ALBERT

Description: We characterize the atmospheric nondust aerosol having the strongest spectral dependence of light absorption (as indicated by the Absorption Angstrom Exponent, AAE) at visible wavelengths in the urban Po Valley. In situ ground measurements of aerosol spectral optical properties, PM1 chemical composition (HR-ToF-AMS), and coarse and fine size distributions, were carried out in Bologna, and data statistically analysed. Findings prove that a "brown" aerosol (AAE from 2.5 to 6) in the ambient atmosphere is composed by "droplet" mode particles enriched in aged organic aerosol (OA) and nitrate. We provide a comprehensive physico-chemical characterisation of this brown aerosol, including its spectral optical signature, and possible sources. To our knowledge, no previous work has considered these issues in the ambient atmosphere. We compared to literature to put findings in a broader perspective. There is consistency with recent "diluted" urban observations (airborne, and AERONET), and combustion chamber observations. Our study adds to these previous ones that the high AAE values featuring the "brown" aerosol depend on the OA to Black Carbon (BC) ratio more than on OA, and that the link between AAE and OA-to-BC (already observed for freshly emitted primary aerosols from biomass burning) does exist in the ambient atmosphere for this aged "brown" aerosol, as well. The comparison with studies on the composition evolution of OA in the atmosphere strengthens the result that this "brown" aerosol is an aged OA, and provides experimental evidence for the aged "brown" OA formation in the ambient atmosphere. Findings will have important atmospheric implications for modeling studies, and remote sensing observations, as regards the parametrization and identification of Brown OA, and Brown Carbon in the atmosphere.

Description: The interaction of aerosol with atmospheric water affects the processing and wet removal of atmospheric particles. Understanding such interaction is mandatory to improve model description of aerosol lifetime and ageing. We analyzed the aerosol-water interaction at high relative humidity during fog events in the Po Valley, in the framework of the ARPA-ER Supersite project. For the first time in this area, the changes in particle chemical composition caused by fog are discussed along with changes in particle microphysics. During the experiment, 14 fog events were observed. The average mass scavenging efficiency was 70% for nitrate, 68% for ammonium, 61% for sulfate, 50% for organics, and 39% for black carbon. After fog formation, the interstitial aerosol was dominated by particles smaller than 200 nm Dva (vacuum aerodynamic diameter) and enriched in carbonaceous aerosol, mainly black carbon and water insoluble organic aerosol (WIOA). For each fog event, the size segregated scavenging efficiency of nitrate and organic aerosol (OA) was calculated by comparing chemical species size distribution before and after fog formation. For both nitrate and OA, the size segregated scavenging efficiency followed a sigmoidal curve, with values close to zero below 100 nm Dva and close to 1 above 700 nm Dva. OA was able to affect scavenging efficiency of nitrate in particles smaller than 300 nm Dva. A linear correlation between nitrate scavenging and particle hygroscopicity (κ) was observed, indicating that 44–51% of the variability of nitrate scavenging in smaller particles (below 300 nm Dva) was explained by changes in particle chemical composition. The size segregated scavenging curves of OA followed those of nitrate, suggesting that organic scavenging was controlled by mixing with water-soluble species. In particular, functional group composition and OA elemental analysis indicated that more oxidized OA was scavenged more efficiently than less oxidized OA. Nevertheless, the small variability of organic functional group composition during the experiment did not allow us to discriminate the effect of different organic functionalities on OA scavenging.

Description: High resolution aerosol mass spectrometer measurements have been performed, for the first time, at the Mt. Cimone Global Atmospheric Watch (GAW) station between June and July 2012, within the EU project PEGASOS and the ARPA–Emilia Romagna project SUPERSITO. Sub-micron aerosol was dominated by organics (63%) with sulphate, ammonium and nitrate contributing for the remaining 20, 9 and 7%, respectively. Organic aerosol (OA) was in general highly oxygenated, consistent with the remote character of the site; our observations suggest that oxidation and secondary organic aerosol (SOA) formation processes occurred during aerosol transport to high altitudes. All of the aerosol component concentrations as well as the OA elemental ratios showed a clear daily trend, driven by the evolution of the planetary boundary layer (PBL) and by the mountain wind regime. Higher loadings and lower OA oxidation levels were observed during the day, when the site was within the PBL, and therefore affected by relatively fresh aerosol transported from lower altitudes. Conversely, lower loadings and higher OA oxidation levels were observed at night, when the top of Mt. Cimone resided in the free troposphere although affected by the transport of residual layers on several days of the campaign. Analysis of the elemental ratios in a Van Krevelen space shows that OA oxidation follows a slope comprised between −0.5 and −1, consistent with addition of carboxylic groups, with or without fragmentation of the parent molecules. The increase of carboxylic groups during OA ageing is confirmed by the increased contribution of organic fragments containing more than one oxygen atom in the free troposphere night-time mass spectra. Finally, positive matrix factorization was able to deconvolve the contributions of relatively fresh OA (OOAa) originating from the PBL, more aged OA (OOAb) present at high altitudes during periods of atmospheric stagnation, and very aged aerosols (OOAc) transported over long distances in the free troposphere.

Description: The interaction of aerosol with atmospheric water affects the processing and wet removal of atmospheric particles. Understanding such interaction is mandatory to improve model description of aerosol lifetime and ageing. We analyzed the aerosol–water interaction at high relative humidity during fog events in the Po Valley within the framework of the Agenzia Regionale per la Prevenzione e l'Ambiente (ARPA) – Emilia Romagna supersite project. For the first time in this area, the changes in particle chemical composition caused by fog are discussed along with changes in particle microphysics. During the experiment, 14 fog events were observed. The average mass scavenging efficiency was 70% for nitrate, 68% for ammonium, 61% for sulfate, 50% for organics, and 39% for black carbon. After fog formation, the interstitial aerosol was dominated by particles smaller than 200 nm Dva (vacuum aerodynamic diameter) and enriched in carbonaceous aerosol, mainly black carbon and water-insoluble organic aerosol. For each fog event, the size-segregated scavenging efficiency of nitrate and organic aerosol (OA) was calculated by comparing chemical species size distribution before and after fog formation. For both nitrate and OA, the size-segregated scavenging efficiency followed a sigmoidal curve, with values close to zero below 100 nm Dva and close to 1 above 700 nm Dva. OA was able to affect scavenging efficiency of nitrate in particles smaller than 300 nm Dva. A linear correlation between nitrate scavenging and particle hygroscopicity (κ) was observed, indicating that 44–51% of the variability of nitrate scavenging in smaller particles (below 300 nm Dva) was explained by changes in particle chemical composition. The size-segregated scavenging curves of OA followed those of nitrate, suggesting that organic scavenging was controlled by mixing with water-soluble species. In particular, functional group composition and OA elemental analysis indicated that more oxidized OA was scavenged more efficiently than less oxidized OA. Nevertheless, the small variability of organic functional group composition during the experiment did not allow us to discriminate the effect of different organic functionalities on OA scavenging.

Description: High-resolution aerosol mass spectrometer measurements were performed, for the first time, at the Mt. Cimone Global Atmosphere Watch (GAW) station between June and July 2012, within the EU project PEGASOS and the ARPA–Emilia-Romagna project SUPERSITO. Submicron aerosol was dominated by organics (63 %), with sulfate, ammonium and nitrate contributing the remaining 20, 9 and 7 %, respectively. Organic aerosol (OA) was in general highly oxygenated, consistent with the remote character of the site; our observations suggest that oxidation and secondary organic aerosol (SOA) formation processes occurred during aerosol transport to high altitudes. All of the aerosol component concentrations as well as the OA elemental ratios showed a clear daily trend, driven by the evolution of the planetary boundary layer (PBL) and by the mountain wind regime. Higher loadings and lower OA oxidation levels were observed during the day, when the site was within the PBL, and therefore affected by relatively fresh aerosol transported from lower altitudes. Conversely, lower loadings and higher OA oxidation levels were observed at night, when the top of Mt. Cimone resided in the free troposphere although affected by the transport of residual layers on several days of the campaign. Analysis of the elemental ratios in a Van Krevelen space shows that OA oxidation follows a slope comprised between −0.5 and −1, consistent with addition of carboxylic groups, with or without fragmentation of the parent molecules. The increase of carboxylic groups during OA ageing is confirmed by the increased contribution of organic fragments containing more than one oxygen atom in the free troposphere night-time mass spectra. Finally, positive matrix factorization was able to deconvolve the contributions of relatively fresh OA (OOAa) originating from the PBL, more aged OA (OOAb) present at high altitudes during periods of atmospheric stagnation, and very aged aerosols (OOAc) transported over long distances in the free troposphere.