General circulation model estimates of aerosol transport and radiative forcing during the Indian Ocean Experiment

Abstract

[1] Aerosol sources, transport, and sinks are simulated, and aerosol direct radiative effects are assessed over the Indian Ocean for the Indian Ocean Experiment (INDOEX) Intensive Field Phase during January to March 1999 using the Laboratoire de Météorologie Dynamique (LMDZT) general circulation model. The model reproduces the latitudinal gradient in aerosol mass concentration and optical depth (AOD). The model-predicted aerosol concentrations and AODs agree reasonably well with measurements but are systematically underestimated during high-pollution episodes, especially in the month of March. The largest aerosol loads are found over southwestern China, the Bay of Bengal, and the Indian subcontinent. Aerosol emissions from the Indian subcontinent are transported into the Indian Ocean through either the west coast or the east coast of India. Over the INDOEX region, carbonaceous aerosols are the largest contributor to the estimated AOD, followed by sulfate, dust, sea salt, and fly ash. During the northeast winter monsoon, natural and anthropogenic aerosols reduce the solar flux reaching the surface by 25 W m À2 , leading to 10-15% less insolation at the surface. A doubling of black carbon (BC) emissions from Asia results in an aerosol single-scattering albedo that is much smaller than in situ measurements, reflecting the fact that BC emissions are not underestimated in proportion to other (mostly scattering) aerosol types. South Asia is the dominant contributor to sulfate aerosols over the INDOEX region and accounts for 60-70% of the AOD by sulfate. It is also an important but not the dominant contributor to carbonaceous aerosols over the INDOEX region with a contribution of less than 40% to the AOD by this aerosol species. The presence of elevated plumes brings significant quantities of aerosols to the Indian Ocean that are generated over Africa and Southeast and east Asia.

Aerosol characteristics at a remote island: Minicoy in southern Arabian Sea

“…Previous studies though confined to the winter period to understand the source regions of aerosol origin during Indian Ocean Experiment intensive field phase (INDOEX-IFP) (January to March 1999) have identified, several regions like Indo-Gangetic Plain, central India, south India, Africa, west Asia and north west India that have a large influence on the aerosol characteristics over the Indian Ocean region (Verma et al 2007). Previous studies using satellite data and models have shown that south Asia and the Indian subcontinent region are major contributors to the aerosol loading over the Indian Ocean region (Nair et al 2003;Reddy et al 2004). The BC mass reported for Port Blair in Andaman and Nicobar Islands in the Bay of Bengal to the east of Indian landmass, was higher than those observed at Minicoy.…”

Exploring the relationship between surface PM<sub>2.5</sub> and meteorology in Northern India

“…While the sources of these emissions are relatively well-known, there are large uncertainties in emission estimates across inventories (e.g., Jena et al, 2015;Zhong et al, 2016). Indeed, correctly simulating the extremely high PM 2.5 abundances in the IGP has proved troublesome for current global chemistry models (Reddy et al, 2004;Mian Chin et al, 2009;Ganguly et al, 2009;Menon et al, 2010;Henriksson et al, 2011;Goto et al, 2011;Nair et al, 2012;Cherian et al, 2013;Moorthy et al, 2013;Sanap et al, 2014;Pan et al, 2015). A multi-model evaluation by Pan et al (2015) concluded that an underestimation of wintertime biofuel emissions was the dominant cause of the models' low biases.…”

Variations in sulphate aerosols concentration during winter monsoon season for two consecutive years using a general circulation model

“…Aerosol transport and its impact on radiative forcing were estimated over the tropical Indian Ocean during INDOEX (Krishnamurty et al, 1998;Podgorny et al, 2000;Rajeev et al, 2001). General circulation model (GCM) simulations over south Asia during the INDOEX period with the high resolution regional emissions information (Reddy et al, 2004;Verma et al, 2008Verma et al, , 2012 indicate the likely contribution of emissions from regions outside India to aerosol loading over the Indian subcontinent and adjoining oceans.…”

“…Elevated aerosols loading was observed in a remote marine environment over the Arabian Sea during INDOEX 1999. The chemical transport models have been used widely for atmospheric simulations over southern Asia during the INDOEX period (Rajeev et al, 2000;Rasch et al, 2001;Podgorny and Ramanathan, 2001;Reddy et al, 2004;Verma et al, 2008Verma et al, , 2012, and have shown a predominance of sulphate in the south Asian region. Most of the INDOEX studies emphasize the underestimated impact of aerosols on climate Ramanathan et al, 2002;UNEP, 2002).…”