Nucleation Active Surface Site Density Research Articles

Measurement report: A comparison of ground-level ice-nucleating-particle abundance and aerosol properties during autumn at contrasting marine and terrestrial locations

Abstract. Ice-nucleating particles (INPs) are an essential class of aerosols found worldwide that have far-reaching but poorly quantified climate feedback mechanisms through interaction with clouds and impacts on precipitation. These particles can have highly variable physicochemical properties in the atmosphere, and it is crucial to continuously monitor their long-term concentration relative to total ambient aerosol populations at a wide variety of sites to comprehensively understand aerosol–cloud interactions in the atmosphere. Hence, our study applied an in situ forced expansion cooling device to measure ambient INP concentrations and test its automated continuous measurements at atmospheric observatories, where complementary aerosol instruments are heavily equipped. Using collocated aerosol size, number, and composition measurements from these sites, we analyzed the correlation between sources and abundance of INPs in different environments. Toward this aim, we have measured ground-level INP concentrations at two contrasting sites, one in the Southern Great Plains (SGP) region of the United States with a substantial terrestrially influenced aerosol population and one in the Eastern North Atlantic Ocean (ENA) region with a primarily marine-influenced aerosol population. These measurements examined INPs mainly formed through immersion freezing and were performed at a ≤ 12 min resolution and with a wide range of heterogeneous freezing temperatures (Ts above −31 °C) for at least 45 d at each site. The associated INP data analysis was conducted in a consistent manner. We also explored the additional offline characterization of ambient aerosol particle samples from both locations in comparison to in situ data. From our ENA data, on average, INP abundance ranges from ≈ 1 to ≈ 20 L−1 (−30 °C ≤ T ≤ −20 °C) during October–November 2020. Backward air mass trajectories reveal a strong marine influence at ENA with 75.7 % of air masses originating over the Atlantic Ocean and 96.6 % of air masses traveling over open water, but analysis of particle chemistry suggests an additional INP source besides maritime aerosols (e.g., sea spray aerosols) at ENA. In contrast, 90.8 % of air masses at the SGP location originated from the North American continent, and 96.1 % of the time, these air masses traveled over land. As a result, organic-rich SGP aerosols from terrestrial sources exhibited notably high INP abundance from ≈ 1 to ≈ 100 L−1 (−30 °C ≤ T ≤ −15 °C) during October–November 2019. The probability density function of aerosol surface area-scaled immersion freezing efficiency (ice nucleation active surface site density; ns) was assessed for selected freezing temperatures. While the INP concentrations measured at SGP are higher than those of ENA, the ns(T) values of SGP (≈ 105 to ≈ 107 m−2 for −30 °C ≤ T ≤ −15 °C) are reciprocally lower than ENA for approximately 2 orders of magnitude (≈ 107 to ≈ 109 m−2 for −30 °C ≤ T ≤ −15 °C). The observed difference in ns(T) mainly stems from varied available aerosol surface areas, Saer, from two sites (Saer,SGP > Saer,ENA). INP parameterizations were developed as a function of examined freezing temperatures from SGP and ENA for our study periods.

Influence of the Neutralization Degree on the Ice Nucleation Ability of Ammoniated Sulfate Particles

AbstractPrevious laboratory measurements suggest that ammonium sulfate crystals (AS, (NH4)2SO4) are efficient ice‐nucleating particles under cirrus conditions. Sulfate particles not completely neutralized by ammonium are less well studied and include two other solids, ammonium bisulfate (AHS, NH4HSO4) and letovicite (LET, (NH4)3H(SO4)2). In this work, we have obtained the first comprehensive data set for the heterogeneous ice nucleation ability of crystallized particles in the AS–LET–AHS system as a function of their degree of neutralization at a temperature of about 220 K. Quantitative data on nucleation onsets, ice‐active fractions, and ice nucleation active surface site densities were derived from expansion cooling experiments in a large cloud chamber and measurements with two continuous flow diffusion chambers. We found a strong dependence of the efficiency and the mode of heterogenous ice nucleation on the degree of neutralization. Ice formation for AS, mixed AS/LET, and LET crystals occurred by the deposition nucleation or pore condensation and freezing mode. The lowest nucleation onset was observed for AS, where 0.1% of the particles became ice‐active at an ice saturation ratio of 1.25. This threshold gradually increased to 1.35 for LET, and abruptly further to 1.45 for mixed LET/AHS crystals, which partially deliquesced and induced ice formation via immersion freezing. Pure AHS crystals did not form due to the inhibition of efflorescence. Our data allow for a more sophisticated treatment of ice formation in the AS–LET–AHS system in future model simulations, which have so far only considered the available data for AS alone.

Heterogeneous ice nucleation ability of aerosol particles generated from Arctic sea surface microlayer and surface seawater samples at cirrus temperatures

Abstract. Sea spray aerosol particles are a recognised type of ice-nucleating particles under mixed-phase cloud conditions. Entities that are responsible for the heterogeneous ice nucleation ability include intact or fragmented cells of marine microorganisms as well as organic matter released by cell exudation. Only a small fraction of sea spray aerosol is transported to the upper troposphere, but there are indications from mass-spectrometric analyses of the residuals of sublimated cirrus particles that sea salt could also contribute to heterogeneous ice nucleation under cirrus conditions. Experimental studies on the heterogeneous ice nucleation ability of sea spray aerosol particles and their proxies at temperatures below 235 K are still scarce. In our article, we summarise previous measurements and present a new set of ice nucleation experiments at cirrus temperatures with particles generated from sea surface microlayer and surface seawater samples collected in three different regions of the Arctic and from a laboratory-grown diatom culture (Skeletonema marinoi). The particles were suspended in the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) cloud chamber and ice formation was induced by expansion cooling. We confirmed that under cirrus conditions, apart from the ice-nucleating entities mentioned above, also crystalline inorganic salt constituents can contribute to heterogeneous ice formation. This takes place at temperatures below 220 K, where we observed in all experiments a strong immersion freezing mode due to the only partially deliquesced inorganic salts. The inferred ice nucleation active surface site densities for this nucleation mode reached a maximum of about 5×1010 m−2 at an ice saturation ratio of 1.3. Much smaller densities in the range of 108–109 m−2 were observed at temperatures between 220 and 235 K, where the inorganic salts fully deliquesced and only the organic matter and/or algal cells and cell debris could contribute to heterogeneous ice formation. These values are 2 orders of magnitude smaller than those previously reported for particles generated from microlayer suspensions collected in temperate and subtropical zones. While this difference might simply underline the strong variability of the number of ice-nucleating entities in the sea surface microlayer across different geographical regions, we also discuss how instrumental parameters like the aerosolisation method and the ice nucleation measurement technique might affect the comparability of the results amongst different studies.

Is Ice Formation by Sea Spray Particles at Cirrus Temperatures Controlled by Crystalline Salts?

The ice nucleating ability of sea spray aerosols (SSA) has been explored in recent years due to the abundance of SSA in the atmosphere. The role of SSA in ice nucleation extends to cirrus clouds, due to processes that loft SSA to the upper troposphere. This is of special relevance because of the frequent occurrence of cirrus in the atmosphere, their role in the Earth’s radiative balance, and uncertainties regarding how aerosols may affect their formation and evolution. In this study, a continuous flow diffusion chamber (CFDC) is used to investigate the ice nucleating ability of size-selected particle distributions of SSA and its primary constituent sodium chloride (NaCl) at temperatures < 235 K. Results show that above ~220 K, the majority of SSA and NaCl particles fully deliquesce and freeze via homogeneous nucleation at or below water relative humidities, RHw, of ~ 95%. However, below 220 K, the onset RHw of freezing for NaCl and SSA is much lower, at ~75%, where strong heterogeneous freezing of 10% of the aerosol population occurs. Similar heterogeneous freezing behavior for NaCl and SSA aerosols, occurring near their predicted deliquescence RHw, points towards SSA freezing at the lowest temperatures being controlled by the crystalline salts. Finally, calculations of ice nucleation active surface site densities show that particle size does not dictate the efficiency of freezing for NaCl and SSA. These results indicate SSA as a potentially significant source of ice nucleating particles at cirrus temperatures, with the ability to contribute to cirrus-mediated climate impacts if sea spray emission and transport scenarios change in the future.

Experimental Study on the Dependency of Ice Nucleation Active Surface Site Density on ATD Aerosol Size

In light of the percentage of Earth’s cloud coverage, heterogeneous ice nucleation in clouds is the most important global-scale pathway. More recent parameterizations of ice nucleation processes in the atmosphere are based on the concept of ice nucleation active surface site density (ns). It is usually assumed that ns is independent of time and aerosol size distribution, i.e. that the surface properties of aerosols of the same species do not vary with size. However, the independence of ns on aerosol size for every species has been questioned. This study presents the results of ice nucleation processes of ATD laboratory-generated aerosol (particle diameters of 0 - 3 μm). Ice nucleation in the condensation mode was performed in a Dynamic Filter Processing Cham- ber at temperatures of -18°C and -22°C, with a saturation ratio with respect to water of 1.02. Results show that ns increased by lowering the nucleation temperature, and was also dependent on the particle size. The ns of particles collected on the filters, after a 0.5 μm D50 cut-off cyclone, resulted statistically higher with respect to the values obtained from the particles collected on total filters. The results obtained suggest the need for further investigation of ns dependence of same composition aerosol particles with a view to support weather and climate predictions.

Concentration and variability of deposition-mode ice nucleating particles from Mt. Tai of China in the early summer

The concentration and chemical composition of aerosol particles were measured and sampled at two altitudes of Mt. Tai, North China Plain in May and June 2017. Concentrations of ice nucleating particles (INP) were analyzed with a static vacuum water vapor diffusion chamber (FRIDGE-NUIST) in deposition mode after being sampled using an electrostatic aerosol collector. The average INP concentration (1.19 to 3.45 L−1) at Mt. Tai matches with the same measured at Tai’ An (foot of the Mt. Tai, 1.57–3.49 L−1, at RHw = 95%, 97%, 99% and t = −20 °C). Correlations between concentrations of aerosol particles and INP were below 0.5 and less than simultaneous measurements made at Tai'An. Clear diurnal variations at the height of the planetary boundary layer (PBL) resulted in changes in aerosol and INP concentrations at the top of the mountain. We found that EC-Aged (aged elemental carbon) and mineral type particles are clearly associated with ice nuclei when the height of the PBL exceeds the altitude of the mountaintop. Furthermore, we found significant correlations between INP concentrations and concentrations of OC (organic carbon) and NaK (sodium/potassium salt) type aerosol particles at night (in the free troposphere). Ice nucleation active surface site densities are higher at night (22:00 LST, local standard time) than during the day (11:00 LST) due to the change in aerosol characteristics. This study shows that OC and NaK type particles may serve as sources of efficient ice nuclei at night.

Solid Ammonium Nitrate Aerosols as Efficient Ice Nucleating Particles at Cirrus Temperatures

AbstractRecent satellite observations and high‐altitude aircraft measurements have demonstrated the presence of solid ammonium nitrate particles in the Asian monsoon upper troposphere. Most likely, these particles are formed by a heterogeneous crystallization mechanism, given that efflorescence is strongly inhibited for pure ammonium nitrate solution droplets. Crystallization can be induced by the presence of ammonium sulfate, which more easily crystallizes and serves as a heterogeneous nucleus for the crystallization of ammonium nitrate. In the present study, we have investigated this crystallization pathway for different amounts of ammonium sulfate admixed to ammonium nitrate solution droplets and measured the heterogeneous ice nucleation ability of the crystallized particles at cirrus temperatures in a cloud chamber and with a continuous flow diffusion chamber. We found that an admixture of only 0.6 mol% of ammonium sulfate was sufficient to induce the crystallization of ammonium nitrate at 223 K and 22% RHw. These almost pure, crystalline ammonium nitrate particles showed the same temperature‐dependent ice nucleation behavior as observed for other inorganic salts like ammonium sulfate and sodium chloride. At temperatures above 230 K, the crystalline ammonium nitrate particles first deliquesced and nucleated ice homogeneously at ice saturation ratios between about 1.45 and 1.50. Below 230 K, heterogeneous ice nucleation was observed to start at an ice saturation ratio as low as 1.13. The deduced ice nucleation active surface site densities were in the range from 1010–1011 m−2 at ice saturation ratios between 1.2 and 1.4, thus similar in magnitude to those of desert dust particles.

Heterogeneous Ice Nucleation Ability of NaCl and Sea Salt Aerosol Particles at Cirrus Temperatures

AbstractIn situ measurements of the composition of heterogeneous cirrus ice cloud residuals have indicated a substantial contribution of sea salt in sampling regions above the ocean. We have investigated the heterogeneous ice nucleation ability of sodium chloride (NaCl) and sea salt aerosol (SSA) particles at cirrus cloud temperatures between 235 and 200 K in the Aerosol Interaction and Dynamics in the Atmosphere aerosol and cloud chamber. Effloresced NaCl particles were found to act as ice nucleating particles in the deposition nucleation mode at temperatures below about 225 K, with freezing onsets in terms of the ice saturation ratio, Sice, between 1.28 and 1.40. Above 225 K, the crystalline NaCl particles deliquesced and nucleated ice homogeneously. The heterogeneous ice nucleation efficiency was rather similar for the two crystalline forms of NaCl (anhydrous NaCl and NaCl dihydrate). Mixed‐phase (solid/liquid) SSA particles were found to act as ice nucleating particles in the immersion freezing mode at temperatures below about 220 K, with freezing onsets in terms of Sice between 1.24 and 1.42. Above 220 K, the SSA particles fully deliquesced and nucleated ice homogeneously. Ice nucleation active surface site densities of the SSA particles were found to be in the range between 1.0 · 1010 and 1.0 · 1011 m−2 at T &lt; 220 K. These values are of the same order of magnitude as ice nucleation active surface site densities recently determined for desert dust, suggesting a potential contribution of SSA particles to low‐temperature heterogeneous ice nucleation in the atmosphere.

Influence of supersaturation on the concentration of ice nucleating particles

There is a consensus on the increase in ice nucleating particles (INP) concentration from subsaturated to supersaturated water conditions typically associated with clouds (1 ÷ 2%). However, it is important to evaluate the INP concentration trend when water supersaturation further increases, as supercooled clouds contain pockets of high water vapor supersaturation. Three laboratory dry-generated aerosols, two biological (microcrystalline and fibrous cellulose) and one mineral (Arizona test dust), and a field aerosol, sampled on filters, were investigated. Atmospheric aerosol (PM1 and PM10 fractions) was sampled at Capo Granitola (CG, coastal site in Sicily) and the National Research Council (CNR) research area in Bologna (urban background site). The dynamic filter processing chamber (DFPC) was used to explore the ice nucleation of the sampled aerosol in the deposition and condensation freezing modes. Experiments were performed from water subsaturated conditions (water saturation ratio Sw = 0.94) to Sw = 1.1, at T = −22 °C. At CG we considered separately events with a prevalent contribution of marine aerosol, and those showing a contribution of both marine and continental aerosols. An increase in INP concentration, the aerosol activated fraction (AF) and ice nucleation active surface site density (ns) from water subsaturated conditions to Sw = 1.02 was measured in both laboratory and field campaigns. This increase is due to the transition from deposition nucleation to condensation freezing. The highest increases in AF and ns from Sw = 1.02 to Sw = 1.1 were obtained for urban and mixed aerosol and the lowest for marine aerosol. Samplings performed in Bologna showed a high increase in the average INP concentration from PM1 to PM10. Our results show the importance of performing measurements of ice nucleation efficiency for continental aerosol even at supersaturation values higher than those typically associated with clouds, and also considering the contribution of coarse aerosol particles.

Modeling immersion freezing with aerosol‐dependent prognostic ice nuclei in Arctic mixed‐phase clouds

AbstractWhile recent laboratory experiments have thoroughly quantified the ice nucleation efficiency of different aerosol species, the resulting ice nucleation parameterizations have not yet been extensively evaluated in models on different scales. Here the implementation of an immersion freezing parameterization based on laboratory measurements of the ice nucleation active surface site density of mineral dust and ice nucleation active bacteria, accounting for nucleation scavenging of ice nuclei, into a cloud‐resolving model with two‐moment cloud microphysics is presented. We simulated an Arctic mixed‐phase stratocumulus cloud observed during Flight 31 of the Indirect and Semi‐Direct Aerosol Campaign near Barrow, Alaska. Through different feedback cycles, the persistence of the cloud strongly depends on the ice number concentration. It is attempted to bring the observed cloud properties, assumptions on aerosol concentration, and composition and ice formation parameterized as a function of these aerosol properties into agreement. Depending on the aerosol concentration and on the ice crystal properties, the simulated clouds are classified as growing, dissipating, and quasi‐stable. In comparison to the default ice nucleation scheme, the new scheme requires higher aerosol concentrations to maintain a quasi‐stable cloud. The simulations suggest that in the temperature range of this specific case, mineral dust can only contribute to a minor part of the ice formation. The importance of ice nucleation active bacteria and possibly other ice formation modes than immersion freezing remains poorly constrained in the considered case, since knowledge on local variations in the emissions of ice nucleation active organic aerosols in the Arctic is scarce.