Two-wavelength thermo-optical determination of Light Absorbing Carbon in atmospheric aerosols

Thermo-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 of several uncertainties and artefacts 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 behaviour 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 thermo-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 set-up, the unit has been modified at the Physics Department of the University of Genoa (IT), making possible the alternative use of the standard laser diode at \u3bb = 635 nm and of a new laser diode at \u3bb = 405 nm. In this way, the optical transmittance through the sample can be monitored at both the wavelengths. Since at shorter wavelengths the BrC absorbance is higher, a better sensitivity to this species is gained. The modified set-up also gives the possibility to quantify the BrC concentration in the sample at both the wavelengths. The new unit has been thoroughly tested, with both artificial and real-world samples: the first experiment, in conjunction with the Multi Wavelength Absorbance Analyzer (MWAA, Massab\uf2 et al., 2013 and 2015), resulted in the first direct determination of the BrC Mass Absorption Coefficient (MAC) at \u3bb = 405 nm: MAC = 23 \ub1 1 m2 g-1

Two-wavelength thermo-optical determination of Organic, Elemental and Brown Carbon.

Introduction Thermo-optical analysis (TOA) is widely adopted for the quantitative determination of Carbonaceous Aerosol in aerosol samples collected on quartz fibre filters. Nevertheless, the quantification of Elemental and Organic Carbon (EC and OC) presents several issues, as the well-known artefacts induced by the formation of pyrolytic carbon (Pyr-C) during the analysis. Furthermore, it is usually neglected the uncertainty due to the possible presence of Brown Carbon (BrC) i.e. the optically active fraction of OC produced by biomass burning and with thermic characteristics intermediate between OC and EC. Methods In (Massab\uf2 et al., 2019), a modified Sunset EC/OC Analyzer was introduced. Briefly, the unit was upgraded making possible the alternative use of a laser diode at \u3bb = 635 nm and at \u3bb = 405 nm. In this way, the optical transmittance through the sample can be monitored at both the wavelengths. Both BrC and Pyr-C absorbance increases at shorter wavelength, so the new set-up has a better sensitivity to these species. First results Massab\uf2 et al., 2019) suggested that the 2-lambda TOA could reduce the discrepancy usually observed between EC/OC quantification by the NIOSH and EUSAAR protocols (Cavalli et al., 2010). Adopting the methodology described in (Massab\uf2 et al., 2016), i.e. the coupled use of the Multi-Wavelength Absorbance Analyzer (MWAA) and of the Sunset EC/OC Analyzer, we therefore performed a new experiment based on a set of samples collected in a rural site. Half of the samples were analysed with the EUSAAR_2 and NIOSH protocols at both the wavelengths looking for OC, EC and BrC concentration values. The other sub-set was instead used to compare the TOA results on untreated and water-washed samples, again using both the laser diodes at \u3bb = 635 nm and \u3bb = 405 nm. The water-wash step removes the water-soluble compounds, which are expected to be the main responsible of the Pyr-C formation in TOA (Piazzalunga P. et al., 2011). Finally, all the samples were also analysed to quantify the Levoglucosan (1,6-Anhydro-betaglucopyranose) content. This step, performed by High Performance Anion Exchange Chromatography coupled with Pulsed Amperometric Detection(HPLC-PAD, more details in Piazzalunga A. et al., 2010), provided the quantification of this biomass burning tracer regardless of BrC thermo-optical properties. The BrC Mass Absorption Cross-section (MAC) at \u3bb = 405 nm and \u3bb = 635 nm was finally determined. Conclusions The data reduction of the described experiment is still in progress: the results will be presented and discussed at the Conference. This work has been partially financed by the National Institute of Nuclear Physics (INFN) in the frame of the TRACCIA experiments. Cavalli, F., Putaud, J. P., Viana, M., Yttri, K. E., & Gemberg, J. (2010). Toward a standardized thermal-optical protocol for measuring atmospheric Organic and Elemental Carbon: the EUSAAR protocol. Atmos. Meas. Tech., 3, 79-89. Massab\uf2, D., Altomari, A., Vernocchi, V., & Prati, P. (2019). Two-wavelength thermo-optical determination of Light Absorbing Carbon in atmospheric aerosols. Atmos. Meas. Tech. Discuss. Massab\uf2, D., Caponi, L., Bove, M. C., & Prati, P. (2016). Brown carbon and thermal-optical analysis: a correction based on optical multiwavelength apportionment of atmospheric aerosols. Atmos. Environ., 125, 119-125. Piazzalunga, A., Fermo, P., Bernardoni, V., Vecchi, R., Valli, G., & De Gregorio M. A. (2010). A simplified method for levoglucosan quantification in wintertime atmospheric particulate matter by high performance anion-exchange chromatography coupled with pulsed amperometric detection. Int. J. Environ. An. Ch., 90, 934-947. Piazzalunga, P., Bernardoni, V., Fermo, P., Valli, G., & Vecchi, R. (2011). On the effect of watersoluble compounds removal on EC quantification by TOT analysis in urban aerosol samples. Atmos. Chem. Phys., 11, 10193-10203

Characterization of soot produced by the mini inverted soot generator with an atmospheric simulation chamber

The performance of a mini inverted soot generator (MISG) has been investigated at ChAMBRe (Chamber for Aerosol Modelling and Bio-aerosol Research) by studying the properties of soot particles generated by ethylene and propane combustion. This work deepens and expands the existing characterization of the MISG, which also exploits an atmospheric simulation chamber (ASC). Different from previous works, MISG performance has been also tested at different fuel flows and higher global equivalence ratios. MISG exhausts were investigated after their injection inside the atmospheric simulation chamber, which is another novelty of this work. Starting from an extensive classification of combustion conditions and resulting flame shapes, the MISG exhaust was characterized in terms of concentration of emitted particles and gases, particle size distribution, and optical properties. Soot particles were also collected on quartz fibre filters and then analysed by optical and thermal\u2013optical techniques to measure the spectral dependence of the absorption coefficient babs and their composition in terms of elemental carbon and organic carbon (EC and OC). Significant differences could be observed when the MISG was fuelled with ethylene and propane in terms of particle size. In particular, the production of super-micrometric aggregates was observed for ethylene combustion. With equal combustion conditions, ethylene produced a higher number concentration of particles and smaller mode diameters. Soot particles produced by propane combustion resulted in higher EC : TC (total carbon) ratios and they were more light absorbing than particles generated by ethylene combustion. Values of the mass absorption cross section (MAC) and of the \uc5ngstr\uf6m absorption exponent (AAE) turned out to be compatible with the literature, even if there were some specific differences. The comprehensive characterization of the MISG soot particles is an important piece of information to design and perform experiments in atmospheric simulation chambers. Particles with well-known properties can be used, for example, to investigate the possible interactions between soot and other atmospheric pollutants, the effects of meteorological variables on soot properties, and the oxidative and toxicological potential of soot particles

Determinazione termo-ottica a pi\uf9 lunghezze d'onda di Elemental, Organic e Brown Carbon

Gli aerosol carboniosi sono largamente presenti in atmosfera ma ancora poco conosciuti. La loro caratterizzazione si basa frequentemente su tecniche termo-ottiche, non esenti da artefatti e sistematici poco compresi. Presentiamo una nuova metodologia basata su uno strumento commerciale (Sunset EC/OC Analyzer) che \ue8 stato modificato rendendo possibile l\u2019analisi a pi\uf9 lunghezze d\u2019onda. Parte integrante dello sviluppo \ue8 un nuovo approccio all\u2019analisi dei dati strumentali. Si ottiene cos\uec una migliore conoscenza delle frazioni in cui viene solitamente diviso l\u2019aerosol carbonioso: Carbonio Elementale (o \u201cBlack\u201d) ed Organico ivi incluso il cosiddetto Brown Carbon, che \ue8 solitamente trascurato nelle analisi termo-ottiche

Mountain glaciers darkening: geochemical characterizazion of cryoconites and their radiative impact on the Vadret da Morteratsch (Swiss Alps)

Mountain glaciers represent an important source of fresh water across the globe. It is well known that these reservoirs are seriously threatened by global climate change, and a widespread reduction of glacier extension has been observed in recent years. Surface processes that promote ice melting are driven both by air temperature/precipitation and surface albedo. This latter is mainly influenced by the growth of snow grains and by the impurities content (such as mineral dust, soot, ash etc.). The origin of these light-absorbing impurities can be local or distal, and often, as a consequence of melting processes, they can aggregate on the glacier tongue, forming characteristics cryoconites, that decrease ice albedo and hence promote the melting. In this contribution, we coupled satellite images (EO1 \u2013 Hyperion and Landsat 8 - OLI) and ground hyperspectral data (ASD field spectrometer) for characterizing ice and snow surface reflectance of the Vadret da Morteratsch glacier (Swiss Alps). On the glacier ablation zone, we sampled ice, snow, surface dust and cryoconite material. To evaluate the possible impact of anthropogenic and natural emissions on cryoconites formation, we determined their geochemical composition (through the Neutron Activation Analysis, NAA) and the concentration of Black Carbon (BC), Organic Carbon (OC), Elemental Carbon (EC) and Levoglucosan. From satellite data, we computed the Snow Darkening Index (SDI), which is non-linearly correlated with dust content in snow. Results showed that, during 2015 summer season, ice albedo in the ablation zone reached very low values of about 0.1-0.2. The darkening of the glacier can be attributed to the impact of surface dust (from lateral moraine and Saharan desert) and cryoconites, coupled with grain growth driven by the extremely warm 2015 summer. The geochemical characterization of non-ice material contained in the cryoconites can provide important information regarding their source and the possible impact of anthropogenic emissions on cryoconites formation and evolution

ChAMBRe \u2013 the development of an atmosferic simulation chamber for bioaerosol studies and aerosol optical properties investigation

Environmental simulation chambers are small to largescale facilities where atmospheric conditions can be monitored in real-time under control to reproduce realistic environments and to study interactions among their constituents. Up to now, they have been used mainly to study chemical and photochemical processes that occur in the atmosphere, but the high versatility of these facilities allows for a wider application covering all fields of atmospheric aerosol science. ChAMBRe (Chamber for Aerosol Modelling and Bioaerosol Research) is the stainless steel atmospheric simulation chamber (volume approximately 3 m3, see Figure 1) recently installed at the National Institute of Nuclear Physics in Genoa (INFN-Genova) in collaboration with the Environmental Physics Laboratory at the Physics Department of Genoa University (www.labfisa.ge.infn.it). The scientific activities at ChAMBRe focus on the following topics: 1) Bioaerosol properties A strong improvement in the understanding of bioaerosol behaviour can be provided by atmospheric chamber experiments, that allow for a scientific intermediate approach between \u201cin vitro\u201d and \u201cin vivo\u201d analysis. Aerosol with realistic composition, including living micro-organisms, can be injected in artificial environments with controlled physical and chemical parameters and then accurately analyzed. In particular, a systematic approach can be used for a better description of micro-organisms viability, of colonies growing modulation and other issues relevant to their spread and their pathogenicity. Very promising results in this direction were obtained by the authors at the CESAM facility at CNRS-LISA (Brotto et al. 2015), while similar results were obtained nearly at the same time at AIDA chamber at KIT (Amato et al. 2015). ChAMBRe experiments are carrying on this path to contribute in getting a deeper understanding of the still unclear mechanisms that control the evolution of bioaerosols in atmosphere and in particular of their bacterial components. 2) Aerosol optical properties \u2013 methodologies and instruments testing The instrumental development efforts at the Environmental Physics Laboratory of the University of Genoa, recently resulted in a new Multi Wavelength Absorbance Analyser (Massab\uf2 et al. 2015) which measure the light absorption on aerosol loaded filters at five wavelengths from UV (absorption bands of organic compounds, mineral dust) to near infrared (carbon soot,\u2026). Furthermore, a new data reduction methodology has been introduced to disentangle the concentration of Black and Brown carbon in atmospheric aerosol, demonstrating the need to mitigate not only exhaust but also non-exhaust emissions, as a potentially important source of PM10. The atmospheric chamber is an effective tool to produce known aerosol mixtures and to test the performance of the optical technology. Actually, there is an on-going collaboration with the CNRS-LISA team working at CESAM following that procedure that will be soon replicated at ChAMBRe facility. ChAMBRe has recently joined the Eurochamp consortium, the European atmospheric chamber facilities network. The network activities have been included in an infrastructure-oriented research project proposal that is going to be submitted within March 2016 to the H2020-INFRAIA call within EU Horizon 2020 Programme. We would like to acknowledge prof. J.F. Doussin and LISA laboratories (http://www.lisa.univ-paris12.fr/en) for providing us part of the chamber structure and for the very useful and fruitful technical discussions

High time-resolved measurements of fine aerosol (PM2.5) in a hot-spot area during wintertime: multi-wavelength optical absorption properties and source apportionment

Black Carbon (BC) is the main absorber of solar radiation among the aerosol components, it influences cloud processes, and alters the melting of snow and ice cover. Although it is one of the most important individual climate-warming components, uncertainties on the radiative forcing related to BC-radiation interaction still cover more than one order of magnitude. Moreover, weakly absorbing organic material (brown carbon, BrC) in the form of particle coating or as particle as-is can be considered a further important contributor to aerosol absorption. The peculiarity of BrC is that it is very effective in the absorption of short-\u3bb radiation whereas its contribution to aerosol absorption is negligible in the red or near-IR bands. It is noteworthy that BC and BrC can also be used for source apportionment purposes (e.g. they can be helpful for the discrimination between fossil fuels combustion vs. biomass burning). Thus, aerosol absorption properties possibly related to mixing and/or size information, and BC content are currently of great interest. Moving in this frame, a multi-\u3bb polar photometer (PP_UniMI) has been developed at the Department of Physics of the University of Milan in the last years. The instrument is based on the measurement on the scattering plane of the light transmitted and scattered in the forward and back hemispheres by unloaded and loaded samples using a rotating photodiode. Data reduction aiming at the determination of the sample absorbance follows Petzold et al. (2004) and therein cited literature. Currently, PP_UNIMI allows performing 4-\u3bb measurements (870, 633, 532, 405 nm) on aerosol collected on different substrates, including aerosol collected with high-time resolution using a streaker sampler. Such sampler collects aerosol segregated in two size-classes (fine and coarse) on a rotating frame with hourly resolution. The set-up of the instrument was validated against independent measurements carried out using a Multi-Angle Absorption Photometer for what concerns the red-light results, considering possible artefact effects shown in Vecchi et al. (2013). The results presented here are related to the analysis of the high time-resolved trends of multiwavelength aerosol absorption properties measured on the fine aerosol fraction during a field campaign performed in Milan (Italy) in November 2015 (see an example in Figure 1). Such data will be used to test the possibility of applying source apportionment models based on optical properties (es. Aethalometer model) using off-line hightime resolved data. It is also noteworthy that equivalent BC can be quantified from the polar photometer measurements at 635 nm using a suitable mass absorption coefficient. Such information will be joined to the elemental components (Na-Pb) detected by Particle-Induced X-ray Emission technique carried out at the INFN-LABEC in Florence to perform receptor modelling analysis (e.g. Positive Matrix Factorization). The results of the source apportionment using such data will be also presented

The effect of Brown Carbon on thermal-optical analysis: a correction based on optical multi-wavelength analysis

Carbonaceous aerosol (CA) plays an important role in environmental issues like air quality, human health and global climate change. It mainly consists of organic carbon (OC) and elemental carbon (EC) although a minor fraction of carbonate carbon could be also present. Thermal-optical methods are presently the most widespread approach to OC/EC speciation. Despite their popularity, there is still a disagreement among the results, especially for what concerns EC as differentthermal protocols can be currently used. The main hypothesis at the basis of the technique is that on their different optical properties: while EC is strongly light absorbing, OC is generally transparent in the visible range. However, another fraction of light-absorbing carbon exists which is not black and it is generally called brown carbon (BrC) (Andreae and Gelencs\ue9r, 2006). We introduced a new way to apportion the absorption coefficient (babs) of carbonaceous atmospheric aerosols starting from a multi-wavelength optical analysis (Massab\uf2 et al., 2015). This analysis was performed thanks to the MWAA, an instrument developed at the Physics Department of University of Genoa (Massab\uf2 et al., 2013). The method uses the information gathered at five different wavelengths in a renewed and upgraded version of the approach usually referred to as Aethalometer model (Sandradewi et al., 2008). Moreover, with some assumptions, also the quantification of OC coming from fossil fuels and wood burning can be obtained. Thermal-optical methods are presently the most widespread approach to OC/EC speciation. Despite their popularity, there is still a disagreement among the results, especially for what concerns EC as different thermal protocols can be used. In fact, the pyrolysis occurring during the analysis can heavily affect OC/EC separation, depending on PM composition in addition to the used protocol. Furthermore, the presence in the sample of BrC can shift the split point since it is light absorbing also @ 635nm, the typical laser wavelength used in this technique (Chen et al., 2015). We have recently introduced a new possibility, based on the apportionment of the absorption coefficient of particleloaded filters, for correcting the thermo-optical analysis of PM samples (Massab\uf2 et al, 2016). We present here the results of an apportionment study of carbonaceous aerosol sources performed in an Alpine area, validated against independent measurements of levoglucosan. We also present developments of the thermo-optical analysis correction (Massab\uf2 et al., 2016) which lead to a better homogeneity between the results obtained with different thermal protocols

Cryoconite as a temporary sink for anthropogenic species stored in glaciers

Cryoconite, the typical sediment found on the surface of glaciers, is mainly known in relation to its role in glacial microbiology and in altering the glacier albedo. But if these aspects are relatively well addressed, the same cannot be said about the geochemical properties of cryoconite and the possible interactions with glacial and peri-glacial environment. Current glacier retreat is responsible for the secondary emission of species deposited in high-altitude regions in the last decades. The role played by cryoconite in relation to such novel geochemical fluxes is largely unknown. Few and scarce observations suggest that it could interact with these processes, accumulating specific substances, but why, how and to what extent remain open questions. Through a multi-disciplinary approach we tried to shed lights. Results reveal that the peculiar composition of cryoconite is responsible for an extreme accumulation capability of this sediment, in particular for some, specific, anthropogenic substances

Multi-wavelength aerosol light absorption measurements in the Amazon rainforest

The most important light-absorbing aerosol is black carbon (BC), which is emitted by incomplete combustion of fossil fuels and biomass. BC is considered the second anthropogenic contributor to global warming. Beyond BC, other aerosols like some organics, dust, and primary biological aerosol particles are able to absorb radiation. In contrast to BC, the light absorption coefficient of these aerosols is wavelength dependent. Therefore, multi-wavelength measurements become important in environments where BC is not the predominant light-absorbing aerosol like in the Amazon. The Amazon Tall Tower Observatory (ATTO) site is located in the remote Amazon rainforest, one of the most pristine continental sites in the world during the wet season. In the dry season, winds coming from the southern hemisphere are loaded with biomass burning aerosol particles originated by farming-related deforestation. BC and aerosol number concentration data from the last two years indicate this is the most polluted period. Two different techniques have been implemented to measure the light absorption at different wavelengths; one of them is the 7-wavelengths Aethalometer, model AE30, an instrument that measures the light attenuation on a filter substrate and requires multiple scattering and filter-loading corrections to retrieve the light absorption coefficient. The other method is an offline technique, the Multi-Wavelength Absorbance Analysis (MWAA), which is able to measure reflectance and absorbance by aerosols collected on a filter and, by means of a radiative model, can retrieve the light absorption coefficient. Filters collected during May-September 2014, comprehending wet-to-dry transition and most of the dry season, were analyzed. The results indicate that the Absorption \uc5ngstr\uf6m Exponent (AAE), a parameter that is directly proportional to the wavelength dependence of the aerosol light absorption, is close to 1.0 during the transition period and slightly decreases in the beginning of the dry season. However, during strong biomass burning episodes in the dry season, the AAE increases significantly, and reaches values higher than 1.3, indicating the presence of wavelength dependent light-absorbing aerosols like organics (brown carbon). The present study is a contribution to the understanding of the optical properties of light-absorbing aerosol particles under pristine and biomass-burning conditions