Altitude distribution of aerosols over Southeast Arabian Sea coast during pre-monsoon season: Elevated layers, long-range transport and atmospheric radiative heating

摘要

Every year, during the pre-monsoon period (March-May), a pronounced increase in aerosol optical depth (AOD) is observed over the eastern Arabian Sea, which is attributed to the transport of continental aerosols. This paper presents the altitude distribution of tropospheric aerosols, characteristics of elevated aerosol layers and aerosol radiative heating of the atmosphere during the pre-monsoon season over Trivandrum (8.5°N, 77°E), a station located at the southwest coast of Indian peninsula which is covered by the eastern Arabian Sea plume. Altitude profiles of aerosol backscatter coefficient (β_a) and linear depolarization ratio (LDR) reveal two distinct aerosol layers persisting between 0-2 km and 2-4 km. The layer at 2-4 km, which contributes about 25% of the AOD during polluted conditions, contains significant amount of non-spherical aerosols. This layer is prominent only when the ad vection of dry airmass occurs from the northern parts of the Indian subcontinent and northern Arabian Sea. Role of long-range transport in the development of this aerosol layer is further confirmed using latitude-altitude cross-section of β_a observed by CALIPSO. Aerosol content in the layer below 2 km is large when advection of air occurs from the north and east Arabian Sea and is significantly small when it occurs from the southwest Arabian Sea or Indian Ocean. During the highly polluted conditions, aerosols tend to increase the diurnal mean atmospheric radiative heating rate by ～0.8 K day~(-1) at 500 m and 0.3 K day~(-1) at 3 km, which are about 80% and 30% of the respective radiative heating in the aerosol-free atmosphere.