Atmospheric brown clouds: Hemispherical and regional variations in long‐range transport, absorption, and radiative forcing

The study uses satellite observations, global assimilated aerosol data sets, Atmospheric Brown Clouds (ABC) observatories, a Monte Carlo aerosol‐cloud‐radiation model and a regional chemical transport model (STEM‐2K) to characterize the spatial extent of brown clouds, regional and megacity ABC hot spots, chemical composition and the direct radiative forcing. It presents the first annual cycle of aerosol observations and forcing from the ABC observatories in the Indo‐Asia‐Pacific regions. East Asia, Indo‐Gangetic Plains, Indonesian region, southern Africa and the Amazon basin are the regional hot spots defined by the criteria that anthropogenic aerosol optical depths (AODs) should exceed 0.3 and absorbing AOD > 0.03. Over these hot spots, as well as in other polluted oceanic regions, the EC mass exceeds 0.5 μ g m −3 , the OC mass exceeds 2 μ g m −3 and sulfate mass exceeds 10 μ g m −3 from the surface to 3 km. The brown clouds also have strong seasonal dependence. In the tropics the seasonal dependence is driven by pollution accumulating during the dry seasons, December to February in Northern Hemisphere tropics and June to August in Southern Hemisphere tropics. In the extratropics the pollution peaks during the summer. The brown cloud problem is not restricted to the tropical regions. Over the eastern half of US and western Europe the AODs exceeds 0.2 and absorption AODs exceed 0.02. Brown clouds also extend well into the western Pacific Ocean, the Indian Ocean reaching as far south as 60°S and the eastern Atlantic Ocean. The largest total SO 2 emission occurs over China and US, while SO 2 emission per unit surface area is maximum over Germany and England. The largest total EC and OC emissions occur over China, but the largest OC emission per unit surface area occur over India. As a result, the maximum negative annual mean TOA direct forcing is over India and Germany. The surface annual‐diurnal mean dimming over the regional hot spots is of the order of −10 W m −2 and −20 W m −2 over megacity hotpots.