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    The influence of the baseline drift on the resulting extinction values of a cavity attenuated phase shift-based extinction monitor (CAPS PMex)
    (Katlenburg-Lindau : Copernicus, 2020) Pfeifer, Sascha; Müller, Thomas; Freedman, Andrew; Wiedensohler, Alfred
    The effect of the baseline drift on the resulting extinction values of three cavity attenuated phase shift-based extinction monitors (CAPS PMex) with different wavelengths and the respective correlation with NO2 was analysed for an urban background station. A drift of more than 0.8 Mm−1min−1 was observed for ambient air, with high probability caused by traffic-emissions-driven changes in carrier gas composition. The baseline drift leads to characteristic measurement artefacts for particle extinction. Artificial particle extinction values of approximately 4 Mm−1 were observed using a baseline period of 5 min. These values can be even higher for longer baseline periods. Two methods are shown to minimize this effect. Modified continuous baseline values are calculated in a post-processing step using simple linear interpolation and cubic smoothing splines. Both methods are useful to reduce artefacts, although the use of cubic smoothing splines gives slightly better results. The extinction artefacts are diminished and the effective scattering of the resulting extinction values is reduced by about 50 %.
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    The effect of rapid relative humidity changes on fast filter-based aerosol-particle light-absorption measurements: Uncertainties and correction schemes
    (Katlenburg-Lindau : Copernicus, 2019) Düsing, Sebastian; Wehner, Birgit; Müller, Thomas; Stöcker, Almond; Wiedensohler, Alfred
    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.

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    Multi-year ACSM measurements at the central European research station Melpitz (Germany)-Part 1: Instrument robustness, quality assurance, and impact of upper size cutoff diameter
    (Katlenburg-Lindau : Copernicus, 2020) Poulain, Laurent; Spindler, Gerald; Grüner, Achim; Tuch, Thomas; Stieger, Bastian; van Pinxteren, Dominik; Petit, Jean-Eudes; Favez, Olivier; Herrmann, Hartmut; Wiedensohler, Alfred
    The aerosol chemical speciation monitor (ACSM) is nowadays widely used to identify and quantify the main components of fine particles in ambient air. As such, its deployment at observatory platforms is fully incorporated within the European Aerosol, Clouds and Trace Gases Research Infrastructure (ACTRIS). Regular intercomparisons are organized at the Aerosol Chemical Monitoring Calibration Center (ACMCC; part of the European Center for Aerosol Calibration, Paris, France) to ensure the consistency of the dataset, as well as instrumental performance and variability. However, in situ quality assurance remains a fundamental aspect of the instrument's stability. Here, we present and discuss the main outputs of long-term quality assurance efforts achieved for ACSM measurements at the research station Melpitz (Germany) since 2012 onwards. In order to validate the ACSM measurements over the years and to characterize seasonal variations, nitrate, sulfate, ammonium, organic, and particle mass concentrations were systematically compared against the collocated measurements of daily offline high-volume PM1 and PM2:5 filter samples and particle number size distribution (PNSD) measurements. Mass closure analysis was made by comparing the total particle mass (PM) concentration obtained by adding the mass concentration of equivalent black carbon (eBC) from the multi-angle absorption photometer (MAAP) to the ACSM chemical composition, to that of PM1 and PM2:5 during filter weighing, as well as to the derived mass concentration of PNSD. A combination of PM1 and PM2:5 filter samples helped identifying the critical importance of the upper size cutoff of the ACSM during such exercises. The ACSM-MAAP-derived mass concentrations systematically deviated from the PM1 mass when the mass concentration of the latter represented less than 60% of PM2:5, which was linked to the transmission efficiency of the aerodynamic lenses of the ACSM. The best correlations are obtained for sulfate (slopeD 0:96; R2 D 0:77) and total PM (slopeD 1:02; R2 D 0:90). Although, sulfate did not exhibit a seasonal dependency, total PM mass concentration revealed a small seasonal variability linked to the increase in non-water-soluble fractions. The nitrate suffers from a loss of ammonium nitrate during filter collection, and the contribution of organo-nitrate compounds to the ACSM nitrate signal make it difficult to directly compare the two methods. The contribution of m=z 44 (f44) to the total organic mass concentration was used to convert the ACSM organic mass (OM) to organic carbon (OC) by using a similar approach as for the aerosol mass spectrometer (AMS). The resulting estimated OCACSM was compared with the measured OCPM1 (slopeD 0:74; R2 D 0:77), indicating that the f44 signal was relatively free of interferences during this period. The PM2:5 filter samples use for the ACSM data quality might suffer from a systematic bias due to a size truncation effect as well as to the presence of chemical species that cannot be detected by the ACSM in coarse mode (e.g., sodium nitrate and sodium sulfate). This may lead to a systematic underestimation of the ACSM particle mass concentration and/or a positive artifact that artificially decreases the discrepancies between the two methods. Consequently, ACSM data validation using PM2:5 filters has to be interpreted with extreme care. The particle mass closure with the PNSD was satisfying (slopeD 0:77; R2 D 0:90 over the entire period), with a slight overestimation of the mobility particle size spectrometer (MPSS)-derived mass concentration in winter. This seasonal variability was related to a change on the PNSD and a larger contribution of the supermicrometer particles in winter. This long-term analysis between the ACSM and other collocated instruments confirms the robustness of the ACSM and its suitability for long-term measurements. Particle mass closure with the PNSD is strongly recommended to ensure the stability of the ACSM. A near-real-time mass closure procedure within the entire ACTRIS-ACSM network certainly represents an optimal quality control and assurance of both warranting the quality assurance of the ACSM measurements as well as identifying cross-instrumental biases. © Author(s) 2020.
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    A global analysis of climate-relevant aerosol properties retrieved from the network of Global Atmosphere Watch (GAW) near-surface observatories
    (Katlenburg-Lindau : Copernicus, 2020) Laj, Paolo; Bigi, Alessandro; Rose, Clémence; Andrews, Elisabeth; Lund Myhre, Cathrine; Collaud Coen, Martine; Lin, Yong; Wiedensohler, Alfred; Schulz, Michael; Ogren, John A.; Fiebig, Markus; Prenni, Anthony; Reisen, Fabienne; Romano, Salvatore; Sellegri, Karine; Sharma, Sangeeta; Schauer, Gerhard; Sheridan, Patrick; Sherman, James Patrick; Schütze, Maik; Schwerin, Andreas; Tuch, Thomas; Sohmer, Ralf; Sorribas, Mar; Steinbacher, Martin; Sun, Junying; Titos, Gloria; Toczko, Barbara; Tulet, Pierre; Tunved, Peter; Vakkari, Ville; Velarde, Fernando; Velasquez, Patricio; Villani, Paolo; Vratolis, Sterios; Wang, Sheng-Hsiang; Weinhold, Kay; Gliß, Jonas; Weller, Rolf; Yela, Margarita; Yus-Diez, Jesus; Zdimal, Vladimir; Zieger, Paul; Zikova, Nadezda; Mortier, Augustin; Pandolfi, Marco; Petäja, Tuukka; Kim, Sang-Woo; Aas, Wenche; Putaud, Jean-Philippe; Mayol-Bracero, Olga; Keywood, Melita; Labrador, Lorenzo; Aalto, Pasi; Ahlberg, Erik; Alados Arboledas, Lucas; Alastuey, Andrés; Andrade, Marcos; Artíñano, Begoña; Ausmeel, Stina; Arsov, Todor; Asmi, Eija; Backman, John; Baltensperger, Urs; Bastian, Susanne; Bath, Olaf; Beukes, Johan Paul; Brem, Benjamin T.; Bukowiecki, Nicolas; Conil, Sébastien; Couret, Cedric; Day, Derek; Dayantolis, Wan; Degorska, Anna; Eleftheriadis, Konstantinos; Fetfatzis, Prodromos; Favez, Olivier; Flentje, Harald; Gini, Maria I.; Gregorič, Asta; Gysel-Beer, Martin; Hallar, A. Gannet; Hand, Jenny; Hoffer, Andras; Hueglin, Christoph; Hooda, Rakesh K.; Hyvärinen, Antti; Kalapov, Ivo; Kalivitis, Nikos; Kasper-Giebl, Anne; Kim, Jeong Eun; Kouvarakis, Giorgos; Kranjc, Irena; Krejci, Radovan; Kulmala, Markku; Labuschagne, Casper; Lee, Hae-Jung; Lihavainen, Heikki; Lin, Neng-Huei; Löschau, Gunter; Luoma, Krista; Marinoni, Angela; Martins Dos Santos, Sebastiao; Meinhardt, Frank; Merkel, Maik; Metzger, Jean-Marc; Mihalopoulos, Nikolaos; Nguyen, Nhat Anh; Ondracek, Jakub; Pérez, Noemi; Perrone, Maria Rita; Petit, Jean-Eudes; Picard, David; Pichon, Jean-Marc; Pont, Veronique; Prats, Natalia
    Aerosol particles are essential constituents of the Earth's atmosphere, impacting the earth radiation balance directly by scattering and absorbing solar radiation, and indirectly by acting as cloud condensation nuclei. In contrast to most greenhouse gases, aerosol particles have short atmospheric residence times, resulting in a highly heterogeneous distribution in space and time. There is a clear need to document this variability at regional scale through observations involving, in particular, the in situ near-surface segment of the atmospheric observation system. This paper will provide the widest effort so far to document variability of climate-relevant in situ aerosol properties (namely wavelength dependent particle light scattering and absorption coefficients, particle number concentration and particle number size distribution) from all sites connected to the Global Atmosphere Watch network. High-quality data from almost 90 stations worldwide have been collected and controlled for quality and are reported for a reference year in 2017, providing a very extended and robust view of the variability of these variables worldwide. The range of variability observed worldwide for light scattering and absorption coefficients, single-scattering albedo, and particle number concentration are presented together with preliminary information on their long-term trends and comparison with model simulation for the different stations. The scope of the present paper is also to provide the necessary suite of information, including data provision procedures, quality control and analysis, data policy, and usage of the ground-based aerosol measurement network. It delivers to users of the World Data Centre on Aerosol, the required confidence in data products in the form of a fully characterized value chain, including uncertainty estimation and requirements for contributing to the global climate monitoring system.
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    On the application and grid-size sensitivity of the urban dispersion model CAIRDIO v2.0 under real city weather conditions
    (Katlenburg-Lindau : Copernicus, 2022) Weger, Michael; Baars, Holger; Gebauer, Henriette; Merkel, Maik; Wiedensohler, Alfred; Heinold, Bernd
    There is a gap between the need for city-wide air-quality simulations considering the intra-urban variability and mircoscale dispersion features and the computational capacities that conventional urban microscale models require. This gap can be bridged by targeting model applications on the gray zone situated between the mesoscale and large-eddy scale. The urban dispersion model CAIRDIO is a new contribution to the class of computational-fluid dynamics models operating in this scale range. It uses a diffuse-obstacle boundary method to represent buildings as physical obstacles at gray-zone resolutions in the order of tens of meters. The main objective of this approach is to find an acceptable compromise between computationally inexpensive grid sizes for spatially comprehensive applications and the required accuracy in the description of building and boundary-layer effects. In this paper, CAIRDIO is applied on the simulation of black carbon and particulate matter dispersion for an entire mid-size city using a uniform horizontal grid spacing of 40gm. For model evaluation, measurements from five operational air monitoring stations representative for the urban background and high-traffic roads are used. The comparison also includes the mesoscale host simulation, which provides the boundary conditions. The measurements show a dominant influence of the mixing layer evolution at background sites, and therefore both the mesoscale and large-eddy simulation (LES) results are in good agreement with the observed air pollution levels. In contrast, at the high-traffic sites the proximity to emissions and the interactions with the building environment lead to a significantly amplified diurnal variability in pollutant concentrations. These urban road conditions can only be reasonably well represented by CAIRDIO while the meosocale simulation indiscriminately reproduces a typical urban-background profile, resulting in a large positive model bias. Remaining model discrepancies are further addressed by a grid-spacing sensitivity study using offline-nested refined domains. The results show that modeled peak concentrations within street canyons can be further improved by decreasing the horizontal grid spacing down to 10gm, but not beyond. Obviously, the default grid spacing of 40gm is too coarse to represent the specific environment within narrow street canyons. The accuracy gains from the grid refinements are still only modest compared to the remaining model error, which to a large extent can be attributed to uncertainties in the emissions. Finally, the study shows that the proposed gray-scale modeling is a promising downscaling approach for urban air-quality applications. The results, however, also show that aspects other than the actual resolution of flow patterns and numerical effects can determine the simulations at the urban microscale.
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    Nano-hygroscopicity tandem differential mobility analyzer (nano-HTDMA) for investigating hygroscopic properties of sub-10nm aerosol nanoparticles
    (Katlenburg-Lindau : Copernicus, 2020) Lei, Ting; Ma, Nan; Hong, Juan; Tuch, Thomas; Wang, Xin; Wang, Zhibin; Pöhlker, Mira; Ge, Maofa; Wang, Weigang; Mikhailov, Eugene; Hoffmann, Thorsten; Pöschl, Ulrich; Su, Hang; Wiedensohler, Alfred; Cheng, Yafang
    Interactions between water and nanoparticles are relevant for atmospheric multiphase processes, physical chemistry, and materials science. Current knowledge of the hygroscopic and related physicochemical properties of nanoparticles, however, is restricted by the limitations of the available measurement techniques. Here, we present the design and performance of a nano-hygroscopicity tandem differential mobility analyzer (nano-HTDMA) apparatus that enables high accuracy and precision in hygroscopic growth measurements of aerosol nanoparticles with diameters less than 10 nm. Detailed methods of calibration and validation are provided. Besides maintaining accurate and stable sheath and aerosol flow rates (1 %), high accuracy of the differential mobility analyzer (DMA) voltage (0:1 %) in the range of 0-50V is crucial for achieving accurate sizing and small sizing offsets between the two DMAs (1:4 %). To maintain a stable relative humidity (RH), the humidification system and the second DMA are placed in a well-insulated and air conditioner housing (0:1 K). We also tested and discussed different ways of preventing predeliquescence in the second DMA. Our measurement results for ammonium sulfate nanoparticles are in good agreement with Biskos et al. (2006b), with no significant size effect on the deliquescence and efflorescence relative humidity (DRH and ERH, respectively) at diameters down to 6 nm. For sodium sulfate nanoparticles, however, we find a pronounced size dependence of DRH and ERH between 20 and 6 nm nanoparticles. © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License.
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    Importance of size representation and morphology in modelling optical properties of black carbon: comparison between laboratory measurements and model simulations
    (Katlenburg-Lindau : Copernicus, 2022) Romshoo, Baseerat; Pöhlker, Mira; Wiedensohler, Alfred; Pfeifer, Sascha; Saturno, Jorge; Nowak, Andreas; Ciupek, Krzysztof; Quincey, Paul; Vasilatou, Konstantina; Ess, Michaela N.; Gini, Maria; Eleftheriadis, Konstantinos; Robins, Chris; Gaie-Levrel, François; Müller, Thomas
    Black carbon (BC) from incomplete combustion of biomass or fossil fuels is the strongest absorbing aerosol component in the atmosphere. Optical properties of BC are essential in climate models for quantification of their impact on radiative forcing. The global climate models, however, consider BC to be spherical particles, which causes uncertainties in their optical properties. Based on this, an increasing number of model-based studies provide databases and parameterization schemes for the optical properties of BC, using more realistic fractal aggregate morphologies. In this study, the reliability of the different modelling techniques of BC was investigated by comparing them to laboratory measurements. The modelling techniques were examined for bare BC particles in the first step and for BC particles with organic material in the second step. A total of six morphological representations of BC particles were compared, three each for spherical and fractal aggregate morphologies. In general, the aggregate representation performed well for modelling the particle light absorption coefficient σabs, single-scattering albedo SSA, and mass absorption cross-section MACBC for laboratory-generated BC particles with volume mean mobility diameters dp,V larger than 100nm. However, for modelling Ångström absorption exponent AAE, it was difficult to suggest a method due to size dependence, although the spherical assumption was in better agreement in some cases. The BC fractal aggregates are usually modelled using monodispersed particles, since their optical simulations are computationally expensive. In such studies, the modelled optical properties showed a 25% uncertainty in using the monodisperse size method. It is shown that using the polydisperse size distribution in combination with fractal aggregate morphology reduces the uncertainty in measured σabs to 10% for particles with dp,V between 60-160nm. Furthermore, the sensitivities of the BC optical properties to the various model input parameters such as the real and imaginary parts of the refractive index (mre and mim), the fractal dimension (Df), and the primary particle radius (app) of an aggregate were investigated. When the BC particle is small and rather fresh, the change in the Df had relatively little effect on the optical properties. There was, however, a significant relationship between app and the particle light scattering, which increased by a factor of up to 6 with increasing total particle size. The modelled optical properties of BC are well aligned with laboratory-measured values when the following assumptions are used in the fractal aggregate representation: mre between 1.6 and 2, mim between 0.50 and 1, Df from 1.7 to 1.9, and app between 10 and 14nm. Overall, this study provides experimental support for emphasizing the importance of an appropriate size representation (polydisperse size method) and an appropriate morphological representation for optical modelling and parameterization scheme development of BC.