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Characteristics of trace gases (O3, CO, CO2, CH4 and N2O) and aerosols (particle size of 2.5 micron) were studied over the Arabian Sea, equatorial Indian Ocean and southwest part of the Bay of Bengal during the monsoon transition period (October-November, 2004). Flow of pollutants is expected from south and southeast Asia during the monsoonal transition period due to the patterns of wind flow which are different from the monsoon period. This is the first detailed report on aerosols and trace gases during the sampled period as the earlier Bay of Bengal Experiment (BOBMEX), Arabian Sea Monsoon Experiment (ARMEX) and Indian Ocean Experiments (INDOEX) were during monsoon seasons. The significant observations during the transition period include: (i) low ozone concentration of the order of 5 ppbv around the equator, (ii) high concentrations of CO2, CH4 and N2O and (iii) variations in PM2.5 of 5-20 µg/m³. [ABSTRACT FROM AUTHOR]

Abstract: Indo-Gangetic (IG) alluvial plains, one of the largest river basins in the world, suffers from the long range transport of mineral dust from the western arid and desert regions of Africa, Arabia and Rajasthan during the summer (pre-monsoon season, April–June). These dust storms influence the aerosol optical depth (AOD) across the IG plains. The Kanpur AERONET (Aerosol Robotic Network) station and Moderate Resolution Imaging Spectro-radiometer (MODIS) data show pronounced effect on the aerosol optical properties and aerosol size distribution during major dust storm events over the IG plains that have significant effect on the aerosol radiative forcing (ARF). The multi-band AOD, from AERONET and MODIS, show contrasting changes in wavelength dependency over dust affected regions. A time collocated (±30min) validation of AERONET AOD with MODIS Terra (level 2 swath product) over Kanpur, at a common wavelength of 550nm for the period 2001–2005 show moderate correlation (R 2∼0.6) during the summer season. The average surface forcing is found to change by −23Wm−2 during dust events and the top of the atmosphere (TOA) forcing change by −11Wm−2 as compared to the non-dusty clear-sky days. A strong correlation is found between AOD at 500nm and the ARF. At surface, the correlation coefficient between AOD and ARF is found to be high (R 2=0.925) and is found to be moderate (R 2=0.628) at the TOA. The slope of the regression line gives the aerosol forcing efficiency at 500nm of about −46±2.6Wm−2 and −17±2.5Wm−2 at the surface and the TOA, respectively. The ARF is found to increase with the advance of the dry season in conjunction with the gradual rise in AOD (at 500nm) from April (0.4–0.5) to June (0.6–0.7) over the IG plains. [Copyright &y& Elsevier]