understanding on the control mechanism of INP concentrations on a time scale longer than that of an individual field study, we present an analysis of INP concentrations and ambient conditions based on a long-term study [58]. We investigate if there are seasonal trends in the correlation between INP concentrations and possibly predictor parameters under different sampling conditions with regard to FT sampling conditions, and also influences from boundary layer injections (BLI).…”