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Mineral dust is an important natural source of aerosols and significantly influences air quality (Querol et al., Environ. Int., 2019) and the global radiation budget (Schepanski, Geosci., 2018). Frequent dust intrusions are observed in the Mediterranean region (Ealo et al., Atmos. Chem. Phys., 2016; Pikridas et al., Atmos. Environ., 2018) and Central Europe (Collaud Coen et al., Atmos. Chem. Phys., 2004; Schauer et al., Aerosol Air Qual. Res., 2016), with high potential to cause exceedances of daily PM10 levels. To separate the influence of anthropogenic and natural contribution to the PM10 levels, the new method was developed within the DNAAP project (Detection of non-anthropogenic air pollution – http://www.aerosol.si/dnaap/). Dust weakly absorbs light in the near ultra-violet and short wavelengths of the visible range, while the light absorption of dust in longer wavelengths from the visible and near infra-red range is negligible. We used filter-based photometer Aethalometer AE33 (Drinovec et al., Atmos. Meas. Tech., 2015) to measure the light absorption at seven wavelengths, from 370 to 950 nm. The mineral dust is not the only light-absorbing aerosol in the air. Black carbon (BC), a unique primary tracer for combustion emissions, strongly absorbs light across the entire visual, near infra-red and near ultra-violet spectral range. Since optical absorption of mineral dust is weaker than the optical absorption of black carbon, the coarse mode mineral particles have to be concentrated using the high-volume virtual impactor (VI). The method is based on the optical absorption measurements of the two sample streams, sampling particle size below 1 µm and sample stream with the concentrated coarse mode particles, where mineral dust contribution is substantial. Experimental configuration includes two Aethalometers AE33 with different size selective inlets: VI inlet for sampling coarse aerosol mode (mostly mineral dust) and PM1 inlet for sampling fine mode of aerosols (mainly BC). The optical absorption of mineral dust can be determined by subtracting the absorption of fine aerosol fraction (PM1) from the absorption of aerosol sampled by the VI, taking into account the enhancement factor of VI setup (Drinovec et al., Atmos. Meas. Tech., 2020). The mineral dust mass concentration is then calculated using mass absorption cross-section (MAC) for dust which could be site and source-region specific. The results from the measurement campaigns performed at six locations in the Mediterranean region will be presented. The measurements took place in NE Spain (Barcelona – BCN, Montseny – MSY, Montsec – MSA), on Cyprus (Nicosia – NI, Agia Marina Xyliatou – AMX), and in Slovenia (Ljubljana – LJ). Two year-long datasets will be presented, focusing on the analyses of aerosol optical properties of PM1 and VI fractions. The results were validated using low time resolution chemical specification of offline filters and a statistical approach where dust was extracted from PM10 measurements for dust intrusions periods determined by models and back-trajectory studies. For better understanding, helium ion microscopy (HIM) was applied to study the microscopic differences between mineral dust and black carbon captured on the AE33 filter tapes. This work was supported by SPIRIT Slovenia – Public Agency for Entrepreneurship, Internationalization, Foreign Investments and Technology, project DNAAP.

The diverse physical and chemical properties of aerosols can cause a diverse impact on air quality, cloud nucleation, planetary radiation balance, public health, etc. Besides carbon particles from incomplete combustion, mineral particles from Saharan dust also present a significant contribution to the changes. It is estimated that 400 to 700 million tons of dust is transported from Sahara every year and with the particular wind directions it is carried to the Mediterranean or even to the north of Europe (Prospero, 1996). It has recently been shown that these particles induce serious problems for asthmatics (Gutierrez et al., 2020). To understand the origin of pollution, necessary input information presents knowing the source apportionment of aerosols. Techniques such as non-destructive particle induced X-ray emission (PIXE) (Lucarelli et al., 2018) and energy dispersive x-ray spectroscopy (SEM-EDS) (Longoria-Rodríguez et al., 2021) have been successfully applied for chemical microanalysis of individual particles. However, knowing both the physical and chemical properties of particles towards nm scales, which would cover all aerosol sizes, is still a challenging task. In our study, we have thus implemented the correlative approach using advanced ion beam techniques to study both physical and chemical properties of mineral particles from Saharan dust collected on quartz fiber filters on Cyprus Atmospheric Observatory (35.04oN,33.06 Eo; 535 m a.s.l.) using a combination of the virtual impactor and Aethalometer AE33 (Aerosol d.o.o.). We have implemented Helium Ion Microscopy (HIM), capable of sub-nm resolution imaging with high depth-of-field contrast, followed by micro-PIXE elemental analysis done on the same filter region. Information from backscattered high-energy ions was found particularly suitable for the registration and overlap of complementary images (Figure 1). The study has revealed the size, shape, architecture, and surface topography of individual mineral particles on nm scale, while micro-PIXE their chemical composition. Additionally, HIM resolution and surface sensitivity enabled the detection and identification of individual black carbon (BC) soot attached to the surface of mineral particles. This information can have a significant impact on our understanding of the optical properties of mineral dust and its relevance to climate changes and health effects. This finding urges for further investigations where additional focused ion beam techniques and instrumentation have been implemented and will be discussed.

The correlative approach of real-time aerosol measurements with offline filter analysis and ParticleInduced X-ray Emission (PIXE) can significantly enhance the scope of aerosol studies. Aerosol particles have diverse physical and chemical properties, thus having a direct impact on air quality, cloud nucleation the planetary radiation balance, public health, etc. Essential information on the chemical composition of aerosol particles can be deduced from the elemental concentrations measured simultaneously for many elements by non-destructive and undemanding PIXE (Lucarelli, 2018). Furthermore, PIXE measurements can be performed with a focused beam allowing the analysis of individual particles (Biancato, 2006). Determining elemental concentrations is also important input information for aerosol source apportionment models and consequently abatement measures in order to improve air quality (Artaxo, 1999). Complementary information of aerosol particles structure down to nm scale, can be obtained with a Helium Ion Microscope (HIM), which, to our knowledge, has never been used before for aerosol characterization. The detection of secondary electrons and backscattered ions enables sub nm lateral resolution and large depth-of-field, also on insulating samples. In addition, a concurrent secondary ion mass spectrometry (ToF-SIMS) integrated in the HIM can provide insights of the composition of elements and molecules with imaging capabilities of sub 8 nm (Klingner, 2019). In this study we coupled real-time measurements of optical properties of aerosols with an Aethalometer (Drinovec, 2015) and their carbon content with a Total Carbon Analyzer (Rigler, 2019) with PIXE and HIM analysis. Ambient aerosols were collected on quartz filters and quartz filters with Teflon coating during different pollution events (traffic or biomass burning dominated, Saharan dust dominated, etc.) at an urban background sampling site in Ljubljana, Slovenia (46o04’17’’N, 14o30’06’’E). The PM2.5 inlet was used for sampling carbonaceous aerosol while a virtual impactor was used for concentrating coarse particles during Saharan dust events. PIXE measurements have been performed on these collected samples and compared to optical properties and source apportionment obtained by the online instruments. The PIXE measurements were performed across several hundred-micrometer regions and on individual points and results are presented including a description of the procedures for quantification. Complementary high-resolution imaging and sputtered ion analysis on single black carbon and Saharian dust aerosol particles was done on the HIM to study their structure and coating composition. The combination of all methods yields a comprehensive view of the aerosol particles.

We have spectra, colours and morphologies from the CFHT for 240 cluster members and 80 field galaxies over a 7 × 46 arcmin field. The cluster galaxies show strong radial gradients in colour, morphology, and spectrum. The central group has only red early-type galaxies of high central concentration, which must have formed at least 5 Gy ago. The population becomes bluer and shows an increasing fraction of disk morphology and recent star-formation with clustocentric radius, and eventually blends into the field. However, the fraction of blue galaxies is significantly lower than in Butcher-Oemler clusters, and there are only two starburst galaxies (one of which is the cluster cD). Fitting of line measures with stellar population models indicates that star-formation has been truncated in the 15-20% of the cluster galaxies that have strong Hδ absorption, and that these galaxies are not all blue. There is evidence that cluster galaxies are dusty compared with the field. The morphology gradient can be fit with models of disk fading. However, there is a small fraction of interacting and merging galaxies, which must play a part in the population evolution. The cluster is accreting from the field in a non-violent way, including some distinct subgroups. This is different from what is seen at both higher and lower redshift, and if generally true, suggests a high Omega universe.These results are being published in detail elsewhere.

In the frame of the EMEP/ACTRIS/COLOSSAL campaign in Milan during winter 2018, equivalent black carbon measurements using the Aethalometer 31 (AE31), the Aethalometer 33 (AE33), and a Multi-Angle Absorption Photometer (MAAP) were carried out together with levoglucosan analyses on 12 h resolved PM2.5 samples collected in parallel. From AE31 and AE33 data, the loading-corrected aerosol attenuation coefficients (bATN) were calculated at seven wavelengths (λ, where λ values are 370, 470, 520, 590, 660, 880, and 950 nm). The aerosol absorption coefficient at 637 nm (babs_MAAP) was determined by MAAP measurements. Furthermore, babs was also measured at four wavelengths (405, 532, 635, 780 nm) on the 12 h resolved PM2.5 samples by a polar photometer (PP_UniMI). After comparing PP_UniMI and MAAP results, we exploited PP_UniMI data to evaluate the filter multiple-scattering enhancement parameter at different wavelengths for AE31 and AE33. We obtained instrument- and wavelength-dependent multiple-scattering enhancement parameters by linear regression of the Aethalometer bATN against the babs measured by PP_UniMI. We found significant dependence of the multiple-scattering enhancement parameter on filter material, hence on the instrument, with a difference of up to 30 % between the AE31 and the AE33 tapes. The wavelength dependence and day–night variations were small – the difference between the smallest and largest value was up to 6 %. Data from the different instruments were used as input to the so-called “Aethalometer model” for optical source apportionment, and instrument dependence of the results was investigated. Inconsistencies among the source apportionment were found fixing the AE31 and AE33 multiple-scattering enhancement parameters to their usual values. In contrast, optimised multiple-scattering enhancement parameters led to a 5 % agreement among the approaches. Also, the component apportionment “MWAA model” (Multi-Wavelength Absorption Analyzer model) was applied to the dataset. It was less sensitive to the instrument and the number of wavelengths, whereas significant differences in the determination of the absorption Ångström exponent for brown carbon were found (up to 22 %).

We experimentally quantified the impact of cloud fraction and cloud type on the heating rate (HR) of black and brown carbon (HRBC and HRBrC). In particular, we examined in more detail the cloud effect on the HR detected in a previous study (Ferrero et al., 2018). High-time-resolution measurements of the aerosol absorption coefficient at multiple wavelengths were coupled with spectral measurements of the direct, diffuse and surface reflected irradiance and with lidar–ceilometer data during a field campaign in Milan, Po Valley (Italy). The experimental set-up allowed for a direct determination of the total HR (and its speciation: HRBC and HRBrC) in all-sky conditions (from clear-sky conditions to cloudy). The highest total HR values were found in the middle of winter (1.43 ± 0.05 K d−1), and the lowest were in spring (0.54 ± 0.02 K d−1). Overall, the HRBrC accounted for 13.7 ± 0.2 % of the total HR, with the BrC being characterized by an absorption Ångström exponent (AAE) of 3.49 ± 0.01. To investigate the role of clouds, sky conditions were classified in terms of cloudiness (fraction of the sky covered by clouds: oktas) and cloud type (stratus, St; cumulus, Cu; stratocumulus, Sc; altostratus, As; altocumulus, Ac; cirrus, Ci; and cirrocumulus–cirrostratus, Cc–Cs). During the campaign, clear-sky conditions were present 23 % of the time, with the remaining time (77 %) being characterized by cloudy conditions. The average cloudiness was 3.58 ± 0.04 oktas (highest in February at 4.56 ± 0.07 oktas and lowest in November at 2.91 ± 0.06 oktas). St clouds were mostly responsible for overcast conditions (7–8 oktas, frequency of 87 % and 96 %); Sc clouds dominated the intermediate cloudiness conditions (5–6 oktas, frequency of 47 % and 66 %); and the transition from Cc–Cs to Sc determined moderate cloudiness (3–4 oktas); finally, low cloudiness (1–2 oktas) was mostly dominated by Ci and Cu (frequency of 59 % and 40 %, respectively). HR measurements showed a constant decrease with increasing cloudiness of the atmosphere, enabling us to quantify for the first time the bias (in %) of the aerosol HR introduced by the simplified assumption of clear-sky conditions in radiative-transfer model calculations. Our results showed that the HR of light-absorbing aerosol was ∼ 20 %–30 % lower in low cloudiness (1–2 oktas) and up to 80 % lower in completely overcast conditions (i.e. 7–8 oktas) compared to clear-sky ones. This means that, in the simplified assumption of clear-sky conditions, the HR of light-absorbing aerosol can be largely overestimated (by 50 % in low cloudiness, 1–2 oktas, and up to 500 % in completely overcast conditions, 7–8 oktas). The impact of different cloud types on the HR was also investigated. Cirrus clouds were found to have a modest impact, decreasing the HRBC and HRBrC by −5 % at most. Cumulus clouds decreased the HRBC and HRBrC by −31 ± 12 % and −26 ± 7 %, respectively; cirrocumulus–cirrostratus clouds decreased the HRBC and HRBrC by −60 ± 8 % and −54 ± 4 %, which was comparable to the impact of altocumulus (−60 ± 6 % and −46 ± 4 %). A higher impact on the HRBC and HRBrC suppression was found for stratocumulus (−63 ± 6 % and −58 ± 4 %, respectively) and altostratus (−78 ± 5 % and −73 ± 4 %, respectively). The highest impact was associated with stratus, suppressing the HRBC and HRBrC by −85 ± 5 % and −83 ± 3 %, respectively. The presence of clouds caused a decrease of both the HRBC and HRBrC (normalized to the absorption coefficient of the respective species) of −11.8 ± 1.2 % and −12.6 ± 1.4 % per okta. This study highlights the need to take into account the role of both cloudiness and different cloud types when estimating the HR caused by both BC and BrC and in turn decrease the uncertainties associated with the quantification of their impact on the climate.

Accurate and temporally consistent measurements of light absorbing aerosol (LAA) heating rate (HR) and of its source apportionment (fossil-fuel, FF; biomass-burning, BB) and speciation (black and brown Carbon; BC, BrC) are needed to evaluate LAA short-term climate forcing. For this purpose, wavelength- and time-dependent accurate LAA absorption coefficients are required. HR was experimentally determined and apportioned (sources/species) in the EMEP/ACTRIS/COLOSSAL-2018 winter campaign in Milan (urban-background site). Two Aethalometers (AE31/AE33) were installed together with a MAAP, CPC, OPC, a low volume sampler (PM2.5) and radiation instruments. AE31/AE33 multiple-scattering correction factors (C) were determined using two reference systems for the absorption coefficient: 1) 5-wavelength PP_UniMI with low time resolution (12 h, applied to PM2.5 samples); 2) timely-resolved MAAP data at a single wavelength. Using wavelength- and time-independent C values for the AE31 and AE33 obtained with the same reference device, the total HR showed a consistency (i.e. reproducibility) with average values comparable at 95% probability. However, if different reference devices/approaches are used, i.e. MAAP is chosen as reference instead of a PP_UniMI, the HR can be overestimated by 23–30% factor (by both AE31/AE33). This became more evident focusing on HR apportionment: AE33 data (corrected by a wavelength- and time-independent C) showed higher HRFF (+24 ± 1%) and higher HRBC (+10 ± 1%) than that of AE31. Conversely, HRBB and HRBrC were −28 ± 1% and −29 ± 1% lower for AE33 compared to AE31. These inconsistencies were overcome by introducing a wavelength-dependent Cλ for both AE31 and AE33, or using multi-wavelength apportionment methods, highlighting the need for further studies on the influence of wavelength corrections for HR determination. Finally, the temporally-resolved determination of C resulted in a diurnal cycle of the HR not statistically different whatever the source- speciation- apportionment used.