The performance and the uncertainty of receptor models (RMs) were assessed in intercomparison exercises\ud employing real-world and synthetic input datasets. To that end, the results obtained by different\ud practitioners using ten different RMs were compared with a reference. In order to explain the differences\ud in the performances and uncertainties of the different approaches, the apportioned mass, the number of\ud sources, the chemical profiles, the contribution-to-species and the time trends of the sources were all\ud evaluated using the methodology described in Belis et al. (2015).\ud In this study, 87% of the 344 source contribution estimates (SCEs) reported by participants in 47\ud different source apportionment model results met the 50% standard uncertainty quality objective\ud established for the performance test. In addition, 68% of the SCE uncertainties reported in the results\ud were coherent with the analytical uncertainties in the input data.\ud The most used models, EPA-PMF v.3, PMF2 and EPA-CMB 8.2, presented quite satisfactory performances\ud in the estimation of SCEs while unconstrained models, that do not account for the uncertainty in\ud the input data (e.g. APCS and FA-MLRA), showed below average performance. Sources with well-defined\ud chemical profiles and seasonal time trends, that make appreciable contributions (>10%), were those\ud better quantified by the models while those with contributions to the PM mass close to 1% represented a\ud challenge.\ud The results of the assessment indicate that RMs are capable of estimating the contribution of the major\ud pollution source categories over a given time window with a level of accuracy that is in line with the\ud needs of air quality management

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Although distinct growth behaviors on different faces of hexagonal ice have long been suggested, their understanding on a molecular scale has been hampered due to experimental difficulties near interfaces. We present a molecular dynamics simulation study to unravel the molecular origin of anisotropy in the growth kinetics of hexagonal ice by visualizing the formation of transient water structures in the growing ice interface. During ice growth, the formation of transient structures and their rearrangement to the final ice configuration are observed irrespective of growth direction. However, we find that their structure and duration differ significantly depending on growth direction. In the direction perpendicular to the basal face of hexagonal ice along which growth occurs most slowly, a two-dimensional transient structure, which is formed by competing hexagonal and cubic arrangements within the same layer, persists for a significant period of time, contrasted with short-lived transient structures in other directions. This observation of such transient water structures and their rearrangement during ice growth provides a clear explanation of different growth rates on each face of hexagonal ice on a molecular scale.

Measurements of 14 polycyclic aromatic hydrocarbons (PAH) have been made in Jeddah, Saudi Arabia, with a view to establishing the concentrations in this major city, and quantifying the contributions of major sources. Particulate and vapour forms have been sampled and analysed separately. The concentrations are compared to measurements from other sites in the Middle Eastern region and are towards the lower end of the range, being far lower than concentrations reported from Riyadh (Saudi Arabia), Assiut (Egypt) and Tehran (Iran) but broadly similar to those measured in Damascus (Syria) and higher than those measured in Kuwait. The partitioning between vapour and particle phases is similar to that in data from Egypt and China, but with many compounds showing a higher particle-associated percentage than in Birmingham (UK) possibly reflecting a higher concentration of airborne particulate matter in the former countries. Concentrations in Jeddah were significantly higher at a site close to the oil refinery and a site close to a major ring road than at a suburban site to the north of the city. Application of positive matrix factorisation to the pooled data elicited three factors accounting respectively for 17%, 33% and 50% of the measured sum of PAH and these are interpreted as arising from gasoline vehicles, industrial sources, particularly the oil refinery, and to diesel/fuel oil combustion.