Near-surface elevated pollution: What we don’t know doesn’t hurt? a numerical study over Mt. Carmel

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 RAMSand 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 100-400mabove 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.