Abstract
Abstract. Southern Asia, extending from Pakistan and Afghanistan to Indonesia and Papua New Guinea, is one of the most heavily populated regions of the world. Biofuel and biomass burning play a disproportionately large role in the emissions of most key pollutant gases and aerosols there, in contrast to much of the rest of the Northern Hemisphere, where fossil fuel burning and industrial processes tend to dominate. This results in polluted air masses which are enriched in carbon-containing aerosols, carbon monoxide, and hydrocarbons. The outflow and long-distance transport of these polluted air masses is characterized by three distinct seasonal circulation patterns: the winter monsoon, the summer monsoon, and the monsoon transition periods. During winter, the near-surface flow is mostly northeasterly, and the regional pollution forms a thick haze layer in the lower troposphere which spreads out over millions of square km between southern Asia and the Intertropical Convergence Zone (ITCZ), located several degrees south of the equator over the Indian Ocean during this period. During summer, the heavy monsoon rains effectively remove soluble gases and aerosols. Less soluble species, on the other hand, are lifted to the upper troposphere in deep convective clouds, and are then transported away from the region by strong upper tropospheric winds, particularly towards northern Africa and the Mediterranean in the tropical easterly jet. Part of the pollution can reach the tropical tropopause layer, the gateway to the stratosphere. During the monsoon transition periods, the flow across the Indian Ocean is primarily zonal, and strong pollution plumes originating from both southeastern Asia and from Africa spread across the central Indian Ocean. This paper provides a review of the current state of knowledge based on the many observational and modeling studies over the last decades that have examined the southern Asian atmospheric pollutant outflow and its large scale effects. An outlook is provided as a guideline for future research, pointing out particularly critical issues such as: resolving discrepancies between top down and bottom up emissions estimates; assessing the processing and aging of the pollutant outflow; developing a better understanding of the observed elevated pollutant layers and their relationship to local sea breeze and large scale monsoon circulations; and determining the impacts of the pollutant outflow on the Asian monsoon meteorology and the regional hydrological cycle, in particular the mountain cryospheric reservoirs and the fresh water supply, which in turn directly impact the lives of over a billion inhabitants of southern Asia.
Highlights
In 1999, several hundred scientists from around the world gathered in the Maldives, located in the northern Indian Ocean, as well as onboard two research ships and at several Indian coastal sites, to carry out the Indian Ocean Experiment (INDOEX)
This is an extension of the comparison that we presented previously in Lelieveld et al (2001), but based on newer information, with different regional definitions, and different emissions classes
An open question is whether this apparent stability is due to the relative interannual constancy of the largely anthropogenic emissions in the regions surrounding the Arabian Sea and northern Indian Ocean, or whether it is more of a balance – perhaps coincidental during the period analyzed by Ramanathan et al (2002) – between processes leading to higher AODs in some parts of the region and lower AODs elsewhere
Summary
In 1999, several hundred scientists from around the world gathered in the Maldives, located in the northern Indian Ocean, as well as onboard two research ships and at several Indian coastal sites, to carry out the Indian Ocean Experiment (INDOEX). Atmospheric Brown Clouds (UNEP Project) Aerosol Index Aerosol Optical Depth Accumulated exposure Over a Threshold of 40 nmol/mol Aerosol Radiative Forcing over India Arabian Sea Monsoon Experiment Arabian Sea above sea level Automated Unmanned Aerial Vehicles Agricultural processes plus waste handling Black Carbon Biofuel Burning Biomass Burning Bay of Bengal Bay of Bengal Experiment Civil Aircraft for Regular Investigation of the atmosphere Based on an Instrument Container Elemental Carbon Emissions Database for Global Atmospheric Research Eastern Tropical Indian Ocean region Fossil Fuel Burning First Field Phase General Circulation Model Global Ozone Monitoring Experiment Integrated Campaign for Aerosol, gases, and Radiation Budget Intensive Field Phase Industrial emissions processes Indian Ocean Experiment Inorganic Oxidized Material Geosphere Biosphere Programme of the Indian Space Research Organization Intertropical Convergence Zone Kaashidhoo Climate Observatory Lower Troposphere Maldives AUAV Campaign Marine boundary layer Maldives Climate Observatory in Hanimadhoo Mediterranean Intensive Oxidants Study Microwave Limb Sounder Measurements Of Pollution In The Troposphere, satellite instrument Middle Troposphere Monsoon Transition Period National Centers for Environmental Prediction – Global Forecasting System Northeast Northern Hemisphere Northern Hemisphere continental Northern Hemisphere continental Tropical Northern Hemisphere continental extratropics Northern Hemisphere maritime equatorial Northern Hemisphere maritime Tropical.
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