Ice-nucleating particles in the free troposphere: long-term observation and first measurements at cirrus formation temperatures using the novel Portable Ice Nucleation Experiment PINEair

Ice formation induced by ice-nucleating particles (INPs) greatly influences the formation, life cycles, and climatic impact of tropospheric clouds, as well as their ability to form precipitation. However, knowledge about the abundance of INPs especially in the free troposphere (FT) is still missing. Therefore, this thesis aimed at observing INPs at the Sonnblick Observatory (SBO, 3106 m a.s.l.) which is frequently located in the FT. The comprehensive measurements were conducted over long term to investigate the seasonal variation, at high time resolution to obtain information on the diurnal and shorter-term variability, and in a wide temperature range to cover mixed-phase cloud (MPC) and cirrus cloud conditions. INPs that impact the ice formation in cirrus clouds were measured with the novel instrument PINEair (Portable Ice Nucleation Experiment airborne), which was developed as part of this PhD thesis. The new aircraft-based INP instrument PINEair was developed especially for use on the HALO (High Altitude and Long Range) research aircraft. It is the first aircraft-based instrument that can perform automated in-situ measurements of INPs to temperatures of −65 °C. PINEair simulates cloud-like conditions by expansion cooling. It consists of three expansion chambers operated in a cycling mode to achieve measurements with higher time and spatial resolution in a fast-flying jet aircraft. Laboratory measurements with a prototype of PINEair have successfully shown that the newly developed instrument can measure the INP concentration in a wide temperature range relevant for MPC and cirrus clouds. The distinction between homogeneous and heterogeneous freezing at cirrus conditions was demonstrated for ground-based conditions with ambient air sampling and for lower pressure (p = 250 mbar) conditions during aircraft flights which were simulated in a series of laboratory experiments. For this purpose, a new expansion procedure was developed where the pressure in the chambers can step-wise be reduced at the beginning of the expansion. This allows the calculation of the peak ice saturation ratio (Sice,p) in the chamber assuming ice-saturated conditions before expansion start and an adiabatic temperature decrease. As part of this PhD thesis, a mobile prototype of PINEair was engineered, built, and tested during a field campaign at the SBO in the Austrian Alps. Measurements were performed quasi continuously during the time period from May 8 - 22, 2023, at both MPC (T = −22.7 °C and T = −27.5 °C) and cirrus (T = −47.8 °C) temperatures. At a temperature of −47.8 °C and Sice,p = 1.49−1.52, a median INP concentration of 1.1 std L^−1 was measured, and a maximum concentration of about 90 std L^−1. Higher Sice,p conditions in the chamber resulted in higher INP concentrations, which is consistent with the literature. During a case study with increased aerosol concentrations and particle mass concentrations, also increased INP concentrations up to 84 std L^−1 were measured at −47.8 °C and Sice, p = 1.52 − 1.55. Another major part of this PhD thesis were long-term INP measurements in the MPC temperature range at the SBO. Due to its location, the site receives both air masses from the FT and the boundary layer (BL). The measurements were performed with the freezing experiment INSEKT (Ice Nucleation Spectrometer of the Karlsruhe Institute of Technology) from August 2019 to August 2022 with a time resolution of one week. To date, this is the longest, continuous INP measurement series. In addition, a 14-month INP time series with a high time resolution of 6 min was performed with PINE, starting in August 2021. Both data sets show a recurrent seasonal sinusoidal trend with the highest INP concentrations in spring/summer and the lowest in winter. From April to September, a daily cycle in the INP concentration is observed with a maximum around noon and a minimum at midnight. In contrast, from October to February, no daily cycle was observed and the INP concentrations were consistently low. The seasonal and diurnal variations of the INP concentration are likely caused by the influence of air masses from the BL, as the concentrations of the tracers (aerosol particles with a diameter larger than 90 nm and 214Polonium concentration) show the same trends. A heat analysis of the filters prior to INP analysis with INSEKT showed reduced INP activity, especially at higher temperatures above −13 °C for all seasons, indicating that biogenic compounds contributed to the INP abundance. Strong sudden increases in the INP concentration are caused by episodically occurring dust events, which were observed by enhanced and correlated concentrations of both the INP number concentration and the aerosol mass concentration. A comparison of the measured INP concentration during a dust event in March 2022 to the parameterization of DeMott et al. (2015) showed a good agreement, the temperature dependence and 98.5 % of the data within a factor of 10 were predicted correctly. The median INP concentration of the whole measurement period was slightly higher during clear-sky periods than during cloudy periods, which could be caused by processes such as wet-removal or pre-activation of INPs. Moreover, the INP concentration was found to be correlated with the aerosol concentration, size, and mass, especially at lower nucleation temperatures. However, the observed variation of the INP concentration was not correlated with meteorological parameters. From this study, it can be concluded that the INP concentration at SBO is mainly influenced by different sources including free tropospheric aerosols, long-range transported dust, and local or regional aerosol sources transported from the BL to the station

How Porosity Influences the Heterogeneous Ice Nucleation Ability of Secondary Organic Aerosol Particles

During processing in deep convective cloud systems, highly viscous or glassy secondary organic aerosol (SOA) particles can develop a porous structure through a process known as atmospheric freeze-drying. This structural modification may enhance their heterogeneous ice nucleation ability under cirrus conditions through the pore condensation and freezing mechanism. Pristine, compact SOA particles, on the other hand, are recommended to be treated as ice-inactive in models. This recommendation also applies to internally mixed particles, where a coating layer of secondary organic matter (SOM) deactivates the intrinsic ice nucleation ability of the core, which may be a mineral dust grain. Ice cloud-processing may also improve the ice nucleation ability of such a composite particle by inducing structural changes in the coating layer, which can release active sites on the mineral surface. In this work, we investigated the change in the ice nucleation ability of pure SOA particles from the ozonolysis of alpha-pinene and two types of internally mixed particles (zeolite and coal fly ash particles coated with SOM) after being subjected to the atmospheric freeze-drying process simulated in an expansion cloud chamber. For pure alpha-pinene SOA, we found only a slight improvement in the ice nucleation ability of the ice cloud-processed, porous particles compared to their pristine, compact counterparts at 221 and 217 K. In contrast, the zeolite and coal fly ash particles, which were initially deactivated by the organic coating, became significantly more ice-active after atmospheric freeze-drying, emphasizing that such composite particles cannot be excluded from model simulations of heterogeneous ice formation. Understanding how ice crystals form in the Earth\u27s atmosphere is important for predicting climate using cloud models. It is believed that certain surface features on atmospheric aerosol particles, such as cracks and pores found on mineral dust grains, can effectively induce ice formation. Ice crystals are formed from small amounts of liquid water that have condensed and frozen in the pores of the particles. As a result, ice formation is less likely to occur on solid particles that lack such pores and have a smooth surface. One type of the latter are particles formed by the condensation of volatile, biogenic and anthropogenic organic compounds. In models of ice formation, such secondary organic aerosol particles are often neglected because of their compact, non-porous shape. In our work, we simulated a pathway by which such particles can acquire a porous structure through cloud-processing in the atmosphere, and investigated how this affects their ability to form ice. We also investigated mixed particle types in which the organic compounds condensed on a mineral surface, blocking the pores that allowed efficient ice formation. The aforementioned cloud-processing partially restructured the organic material on the mineral surface, and the particles became more effective at forming ice again. Regardless of particle morphology, pure secondary organic aerosol particles are inefficient at nucleating ice under cirrus conditions A pristine coating layer of secondary organic matter deactivates efficient ice nucleating particles such as mineral dust and coal fly ash Ice cloud-processing changes the structure of the coating layer and partially restores the ice nucleation ability of such mixed particle

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

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.Peer reviewe

Synergistic HNO3_{3}–H2_{2}SO4_{4}–NH3_{3} upper tropospheric particle formation

New particle formation in the upper free troposphere is a major global source of cloud condensation nuclei (CCN)$^{1,2,3,4}$. However, the precursor vapours that drive the process are not well understood. With experiments performed under upper tropospheric conditions in the CERN CLOUD chamber, we show that nitric acid, sulfuric acid and ammonia form particles synergistically, at rates that are orders of magnitude faster than those from any two of the three components. The importance of this mechanism depends on the availability of ammonia, which was previously thought to be efficiently scavenged by cloud droplets during convection. However, surprisingly high concentrations of ammonia and ammonium nitrate have recently been observed in the upper troposphere over the Asian monsoon region5,6. Once particles have formed, co-condensation of ammonia and abundant nitric acid alone is sufficient to drive rapid growth to CCN sizes with only trace sulfate. Moreover, our measurements show that these CCN are also highly efficient ice nucleating particles—comparable to desert dust. Our model simulations confirm that ammonia is efficiently convected aloft during the Asian monsoon, driving rapid, multi-acid HNO$_{3}$–H$_{2}$SO$_{4}$–NH$_{3}$ nucleation in the upper troposphere and producing ice nucleating particles that spread across the mid-latitude Northern Hemisphere

Synergistic HNO3-H2SO4-NH3 upper tropospheric particle formation

New particle formation in the upper free troposphere is a major global source of cloud condensation nuclei (CCN)1-4. However, the precursor vapours that drive the process are not well understood. With experiments performed under upper tropospheric conditions in the CERN CLOUD chamber, we show that nitric acid, sulfuric acid and ammonia form particles synergistically, at rates that are orders of magnitude faster than those from any two of the three components. The importance of this mechanism depends on the availability of ammonia, which was previously thought to be efficiently scavenged by cloud droplets during convection. However, surprisingly high concentrations of ammonia and ammonium nitrate have recently been observed in the upper troposphere over the Asian monsoon region5,6. Once particles have formed, co-condensation of ammonia and abundant nitric acid alone is sufficient to drive rapid growth to CCN sizes with only trace sulfate. Moreover, our measurements show that these CCN are also highly efficient ice nucleating particles-comparable to desert dust. Our model simulations confirm that ammonia is efficiently convected aloft during the Asian monsoon, driving rapid, multi-acid HNO3-H2SO4-NH3 nucleation in the upper troposphere and producing ice nucleating particles that spread across the mid-latitude Northern Hemisphere

Synergistic HNO3–H2SO4–NH3 upper tropospheric particle formation

New particle formation in the upper free troposphere is a major global source of cloud condensation nuclei (CCN). However, the precursor vapours that drive the process are not well understood. With experiments performed under upper tropospheric conditions in the CERN CLOUD chamber, we show that nitric acid, sulfuric acid and ammonia form particles synergistically, at rates that are orders of magnitude faster than those from any two of the three components. The importance of this mechanism depends on the availability of ammonia, which was previously thought to be efficiently scavenged by cloud droplets during convection. However, surprisingly high concentrations of ammonia and ammonium nitrate have recently been observed in the upper troposphere over the Asian monsoon region. Once particles have formed, co-condensation of ammonia and abundant nitric acid alone is sufficient to drive rapid growth to CCN sizes with only trace sulfate. Moreover, our measurements show that these CCN are also highly efficient ice nucleating particles—comparable to desert dust. Our model simulations confirm that ammonia is efficiently convected aloft during the Asian monsoon, driving rapid, multi-acid HNO3–H2SO4–NH3 nucleation in the upper troposphere and producing ice nucleating particles that spread across the mid-latitude Northern Hemisphere

Synergistic HNO3–H2SO4–NH3 upper tropospheric particle formation

New particle formation in the upper free troposphere is a major global source of cloud condensation nuclei (CCN)1,2,3,4. However, the precursor vapours that drive the process are not well understood. With experiments performed under upper tropospheric conditions in the CERN CLOUD chamber, we show that nitric acid, sulfuric acid and ammonia form particles synergistically, at rates that are orders of magnitude faster than those from any two of the three components. The importance of this mechanism depends on the availability of ammonia, which was previously thought to be efficiently scavenged by cloud droplets during convection. However, surprisingly high concentrations of ammonia and ammonium nitrate have recently been observed in the upper troposphere over the Asian monsoon region5,6. Once particles have formed, co-condensation of ammonia and abundant nitric acid alone is sufficient to drive rapid growth to CCN sizes with only trace sulfate. Moreover, our measurements show that these CCN are also highly efficient ice nucleating particles—comparable to desert dust. Our model simulations confirm that ammonia is efficiently convected aloft during the Asian monsoon, driving rapid, multi-acid HNO3–H2SO4–NH3 nucleation in the upper troposphere and producing ice nucleating particles that spread across the mid-latitude Northern Hemisphere

Molecular Understanding of the Enhancement in Organic Aerosol Mass at High Relative Humidity

The mechanistic pathway by which high relative humidity (RH) affects gas–particle partitioning remains poorly understood, although many studies report increased secondary organic aerosol (SOA) yields at high RH. Here, we use real-time, molecular measurements of both the gas and particle phase to provide a mechanistic understanding of the effect of RH on the partitioning of biogenic oxidized organic molecules (from α-pinene and isoprene) at low temperatures (243 and 263 K) at the CLOUD chamber at CERN. We observe increases in SOA mass of 45 and 85% with increasing RH from 10–20 to 60–80% at 243 and 263 K, respectively, and attribute it to the increased partitioning of semi-volatile compounds. At 263 K, we measure an increase of a factor 2–4 in the concentration of C10H16O2–3, while the particle-phase concentrations of low-volatility species, such as C10H16O6–8, remain almost constant. This results in a substantial shift in the chemical composition and volatility distribution toward less oxygenated and more volatile species at higher RH (e.g., at 263 K, O/C ratio = 0.55 and 0.40, at RH = 10 and 80%, respectively). By modeling particle growth using an aerosol growth model, which accounts for kinetic limitations, we can explain the enhancement in the semi-volatile fraction through the complementary effect of decreased compound activity and increased bulk-phase diffusivity. Our results highlight the importance of particle water content as a diluting agent and a plasticizer for organic aerosol growth