Aircraft observations of elevated pollution layers near the foothills of the Himalayas during CAIPEEX-2009

Pre-monsoon aerosols in the northern part of India may play an important role in the advancement of the monsoon. This study investigates the properties of aerosols and their spatial and vertical distribution near the foothills of the Himalayas using data from an instrumented aircraft during the Cloud Aerosol Interactions and Precipitation Enhancement Experiment (CAIPEEX), 2009. CAIPEEX was conducted over five days during May 2009 in the northern part of India near the foothills of the Himalayas. On all the flight days thick haze was observed, with elevated aerosol layers up to 4 km with varying concentrations. The sources are identified as being from local anthropogenic activities such as biomass burning, as inferred from MODIS fire maps and dust from local as well as from long-range transport, as suggested by trajectory analysis. The aerosol size distributions depict the increases in both fine and coarse mode aerosols in polluted layers. This indicates that aerosols over this region are well mixed and the vertical distribution is a mixture of both biomass burning and dust aerosols at different altitudes, as also observed by CALIPSO and inferred from Mie calculations. Clouds observed above the elevated aerosol layers showed higher droplet concentrations (200-1400 cm -3) with small effective radii (3.5 to < 6 μm). Ice phase observed above 6 km at temperatures lower than -14°C might be due to the presence of dust aerosol acting as potential ice nuclei