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Biomass burning is a major source of atmospheric brown carbon (BrC), and through its absorption of UV/VIS radiation, it can play an important role in the planetary radiative balance and atmospheric photochemistry. The considerable uncertainty of BrC impacts is associated with its poorly constrained sources, transformations, and atmospheric lifetime. Here we report laboratory experiments that examined changes in the optical properties of the water-soluble (WS) BrC fraction of laboratory-generated biomass burning particles from hardwood pyrolysis. Effects of direct UVB photolysis and OH oxidation in the aqueous phase on molecular-weight-separated BrC were studied. Results indicated that the majority of low-molecular-weight (MW) BrC (<400 Da) was rapidly photobleached by both direct photolysis and OH oxidation on an atmospheric timescale of approximately 1 h. High MW BrC (≥400 Da) underwent initial photoenhancement up to ∼15 h, followed by slow photobleaching over ∼10 h. The laboratory experiments were supported by observations from ambient BrC samples that were collected during the fire seasons in Greece. These samples, containing freshly emitted to aged biomass burning aerosol, were analyzed for both water- and methanol-soluble BrC. Consistent with the laboratory experiments, high-MW BrC dominated the total light absorption at 365 nm for both methanol and water-soluble fractions of ambient samples with atmospheric transport times of 1 to 68 h. These ambient observations indicate that overall, biomass burning BrC across all molecular weights has an atmospheric lifetime of 15 to 28 h, consistent with estimates from previous field studies – although the BrC associated with the high-MW fraction remains relatively stable and is responsible for light absorption properties of BrC throughout most of its atmospheric lifetime. For ambient samples of aged (>10 h) biomass burning emissions, poor linear correlations were found between light absorptivity and levoglucosan, consistent with other studies suggesting a short atmospheric lifetime for levoglucosan. However, a much stronger correlation between light absorptivity and total hydrous sugars was observed, suggesting that they may serve as more robust tracers for aged biomass burning emissions. Overall, the results from this study suggest that robust model estimates of BrC radiative impacts require consideration of the atmospheric aging of BrC and the stability of high-MW BrC. [ABSTRACT FROM AUTHOR]

An upgraded three-dimensional (3D) version of the back-trajectory based Concentration Weighted Trajectory (CWT) model, segregating distinct vertical layers of atmospheric transport, was combined with two years of PM 10 chemical composition measurements from the Finokalia superstation on the island of Crete, Greece, in the Eastern Mediterranean basin. Three altitudinal transport layers were studied: a) 0 m ≤ Layer 1 < 1000 m b) 1000 m ≤ Layer 2 < 2000 m c) 2000 m ≤ Layer 3, and the analysis was conducted separately for the cold (16 October – 15 April) and the warm (16 April – 15 October) periods. Mediterranean airflows in all layers enriched the concentrations of sea salt water soluble ions (Na+, Cl−, Mg2+ and K+) whilst during the warm period long-range northeastern airflows in layer 2 and layer 3 increased the levels of carbonaceous aerosols (OC and EC) and K+ due to intensive biomass burning in areas around the Black Sea. Peak concentrations of PM 10 -bound SO 4 2− and NH 4 + were associated with continental Northern airflows in layer 2 and layer 3, and were attributed to emissions in Northern Greece, Turkey, Balkan Peninsula and Eastern Europe. In addition, episodes of crustal constituents such as Ca2+, Mn , Al and Fe , were triggered by Saharan dust intrusions occurring mainly during cold period through all altitudinal layers. Moreover, the impact of heavy fuel oil combustion from shipping during warm period was reflected by maritime flows enhancing V concentrations, whilst a Saharan dust source for V was observed in all layers during cold period. [Display omitted] • 3D-CWT model revealed spatial and altitudinal PM 10 transport pathways in Finokalia. • Crustal PM 10 species are strongly affected by Saharan dust intrusions. • Sea salt PM 10 constituents were enhanced mainly by Mediterranean airflows. • Biomass burning around the Black Sea affected K+, OC and EC levels in warm period. • North-Northeastern continental flows enriched the SO 4 2− and NH 4 + PM 10 constituents. [ABSTRACT FROM AUTHOR]

The Mediterranean Sea is an oligotrophic semi-closed environment where atmospheric deposition is expected to be an important driver for biological activity. The present study uses a three-dimensional atmospheric chemistry transport model to evaluate the nitrogen (N), phosphorus (P) and iron (Fe) atmospheric deposition fluxes to the Mediterranean Sea. The model takes into account both the inorganic and organic fractions of these fluxes and compares them to other sources of these nutrients that are external to the ocean. The estimated atmospheric deposition fluxes of soluble nutrients amount to 1281 Gg-N y−1, 4.31 Gg-P y−1 and 6.32 Gg-Fe y−1 for the present day, and are within the range of the few estimates available in literature. An almost 6-fold increase in the atmospheric deposition of soluble N is also calculated to be the result of the increase in anthropogenic and biomass burning emissions since 1850, while soluble P and Fe deposition fluxes have increased by 59% and 114%, respectively. For future (2100) emissions, however, N deposition is projected to increase only slightly (4%) while soluble P and Fe fluxes will decrease by 34% and 32% compared to current estimates. The soluble organic N and P annual deposition fluxes are calculated to be 12% and 28–83% of total soluble N and P present-day annual deposition fluxes into the Mediterranean Sea, respectively. However, to reconcile with the observed fluxes in the west and the east Mediterranean, ∼14 times higher flux of soluble P, in particular organic P, and at least 2.5 times higher flux of soluble Fe need to be considered in the model. Such high fluxes can be due to higher combustion emissions of soluble Fe and P, to higher dust emissions or solubilisation of Fe and P contained in dust aerosols, and also higher organic P flux associated with bioaerosols than currently used in the global models. Overall, the calculated deposition fluxes provide an integrated spatially complete picture of the atmospheric inputs to the Mediterranean marine ecosystem, which are potentially important for net primary production and the ocean carbon cycle. [ABSTRACT FROM AUTHOR]