The effect of rapid relative humidity changes on fast filter-based aerosol-particle light-absorption measurements: Uncertainties and correction schemes

Measuring vertical profiles of the particle light-absorption coefficient by using absorption photometers may face the challenge of fast changes in relative humidity (RH). These absorption photometers determine the particle light-absorption coefficient due to a change in light attenuation through a particle-loaded filter. The filter material, however, takes up or releases water with changing relative humidity (RH in %), thus influencing the light attenuation. A sophisticated set of laboratory experiments was therefore conducted to investigate the effect of fast RH changes (dRH/dt) on the particle light-absorption coefficient (σabs in Mm-1) derived with two absorption photometers. The RH dependence was examined based on different filter types and filter loadings with respect to loading material and areal loading density. The Single Channel Tricolor Absorption Photometer (STAP) relies on quartz-fiber filter, and the microAeth® MA200 is based on a polytetrafluoroethylene (PTFE) filter band. Furthermore, three cases were investigated: clean filters, filters loaded with black carbon (BC), and filters loaded with ammonium sulfate. The filter areal loading densities (ρ∗) ranged from 3.1 to 99.6 mg m-2 in the case of the STAP and ammonium sulfate and 1.2 to 37.6 mg m-2 in the case the MA200. Investigating BC-loaded cases, M8 scroll mrow miBCm 15pt was in the range of 2.9 to 43.0 and 1.1 to 16.3 mg m-2 for the STAP and MA200, respectively. Both instruments revealed opposing responses to relative humidity changes ("RH) with different magnitudes. The STAP shows a linear dependence on relative humidity changes. The MA200 is characterized by a distinct exponential recovery after its filter was exposed to relative humidity changes. At a wavelength of 624 nm and for the default 60 s running average output, the STAP reveals an absolute change in σabs per absolute change of RH ("σabsĝ•"RH) of 0.14 Mm-1 %-1 in the clean case, 0.29 Mm-1 %-1 in the case of BC-loaded filters, and 0.21 Mm-1 %-1 in the case filters loaded with ammonium sulfate. The 60 s running average of the particle light-absorption coefficient at 625 nm measured with the MA200 revealed a response of around -0.4 Mm-1 %-1 for all three cases. Whereas the response of the STAP varies over the different loading materials, in contrast, the MA200 was quite stable. The response was, for the STAP, in the range of 0.17 to 0.24 Mm-1 %-1 and, in the case of ammonium sulfate loading and in the BC-loaded case, 0.17 to 0.62 Mm-1 %-1. In the ammonium sulfate case, the minimum response shown by the MA200 was -0.42 with a maximum of -0.36 Mm-1 %-1 and a minimum of -0.42 and maximum -0.37 Mm-1 %-1 in the case of BC. A linear correction function for the STAP was developed here. It is provided by correlating 1 Hz resolved recalculated particle light-absorption coefficients and RH change rates. The linear response is estimated at 10.08 Mm-1 s-1 %-1. A correction approach for the MA200 is also provided; however, the behavior of the MA200 is more complex. Further research and multi-instrument measurements have to be conducted to fully understand the underlying processes, since the correction approach resulted in different correction parameters across various experiments. However, the exponential recovery after the filter of the MA200 experienced a RH change could be reproduced. However, the given correction approach has to be estimated with other RH sensors as well, since each sensor has a different response time. And, for the given correction approaches, the uncertainties could not be estimated, which was mainly due to the response time of the RH sensor. Therefore, we do not recommend using the given approaches. But they point in the right direction, and despite the imperfections, they are useful for at least estimating the measurement uncertainties due to relative humidity changes. Due to our findings, we recommend using an aerosol dryer upstream of absorption photometers to reduce the RH effect significantly. Furthermore, when absorption photometers are used in vertical measurements, the ascending or descending speed through layers of large relative humidity gradients has to be low to minimize the observed RH effect. But this is simply not possible in some scenarios, especially in unmixed layers or clouds. Additionally, recording the RH of the sample stream allows correcting for the bias during post-processing of the data. This data correction leads to reasonable results, according to the given example in this study. © Author(s) 2019

CAMP: An instrumented platform for balloon-borne aerosol particle studies in the lower atmosphere

Airborne observations of vertical aerosol particle distributions are crucial for detailed process studies and model improvements. Tethered balloon systems represent a less expensive alternative to aircraft to probe shallow atmospheric boundary layers (ABLs). This study presents the newly developed cubic aerosol measurement platform (CAMP) for balloon-borne observations of aerosol particle microphysical properties. With an edge length of 35 cm and a weight of 9 kg, the cube is an environmentally robust instrument platform intended for measurements at low temperatures, with a particular focus on applications in cloudy Arctic ABLs. The aerosol instrumentation on board CAMP comprises two condensation particle counters with different lower detection limits, one optical particle size spectrometer, and a miniaturized absorption photometer. Comprehensive calibrations and characterizations of the instruments were performed in laboratory experiments. The first field study with a tethered balloon system took place at the Leibniz Institute for Tropospheric Research (TROPOS) station in Melpitz, Germany, in the winter of 2019. At ambient temperatures between-8 and 15 C, the platform was operated up to a 1.5 km height on 14 flights under both clear-sky and cloudy conditions. The continuous aerosol observations at the ground station served as a reference for evaluating the CAMP measurements. Exemplary profiles are discussed to elucidate the performance of the system and possible process studies. Based on the laboratory instrument characterizations and the observations during the field campaign, CAMP demonstrated the capability to provide comprehensive aerosol particle measurements in cold and cloudy ABLs

Meteorologische Einflüsse auf Stickstoffdioxid: Einfluss von Wetterlagen und Witterung auf die Stickstoffdioxid-Konzentrationen in der Außenluft 2015 bis 2018

Im Rahmen eines Forschungsvorhabens wurden die Einflüsse von Ozon und meteorologischen Bedingungen sowie verschiedenen Maßnahmen auf die NO2-Konzentrationen in der Außenluft an ausgewählten sächsischen Luftgüte-Messstationen untersucht. Für die statistische Analyse wurde das Verfahren „boosted regression trees“ angewendet. Die Broschüre richtet sich an Fachbehörden und Wissenschaftler, die sich mit der Analyse und Überwachung der Luftgüte beschäftigen. Redaktionsschluss: 22.01.202

The HD(CP)² Observational Prototype Experiment (HOPE) – an overview

The HD(CP)2 Observational Prototype Experiment (HOPE) was performed as a major 2-month field experiment in Jülich, Germany, in April and May 2013, followed by a smaller campaign in Melpitz, Germany, in September 2013. HOPE has been designed to provide an observational dataset for a critical evaluation of the new German community atmospheric icosahedral non-hydrostatic (ICON) model at the scale of the model simulations and further to provide information on land-surface–atmospheric boundary layer exchange, cloud and precipitation processes, as well as sub-grid variability and microphysical properties that are subject to parameterizations. HOPE focuses on the onset of clouds and precipitation in the convective atmospheric boundary layer. This paper summarizes the instrument set-ups, the intensive observation periods, and example results from both campaigns. HOPE-Jülich instrumentation included a radio sounding station, 4 Doppler lidars, 4 Raman lidars (3 of them provide temperature, 3 of them water vapour, and all of them particle backscatter data), 1 water vapour differential absorption lidar, 3 cloud radars, 5 microwave radiometers, 3 rain radars, 6 sky imagers, 99 pyranometers, and 5 sun photometers operated at different sites, some of them in synergy. The HOPE-Melpitz campaign combined ground-based remote sensing of aerosols and clouds with helicopter- and balloon-based in situ observations in the atmospheric column and at the surface. HOPE provided an unprecedented collection of atmospheric dynamical, thermodynamical, and micro- and macrophysical properties of aerosols, clouds, and precipitation with high spatial and temporal resolution within a cube of approximately 10  ×  10  ×  10 km3. HOPE data will significantly contribute to our understanding of boundary layer dynamics and the formation of clouds and precipitation. The datasets have been made available through a dedicated data portal. First applications of HOPE data for model evaluation have shown a general agreement between observed and modelled boundary layer height, turbulence characteristics, and cloud coverage, but they also point to significant differences that deserve further investigations from both the observational and the modelling perspective

Investigation of physio-optical aerosol properties with in-situ and remote-sensing techniques

Aerosol particles directly influence the radiative transfer within the atmosphere by scattering and absorption of solar radiation and indirectly in their role as cloud condensation nuclei (CCN). The influence is subject to uncertainties, which can be reduced by a better understanding of their vertical distribution. The vertical aerosol distribution can be determined by airborne in-situ measurements, e.g., helicopter-borne probes, and ground-based remote sensing methods, e.g., lidar. Using algorithms with underlying assumptions and simplifications, physical and optical aerosol properties can be retrieved from lidar measurements. These derived aerosol properties, such as the CCN number concentration (NCCN), can be validated using direct in-situ measurements. In addition, the optical properties underlying the retrieval algorithms can be validated using Mie theory-based modeling. However, here the ambient humidity of the aerosol must be considered since the hygroscopic growth of the aerosol particles changes their optical properties. In the frame of this dissertation, three peer-reviewed scientific papers were published. The goals were to validate lidar-retrieval-based aerosol properties like NCCN, identify sources of uncertainty in Mie theory-based validation studies, and improve the quality of in-situ measurements, such as those of the aerosol particle light absorption coefficient conducted with filter-based absorption photometers. To this end, a Mie theory-based model was developed to calculate aerosol particles' optical properties in the dry and ambient states. The model input parameters were determined with sophisticated instrumentation deployed on the ground and within airborne measurement platforms during three conducted field campaigns. Lidar-based NCCN and the model were compared with corresponding direct in-situ measurements of aerosol optical and microphysical properties with satisfactory results. Sources of deviations concerning the modeled aerosol particle light absorption were identified; among others, the assumed aerosol mixing state and the consideration of light-absorbing organic aerosol components are key parameters. In addition, the representation of aerosol hygroscopicity in the model was investigated based on different measurement techniques. Possible further sources of uncertainty regarding the comparison with aerosol optical properties measured by lidar were discussed. Regarding the aerosol particle light extinction-to-backscatter ratio, also known as the lidar ratio, the dependence on the ambient relative humidity is shown for the first time by employing in-situ measurements and the developed Mie model. A corresponding parameterization was determined. Previous theoretical considerations from other studies were qualitatively confirmed. In addition, the lidar ratio was determined for the first time based on airborne in-situ measurements for the light of wavelength 1064 nm. Also, employing a laboratory study, the influence of rapid changes in relative humidity on filter-based absorption photometers was quantified. Surprisingly, an opposite effect was observed for two different filter materials. First approaches to correct the observed humidity effect were provided using two parameterizations. The findings are crucial, especially in environments with low aerosol particle light absorption, because the effect can exceed the measured values. The results of this work are an important contribution to the improvement, interpretation, and application of the vertical measurement of aerosol properties.:List of Figures I List of Tables I List of Acronyms II List of Symbols IV 1 Introduction 1 2 Theoretical background 5 2.1 Aerosol particles 5 2.1.1 Microphysical properties 5 2.1.2 Aerosol optical properties 8 2.1.3 Aerosol particles under humidified conditions 11 2.2 Lidar theory 11 2.3 Filter-based aerosol particle light absorption measurements and eBC mass concentration 15 2.4 Mie theory 17 3 Methodology 19 3.1 Measurement site 19 3.2 Mie-model 20 3.3 Field campaign instrumentation 20 3.3.1 Aerosol microphysical properties 22 3.3.2 Aerosol Hygroscopicity and complex refractive index 22 3.3.3 Aerosol optical properties 24 3.4 Filter-based particle light absorption measurements and RH 24 4 Results and Discussion 27 4.1 First publication 27 4.1.1 Re-usage of parts of Master thesis 27 4.1.2 Helicopter-borne observations of continental background aerosol in combination with remote sensing and ground-based measurements 27 4.2 Second publication 57 4.2.1 The effect of rapid relative humidity changes on filter-based aerosol-particle light-absorption measurements: uncertainties and correction schemes 57 4.3 Third publication 75 4.3.1 Measurement report: Comparison of airborne, in situ measured, lidar-based, and modeled aerosol optical properties in the central European background – identifying sources of deviations 75 4.3.2 Supplementary material of “Measurement report: Comparison of airborne, in situ measured, lidar-based, and modeled aerosol optical properties in the central European background – identifying sources of deviations” 105 5 Summary, Conclusions, and Outlook 117 Appendix A 121 Bibliography 123 Acknowledgments i Declaration of Independence iiiAerosolpartikel beeinflussen den Strahlungstransport in der Atmosphäre durch Streuung und Absorption von solarer Strahlung direkt und indirekt in ihrer Wirkung als Wolkenkondensationskerne (CCN, cloud condensation nuclei). Der Einfluss ist mit Unsicherheiten behaftet, welche unter anderem durch ein besseres Verständnis über deren vertikale Verteilung reduziert werden kann. Die vertikale Verteilung der Aerosole kann mittels luftgetragener in-situ Messungen, z.B. mit helikoptergetragenen Sonden, und bodengebundener Fernerkundungsmethoden, z.B. Lidar, bestimmt werden. Mittels verschiedener Algorithmen, denen jedoch diverse Annahmen und Vereinfachungen zu Grunde liegen, können aus Lidarmessungen mikrophysikalische und optische Aerosoleigenschaften abgeleitet werden. Mittels direkter Messungen können diese abgeleiteten Aerosoleigenschaften, wie z.B. die CCN Anzahlkonzentration (NCCN), überprüft werden. Zudem können die, den Ableitungsalgorithmen zu Grunde liegenden, optischen Eigenschaften mittels Mie-Theorie basierter Modellierung validiert werden. Die Umgebungsfeuchte des Aerosols muss dabei jedoch berücksichtig werden, da auf Grund des hygroskopischen Wachstums die optischen Eigenschaften von Aerosolpartikeln verändert werden. Im Rahmen dieser Dissertation wurden drei begutachteten wissenschaftlichen Artikel publiziert. Die Ziele waren aus Lidarretrievals abgeleitete Aerosolgrößen wie z.B. NCCN zu validieren, Unsicherheitsquellen in Mie-Theorie basierten Validierungsstudien zu identifizieren und die Qualität von in-situ Messungen, wie z.B. die des Lichtabsorptionskoeffizienten von Aerosolpartikeln mittels filterbasierter Absorptionsphotometern, zu steigern. Dazu wurde ein Mie-Theorie basiertes Model entwickelt, welches die optischen Eigenschaften von Aerosolpartikeln im trockenen und Umgebungszustand berechnen kann. Dessen Eingangsparameter wurden mit hochqualitativen Messungen am Boden und in der Luft während dreier Feldkampagnen ermittelt. Korrespondierende in-situ Messungen am Boden und in der Luft verifizierten neben Lidar-basierten NCCN die Modelqualität mit zufriedenstellendem Resultat. Bezüglich der Aerosolpartikellichtabsorption konnte die Wichtigkeit des angenommen Aerosolmischungszustandes und der Berücksichtigung von lichtabsorbierenden organischen Aerosolkomponenten identifiziert werden. Zudem wurde die Repräsentierung der Aerosolhygroskopizität im Model auf Grundlage von verschiedenen Messtechniken untersucht. Mögliche weitere Unsicherheitsquellen bezüglich des Vergleiches mit den von Lidar gemessenen optischen Aerosoleigenschaften wurden diskutiert. Bezüglich des Lichtextinktions-zu-Rückstreuverhältnisses von Aerosolpartikeln, auch Lidarverhältnis (LR), konnte erstmals mit in-situ Messungen und dem entwickelten Mie-Model die Abhängigkeit dieses Parameters gegenüber der relativen Umgebungsfeuchte gezeigt und parametrisiert werden. Theoretische Betrachtungen aus vorherigen Studien wurden qualitativ bestätigt. Auf Basis von luftgetragenen in-situ Messungen konnte erstmalig das LR für Licht der Wellenlänge 1064 nm bestimmt werden. Mittels einer Laborstudie konnte der Einfluss von schnellen Änderungen der relativen Feuchte auf filterbasierte Absorptionsphotometer quantifiziert werden. Überraschenderweise zeigten zwei verschieden Filtermaterialen ein gegensätzlicher Effekt. Erste Ansätze zu der Korrektur des Feuchteeffekts wurden mit zwei Parametrisierungen geliefert. Gerade in Umgebungen mit niedriger Lichtabsorption durch Aerosol kann dies wichtig sein, da der Effekt die Messwerte übersteigen kann. Die Ergebnisse diese Arbeit sind ein wichtiger Beitrag zur Verbesserung, Interpretation und Anwendung vertikaler Messungen von Aerosoleigenschaften.:List of Figures I List of Tables I List of Acronyms II List of Symbols IV 1 Introduction 1 2 Theoretical background 5 2.1 Aerosol particles 5 2.1.1 Microphysical properties 5 2.1.2 Aerosol optical properties 8 2.1.3 Aerosol particles under humidified conditions 11 2.2 Lidar theory 11 2.3 Filter-based aerosol particle light absorption measurements and eBC mass concentration 15 2.4 Mie theory 17 3 Methodology 19 3.1 Measurement site 19 3.2 Mie-model 20 3.3 Field campaign instrumentation 20 3.3.1 Aerosol microphysical properties 22 3.3.2 Aerosol Hygroscopicity and complex refractive index 22 3.3.3 Aerosol optical properties 24 3.4 Filter-based particle light absorption measurements and RH 24 4 Results and Discussion 27 4.1 First publication 27 4.1.1 Re-usage of parts of Master thesis 27 4.1.2 Helicopter-borne observations of continental background aerosol in combination with remote sensing and ground-based measurements 27 4.2 Second publication 57 4.2.1 The effect of rapid relative humidity changes on filter-based aerosol-particle light-absorption measurements: uncertainties and correction schemes 57 4.3 Third publication 75 4.3.1 Measurement report: Comparison of airborne, in situ measured, lidar-based, and modeled aerosol optical properties in the central European background – identifying sources of deviations 75 4.3.2 Supplementary material of “Measurement report: Comparison of airborne, in situ measured, lidar-based, and modeled aerosol optical properties in the central European background – identifying sources of deviations” 105 5 Summary, Conclusions, and Outlook 117 Appendix A 121 Bibliography 123 Acknowledgments i Declaration of Independence ii

Meteorologische Einflüsse auf Stickstoffdioxid: Einfluss von Wetterlagen und Witterung auf die Stickstoffdioxid-Konzentrationen in der Außenluft 2015 bis 2018

Im Rahmen eines Forschungsvorhabens wurden die Einflüsse von Ozon und meteorologischen Bedingungen sowie verschiedenen Maßnahmen auf die NO2-Konzentrationen in der Außenluft an ausgewählten sächsischen Luftgüte-Messstationen untersucht. Für die statistische Analyse wurde das Verfahren „boosted regression trees“ angewendet. Die Broschüre richtet sich an Fachbehörden und Wissenschaftler, die sich mit der Analyse und Überwachung der Luftgüte beschäftigen. Redaktionsschluss: 22.01.202

Meteorologische Einflüsse auf Stickstoffdioxid: Einfluss von Wetterlagen und Witterung auf die Stickstoffdioxid-Konzentrationen in der Außenluft 2015 bis 2018

Im Rahmen eines Forschungsvorhabens wurden die Einflüsse von Ozon und meteorologischen Bedingungen sowie verschiedenen Maßnahmen auf die NO2-Konzentrationen in der Außenluft an ausgewählten sächsischen Luftgüte-Messstationen untersucht. Für die statistische Analyse wurde das Verfahren „boosted regression trees“ angewendet. Die Broschüre richtet sich an Fachbehörden und Wissenschaftler, die sich mit der Analyse und Überwachung der Luftgüte beschäftigen. Redaktionsschluss: 22.01.202

Measurement report: Comparison of airborne, in situ measured, lidar-based, and modeled aerosol optical properties in the central European background – identifying sources of deviations

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Helicopter borne probe HELiPOD measurements during MOSAiC - Flight 4

The helicopter borne probe HELiPOD was deployed during the MOSAiC expedition on Leg 3 and 4 from the research vessel Polarstern to investigate the ocean-ice-atmosphere exchange. During five flights, a variety of parameters were measured and calculated, concerning atmospheric dynamics (pressure, temperature, humidity, wind vector), aerosol particles (number concentrations in different size classes, absorption coefficients for three different wavelengths), trace gas concentrations (carbon dioxide, methane, ozone), radiation (solar and terrestrial, upward and downward), surface properties (temperature, images) as well as flight state parameters (position, altitude, attitude). All data were re-sampled at 100 Hz to the same time grid, if not indicated differently. The probe enabled the spatial extension of MOSAiC observations in a range of 25 – 60 km distance to Polarstern. After dedicated postprocessing of the complex data set, two out of the five flights, which were performed on 22 July 2020, and with a flight duration of around 1 h per flight, are initially uploaded to the PANGAEA data base. A technical overview of the HELiPOD is given in Pätzold et al. (2023)