Aerosol Characteristics at a high-altitude station Nainital during the ISRO-GBP Land Campaign-II

During the second land campaign (LC-II) organised by ISRO-GBP, extensive ground-based measurements of aerosol characteristics were carried out over Manora Peak (29.4oN; 79.5oE; 1951 metres above mean sea level), Nainital (a high altitude station located in the Shivalik ranges of Central Himalayas) during the dry, winter season (December) of 2004. These measurements included the spectral aerosol optical depths (AOD), columnar water vapour content (W), Total Columnar Ozone (TCO), total number concentration (NT) of near surface aerosols, mass concentration of black carbon (MB), aerosol mass loading (MT), and Global Solar Radiation. Based on these measured parameters, we present the results on the near-surface and columnar properties of atmospheric aerosols at Nainital.Comment: Published in the Proceedings of the ISRO-GBP Land-Campaign-II meeting, Physical Research Laboratory, Ahmadabad (Inida), March 200

Large latitudinal gradients and temporal heterogeneity in aerosol black carbon and its mass mixing ratio over southern and northern oceans observed during a trans-continental cruise experiment

Extensive, and collocated measurements of the mass concentrations (MB) of aerosol black carbon (BC) and (MT) of composite aerosols were made over the Arabian Sea, tropical Indian Ocean and the Southern Ocean during a trans-continental cruise experiment. Our investigations show that MB remains extremely low (<50 ng m-3) and remarkably steady (in space and time) in the Southern Ocean (20°S to 56°S). In contrast, large latitudinal gradients exist north of ~20°S; MB increasing exponentially to reach as high as 2000 ng m-3 in the Arabian Sea (~8°N). Interestingly, the share of BC showed a distinctly different latitudinal variation, with a peak close to the equator and decreasing on either side. Large fluctuations were seen in MT over Southern Ocean associated with enhanced production of sea-salt aerosols in response to sea-surface wind speed. These spatio-temporal changes in MB and its mixing ratio have important implications to regional and global climate

Optical properties of atmospheric aerosols over the Arabian Sea and Indian Ocean: North-South contrast across the ITCZ

Extensive estimates of aerosol spectral optical depths are made over the Arabian Sea and south-western Indian Ocean, using a 10-channel multi-wavelength solar radiometer (MWR) and a 4- channel hand held EKO sun-photometer (ESP) on-board the cruise # 133 of ORV Sagar Kanya during the First Field Phase (FFP- 98) of the Indian Ocean experiment (INDOEX) in February and March 1998. High values of optical depths, particularly in the visible wavelengths, are encountered in the coastal areas, with a gradual increase from Goa to Male, The optical depths decrease sharply as the ship moves out to the south western Indian Ocean across the equator; the effect is more significant at shorter wavelengths. Over the pris-tine environment south of the ITCZ, extremely low optical depths appear at the visible wavelengths, while at the NIR wavelengths, the optical depths remain nearly the same as on the northern side. On the return Ieg, again higher optical depths are encountered north of the ITCZ with those at the visible wavelengths sharply increased. Over the north-western Arabian Sea, higher optical depthvalues occur farther away from the coast, suggesting additional input of aerosols over mid ocean, possibly transported by various wind trajectories from the west Asian deserts. Comparing air trajectories both at the surface and 850 h Pa reveals that in addition to those advected from continental India, winds transporting aerosols from various north/west Asian regions contribute significantly to the aerosol optical depths over the Arabian Sea

Aerosol Characteristics at a High Altitude Location in Central Himalayas: Optical Properties and Radiative Forcing

Collocated measurements of the mass concentrations of aerosol black carbon (BC) and composite aerosols near the surface were carried out along with spectral aerosol optical depths (AODs) from a high altitude station, Manora Peak in Central Himalayas, during a comprehensive aerosol field campaign in December 2004. Despite being a pristine location in the Shivalik Ranges of Central Himalayas, and having a monthly mean AOD (at 500 nm) of 0.059 $\pm$ 0.033 (typical to this site), total suspended particulate (TSP) concentration was in the range 15 - 40 micro g m^(-3) (mean value 27.1 $\pm$ 8.3 micro g m^(-3)). Interestingly, aerosol BC had a mean concentration of 1.36 $\pm$ 0.99 micro g m^(-3), contributed to ~5.0 $\pm$ 1.3 % to the composite aerosol mass. This large abundance of BC is found to have linkages to the human activities in the adjoining valley and to the boundary layer dynamics. Consequently, the inferred single scattering albedo lies in the range of 0.87 to 0.94 (mean value 0.90 $\pm$ 0.03), indicating significant aerosol absorption. The estimated aerosol radiative forcing was as low as 4.2 W m^(-2) at the surface, +0.7 W m^(-2) at the top of the atmosphere, implying an atmospheric forcing of +4.9 W m^(-2). Though absolute value of the atmospheric forcing is quite small, which arises primarily from the very low AOD (or the column abundance of aerosols), the forcing efficiency (forcing per unit optical depth) was $\sim$88 W m^(-2), which is attributed to the high BC mass fraction.Comment: 32 Pages, Accepted in JGR (Atmosphere

Aerosol study during INDOEX: observation of enhanced aerosol activity over the Mid Arabian Sea during the Northern winter

The spatial distribution of the aerosol optical depth during the northern winter, along the West coast of India and over the oceanic environments of the Arabian Sea and the South-West Indian Ocean (between 60°E–78°E and 15°N–20°S), has been investigated using co-ordinated ground-based and ship-borne measurements carried out during January–March of 1998 and 1999 under the Indian Ocean Experiment (INDOEX). In this paper apart from the expected results, an interesting observation is presented of enhanced aerosol activity in the mid Arabian Sea, far removed from the continent. Its implications are discussed

Large latitudinal gradients and temporal heterogeneity in aerosol black carbon and its mass mixing ratio over southern and northern oceans observed during a trans-continental cruise experiment

[1] Extensive, and collocated measurements of the mass concentrations (M B ) of aerosol black carbon (BC) and (M T ) of composite aerosols were made over the Arabian Sea, tropical Indian Ocean and the Southern Ocean during a trans-continental cruise experiment. Our investigations show that M B remains extremely low (<50 ng m À3 ) and remarkably steady (in space and time) in the Southern Ocean (20°S to 56°S). In contrast, large latitudinal gradients exist north of $20°S; M B increasing exponentially to reach as high as 2000 ng m À3 in the Arabian Sea ($8°N). Interestingly, the share of BC showed a distinctly different latitudinal variation, with a peak close to the equator and decreasing on either side. Large fluctuations were seen in M T over Southern Ocean associated with enhanced production of sea-salt aerosols in response to sea-surface wind speed. These spatio-temporal changes in M B and its mixing ratio have important implications to regional and global climate. Citation: Moorthy, K. K., S. K. Satheesh, S. S. Babu, and A. Saha (2005), Large latitudinal gradients and temporal heterogeneity in aerosol black carbon and its mass mixing ratio over southern and northern oceans observed during a trans-continental cruise experiment, Geophys. Res. Lett., 32, L14818

One year measurements of aerosol optical properties over an urban coastal site: Effect on local direct radiative forcing

International audienceWe present results of direct aerosol radiative forcing over a French Mediterranean coastal zone based on one year of continuous observations of aerosol optical properties during 2005–2006. Monthly-mean aerosol optical depth at 440 nm ranged between 0.1 and 0.34, with high Angstrom coefficient (α > 1.2). The single scattering albedo (at 525 nm) estimated at the surface ranged between 0.7 and 0.8, indicating significant absorption. The presence of aerosols over the Mediterranean zone during summer decreases the shortwave radiation reaching the surface by as much as 26 ± 3.9 W m−2, and increases the top of the atmosphere reflected radiation by as much as 5.2 ± 1.0 W m−2. The shortwave atmospheric absorption translates to an atmospheric heating of 2.5 to 4.6 K day−1. Concerted efforts are needed for investigating the possible impact of the increase in heating rate on the maintenance of heat-waves frequently occurring over this coastal region during summer time

Aerosol size characteristics over the Arabian Sea and Indian Ocean: extensive sub-micron aerosol loading in the Northern Hemisphere

Aerosol optical depth (AOD) measurements in the spectral range 0.3–1.6 mm have been carried out using a multi-channel solar radiometer onboard ORV Sagar Kanya to understand the interaction between continental and marine aerosols. Experiments have been carried out under clear sky conditions for 22 days during 20 January–12 March 1999 covering the latitude belt between 17°N and 20°S, and longitude belt between 71°E and 59°E, over Indian Ocean and Arabian Sea as part of INDOEX IFP-99. Results show larger particle abundance, close to the Indian coast during forward leg compared to that of return leg of the cruise. AODs, in general, exhibit larger values north of ITCZ compared to that of south of ITCZ. The spatial distribution of AOD at 0.502 mm shows greater values north of ITCZ with significant latitudinal gradient compared to that of south of ITCZ. Aerosol size spectrum, derived from spectral AODs, shows monomodal distribution for both pristine oceanic environment (south of ITCZ) and for polluted environment (north of ITCZ)

Aerosol optical depths over peninsular India and adjoining oceans during the INDOEX campaigns: spatial, temporal, and spectral characteristics

The spatial, temporal, and spectral characteristics of aerosol optical depths τ p λ for the Indian Ocean Experiment (INDOEX) study period (January to April) are examined using data collected through a ground-based network of multiwavelength solar radiometers (MWR) over coastal regions of peninsular India; two island locations, one in the Arabian Sea and another in the southern Indian Ocean at 20°S; in conjunction with estimates made over various locations over the Arabian Sea and Indian Ocean during the INDOEX cruises of 1996, 1998, and 1999. Spatial variations show extremely low values of τ p at the shorter (visible) wavelengths (λ<750 nm) to the south of the Intertropical Convergence Zone (ITCZ), but increases substantially at locations due north of the ITCZ due to increased source impact and advection by favorable winds. An enhancement in τ p is seen in the central Arabian Sea, which is attributed to air trajectory effects. Ångström parameters, deduced from optical depth spectra, reveal a high value of α (∼0.9) for north of the ITCZ, while for the south α is negative, indicating a change in the aerosol size distribution. Accumulation aerosols dominate in the north, while concentration of coarse aerosols remain nearly about the same, except very close to the coast. A north-south gradient in aerosol optical depth, with scaling distance of ∼1000 to 2000 km at shorter wavelengths and much higher at longer wavelengths, is observed. The gradient becomes shallower at high wind speeds. The large-scale dynamics associated with the movement of the ITCZ and its interannual variation appears to significantly influence the aerosol characteristics. As the southwest monsoon sets in over India, considerable wet removal and change in air mass characteristics cause a significant depletion in optical depths, which then became comparable to those prevailing in the southern hemisphere