Cloud Condensation Nuclei and Ice-Nucleating Particles Over Tropical and Subtropical Regions in the Northern Hemisphere

A change in atmospheric aerosol particles, especially cloud condensation nuclei (CCN) and ice-nucleating particles (INPs), is bound to impact cloud properties, precipitation and cloud radiative effects. In this thesis, two field campaigns were carried out in two representative locations, i.e. the anthropogenic polluted environment at Cyprus and the marine-dust intersect environment at Cabo Verde (a.k.a. Cape Verde) to understand the role of CCN and INPs over the tropical and subtropical regions in the northern hemisphere. On-line aerosol physical measurements were performed and samples from different environ- mental compartments were examined with respect to INPs: the oceanic sea surface microlayer (SML), underlying water (ULW), cloud water and atmospheric filters. Both measurement sites differ in aerosol properties, such as particle number size distribution, CCN and INP concentrations and CCN-derived particle hygroscopicity, due to different environment backgrounds and air mass origins. Aerosol particles at Cyprus were dominated by anthropogenic pollution, with small contributions of sea spray aerosol (SSA) and mineral dust. Particle aging process were observed through changes in CCN-derived particle hygroscopicity. New particle formation events with subsequent growth of the particles into the CCN size range were observed. INPs mainly originated from long-range transport. And anthropogenic pollution were found to be inefficient INPs at temperature range >−25 ◦C. However, aerosol particles at Cabo Verde featured a marine background with intrusions of dust. Dust and marine aerosols featured clearly different PNSDs. CCN number concentration at a supersaturation of 0.30% during the strongest observed dust periods was about 2.5 times higher than during marine periods. However, the CCN-derived hygroscopicity for marine and dust periods shows no significant difference. INPs at Cabo Verde were mainly in the supermicron size range, with a large contribution of biological particles. When comparing atmospheric INP number concentration to those found in seawater, it can be concluded that SSA only contributed a minor fraction to the atmospheric INP population.:1 Introduction 2 Methodology 3 Results and Discussion 4 Summary and Conclusions 5 Outlook Appendix BibliographyVeränderungen im atmosphärischen Aerosol, speziell bei Wolkenkondensationskernen (CCN) und eisnukleierenden Partikeln (INPs), haben Auswirkungen auf Wolkeneigenschaften wie Niederschlagsbildung und Strahlung. Für die hier vorgelegte Arbeit wurden zwei Feldmesskampagnen durchgeführt, im anthropogen verschmutzten Zypern und auf Cabo Verde (alias Kap Verde), einer Schnittstelle zwischen Meer und Wüste. Ziel war es, die Rolle von CCN und INPs in den tropischen und subtropischen Regionen der nördlichen Hemisphäre besser zu verstehen. Es wurden aerosol-physikalische online Messungen durchgeführt und verschiedene Proben auf INPs hin untersucht: die Meeresoberflächen-Mikroschicht (SML), das darunter liegende Wasser (ULW), das Wolkenwasser und atmosphärische Filter. Die beiden verschiedenen Orte an denen die Messkampagnen stattfanden unterscheiden sich in den Aerosoleigenschaften wie z.B. Partikelanzahlgrößenverteilung (PNSD), CCN- und INP-Konzentration und der von CCN abgeleiteten Partikelhygroskopizität. Grund hierfür sind Unterschiede in der Umgebung und der Luftmassenherkunft. Die Aerosolpartikel auf Zypern wurden von anthropogener Verschmutzung dominiert, mit kleinen Beiträgen von Partikeln aus Meeres-Gischt (SSA) und Mineralstaub. Partikelalterung ging einher mit einer Veränderung der Hygroskopizität der CCN. Partikelneubildung wurde beobachtet, mit anschließendem Wachstum der Partikel bis in den CCN-Größenbereich. INPs stammen hauptsächlich aus Ferntransport, und Partikel aus anthropogener Verschmutzung waren ineffiziente INPs im Temperaturbereich >−25 ◦C. Das Aerosol in Cabo Verde speiste sich sowohl aus marinen Quellen als auch aus Wüstenstaub. Staub und marines Aerosol wiesen sehr verschiedene PNSDs auf. Die CCN-Anzahlkonzentration bei 0,30% Übersättigung war während der stärksten Staubperioden etwa 2,5 Mal höher als während der marinen Perioden. Die aus CCN abgeleitete Hygroskopizität zeigte jedoch keinen signifikanten Unterschied für marine und Staubperioden. Die INPs in Cabo Verde waren zum Großteil größer als ein Mikrometer, und waren zum Großteil biogenen Ursprungs. Aus dem Vergleich der atmosphärischen INP-Anzahlkonzentration mit der im Meerwasser gefundenen kann man schließen, dass SSA nur einen geringen Anteil zur atmosphärischen INP-Population beitrug.:1 Introduction 2 Methodology 3 Results and Discussion 4 Summary and Conclusions 5 Outlook Appendix Bibliograph

Understanding aerosol microphysical properties from 10 years of data collected at Cabo Verde based on an unsupervised machine learning classification

The Cape Verde Atmospheric Observatory (CVAO), which is influenced by both marine and desert dust air masses, has been used for long-term measurements of different properties of the atmospheric aerosol from 2008 to 2017. These properties include particle number size distributions (PNSD), light-absorbing carbon (LAC) and concentrations of cloud condensation nuclei (CCN) together with their hygroscopicity. Here we summarize the results obtained for these properties and use an unsupervised machine learning algorithm for the classification of aerosol types. Five types of aerosols, i.e., marine, freshly formed, mixture, moderate dust and heavy dust, were classified. Air masses during marine periods are from the Atlantic Ocean and during dust periods are from the Sahara Desert. Heavy dust was more frequently present during wintertime, whereas the clean marine periods were more frequently present during springtime. It was observed that during the dust periods CCN number concentrations at a supersaturation of 0.30g% were roughly 2.5 times higher than during marine periods, but the hygroscopicity (κ) of particles in the size range from g1/4g30 to g1/4g175gnm during marine and dust periods were comparable. The long-term data presented here, together with the aerosol classification, can be used as a basis to improve our understanding of annual cycles of the atmospheric aerosol in the eastern tropical Atlantic Ocean and on aerosol-cloud interactions and it can be used as a basis for driving, evaluating and constraining atmospheric model simulations

New particle formation and sub-10nm size distribution measurements during the A-LIFE field experiment in Paphos, Cyprus

Atmospheric particle size distributions were measured in Paphos, Cyprus, during the A-LIFE (absorbing aerosol layers in a changing climate: ageing, lifetime and dynamics) field experiment from 3 to 30 April 2017. The newly developed differential mobility analyser train (DMAtrain) was deployed for the first time in an atmospheric environment for the direct measurement of the nucleation mode size range between 1.8 and 10 nm diameter. The DMA-train set-up consists of seven size channels, of which five are set to fixed particle mobility diameters and two additional diameters are obtained by alternating voltage settings in one DMA every 10 s. In combination with a conventional mobility particle size spectrometer (MPSS) and an aerodynamic particle sizer (APS) the complete atmospheric aerosol size distribution from 1.8 nm to 10 μ m was covered. The focus of the A-LIFE study was to characterize new particle formation (NPF) in the eastern Mediterranean region at a measurement site with strong local pollution sources. The nearby Paphos airport was found to be a large emission source for nucleation mode particles, and we analysed the size distribution of the airport emission plumes at approximately 500 m from the main runway. The analysis yielded nine NPF events in 27 measurement days from the combined analysis of the DMAtrain, MPSS and trace gas monitors. Growth rate calculations were performed, and a size dependency of the initial growth rate (< 10 nm) was observed for one event case. Fast changes of the sub-10 nm size distribution on a timescale of a few minutes were captured by the DMA-train measurement during early particle growth and are discussed in a second event case. In two cases, particle formation and growth were detected in the nucleation mode size range which did not exceed the 10 nm threshold. This finding implies that NPF likely occurs more frequently than estimated from studies where the lower nanometre size regime is not covered by the size distribution measurements. © 2020 Author(s)

Significant continental source of ice-nucleating particles at the tip of Chile's southernmost Patagonia region

The sources and abundance of ice-nucleating particles (INPs) that initiate cloud ice formation remain understudied, especially in the Southern Hemisphere. In this study, we present INP measurements taken close to Punta Arenas, Chile, at the southernmost tip of South America from May 2019 to March 2020, during the Dynamics, Aerosol, Cloud, And Precipitation Observations in the Pristine Environment of the Southern Ocean (DACAPO-PESO) campaign. The highest ice nucleation temperature was observed at −3◦C, and from this temperature down to ∼ −10◦C, a sharp increase of INP number concentration (NINP) was observed. Heating of the samples revealed that roughly 90 % and 80 % of INPs are proteinaceous-based biogenic particles at > −10 and −15◦C, respectively. The NINP at Punta Arenas is much higher than that in the Southern Ocean, but it is comparable with an agricultural area in Argentina and forestry environment in the US. Ice active surface site density (ns) is much higher than that for marine aerosol in the Southern Ocean, but comparable to English fertile soil dust. Parameterization based on particle number concentration in the size range larger than 500 nm (N>500 nm) from the global average (DeMott et al., 2010) overestimates the measured INP, but the parameterization representing biological particles from a forestry environment (Tobo et al., 2013) yields NINP comparable to this study. No clear seasonal variation of NINP was observed. High precipitation is one of the most important meteorological parameters to enhance the NINP in both cold and warm seasons. A comparison of data from in situ and lidar measurements showed good agreement for concentrations of large aerosol particles (> 500 nm) when assuming continental conditions for retrieval of the lidar data, suggesting that these particles were well mixed within the planetary boundary layer (PBL). This corroborates the continental origin of these particles, consistent with the results from our INP source analysis. Overall, we suggest that a high NINP of biogenic INPs originated from terrestrial sources and were added to the marine air masses during the overflow of a maximum of roughly 150 km of land before arriving at the measurement station

Characterization of aerosol properties at Cyprus, focusing on cloud condensation nuclei and ice-nucleating particles

As part of the A-LIFE (Absorbing aerosol layers in a changing climate: aging, LIFEtime and dynamics) campaign, ground-based measurements were carried out in Paphos, Cyprus, to characterize the abundance, properties, and sources of aerosol particles in general and cloud condensation nuclei (CCN) and ice-nucleating particles (INP) in particular. New particle formation (NPF) events with subsequent growth of the particles into the CCN size range were observed. Aitken mode particles featured κ values of 0.21 to 0.29, indicating the presence of organic materials. Accumulation mode particles featured a higher hygroscopicity parameter, with a median κ value of 0.57, suggesting the presence of sulfate and maybe sea salt particles mixed with organic carbon. A clear downward trend of κ with increasing supersaturation and decreasing dcrit was found. Super-micron particles originated mainly from sea-spray aerosol (SSA) and partly from mineral dust. INP concentrations (NINP) were measured in the temperature range from -6.5 to -26.5 °C, using two freezing array-type instruments. NINP at a particular temperature span around 1 order of magnitude below -20 °C and about 2 orders of magnitude at warmer temperatures (T > -18 °C). Few samples showed elevated concentrations at temperatures > -15 °C, which suggests a significant contribution of biological particles to the INP population, which possibly could originate from Cyprus. Both measured temperature spectra and NINP probability density functions (PDFs) indicate that the observed INP (ice active in the temperature range between -15 and -20 °C) mainly originate from long-range transport. There was no correlation between NINP and particle number concentration in the size range> 500 nm (N>500 nm). Parameterizations based on N>500 nm were found to overestimate NINP by about 1 to 2 orders of magnitude. There was also no correlation between NINP and particle surface area concentration. The ice active surface site density (ns) for the polluted aerosol encountered in the eastern Mediterranean in this study is about 1 to 3 orders of magnitude lower than the ns found for dust aerosol particles in previous studies. This suggests that observed NINP PDFs such as those derived here could be a better choice for modeling NINP if the aerosol particle composition is unknown or uncertain

Concerted measurements of lipids in seawater and on submicrometer aerosol particles at the Cabo Verde islands: biogenic sources, selective transfer and high enrichments

In the marine environment, measurements of lipids as representative species within different lipid classes have been performed to characterize their oceanic sources and their transfer from the ocean into the atmosphere to marine aerosol particles. The set of lipid classes includes hydrocarbons (HC); fatty acid methyl esters (ME); free fatty acids (FFA); alcohols (ALC); 1,3-diacylglycerols (1,3 DG); 1,2-diacylglycerols (1,2 DG); monoacylglycerols (MG); wax esters (WE); triacylglycerols (TG); and phospholipids (PP) including phosphatidylglycerols (PG), phosphatidylethanolamine (PE), phosphatidylcholines (PC), as well as glycolipids (GL) which cover sulfoquinovosyldiacylglycerols (SQDG), monogalactosyl-diacylglycerols (MGDG), digalactosyldiacylglycerols (DGDG) and sterols (ST). These introduced lipid classes have been analyzed in the dissolved and particulate fraction of seawater, differentiating between underlying water (ULW) and the sea surface microlayer (SML) on the one hand. On the other hand, they have been examined on ambient submicrometer aerosol particle samples (PM1) which were collected at the Cape Verde Atmospheric Observatory (CVAO) by applying concerted measurements. These different lipids are found in all marine compartments but in different compositions. Along the campaign, certain variabilities are observed for the concentration of dissolved (∑DLULW: 39.8–128.5 µg L−1, ∑DLSML: 55.7–121.5 µg L−1) and particulate (∑PLULW: 36.4–93.5 µg L−1, ∑PLSML: 61.0–118.1 µg L−1) lipids in the seawater of the tropical North Atlantic Ocean. Only slight SML enrichments are observed for the lipids with an enrichment factor EFSML of 1.1–1.4 (DL) and 1.0–1.7 (PL). On PM1 aerosol particles, a total lipid concentration between 75.2–219.5 ng m−3 (averaged: 119.9 ng m−3) is measured. As also bacteria – besides phytoplankton sources – influence the lipid concentrations in seawater and on the aerosol particles, the lipid abundance cannot be exclusively explained by the phytoplankton tracer (chlorophyll a). The concentration and enrichment of lipids in the SML are not related to physicochemical properties which describe the surface activity. On the aerosol particles, an EFaer (the enrichment factor on the submicrometer aerosol particles compared to the SML) between 9×104–7×105 is observed. Regarding the individual lipid groups on the aerosol particles, a statistically significant correlation (R2=0.45, p=0.028) was found between EFaer and lipophilicity (expressed by the KOW value), which was not present for the SML. But simple physicochemical descriptors are overall not sufficient to fully explain the transfer of lipids. As our findings show that additional processes such as formation and degradation influence the ocean–atmosphere transfer of both OM in general and of lipids in particular, they have to be considered in OM transfer models. Moreover, our data suggest that the extent of the enrichment of the lipid class constituents on the aerosol particles might be related to the distribution of the lipid within the bubble–air–water interface. The lipids TG and ALC which are preferably arranged within the bubble interface are transferred to the aerosol particles to the highest extent. Finally, the connection between ice nucleation particles (INPs) in seawater, which are already active at higher temperatures (−10 to −15 ∘C), and the lipid classes PE and FFA suggests that lipids formed in the ocean have the potential to contribute to (biogenic) INP activity when transferred into the atmosphere

Characterization of aerosol particles at Cabo Verde close to sea level and at the cloud level – Part 2: Ice-nucleating particles in air, cloud and seawater

Ice-nucleating particles (INPs) in the troposphere can form ice in clouds via heterogeneous ice nucleation. Yet, atmospheric number concentrations of INPs (NINP) are not well characterized, and, although there is some understanding of their sources, it is still unclear to what extend different sources contribute or if all sources are known. In this work, we examined properties of INPs at Cabo Verde (a.k.a. Cape Verde) from different environmental compartments: the oceanic sea surface microlayer (SML), underlying water (ULW), cloud water and the atmosphere close to both sea level and cloud level. Both enrichment and depletion of NINP in SML compared to ULW were observed. The enrichment factor (EF) varied from roughly 0.4 to 11, and there was no clear trend in EF with ice-nucleation temperature. NINP values in PM10 sampled at Cape Verde Atmospheric Observatory (CVAO) at any particular ice-nucleation temperature spanned around 1 order of magnitude below −15 ∘C, and about 2 orders of magnitude at warmer temperatures (>−12  ∘C). Among the 17 PM10 samples at CVAO, three PM10 filters showed elevated NINP at warm temperatures, e.g., above 0.01 L−1 at −10 ∘C. After heating samples at 95 ∘C for 1 h, the elevated NINP at the warm temperatures disappeared, indicating that these highly ice active INPs were most likely biological particles. INP number concentrations in PM1 were generally lower than those in PM10 at CVAO. About 83±22 %, 67±18 % and 77±14 % (median±standard deviation) of INPs had a diameter >1 µm at ice-nucleation temperatures of −12, −15 and −18 ∘C, respectively. PM1 at CVAO did not show such elevated NINP at warm temperatures. Consequently, the difference in NINP between PM1 and PM10 at CVAO suggests that biological ice-active particles were present in the supermicron size range. NINP in PM10 at CVAO was found to be similar to that on Monte Verde (MV, at 744 m a.s.l.) during noncloud events. During cloud events, most INPs on MV were activated to cloud droplets. When highly ice active particles were present in PM10 filters at CVAO, they were not observed in PM10 filters on MV but in cloud water samples instead. This is direct evidence that these INPs, which are likely biological, are activated to cloud droplets during cloud events. For the observed air masses, atmospheric NINP values in air fit well to the concentrations observed in cloud water. When comparing concentrations of both sea salt and INPs in both seawater and PM10 filters, it can be concluded that sea spray aerosol (SSA) only contributed a minor fraction to the atmospheric NINP. This latter conclusion still holds when accounting for an enrichment of organic carbon in supermicron particles during sea spray generation as reported in literature

Automated identification of local contamination in remote atmospheric composition time series

Atmospheric observations in remote locations offer a possibility of exploring trace gas and particle concentrations in pristine environments. However, data from remote areas are often contaminated by pollution from local sources. Detecting this contamination is thus a central and frequently encountered issue. Consequently, many different methods exist today to identify local contamination in atmospheric composition measurement time series, but no single method has been widely accepted. In this study, we present a new method to identify primary pollution in remote atmospheric datasets, e.g., from ship campaigns or stations with a low background signal compared to the contaminated signal. The pollution detection algorithm (PDA) identifies and flags periods of polluted data in five steps. The first and most important step identifies polluted periods based on the derivative (time derivative) of a concentration over time. If this derivative exceeds a given threshold, data are flagged as polluted. Further pollution identification steps are a simple concentration threshold filter, a neighboring points filter (optional), a median, and a sparse data filter (optional). The PDA only relies on the target dataset itself and is independent of ancillary datasets such as meteorological variables. All parameters of each step are adjustable so that the PDA can be "tuned" to be more or less stringent (e.g., flag more or fewer data points as contaminated). The PDA was developed and tested with a particle number concentration dataset collected during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition in the central Arctic. Using strict settings, we identified 62 % of the data as influenced by local contamination. Using a second independent particle number concentration dataset also collected during MOSAiC, we evaluated the performance of the PDA against the same dataset cleaned by visual inspection. The two methods agreed in 94 % of the cases. Additionally, the PDA was successfully applied to a trace gas dataset (CO2), also collected during MOSAiC, and to another particle number concentration dataset, collected at the high-altitude background station Jungfraujoch, Switzerland. Thus, the PDA proves to be a useful and flexible tool to identify periods affected by local contamination in atmospheric composition datasets without the need for ancillary measurements. It is best applied to data representing primary pollution. The user-friendly and open-access code enables reproducible application to a wide suite of different datasets. It is available at https://doi.org/10.5281/zenodo.5761101 (Beck et al., 2021).Peer reviewe

Ship-based measurements of ice nuclei concentrations over the Arctic, Atlantic, Pacific and Southern oceans

Ambient concentrations of ice-forming particles measured during ship expeditions are collected and summarised with the aim of determining the spatial distribution and variability in ice nuclei in oceanic regions. The presented data from literature and previously unpublished data from over 23 months of ship-based measurements stretch from the Arctic to the Southern Ocean and include a circumnavigation of Antarctica. In comparison to continental observations, ship-based measurements of ambient ice nuclei show 1 to 2 orders of magnitude lower mean concentrations. To quantify the geographical variability in oceanic areas, the concentration range of potential ice nuclei in different climate zones is analysed by meridionally dividing the expedition tracks into tropical, temperate and polar climate zones. We find that concentrations of ice nuclei in these meridional zones follow temperature spectra with similar slopes but vary in absolute concentration. Typically, the frequency with which specific concentrations of ice nuclei are observed at a certain temperature follows a log-normal distribution. A consequence of the log-normal distribution is that the mean concentration is higher than the most frequently measured concentration. Finally, the potential contribution of ship exhaust to the measured ice nuclei concentration on board research vessels is analysed as function of temperature. We find a sharp onset of the influence at approximately 36 C but none at warmer temperatures that could bias ship-based measurements. © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License

Concerted measurements of lipids in seawater and on submicron aerosol particles at the Cape Verde Islands: biogenic sources, selective transfer and high enrichments

Measurements of lipids as representative species for different lipid classes in the marine environment have been performed to characterize their oceanic sources and their transfer from the ocean into the atmosphere to marine aerosol particles. To this end, a set of lipid classes (hydrocarbons (HC), fatty acid methyl esters (ME), free fatty acids (FFA), alcohols (ALC), 1, 3-diacylglycerols (1, 3 DG), 1, 2-diacylglycerols (1, 2 DG), monoacylglycerols (MG), wax esters (WE), triacylglycerols (TG), phospholipids (PP) including phosphatidylglycerols (PG), phosphatidylethanolamine (PE), phosphatidylcholines (PC), glycolipids (GL) including sulfoquinovosyldiacylglycerols (SQDG), monogalactosyl-diacylglycerols (MGDG), digalactosyldiacylglycerols (DGDG) and sterols (ST)) is investigated in both the dissolved and particulate fraction in seawater, differentiated between underlying water (ULW) and the sea surface microlayer (SML), and in ambient submicron aerosol particle samples (PM1) at the Cape Verde Atmospheric Observatory (CVAO) applying concerted measurements. The different lipids are found in all marine compartments but in different compositions. At this point, a certain variability is observed for the concentration of dissolved (∑DLULW: 39.8–128.5 μg L−1, ∑DLSML: 55.7–121.5 μg L−1) and particulate (∑PLULW: 36.4–93.5 μg L−1, ∑PLSML: 61.0–118.1 μg L−1) lipids in seawater of the tropical North Atlantic Ocean along the campaign. Only slight SML enrichments are observed for the lipids with an enrichment factor EFSML of 1.1–1.4 (DL) and 1.0–1.7 (PL). On PM1 aerosol particles, a total lipid concentration between 75.2–219.5 ng m−3 (averaged: 119.9 ng m−3) is measured with high atmospheric concentration of TG (averaged: 21.9 ng m−3) as a potential indicator for freshly emitted sea spray. Besides phytoplankton sources, bacteria influence the lipid concentrations in seawater and on the aerosol particles, so that the phytoplankton tracer (chlorophyll-a) cannot sufficiently explain the lipid abundance. The concentration and enrichment of lipids in the SML is not related to physicochemical properties describing the surface activity. For aerosol, however, the high enrichment of lipids (as a sum) corresponds well with the consideration of their high surface activity, thus the EFaer (enrichment factor on submicron aerosol particles compared to SML) ranges between 9 × 104–7 × 105. Regarding the single lipid groups on the aerosol particles, a weak relation between EFaer and lipophilicity (expressed by the KOW value) was identified, which was absent for the SML. However, overall simple physico- chemical descriptors are not sufficient to fully explain the transfer of lipids. As our findings show that additional processes such as formation and degradation influence the ocean- atmosphere transfer of both OM in general and of lipids in particular, they have to be considered in OM transfer models. Moreover, our data suggest that the extend of enrichment of lipid classes constituents on the aerosol particles might be related to the distribution of the lipid within the bubble-air-water- interface. Lipids, which are preferably arranged within the bubble interface, namely TG and ALC, are transferred to the aerosol particles to the highest extend. Finally, the connection between ice nucleation particles (INP) in seawater, which are active already at higher temperatures (−10 °C to −15 °C), and the lipid classes PE and FFA suggests that lipids formed in the ocean have the potential to contribute to (biogenic) INP activity when transferred to the atmosphere