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Residents of urban areas are disproportionately affected by heat stress due to the combination of global warming and increasing urbanisation[1]. This not only affects the quality of life, but also poses a significant health risk and has been shown to lead to increased mortality rates[2]. However, due to the complex nature of urban climate, the impact of such interventions can vary depending on the local conditions and are thus hard to predict.Figure 1) Left: Image of the misting system. Right: Locations of the misting system (blue circle) and the nearby measuring stations. Station 110 serves as a reference. Left photo from Tabea Vogel, https://www.stadt-zuerich.ch/ted/de/index/gsz/aktuell/aktuelle-projekte/Nebelwolke-Turbinenplatz.html, accessed: 04.01.2023. Right aerial image from www.geo.admin.ch, Swiss federal authorities.In this study a real-world implementation of a spray mist cooling system in the city of Zurich is investigated. A ring carrying 180 high pressure nozzles was installed on a public square, as shown in figure 1a. Studies on spray mist cooling are scarce and inconclusive, as it depends on various operational, environmental and experimental factors[3]. State-of-the-art measuring stations[4] have been deployed for continuous measurements of temperature, humidity and other parameters during summer 2022, as shown in figure 1b. The measurements showed a weak cooling effect that was most pronounced south of the cloud, as shown in figure 2. A mean effect of -0.7°C was measured, with the strongest cooling of up to -2.5°C. The impact of the cloud was most pronounced at 25°C. A dependency on relative humidity and wind direction was measured, with the largest effect measured at low relative humidity downwind of the misting system. Outside of the operational hours no temperature difference was observed.Figure 2) Mean difference between the mean temperatures of the measurement sites and the reference station from 12 July to 26 September during operating hours The field experiment supports model development as an ideal case for model validation. The effect of the misting systems on heat and moisture fluxes have been implemented into the urban LES model PALM[1]. The parameterised cooling system in PALM was then used to investigate variations in placement, weather conditions and amount of sprayed water. [1] Keith Oleson u. a., „Interactions between urbanization, heat stress, and climate change“, Climatic Change 129 (1. April 2013), https://doi.org/10.1007/s10584-013-0936-8. [2] Sally Howard und Geetanjali Krishna, „How Hot Weather Kills: The Rising Public Health Dangers of Extreme Heat“, BMJ 378 (14. Juli 2022): o1741, https://doi.org/10.1136/bmj.o1741. [3] Giulia Ulpiani, „Water Mist Spray for Outdoor Cooling: A Systematic Review of Technologies, Methods and Impacts“, Applied Energy 254 (November 2019): 113647, https://doi.org/10.1016/j.apenergy.2019.113647. [4] BARANI DESIGN Technologies, „MeteoHelix IoT Pro Datasheet“, 18. August 2022, https://www.baranidesign.com/meteohelix-pro-weather-station. [5] Maronga et al., Geosci. Model Dev., 13, 1335–1372, https://doi.org/10.5194/gmd-13-1335-2020, 2020

The formation of ice particles in Earth's atmosphere strongly influences the dynamics and optical properties of clouds and their impacts on the climate system. Ice formation in clouds is often triggered heterogeneously by ice-nucleating particles (INPs) that represent a very low number of particles in the atmosphere. To date, many sources of INPs, such as mineral and soil dust, have been investigated and identified in the low and mid latitudes. Although less is known about the sources of ice nucleation at high latitudes, efforts have been made to identify the sources of INPs in the Arctic and boreal environments. In this study, we investigate the INP emission potential from high-latitude boreal forests in the mixed-phase cloud regime. We introduce the HyICE-2018 measurement campaign conducted in the boreal forest of Hyytiala, Finland, between February and June 2018. The campaign utilized the infrastructure of the Station for Measuring Ecosystem-Atmosphere Relations (SMEAR) II, with additional INP instruments, including the Portable Ice Nucleation Chamber I and II (PINC and PINCii), the SPectrometer for Ice Nuclei (SPIN), the Portable Ice Nucleation Experiment (PINE), the Ice Nucleation SpEctrometer of the Karlsruhe Institute of Technology (INSEKT) and the Microlitre Nucleation by Immersed Particle Instrument (mu L-NIPI), used to quantify the INP concentrations and sources in the boreal environment. In this contribution, we describe the measurement infrastructure and operating procedures during HyICE-2018, and we report results from specific time periods where INP instruments were run in parallel for inter-comparison purposes. Our results show that the suite of instruments deployed during HyICE-2018 reports consistent results and therefore lays the foundation for forthcoming results to be considered holistically. In addition, we compare measured INP concentrations to INP parameterizations, and we observe good agreement with the Tobo et al. (2013) parameterization developed from measurements conducted in a ponderosa pine forest ecosystem in Colorado, USA., Atmospheric Chemistry and Physics, 22 (8), ISSN:1680-7375, ISSN:1680-7367

Changes in the ambient aerosol concentration are known to affect the microphysical properties of clouds. Especially regarding precipitation formation, increasing aerosol concentrations are assumed to delay the precipitation onset, but may increase precipitation rates via convective invigoration and orographic spillover further downstream. In this study, we analyse the effect of increased aerosol concentrations on a heavy precipitation event observed in summer 2017 over northeastern Switzerland, an event which was considerably underestimated by the operational weather forecast model. Preceding the precipitation event, Saharan dust was advected towards the Alps, which could have contributed to increased precipitation rates north of the Alpine ridge. To investigate the potential impact of the increased ambient aerosol concentrations on surface precipitation, we perform a series of sensitivity simulations using the Consortium for Small-scale Modeling (COSMO) model with different microphysical parametrizations and prognostic aerosol perturbations. The results show that the choice of the microphysical parametrization scheme in terms of a one- or two-moment scheme has the relatively largest impact on surface precipitation rates. In the one-moment scheme, surface precipitation is strongly reduced over the Alpine ridge and increased further downstream. Simulated changes in surface precipitation in response to aerosol perturbations remain smaller in contrast to the impact of the microphysics scheme. Elevated cloud condensation nuclei (CCN) concentrations lead to increased cloud water and decreased cloud ice mass, especially in regions of high convective activity south of the Alps. These altered cloud properties indeed increase surface precipitation further downstream, but the simulated change is too small to explain the observed heavy precipitation event. Additional ice-nucleating particles (INPs) increase cloud ice mass, but only trigger local changes in downstream surface precipitation. Thus, increased aerosol number concentrations during the Saharan dust outbreak are unlikely to have caused the heavy precipitation event in summer 2017., Quarterly Journal of the Royal Meteorological Society, 148 (743), ISSN:0035-9009, ISSN:1477-870X

The formation of ice particles in Earth’s atmosphere strongly influences the dynamics and optical properties of clouds and their impacts on the climate system. Ice formation in clouds is often triggered heterogeneously by ice nucleating particles (INPs) that represent a very low number of particles in the atmosphere. To date, many sources of INPs, such as mineral and soil dust, have been investigated and identified in the lower latitudes. Although less is known about the sources of ice nucleation at higher latitudes, efforts have been made to identify the sources of INPs in the Arctic and boreal environments. In this study, we investigate the INP emission potential from high latitude boreal forests. We introduce the HyICE-2018 measurement campaign conducted in the boreal forest of Hyytiälä, Finland between February and June 2018. The campaign utilized the infrastructure of the SMEAR II research station with additional instrumentation for measuring INPs to quantify the concentrations and sources of INPs in the boreal environment. In this contribution, we describe the measurement infrastructure and operating procedures during HyICE-2018 and we report results from specific time periods where INP instruments were run in parallel for inter-comparison purposes. Our results show that the suite of instruments deployed during HyICE-2018 reports consistent results and therefore lays the foundation for forthcoming results to be considered holistically. In addition, we compare the INP concentration we measured to INP parameterizations, and we show a very good agreement with the Tobo et al. (2013) parameterization developed from measurements conducted in a ponderosa pine forest ecosystem in Colorado, USA.

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] spanned 3 orders of magnitude and showed a general increase from mid-February until early April. No persistent local or regional sources of INPs in the boreal environment of southern Finland could be identified. Rather, it is hypothesised that the INPs at SMEAR II are a result of dilution 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, most likely nanoscale biological fragments such as ice nucleating macromolecules (INMs), 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. Limitations of the instrumental setup and the necessity of the future field INP studies are addressed.

The largest contributors to the uncertainty in assessing the anthropogenic contribution in radiative forcing are the direct and indirect effects of aerosol particles on the Earth's radiative budget. Soot particles are of special interest since their properties can change significantly due to aging processes once they are emitted to the atmosphere. Probably the largest obstacle for the investigation of these processes in the laboratory is the long atmospheric lifetime of one week, demanding tailored experiments that cover this time span. This work presents results on the ability of two types of soot to act as cloud condensation nuclei (CCN) after exposure to atmospherically relevant levels of ozone and humidity. Aging times of up to 12 h were achieved by successful application of the continuous-flow stirred tank reactor (CSTR) concept while allowing for size-selection of particles prior to the aging step. 100 nm particles rich in organic carbon (OC) that were initially CCN-inactive showed significant CCN-activity at supersaturations (SS) down to 0.3 % after 10 h of exposure to 200 ppb of ozone. While this process was not affected by different levels of relative humidity in the range 5–75 %, a high sensitivity towards the ambient/reaction temperature was observed. Soot particles with a lower OC-content demanded an approximately four-fold longer aging duration to show CCN-activity for the same SS. Prior to the slow change in the CCN-activity, a rapid increase in the particle diameter was detected which occurred within several minutes. This study highlights the applicability of the CSTR-approach for the simulation of atmospheric aging processes, as aging durations beyond 12 h can be achieved in comparably small aerosol chamber volumes (