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Carbonaceous soot particles are formed during incomplete combustion of fossil fuels, biofuels or biomass and are considered to be a significant proportion of aerosol emission, especially in polluted areas, and to contain light-absorbing carbon fractions. The light-absorbing carbon components make these particles exhibit positive radiative forcing and thus they contribute to atmospheric warming processes. The exact contribution to this process however still has significant uncertainties. One of the sources of uncertainty is related to the capacity to accurately describe soot spectral optical properties (MAC/MSC/MEC - mass absorption/scattering/extinction cross-sections in m2g-1; CRI - complex refractive index), due to significant uncertainties for key measurements like the absorption coefficient or absorbing mass fraction. A second source is considered to be the change of the optical properties with the variable physico-chemical state of soot (e.g. chemical composition, morphology, primary particle size, aggregate size distribution, coating and mixing state) which depends on (i) the combustion conditions/sources and is (ii) known to change during atmospheric lifetime due to ageing and mixing processes.The present work aims at providing new measurements of soot spectral optical properties and investigating their dependence on particles’ physico-chemical state and the role of measurement uncertainties. For this, a set of original experiments was performed in the 4.3m3 CESAM atmospheric simulation chamber (https://cesam.cnrs.fr/) on soot aerosols generated by a commercial propane diffusion flame soot generator (miniCAST model 6204 TYPE C, JING). In these experiments, the variability of soot properties due to (i) generation and (ii) atmospheric ageing was explored. Different combustion conditions going from fuel–leaner to fuel-richer were set to produce soot aerosol with different effective densities, EC/TC-ratios ranging from 0.8 ± 0.1 to 0.0 ± 0.1 and size distributions with median diameters between 30 and 120 nm. Selected aerosols were subjected to ageing in a N2/O2 atmosphere under simulated atmospheric conditions (humid, with/without illumination, up to 26 hours lifetime). In these conditions, physical and chemical ageing, under the presence of different gaseous phases (O3, SO2) and addition of a second aerosol phase produced by photo-oxidation of SO2 or the ozonolysis of α-pinene, led to changes in the physico-chemical properties of the soot.State-of-the-art techniques were used to generate an extensive dataset of physico-chemical parameters (mass concentration, morphology, effective density, composition, size distribution) and spectral optical properties (absorption, scattering, extinction coefficients) for different soot aerosols and different ageing states of these. The absorption coefficient in particular was measured by both filter-based (AE - Aethalometer, MAAP - Multi-Angle Absorption Photometer, MWAA – Multi-Wavelength Absorbance Analyzer, PP_UniMI - Polar Photometer by University of Milano) and extinction minus scattering techniques. The MAC, MSC and MEC datasets, retrieved by combining these measurements, will be presented for the ensemble of chamber experiments and the variability of these parameters in link with variations in the particles’ physico-chemical properties will be discussed together with key relevant uncertainties.

Bio-aerosols consist of airborne particles such as pollens, fungi, bacteria, viruses and debris from biological matter, such as metabolites and toxins, that are present ubiquitously in the environment. In last decades, the interest in bioaerosols has increased rapidly to broaden the pool of knowledge on their identification, quantification, distribution, but also to understand how bioaerosols impact human health in both indoor and outdoor settings. Experiments conducted inside confined artificial environments, such as the Atmospheric Simulations Chambers (ASCs), where atmospheric conditions and composition are controlled, can provide valuable information on bio-aerosol viability, dispersion, and impact. At ChAMBRe (Chamber for Aerosol Modelling and Bio-aerosol Research), a 2.3 m3 stainless steel ASC, managed by INFN at the Physics Department of the University of Genoa, Italy, the research on bioaerosol is focused on the investigation of the airborne bacteria behavior in different atmospheric conditions (Massabò et al., AMT, 2018). Our experiments were performed with two types of bacteria, Bacillus Subtilis and Escherichia Coli evolutionarily divergent model organisms, Gram-negative and Gram-positive respectively, to compare cellular viability by varying of the atmospheric conditions in the simulation chambre. A great effort has been put in the assessment of an experimental protocol which includes bacteria cultivation, injection in the chamber of viable cells, exposure to peculiar environmental conditions and final verification of the loss/gain in viability. With first experiments, bacteria were exposed to high concentration of characterized soot particles (Black carbon ~ 600 μg m-3) and NOx exhausts, both produced by propane combustion (Vernocchi et al., 2021). The effects of NOx only were also separately investigated, in another set of experiments, in which different concentration of NOx were tested. Evidence of an impact of NOx concentration on the viability of the Gram-negative Bacillus Subtilis were observed and will be fully reported at the conference.

Thermal–optical analysis is widely adopted for the quantitative determination of total (TC), organic (OC), and elemental (EC) carbon in atmospheric aerosol sampled by suitable filters. Nevertheless, the methodology suffers from several uncertainties and artifacts such as the well-known issue of charring affecting the OC–EC separation. In the standard approach, the effect of the possible presence of brown carbon, BrC, in the sample is neglected. BrC is a fraction of OC, usually produced by biomass burning with a thermic behavior intermediate between OC and EC. BrC is optically active: it shows an increasing absorbance when the wavelength moves to the blue–UV region of the electromagnetic spectrum. Definitively, the thermal–optical characterization of carbonaceous aerosol should be reconsidered to address the possible BrC content in the sample under analysis. We introduce here a modified Sunset Lab Inc. EC–OC analyzer. Starting from a standard commercial instrument, the unit has been modified at the physics department of the University of Genoa (Italy), making possible the alternative use of the standard laser diode at λ=635 nm and of a new laser diode at λ=405 nm. In this way, the optical transmittance through the sample can be monitored at both wavelengths. Since at shorter wavelengths the BrC absorbance is higher, a better sensitivity to this species is gained. The modified instrument also gives the possibility to quantify the BrC concentration in the sample at both wavelengths. The new unit has been thoroughly tested, with both artificial and real-world aerosol samples: the first experiment, in conjunction with the multi-wavelength absorbance analyzer (MWAA; Massabò et al., 2013, 2015), resulted in the first direct determination of the BrC mass absorption coefficient (MAC) at λ=405 nm: MAC =23±1 m2 g−1.

Accounting for the wavelength- and source-dependent optical absorption properties of the abundant light-absorbing organic (brown) carbon (BrC) and the mixing state of atmospheric black carbon (BC) are essential to reduce the large uncertainty in aerosol radiative forcing. Estimation of BrC absorption online by subtraction is highly uncertain and may be biased if not decoupled from the potential BC absorption enhancement (lensing) due to non-refractory (organic and inorganic) coating acquisition.Here, the reported total particulate absorption is based on long-term, filter-based seven-wavelength Aethalometer (AE33 model) data, corrected for multiple scattering effects with Multi-Wavelength Absorbance Analyzer (5λ MWAA) measurements. Using ultraviolet-visible spectroscopy absorbance measurements along with particle size distributions obtained by a scanning mobility particle sizer, we have conducted Mie calculations to assess the importance of source-specific extractable particulate BrC (Moschos et al., 2018) versus BC absorption.For the species-specific optical closure, the wavelength dependence of bare BC absorption is estimated using MWAA measurements upon successive filter extractions to remove the influence of BrC/coatings. The lensing contribution, supported by observations from field-emission scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, is estimated at longer wavelengths using a refined proxy for the BC coating thickness. The approach is validated independently by applying a novel positive matrix factorization-based approach on the calibrated total AE33 absorption data.Based on the observational constraints established in this study, we demonstrate for various distinct case studies that the interplay between lensing and BrC absorption results in lower than expected BC absorption at shorter wavelengths. This indicates that the volume additivity assumption is not valid for particulate absorption by internally mixed heterogeneous atmospheric aerosol populations. These comprehensive experimental analyses verify the BC lensing suppression predicted for simplified core-shell structures containing moderately absorbing BrC (Lack & Cappa, 2010). The implications discussed in this work are relevant for co-emitted species from biomass burning or aged plumes with high BrC to BC mass/absorption ratio. ReferencesMoschos, V., Kumar, N. K., Daellenbach, K. R., Baltensperger, U., Prévôt, A. S. H., and El Haddad, I.: Source apportionment of brown carbon absorption by coupling ultraviolet-visible spectroscopy with aerosol mass spectrometry, Environ. Sci. Tech. Lett., 5, 302-308, https://doi.org/10.1021/acs.estlett.8b00118, 2018.Lack, D. A. and Cappa, C. D.: Impact of brown and clear carbon on light absorption enhancement, single scatter albedo and absorption wavelength dependence of black carbon, Atmos. Chem. Phys., 10, 4207–4220, https://doi.org/10.5194/acp-10-4207-2010, 2010.

This paper presents new laboratory measurements of the mass absorption efficiency (MAE) between 375 and 850 nm for 12 individual samples of mineral dust from different source areas worldwide and in two size classes: PM10. 6 (mass fraction of particles of aerodynamic diameter lower than 10.6 µm) and PM2. 5 (mass fraction of particles of aerodynamic diameter lower than 2.5 µm). The experiments were performed in the CESAM simulation chamber using mineral dust generated from natural parent soils and included optical and gravimetric analyses. The results show that the MAE values are lower for the PM10. 6 mass fraction (range 37–135 × 10−3 m2 g−1 at 375 nm) than for the PM2. 5 (range 95–711 × 10−3 m2 g−1 at 375 nm) and decrease with increasing wavelength as λ−AAE, where the Ångström absorption exponent (AAE) averages between 3.3 and 3.5, regardless of size. The size independence of AAE suggests that, for a given size distribution, the dust composition did not vary with size for this set of samples. Because of its high atmospheric concentration, light absorption by mineral dust can be competitive with black and brown carbon even during atmospheric transport over heavy polluted regions, when dust concentrations are significantly lower than at emission. The AAE values of mineral dust are higher than for black carbon (∼ 1) but in the same range as light-absorbing organic (brown) carbon. As a result, depending on the environment, there can be some ambiguity in apportioning the aerosol absorption optical depth (AAOD) based on spectral dependence, which is relevant to the development of remote sensing of light-absorbing aerosols and their assimilation in climate models. We suggest that the sample-to-sample variability in our dataset of MAE values is related to regional differences in the mineralogical composition of the parent soils. Particularly in the PM2. 5 fraction, we found a strong linear correlation between the dust light-absorption properties and elemental iron rather than the iron oxide fraction, which could ease the application and the validation of climate models that now start to include the representation of the dust composition, as well as for remote sensing of dust absorption in the UV–vis spectral region.

Cryoconite is extremely rich in natural and artificial radionuclides, but a comprehensive discussion about its ability to accumulate radioactivity is lacking. A characterization of cryoconite from two Alpine glaciers is presented and discussed. Results confirm that cryoconite is among the most radioactive environmental matrices, with activity concentrations exceeding 10,000 Bq kg−1 for single radionuclides. Atomic and activity ratios of Pu and Cs radioactive isotopes reveal that the artificial radioactivity of Alpine cryoconite is mostly related to the stratospheric fallout from nuclear weapon tests and to the 1986 Chernobyl accidents. The signature of cryoconite radioactivity is thus influenced by both local and more widespread events. The extreme accumulation of radioactivity in cryoconite can be explained only considering the glacial environment as a whole, and particularly the interaction between ice, meltwater, cryoconite and atmospheric deposition. Cryoconite is an ideal monitor to investigate the deposition and occurrence of natural and artificial radioactive species in glacial environment.

This paper presents new laboratory measurements of the mass absorption efficiency (MAE) between 375 and 850 nm for mineral dust of different origin in two size classes: PM10.6 (mass fraction of particles of aerodynamic diameter lower than 10.6 µm) and PM2.5 (mass fraction of particles of aerodynamic diameter lower than 2.5 µm). Experiments have been performed in the CESAM simulation chamber using generated mineral dust from natural parent soils, and optical and gravimetric analyses. Results show that the MAE values are lower for the PM10.6 mass fraction (range 37–135 × 10−3 m2 g−1 at 375 nm) than for the PM2.5 (range 95–711 × 10−3 m2 g−1 at 375 nm), and decrease with increasing wavelength as λ-AAE, where Angstrom Absorption Exponent (AAE) averages between 3.3–3.5, regardless of size. The size-independence of AAE suggests that, for a given size distribution, the possible variation of dust composition with size would not affect significantly the spectral behavior of shortwave absorption. Because of its high atmospheric concentration, light-absorption by mineral dust can be competitive to black and brown carbon even during atmospheric transport over heavy polluted regions, when dust concentrations are significantly lower than at emission. The AAE values of mineral dust are higher than for black carbon (~ 1), but in the same range as light-absorbing organic (brown) carbon. As a result, depending on the environment, there can be some ambiguity in apportioning the AAOD based on spectral dependence, which is relevant to the development of remote sensing of light-absorption aerosols from space, and their assimilation in climate models. We suggest that the sample-to-sample variability in our dataset of MAE values is related to regional differences of the mineralogical composition of the parent soils. Particularly in the PM2.5 fraction, we found a strong linear correlation between the dust light-absorption properties and elemental iron rather than the iron oxide fraction, which could ease the application and the validation of climate models that now start to include the representation of the dust composition, as well as for remote sensing of dust absorption in the UV-VIS spectral region.