[1] Strong contrasts in daytime mixing height (boundary layer [BL] height or z i ) between urban and rural areas were observed during the 1999 Nashville Summer Intensive field campaign of the Southern Oxidants Study. On occasion, the urban mixing height was as much as 45% (700 m) higher than that over the rural areas. The difference was quite persistent, showing strongly in statistical comparisons, with a mean difference over all hours available for comparison of 160 m. Clouds had higher bases and were more common over the urban area as well. In this paper, measurements from wind profiling radars, lidars, and aircraft are used to characterize mixing height and clouds. The urbanrural contrasts have important implications for regional air quality. The mixing height is a first-order control on pollutant concentrations. The urban-rural contrast also results in the venting of urban pollutants, affecting the local concentrations and the regional background. Clouds affect air quality by changing the radiative input for photochemistry and through changes in mixing and venting.

Soiling of Photovoltaic (PV) modules is a growing area of concern due to the adverse effect of dust accumulation on PV performance and reliability. In this work, we report on four fundamental adhesion forces that take place at the first stage of soiling process. These are capillary, van der Waal, electrostatic and gravitational forces. It is found that under high relative humidity, the adhesion mechanism between dust particles and PV module surfaces is dominated by capillary force, while van der Waal force dominates under dry conditions. Moreover, real field data for long soiling periods over solar panels in Qatar were investigated and resulted in proposing a novel modified sigmoid function that predicts a relative humidity inflexion value at which transition in the particulate matter deposition rate takes place from low to high values. Moreover, the effect of surface roughness was investigated by measuring adhesion force over clean glass versus substrates that are coated with in-house developed anti-dust titania thin films.

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This study examines the impact of two of the most advanced dust emission schemes on the predictions of the weather research and forecasting model with chemistry (WRF-Chem) over the Middle East during a summer time period. Results show significant differences between the two simulations in the spatial distribution of dust emissions as well as in their size-resolved mass discretization. The AFWA scheme simulation predicts 30 % higher dust emission fluxes than the S11 module over the Arabian Peninsula (6.7 lg m -2 s -1 compared to 4.5 lg m -2 s -1 , respectively). In the S11 simulation 70 % of the emitted dust is in the 10-20 lm size range while the AFWA simulation assigns 50 % of dust emitted particles in the 6-12 lm size section. Both simulations reproduce the majority of the ambient PM 10 data (more than 70 %) within a factor of two. However, the S11 simulation predicts, on average, 50 % lower PM 10 concentrations compared to AFWA over the high resolution (2 9 2 km 2 ) domain of Qatar. Previous applications of WRF-Chem may have substantially overestimated the simulated dust in this region.