Modulation of the aerosol absorption and single-scattering albedo in Southwest Asia due to large scale and sea breeze circulations.

The spectral aerosol absorption properties in the Arabian Gulf region were observed during the United Arab Emirates Unified Aerosol Experiment (UAE2). Measurements were taken at a coastal region of the Arabian Gulf located 60 km northeast of Abu Dhabi, the capital of the United Arab Emirates, allowing characterization of pollution and dust particle absorption properties in a highly heterogeneous environment. A large observed modulation of the diurnal signal was due to (a) strong sea- and land-breeze and (b) changes in prevailing large scale flow. During the night, stagnating air resulted in gradual accumulation of pollution with maximum absorption in the early morning hours. The rising sun increased both the depth of the boundary layer and the temperature of the interior desert, resulting in strong and sudden sea-breeze onset which ventilated the polluted air accumulated during the night. Our observations show that the onshore winds brought cleaner air resulting in decreasing values of the absorption coefficient and increasing values of the single-scattering albedo (wo). The mean value of the absorption coefficient at 550 nm measured during the sea breeze was 10.2 Mm-1 while during the land breeze it was 13.8 Mm-1. Large scale transport also strongly influenced particle fine/coarse partition with “northern” flow bringing pollution particles and “southern” flow bringing more dust.