Aerosol Studies

Instruments were flown that measured cloud microphysical parameters, aerosol characteristics, and trace gas concentrations. This suite of instruments combined to provide information about natural conditions important to cloud and precipitation development. For example, emissions of SO2 into the atmosphere result in the production of aerosols through oxidation. Highest concentrations of these particles were expected to occur in the vicinity of the highest emissions of SO2 in the UAE region. This is due to the fact that the synoptic conditions over the UAE are dominated by anticyclonic circulation, with low wind speeds and stagnant air. In addition, the afternoon and evening sea breeze could bring many of these pollutants onshore. CN aerosol concentrations were indeed found to be highest at identified SO2 sources. These aerosols can be very effective CCN and therefore have the potential to influence cloud microphysical processes and rainfall production efficiencies.

Measurements of CCN showed that background levels were enhanced due to local pollution sources in the region, which resulted in higher cloud droplet concentrations in clouds, making them more continental in nature. These are conditions generally thought to be more amenable to seeding with hygroscopic flares to enhance rainfall, since the polluted background particles generally work to suppress precipitation development. When mineral (desert) dust exists in conjunction with sulfates (and possibly other pollutants), the dust particles can become coated with sulfate, making them more active as cloud condensation nuclei. This is especially important in the UAE region where both sources exist and likely have an impact on the precipitation processes in clouds. In 2002, filters for detailed examination under electron microscopes were collected to determine the composition of the aerosols and assess whether sulfate coating of dust particles occurred and was significant. Analysis is still ongoing, but preliminary results suggest that sulfate coating is not a significant process and therefore may not be a complicating mechanism in determining the natural variability of CCN.

Vertical profiles of the aerosols show that the top of the boundary layer was at about 1500 m for the experimental periods in the winter and much higher (to 3500 m or more) in the summer. This is the layer that is influenced by surface sources and processes. Concentrations of aerosols in all size ranges as well as most trace gases decreased rapidly above the boundary layer. The large difference between the CN and aerosol (PCASP) concentrations indicates that the UAE region is a large source of nucleation mode aerosols. This is most obvious near specific source regions within the UAE. The aerosol measurements above the boundary layer also suggest that nucleation mode aerosols are as important a source of CCN as the accumulation mode aerosols.