Capping inversions act as barriers to the vertical diffusion of pollutants, occasionally leading to significant low-level air pollution episodes in the lower troposphere. Here, we conducted two summer campaigns where global positioning system radiosondes were operated in Haifa Bay on the eastern Mediterranean coast, a region of steep terrain with significant pollution. The campaigns provided unique high resolution measurements related to capping inversions. It was found that the classical definition of a capping inversion was insufficient for an explicit identification of a layer; hence additional criteria are required for a complete spatial analysis of inversion evolution. Based on the vertical temperature derivative, an inner fine structure of inversion layers was explored, and was then used to track inversion layers spatially and to investigate their evolution. The exploration of the inner structure of inversion layers revealed five major patterns: symmetric peak, asymmetric peak, double peak, flat peak, and the zig-zag pattern. We found that the symmetric peak is related to the strongest inversions, double peak inversions tended to break apart into two layers, and the zig-zag pattern was related to the weakest inversions. Employing this classification is suggested for assistance in following the evolution of inversion layers.
Many air pollution events are occasionally difficult to explain. While most monitoring-based air pollution assessment studies deal with surface analysis, the near-surface elevated pollutants are challenging. The lack of data and understanding of those elevated layers, leaves us ‘blind’ and with no clue where, when and how intensively these pollutants may hit the surface. Here, this challenge at the specific domain of Mt. Carmel is addressed. The atmospheric numerical models RAMS and HYPACT were employed on Haifa Bay in the Eastern Mediterranean with nested horizontal grids down to 0.5 km, in order to resolve the fine-scale flow, along an air pollution episode which serves as a case study. Sixteen locations were determined, representing monitored and non-monitored sites in the complex terrain sub-domains. Results show multi-inversion profiles, which are consistent with an earlier observational study over the region. Concentration differences up to an order of magnitude between adjacent sites (∼2 km) were found, often associated with near-zero surface values, while some simulated peaks were at elevations of 100–400 m above ground level (AGL). The current event offers a view on the near-surface elevated layers, and points at limitations of ground-level monitoring as an indicator of air pollution. This study highlights the importance of near-surface pollution, which is often an unknown source for surface pollution. Overall, steep vertical gradient of pollution as shown here is associated with a combination of deep inversion (or multi-inversion profile), vertical circulation due to topography or synoptic flow, and small scale circulation induced by the complex topography. Since monitoring of the elevated layers is limited by the technology, it is suggested that high resolution advanced models should be used for further exploration of the near-surface pollution.
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