Sumita Kedia

Aerosol radiative effects over two environmentally distinct locations, Kanpur (urban site) and Gandhi College (rural location) in the Indo-Gangetic plain (IGP), a regional aerosol hot spot, utilizing the measured optical and physical characteristics of aerosols, an aerosol optical properties model and a radiative transfer model, are examined. Shortwave aerosol radiative forcing (ARF) at the top of the atmosphere (TOA) is < −12 W m^− 2 over Kanpur and Gandhi College. ARF at the surface is ≥ −30 W m^− 2. Atmospheric warming is maximum during premonsoon (>30 W m^− 2). Shortwave atmospheric heating due to aerosols is >0.4 K/day over IGP and peaks during premonsoon at >0.6 K/day due to lower single scattering albedo (SSA) and higher surface albedo. TOA forcing is always less negative over Kanpur when compared to Gandhi College due to lower surface albedo except in postmonsoon owing to higher SSA. This happens as TOA forcing depends on SSA and surface albedo in addition to aerosol optical depth. The magnitude of longwave forcing and atmospheric cooling in an absolute sense is significantly small and contributes only about 20% or less to the net (shortwave + longwave) forcing. Aerosol radiative effects over these two locations, despite differences in aerosol characteristics, are similar, thus confirming that aerosols and their radiative influence get transported due to circulation. ARF over Kanpur and Gandhi College is an order of magnitude higher when compared to greenhouse gas forcing. A large reduction in surface reaching solar irradiance accompanied by large atmospheric warming can have implications on precipitation and hydrological cycle, and these aerosol radiative effects should be included while performing regional-scale aerosol climate assessments.

Spectral aerosol optical depths (AOD) measured on board a cruise over the Bay of Bengal and the Arabian Sea during March–May 2006 are analyzed. Mean 0.5 μm AOD over the Bay of Bengal is higher (0.36) than the Arabian Sea (0.25). AODs obtained from MODIS Terra and Aqua are found to track well the Sun photometer AODs. A comparison between Sun photometer and MODIS AODs yielded a correlation coefficient of 0.96. MODIS fine mode fraction over the Bay of Bengal is higher (0.71) when compared to the Arabian Sea value of 0.60. Ångström exponent (α) over the Bay of Bengal is higher at 1.12, indicating the dominance of smaller size aerosols in the aerosol columnar distribution than the Arabian Sea (0.73), while Ångström coefficient (β) is comparable at 0.15 and 0.16 for the Bay of Bengal and the Arabian Sea. Background AODs are higher over the Bay of Bengal and the Arabian Sea indicating a strong continental influence. The scaling distance is lower over the Bay of Bengal due to nearly similar AODs, while the scaling distance over the Arabian Sea is about 2000 km. The anthropogenic influence in AODs over the Bay of Bengal and the Arabian Sea estimated using wind speed dependent AODs and maritime clean AODs are found to agree within ±1σ of the anthropogenic fraction obtained from MODIS AODs and FMFs. The mean anthropogenic contribution to the AODs estimated from the three methods is higher over the Bay of Bengal (68–75%) than the Arabian Sea (51–65%).