scholarly journals Effects of relative humidity on aerosol light scattering: results from different European sites

2013 ◽  
Vol 13 (21) ◽  
pp. 10609-10631 ◽  
Author(s):  
P. Zieger ◽  
R. Fierz-Schmidhauser ◽  
E. Weingartner ◽  
U. Baltensperger
Keyword(s):  
Light Scattering ◽  
Relative Humidity ◽  
Water Uptake ◽  
Swiss Alps ◽  
The Arctic ◽  
Rural Site ◽  
Hygroscopic Growth ◽  
Aerosol Model ◽  
Arctic Aerosol ◽  
Measurement Results

Abstract. The effect of aerosol water uptake on the aerosol particle light scattering coefficient (σsp) is described in this study by comparing measurements from five European sites: the Jungfraujoch, located in the Swiss Alps at 3580 m a.s.l.; Ny-Ålesund, located on Spitsbergen in the Arctic; Mace Head, a coastal site in Ireland; Cabauw, a rural site in the Netherlands; and Melpitz, a regional background site in Eastern Germany. These sites were selected according to the aerosol type usually encountered at that location. The scattering enhancement factor f(RH, λ) is the key parameter to describe the effect of water uptake on the particle light scattering. It is defined as the σsp(RH) at a certain relative humidity (RH) and wavelength λ divided by its dry value. f(RH) at the five sites varied widely, starting at very low values of f(RH = 85%, λ = 550 nm) around 1.28 for mineral dust, and reaching up to 3.41 for Arctic aerosol. Hysteresis behavior was observed at all sites except at the Jungfraujoch (due to the absence of sea salt). Closure studies and Mie simulations showed that both size and chemical composition determine the magnitude of f(RH). Both parameters are also needed to successfully predict f(RH). Finally, the measurement results were compared to the widely used aerosol model, OPAC (Hess et al., 1998). Significant discrepancies were seen, especially at intermediate RH ranges; these were mainly attributed to inappropriate implementation of hygroscopic growth in the OPAC model. Replacement of the hygroscopic growth with values from the recent literature resulted in a clear improvement of the OPAC model.

scholarly journals Effects of relative humidity on aerosol light scattering: results from different European sites

2013 ◽  
Vol 13 (4) ◽  
pp. 8939-8984 ◽  
Author(s):  
P. Zieger ◽  
R. Fierz-Schmidhauser ◽  
E. Weingartner ◽  
U. Baltensperger
Keyword(s):  
Light Scattering ◽  
Relative Humidity ◽  
Water Uptake ◽  
Swiss Alps ◽  
The Arctic ◽  
Rural Site ◽  
Hygroscopic Growth ◽  
Aerosol Model ◽  
Arctic Aerosol ◽  
Measurement Results

Abstract. The effect of aerosol water uptake on the aerosol particle light scattering coefficient (σsp) is described in this study by comparing measurements from five European sites: the Jungfraujoch, located in the Swiss Alps at 3580 m a.s.l., Ny-Ålesund, located on Spitsbergen in the Arctic, Mace Head, a coastal site in Ireland, Cabauw, a rural site in the Netherlands and Melpitz, a regional background site in Eastern Germany. These sites were selected according to the aerosol type usually encountered at that location. The scattering enhancement factor f(RH,λ) is the key parameter to describe the effect of water uptake on the particle light scattering. It is defined as the σsp(RH) at a certain relative humidity (RH) and wavelength λ divided by its dry value. f(RH) largely varied at the five sites starting from very low values of f(RH = 85%,λ = 550 nm) around 1.28 for mineral dust to 3.41 for Arctic aerosol. Hysteresis behavior was observed at all sites except at the Jungfraujoch due to the absence of sea salt. Closure studies and Mie simulations showed that both size and chemical composition determine the magnitude of f(RH). Both parameters are also needed to successfully predict f(RH). Finally, the measurement results were compared to the widely used aerosol model OPAC (Hess et al., 1998). Significant discrepancies were seen especially at intermediate RH ranges, which were mainly attributed to inappropriate implemented hygroscopic growth within OPAC. Replacement of the hygroscopic growth with recent literature values showed a clear improvement of the OPAC model.

scholarly journals Linking variations in sea spray aerosol particle hygroscopicity to composition during two microcosm experiments

2016 ◽  
Author(s):  
Sara D. Forestieri ◽  
Gavin C. Cornwell ◽  
Taylor M. Helgestad ◽  
Kathryn A. Moore ◽  
Christopher Lee ◽  
...  
Keyword(s):  
Light Scattering ◽  
Relative Humidity ◽  
Salt Content ◽  
Sea Salt ◽  
Sea Spray ◽  
Hygroscopic Growth ◽  
Phytoplankton Blooms ◽  
Aerosol Composition ◽  
Sea Spray Aerosol ◽  
Indoor And Outdoor

Abstract. The extent to which water uptake influences the light scattering ability of marine sea spray aerosol (SSA) particles depends critically on SSA chemical composition. The organic fraction of SSA can increase during phytoplankton blooms, decreasing the salt content and therefore the hygroscopicity of the particles. In this study, subsaturated hygroscopic growth factors at 85 % relative humidity (GF(85 %)) of SSA particles were quantified during two induced phytoplankton blooms in marine aerosol reference tanks (MARTs). One MART was illuminated with fluorescent lights and the other was illuminated with sunlight, referred to as the "indoor" and "outdoor" MARTs, respectively. GF(85 %) values for SSA particles were derived from measurements of light scattering and particle size distributions, concurrently with online single particle and bulk aerosol composition measurements. During both microcosm experiments, the observed bulk average GF(85 %) values were depressed substantially relative to pure, inorganic sea salt, by 10 to 19 %, with a one (indoor MART) and six (outdoor MART) day lag between GF(85 %) depression and the peak chlorophyll-a concentrations. The fraction of organiccontaining SSA particles generally increased after the peak of the phytoplankton blooms. The GF(85 %) values were inversely correlated with the fraction of particles containing organic or other biological markers. This indicates these particles were less hygroscopic than the particles identified as predominately sea salt containing and demonstrates a clear relationship between SSA particle composition and the sensitivity of light scattering to variations in relative humidity. The implications of these observations to the direct climate effects of SSA particles are discussed.

First global assessment of modelled aerosol hygroscopicity in the context of other aerosol optical properties

2021 ◽  
Author(s):  
Maria Ángeles Burgos Simón ◽  
Elisabeth Andrews ◽  
Gloria Titos ◽  
Angela Benedetti ◽  
Huisheng Bian ◽  
...  
Keyword(s):  
Optical Properties ◽  
Light Scattering ◽  
Relative Humidity ◽  
Water Uptake ◽  
Chem Phys ◽  
Earth System ◽  
Aerosol Optical Properties ◽  
System Models ◽  
Earth System Models

<p>The particle hygroscopic growth impacts the optical properties of aerosols and, in turn, affects the aerosol-radiation interaction and calculation of the Earth’s radiative balance. The dependence of particle light scattering on relative humidity (RH) can be described by the scattering enhancement factor f(RH), defined as the ratio between the particle light scattering coefficient at a given RH divided by its dry value.</p><p>The first effort of the AeroCom Phase III – INSITU experiment was to develop an observational dataset of scattering enhancement values at 26 sites to study the uptake of water by atmospheric aerosols, and evaluate f(RH) globally (Burgos et al., 2019). Model outputs from 10 Earth System Models (CAM, CAM-ATRAS, CAM-Oslo, GEOS-Chem, GEOS-GOCART, MERRAero, TM5, OsloCTM3, IFS-AER, and ECMWF) were then evaluated against this in-situ dataset. Building on these results, we investigate f(RH) in the context of other aerosol optical and chemical properties, making use of the same 10 Earth System Models (ESMs) and in-situ measurements as in Burgos et al. (2020) and Titos et al. (2021).</p><p>Given the difficulties of deploying and maintaining instrumentation for long-term, accurate and comprehensive f(RH) observations, it is desirable to find an observational proxy for f(RH). This observation-based proxy would also need to be reproduced in modelling space. Our aim here is to evaluate how ESMs currently represent the relationship between f(RH), scattering Ångström exponent (SAE), and single scattering albedo (SSA). This work helps to identify current challenges in modelling water-uptake by aerosols and their impact on aerosol optical properties within Earth system models.</p><p>We start by analyzing the behavior of SSA with RH, finding the expected increase with RH for all site types and models. Then, we analyze the three variables together (f(RH)-SSA-SAE relationship). Results show that hygroscopic particles tend to be bigger and scatter more than non-hygroscopic small particles, though variability within models is noticeable. This relationship can be further studied by relating SAE to model chemistry, by selecting those grid points dominated by a single chemical component (mass mixing ratios > 90%). Finally, we analyze model performance at three specific sites representing different aerosol types: Arctic, marine and rural. At these sites, the model data can be exactly temporally and spatially collocated with the observations, which should help to identify the models which exhibit better agreement with measurements and for which aerosol type.</p><p> </p><p>Burgos, M.A. et al.: A global view on the effect of water uptake on aerosol particle light scattering. Sci Data 6, 157. https://doi.org/10.1038/s41597-019-0158-7, 2019.</p><p>Burgos, M.A. et al.: A global model–measurement evaluation of particle light scattering coefficients at elevated relative humidity, Atmos. Chem. Phys., 20, 10231–10258, https://doi.org/10.5194/acp-20-10231-2020, 2020.</p><p>Titos, G. et al.: A global study of hygroscopicity-driven light scattering enhancement in the context of other in-situ aerosol optical properties, Atmos. Chem. Phys. Discuss. [preprint], https://doi.org/10.5194/acp-2020-1250, in review, 2020.</p>

scholarly journals Aerosol water parameterisation: a single parameter framework

2016 ◽  
Vol 16 (11) ◽  
pp. 7213-7237 ◽  
Author(s):  
Swen Metzger ◽  
Benedikt Steil ◽  
Mohamed Abdelkader ◽  
Klaus Klingmüller ◽  
Li Xu ◽  
...  
Keyword(s):  
Ammonium Nitrate ◽  
Water Uptake ◽  
Climate Model ◽  
Mixed Solution ◽  
Hygroscopic Growth ◽  
Simple Method ◽  
Aerosol Model ◽  
Kohler Theory ◽  
Comprehensive Comparison ◽  
Term Evaluation

Abstract. We introduce a framework to efficiently parameterise the aerosol water uptake for mixtures of semi-volatile and non-volatile compounds, based on the coefficient, νi. This solute-specific coefficient was introduced in Metzger et al. (2012) to accurately parameterise the single solution hygroscopic growth, considering the Kelvin effect – accounting for the water uptake of concentrated nanometer-sized particles up to dilute solutions, i.e. from the compounds relative humidity of deliquescence (RHD) up to supersaturation (Köhler theory). Here we extend the νi parameterisation from single to mixed solutions. We evaluate our framework at various levels of complexity, by considering the full gas–liquid–solid partitioning for a comprehensive comparison with reference calculations using the E-AIM, EQUISOLV II and ISORROPIA II models as well as textbook examples. We apply our parameterisation in the EQuilibrium Simplified Aerosol Model V4 (EQSAM4clim) for climate simulations, implemented in a box model and in the global chemistry–climate model EMAC. Our results show (i) that the νi approach enables one to analytically solve the entire gas–liquid–solid partitioning and the mixed solution water uptake with sufficient accuracy, (ii) that ammonium sulfate mixtures can be solved with a simple method, e.g. pure ammonium nitrate and mixed ammonium nitrate and (iii) that the aerosol optical depth (AOD) simulations are in close agreement with remote sensing observations for the year 2005. Long-term evaluation of the EMAC results based on EQSAM4clim and ISORROPIA II will be presented separately.

scholarly journals Effects of relative humidity on aerosol light scattering in the Arctic

2010 ◽  
Vol 10 (8) ◽  
pp. 3875-3890 ◽  
Author(s):  
P. Zieger ◽  
R. Fierz-Schmidhauser ◽  
M. Gysel ◽  
J. Ström ◽  
S. Henne ◽  
...  
Keyword(s):  
Standard Deviation ◽  
Light Scattering ◽  
Relative Humidity ◽  
Aerosol Particles ◽  
Aerosol Size Distribution ◽  
Hygroscopic Growth ◽  
Scattering Coefficients ◽  
Dry Conditions ◽  
Hygroscopic Particles

Abstract. Aerosol particles experience hygroscopic growth in the ambient atmosphere. Their optical properties – especially the aerosol light scattering – are therefore strongly dependent on the ambient relative humidity (RH). In-situ light scattering measurements of long-term observations are usually performed under dry conditions (RH>30–40%). The knowledge of this RH effect is of eminent importance for climate forcing calculations or for the comparison of remote sensing with in-situ measurements. This study combines measurements and model calculations to describe the RH effect on aerosol light scattering for the first time for aerosol particles present in summer and fall in the high Arctic. For this purpose, a field campaign was carried out from July to October 2008 at the Zeppelin station in Ny-Ålesund, Svalbard. The aerosol light scattering coefficient σsp(λ) was measured at three distinct wavelengths (λ=450, 550, and 700 nm) at dry and at various, predefined RH conditions between 20% and 95% with a recently developed humidified nephelometer (WetNeph) and with a second nephelometer measuring at dry conditions with an average RH<10% (DryNeph). In addition, the aerosol size distribution and the aerosol absorption coefficient were measured. The scattering enhancement factor f(RH, λ) is the key parameter to describe the RH effect on σsp(λ) and is defined as the RH dependent σsp(RH, λ) divided by the corresponding dry σsp(RHdry, λ). During our campaign the average f(RH=85%, λ=550 nm) was 3.24±0.63 (mean ± standard deviation), and no clear wavelength dependence of f(RH, λ) was observed. This means that the ambient scattering coefficients at RH=85% were on average about three times higher than the dry measured in-situ scattering coefficients. The RH dependency of the recorded f(RH, λ) can be well described by an empirical one-parameter equation. We used a simplified method to retrieve an apparent hygroscopic growth factor g(RH), defined as the aerosol particle diameter at a certain RH divided by the dry diameter, using the WetNeph, the DryNeph, the aerosol size distribution measurements and Mie theory. With this approach we found, on average, g(RH=85%) values to be 1.61±0.12 (mean±standard deviation). No clear seasonal shift of f(RH, λ) was observed during the 3-month period, while aerosol properties (size and chemical composition) clearly changed with time. While the beginning of the campaign was mainly characterized by smaller and less hygroscopic particles, the end was dominated by larger and more hygroscopic particles. This suggests that compensating effects of hygroscopicity and size determined the temporal stability of f(RH, λ). During sea salt influenced periods, distinct deliquescence transitions were observed. At the end we present a method on how to transfer the dry in-situ measured aerosol scattering coefficients to ambient values for the aerosol measured during summer and fall at this location.

scholarly journals Effect of mixing structure on the water uptake of mixtures of ammonium sulfate and phthalic acid particles

2021 ◽  
Vol 21 (3) ◽  
pp. 2179-2190
Author(s):  
Weigang Wang ◽  
Ting Lei ◽  
Andreas Zuend ◽  
Hang Su ◽  
Yafang Cheng ◽  
...  
Keyword(s):  
Growth Factor ◽  
Relative Humidity ◽  
Water Uptake ◽  
Phthalic Acid ◽  
Aerosol Particles ◽  
Core Shell ◽  
Hygroscopic Growth ◽  
The Core ◽  
Mass Fractions ◽  
Mixing States

Abstract. Aerosol mixing state regulates the interactions between water molecules and particles and thus controls aerosol activation and hygroscopic growth, which thereby influences visibility degradation, cloud formation, and its radiative forcing. There are, however, few current studies on the mixing structure effects on aerosol hygroscopicity. Here, we investigated the hygroscopicity of ammonium sulfate / phthalic acid (AS / PA) aerosol particles with different mass fractions of PA in different mixing states in terms of initial particle generation. Firstly, the effect of PA coatings on the hygroscopic behavior of the core-shell-generated mixtures of AS with PA was studied using a coating hygroscopicity tandem differential mobility analyzer (coating HTDMA). The slow increase in the hygroscopic growth factor of core-shell-generated particles is observed with increasing thickness of the coating PA prior to the deliquescence relative humidity (DRH) of AS. At relative humidity (RH) above 80 %, a decrease in the hygroscopic growth factor of particles occurs as the thickness of the PA shell increases, which indicates that the increase of PA mass fractions leads to a reduction of the overall core-shell-generated particle hygroscopicity. In addition, the use of the Zdanovskii–Stokes–Robinson (ZSR) relation leads to the underestimation of the measured growth factors of core-shell-generated particles without consideration of the morphological effect of core-shell-generated particles, especially at higher RH. Secondly, in the case of the AS / PA initially well-mixed particles, a shift of the DRH of AS (∼80 %, Tang and Munkelwitz, 1994) to lower RH is observed due to the presence of PA in the initially well-mixed particles. The predicted hygroscopic growth factor using the ZSR relation is consistent with the measured hygroscopic growth factor of the initially well-mixed particles. Moreover, we compared and discussed the influence of mixing states on the water uptake of AS / PA aerosol particles. It is found that the hygroscopic growth factor of the core-shell-generated particles is slightly higher than that of the initially well-mixed particles with the same mass fractions of PA at RH above 80 %. The observation of AS / PA particles may contribute to a growing field of knowledge regarding the influence of coating properties and mixing structure on water uptake.

scholarly journals Aerosol water parameterization: long-term evaluation and importance for climate studies

2018 ◽  
Vol 18 (22) ◽  
pp. 16747-16774 ◽  
Author(s):  
Swen Metzger ◽  
Mohamed Abdelkader ◽  
Benedikt Steil ◽  
Klaus Klingmüller
Keyword(s):  
Water Uptake ◽  
Climate Model ◽  
Climate Modeling ◽  
Mixed Solution ◽  
Hygroscopic Growth ◽  
Computationally Efficient ◽  
Aerosol Model ◽  
Term Evaluation ◽  
Climate Studies

Abstract. We scrutinize the importance of aerosol water for the aerosol optical depth (AOD) calculations using a long-term evaluation of the EQuilibrium Simplified Aerosol Model v4 for climate modeling. EQSAM4clim is based on a single solute coefficient approach that efficiently parameterizes hygroscopic growth, accounting for aerosol water uptake from the deliquescence relative humidity up to supersaturation. EQSAM4clim extends the single solute coefficient approach to treat water uptake of multicomponent mixtures. The gas–aerosol partitioning and the mixed-solution water uptake can be solved analytically, preventing the need for iterations, which is computationally efficient. EQSAM4clim has been implemented in the global chemistry climate model EMAC and compared to ISORROPIA II on climate timescales. Our global modeling results show that (I) our EMAC results of the AOD are comparable to modeling results that have been independently evaluated for the period 2000–2010, (II) the results of various aerosol properties of EQSAM4clim and ISORROPIA II are similar and in agreement with AERONET and EMEP observations for the period 2000–2013, and (III) the underlying assumptions on the aerosol water uptake limitations are important for derived AOD calculations. Sensitivity studies of different levels of chemical aging and associated water uptake show larger effects on AOD calculations for the year 2005 compared to the differences associated with the application of the two gas–liquid–solid partitioning schemes. Overall, our study demonstrates the importance of aerosol water for climate studies.

scholarly journals Seasonal variation of aerosol water uptake and its impact on the direct radiative effect at Ny-Ålesund, Svalbard

2014 ◽  
Vol 14 (14) ◽  
pp. 7445-7460 ◽  
Author(s):  
N. Rastak ◽  
S. Silvergren ◽  
P. Zieger ◽  
U. Wideqvist ◽  
J. Ström ◽  
...  
Keyword(s):  
Seasonal Variation ◽  
Water Uptake ◽  
Aerosol Particles ◽  
The Arctic ◽  
Ambient Atmosphere ◽  
Radiative Effect ◽  
Hygroscopic Growth ◽  
Scattering Coefficients ◽  
Aerosol Scattering ◽  
July August September

Abstract. In this study we investigated the impact of water uptake by aerosol particles in ambient atmosphere on their optical properties and their direct radiative effect (ADRE, W m−2) in the Arctic at Ny-Ålesund, Svalbard, during 2008. To achieve this, we combined three models, a hygroscopic growth model, a Mie model and a radiative transfer model, with an extensive set of observational data. We found that the seasonal variation of dry aerosol scattering coefficients showed minimum values during the summer season and the beginning of fall (July-August-September), when small particles (< 100 nm in diameter) dominate the aerosol number size distribution. The maximum scattering by dry particles was observed during the Arctic haze period (March-April-May) when the average size of the particles was larger. Considering the hygroscopic growth of aerosol particles in the ambient atmosphere had a significant impact on the aerosol scattering coefficients: the aerosol scattering coefficients were enhanced by on average a factor of 4.30 ± 2.26 (mean ± standard deviation), with lower values during the haze period (March-April-May) as compared to summer and fall. Hygroscopic growth of aerosol particles was found to cause 1.6 to 3.7 times more negative ADRE at the surface, with the smallest effect during the haze period (March-April-May) and the highest during late summer and beginning of fall (July-August-September).

scholarly journals Aerosol water parameterization: long-term evaluation and importance

2018 ◽  
Author(s):  
Swen Metzger ◽  
Mohamed Abdelkader ◽  
Benedikt Steil ◽  
Klaus Klingmüller
Keyword(s):  
Aerosol Optical Depth ◽  
Optical Depth ◽  
Water Uptake ◽  
Climate Model ◽  
Climate Modeling ◽  
Mixed Solution ◽  
Hygroscopic Growth ◽  
Aerosol Model ◽  
Term Evaluation

Abstract. We scrutinize the importance of aerosol water for the aerosol optical depth (AOD) calculations by a long-term evaluation of the EQuilibrium Simplified Aerosol Model V4 for climate modeling, which was introduced by Metzger et al. (2016a). EQSAM4clim is based on a sin-gle solute coefficient approach that efficiently parameterizes hygroscopic growth, account- ing for aerosol water uptake from the deliquescence relative humidity up to supersaturation. EQSAM4clim extends the single solute coefficient approach to treat water uptake of multi- component mixtures. The gas-aerosol partitioning and the mixed solution water uptake can be solved analytically, preventing the need for iterations, which is computationally efficient. EQSAM4clim has been implemented in the global chemistry climate model EMAC and com- pared to ISORROPIA II (Fountoukis and Nenes, 2007) at climate time-scales. Our global modeling results show that (I) our EMAC results of the aerosol optical depth (AOD) are comparable to independent results of Pozzer et al. (2015) for the period 2000–2010, (II) the results of various aerosol properties of EQSAM4clim and ISORROPIA II are similar and in agreement with AERONET and EMEP observations for the period 2000–2013, and (III) that the underlying assumptions on the aerosol water uptake limitations are important for derived AOD calculations. Sensitivity studies of different levels of chemical aging and associated water uptake show larger effects on AOD calculations for the year 2005 compared to the differences associated with the application of the two gas-liquid-solid partitioning schemes. Altogether, our study reveals the importance of the aerosol water for climate applications.

scholarly journals Aerosol-cloud interaction inferred from MODIS satellite data and global aerosol models

2006 ◽  
Vol 6 (5) ◽  
pp. 9351-9388 ◽  
Author(s):  
G. Myhre ◽  
F. Stordal ◽  
M. Johnsrud ◽  
Y. J. Kaufman ◽  
D. Rosenfeld ◽  
...  
Keyword(s):  
Relative Humidity ◽  
Optical Depth ◽  
Water Uptake ◽  
Satellite Data ◽  
Cloud Cover ◽  
Hygroscopic Growth ◽  
Modis Data ◽  
Aerosol Cloud ◽  
Cloud Parameters ◽  
Ångström Exponent

Abstract. We have used the Modis satellite data and two global aerosol models to investigate relationships between aerosol optical depth (AOD) and cloud parameters that may be affected by the aerosol concentration. The relationships that are studied are mainly between AOD on the one hand and cloud cover, cloud liquid water path, and water vapour on the other. Additionally, cloud droplet effective radius, cloud optical depth, cloud top pressure and aerosol Ångström exponent, have been analysed in a few cases. In the Modis data we found as in earlier studies an enhancement in the cloud cover with increasing AOD. We find it likely that most of the strong increase in cloud cover with AOD, at least for AOD<0.2, is a result of aerosol-cloud interactions and prolonged cloud lifetime. Meteorology seems not to be a cause for the increase in cloud cover with AOD in this range. When water uptake of the aerosols is not taken into account in the models the modelled cloud cover actually decreases with AOD. Part of the relationship found in the Modis data for AOD>0.2 can be explained by larger water uptake close to clouds since relative humidity is higher in regions with higher cloud cover. The efficiency of the hygroscopic growth depends on aerosol type, hygroscopic nature of the aerosol, the relative humidity, and to some extent the cloud screening. By analysing the Ångström exponent we find that the hygroscopic growth of the aerosol is not likely to be a main contributor to the cloud cover increase with AOD. Since the largest increase in cloud cover with AOD is for low AOD (~0.2) and thus also for low cloud cover, cloud contamination is not likely to play a large role. However, interpretation of the complex relationships between AOD and cloud parameters should be made with great care and further work is clearly needed.

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