New Directions: Atmospheric Brown “Clouds”☆

By Veerabhadran Ramanathan, P. Crutzen. 2003

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In February 1999, over 200 scientists from India, Europe and USA gathered in the islands of the Maldives to conduct the Indian Ocean Experiment (INDOEX). As reported by Ramanathan and coworkers (Journal of Geophysical Research, 106 (2001), 371–399), INDOEX revealed the so-called ‘‘brown cloud’’ phenomenon over the Northern Indian Ocean region with a large impact on the solar radiative heating of the region. The impact on the regional gas phase chemistry of the region, as reported by Lelieveld and coworkers (Science, 291 (2001), 1031–1036), was also large. The fundamental message from INDOEX is that due to long-range transport, what we normally associate with urban haze can span an entire sub-continent plus an ocean basin. The persistence of the haze during the 6 month long dry season, its black carbon content and the large perturbation to the radiative energy budget of the region have significant implications to the regional and global water budget, agriculture and health. The logical implication is that air pollution and climate changes are intricately linked and should be addressed under one common framework (see the ABC concept paper by Ramanathan and Crutzen in http://www-ABC-ASIA.ucsd.edu). In a related but independent development, Kaufman et al. (Nature, 419 (2002), 215–223) published satellite results for the aerosol optical depth over both land and ocean. These data revealed that pollution aerosols are found in and downwind of all inhabited regions of the planet and each of these hazes spreads over a large region. The data showed a Northern American haze extending all the way into the mid-Atlantic Ocean and Europe; heavy aerosol loading in north and eastern Europe and the Mediterranean Sea; dust plume from Sahara extending across tropical Atlantic; Dust and anthropogenic haze from Mongolia and China extending all the way across the Pacific; and the biomass burning aerosols from Africa and the Amazon region extending into the Atlantic. Thus we should think of the brown clouds of Asia, Africa, North and South America and Europe. Accordingly, we now propose that the phenomenon be referred to as ‘‘Atmospheric Brown Clouds’’ (ABC). Haze instead of cloud is a more correct designation from a purely meteorological definition, but the distinction is subjective, for a haze will become a cloud if the relative humidity reaches saturation. Under the sponsorship of the United Nations Environmental Program (UNEP) an international program entitled ABC, starting with Asia has been created (http://www-ABC-ASIA.ucsd.edu). In what follows we will summarize INDOEX findings and discuss ABC and future research that is needed in this important area. The equatorial Indian Ocean is a unique natural laboratory for studying the impact of anthropogenic aerosols on climate, because pollutants from South and S.E. Asia is directly connected to the pristine air from the southern hemisphere by a low altitude monsoonal flow into the inter tropical convergence zone (ITCZ). This fact provided the main motivation for INDOEX (http://www-indoex.ucsd.edu/publications/white paper/). The South Asian brown haze covers most of the Arabian Sea, Bay of Bengal, the Northern Indian Ocean and the South Asian region and extends from about November to May and possibly longer. The brownish haze consists of a mixture of anthropogenic sulfate, nitrate, organics, black carbon, dust and fly ash particles and natural aerosols such as sea salt and mineral dust. Biomass burning and fossil fuel combustion contribute as much as 75% to the observed aerosol. Why is the haze problem so severe in the tropics? The large increase in emissions of aerosols and their precursors is one important reason. The other important contributor is the unique meteorology of the tropics and the subtropics (including Southern Asia) which leads to a long dry season with minimal rainfall extending from late fall and winter until spring. The dryness precludes the wet removal of haze particles by rain. On the other hand, in the mid and high latitudes, the absence of a long dry season, and seasonally distributed rainfall (and snow fall) cleans the atmosphere more efficiently. Aerosols, by scattering/absorbing solar and emitting/ absorbing long wave (IR) radiation, change the radiation fluxes at the surface and the top of the atmosphere, thereby especially perturbing the atmospheric absorption of solar radiation. These aerosol-induced changes in the radiation budget are referred to as the direct forcing. Black carbon, which strongly absorbs solar radiation, plays a major role in the forcing by partially shielding ARTICLE IN PRESS

Subfields: Atmospheric Science, Earth-Surface Processes, Global and Planetary Change

View Paper: https://www.semanticscholar.org/paper/74e35c58dd2fbb2076521925b0577dd13d27a338

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Citations in the corpus

Wintertime aerosol properties during foggy and nonfoggy days over urban center Delhi and their implications for shortwave radiative forcing (2006, background): “This persistent haze with frequent occurrence of fog is capable of producing a large imbalance between the incoming solar and outgoing IR radiation, which can have climate impacts not only on a regional scale but also globally [ Ramanathan and Crutzen, 2003; Ramanathan et al., 2005].”
Overview of the Atmospheric Brown Cloud East Asian Regional Experiment 2005 and a study of the aerosol direct radiative forcing in east Asia (2007, background): “[4] Under the abovementioned background, UNEP established the Atmospheric Brown Cloud (ABC) project to study aerosols and air pollution in Asia for understanding their impact to the climate, human health, agriculture, and water resources [Ramanathan and Crutzen, 2003].”
Impact of monsoon transitions on the physical and optical properties of aerosols (2006, background): “(ABC-Asia) was designed to pick up where the INDOEX project left off and was intended to provide continuous monitoring of atmospheric pollution, radiative effects and climate change in the Asian-Pacific region [Ramanathan and Crutzen, 2003].”