K. Krishna Moorthy

Maritime Aerosol Network as a component of Aerosol Robotic Network

The paper presents the current status of the Maritime Aerosol Network (MAN), which has been developed as a component of the Aerosol Robotic Network (AERONET). MAN deploys Microtops handheld Sun photometers and utilizes the calibration procedure and data processing (Version 2) traceable to AERONET. A web site dedicated to the MAN activity is described. A brief historical perspective is given to aerosol optical depth (AOD) measurements over the oceans. A short summary of the existing data, collected on board ships of opportunity during the NASA Sensor Intercomparison and Merger for Biological and Interdisciplinary Oceanic Studies (SIMBIOS) Project is presented. Globally averaged oceanic aerosol optical depth (derived from island-based AERONET measurements) at 500 nm is similar to 0.11 and Angstrom parameter (computed within spectral range 440-870 nm) is calculated to be similar to 0.6. First results from the cruises contributing to the Maritime Aerosol Network are shown. MAN ship-based aerosol optical depth compares well to simultaneous island and near-coastal AERONET site AOD.

Investigations of marine aerosols over the tropical Indian Ocean

Results of the studies on the characteristics of marine aerosol optical depths made in remote regions of the tropical Indian Ocean using a multiwavelength solar radiometer, taken on a scientific cruise, are presented. The dependence of the aerosol optical depths on prevailing winds in marine boundary layer and on deck level relative humidity (RH) are examined. Aerosol optical depths (τp) in far oceanic regions with marine air mass prevailing, in general, are found to increase nearly exponentially with average wind speed (Ua) as τp=τo exp(a Ua), where a = 0.16 ± 0.04. No association is seen with deck level relative humidity, as the effects of wind appear to offset the effects due to RH. The columnar size distributions retrieved from aerosol spectral optical depths reveal two modes, one at ∼0.04 μm and a secondary mode at ∼0.8 μm. The columnar mass loading estimated from the size distributions increases exponentially with wind speed and has a wind independent value of ∼28 mg m−2.

Aerosol characteristics and radiative impacts over the Arabian Sea during the intermonsoon season: Results from armex field campaign

During the second phase of the Arabian Sea Monsoon Experiment (ARMEX-II), extensive measurements of spectral aerosol optical depth, mass concentration, and mass size distribution of ambient aerosols as well as mass concentration of aerosol black carbon (BC) were made onboard a research vessel during the intermonsoon period (i.e., when the monsoon winds are in transition from northeasterlies to westerlies/ southwesterlies) over the Arabian Sea (AS) adjoining the Indian Peninsula. Simultaneous measurements of spectral aerosol optical depths (AODs) were made at different regions over the adjoining Indian landmass. Mean AODs (at 500-nm wavelength) over the ocean (similar to0.44) were comparable to those over the coastal land (similar to0.47), but were lower than the values observed over the plateau regions of central Indian Peninsula (similar to0.61). The aerosol properties were found to respond distinctly with respect to change in the trajectories, with higher optical depths and flatter AOD spectra associated with trajectories indicating advection from west Asia, and northwest and west-coastal India. On average, BC constituted only similar to2.2% to total aerosol mass compared to the climatological values of similar to6% over the coastal land during the same season. These data are used to characterize the physical properties of aerosols and to assess the resulting short-wave direct aerosol forcing. The mean values were similar to27 W m(-2) at the surface and -12 W m(-2) at the top of the atmosphere (TOA), resulting in a net atmospheric forcing of +15 W m(-2). The forcing also depended on the region from where the advection predominates. The surface and atmospheric forcing were in the range -40 to -57 W m(-2) and +27 to +39 W m(-2), respectively, corresponding to advection from the west Asian and western coastal India where they were as low as -19 and +10 W m(-2), respectively, when the advection was mainly from the Bay of Bengal and from central/peninsular India. In all these cases, the net atmospheric forcing (heating) efficiency was lower than the values reported for northern Indian Ocean during northern winter, which is attributed to the reduced BC mass fraction.

Climate implications of large warming by elevated aerosol over India

Wide-ranging multi-platform data from a major field campaign conducted over Indian region was used to estimate the energy absorbed in ten layers of the atmosphere. We found that during pre-monsoon season, most of Indian region is characterized by elevated aerosol layers.Three-fold increase in aerosol extinction coefficient was observed at higher atmospheric layers (>2 km) compared to that near the surface and a substantial fraction (as much as 50 to 70%) of aerosol optical depth was found contributed by aerosols above (reflecting) clouds. Consequent absorption and hence strong warming above clouds was found larger by several degrees (K) compared to that near the surface. The aerosol-induced elevated warming was mostly confined below 2 km over northern Indian Ocean while found up to 4 km over central India, thus exhibiting strong meridional gradients (similar to 4 K) at atmospheric levels above 2 km. Climate implications of the large elevated warming are discussed.

Radiative forcing by aerosols over the Bay of Bengal region derived from shipborne, island‐based, and satellite (Moderate‐Resolution Imaging Spectroradiometer) observations

Measurements of spectral aerosol optical depths (AODs) were made over the Bay of Bengal region (adjacent to the Indian landmass) on board the oceanographic research vessel Sagar Kanya during February 2003. Simultaneous measurements of spectral AODs and mass concentrations of the composite aerosols and aerosol black carbon (BC) were made at an island location, Port Blair (11.63degreesN, 92.71degreesE), also in the Bay of Bengal. At the cruise locations the AODs were in the range of similar to0.3-0.6 at 500 nm (with a mean value of 0.41 +/- 0.14) and Angstrom wavelength exponent of similar to1.1 +/- 0.1; while at Port Blair the AODs were in the range of 0.11-0.48 at 500 nm and Angstrom wavelength exponent of 0.98 +/- 0.07. Aerosol BC constituted 5.8 +/- 0.6% of the composite aerosol mass concentration with a single-scattering albedo of similar to0.88, indicating the presence of a significant amount of submicron absorbing aerosols. Comparisons of AODs measured at Port Blair during cruise 188 and an earlier cruise (cruise 161B) during March 2001 (over the Bay of Bengal, Arabian Sea, and Indian Ocean) with those derived from Moderate-Resolution Imaging Spectroradiometer (MODIS) (on board the TERRA platform) showed excellent agreement with a mean difference of similar to0.01 and a root-mean-square difference of similar to0.03. Regionally averaged aerosol (net) forcing over the Bay of Bengal was in the range -15 to -24 W m(-2) at the surface and -2 to -4 W m(-2) at the top of the atmosphere in February 2003; these values were smaller in magnitude than those observed over this region during March 2001 and larger than that observed over the Arabian Sea and the Indian Ocean. The resulting atmospheric heating due to aerosol absorption was similar to0.5degreesK/d.

Chemical, microphysical, and radiative effects of Indian Ocean aerosols

Received 19 April 2002; revised 2 August 2002; accepted 6 August 2002; published 14 December 2002. [1] Extensive and long-term multistation measurements of aerosol properties and radiative fluxes were carried out in the haze plume off the South Asian continent. These experiments are carried out at Kaashidhoo Climate Observatory (KCO) (4.95� N, 73.5� E), Minicoy (8.5� N, 73.0� E), and Trivandrum (8.5� N, 77.0� E). In addition, the top of the atmosphere fluxes were measured simultaneously by the CERES radiation budget instrument. Long-term observations (more than 15 years) over Trivandrum show that there is a gradual increase in aerosol visible optical depth from � 0.2 in 1986 to � 0.4 in 1999. Pre- and post-monsoon aerosol characteristics are examined to study the seasonal variations. The impact of aerosols on short-wave radiation budget is estimated using direct observations of solar radiation using several independent ground-based radiometers and satellite data as well as from modeled aerosol properties. It was observed that ‘‘excess absorption’’ is not needed to model diffuse fluxes. The lower atmospheric heating due to absorbing aerosols was as high as � 20 W m � 2 which translates to a heating rate perturbation of � 0.5� K/day. The effect of aerosol mixing state (internally and externally) on aerosol forcing appears to be negligible. A sensitivity study of the effect of aerosols over land in contrast to that over the ocean shows an enhancement in lower atmosphere heating by about 40% simultaneous with a reduction of � 33% in surface cooling. Increasing sea-surface winds increase aerosol cooling due to increased sea salt aerosol concentrations, which can partly offset the heating effect due to absorbing aerosols. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 1610 Global Change: Atmosphere (0315, 0325); 1704 History of Geophysics: Atmospheric sciences; 4801 Oceanography: Biological and Chemical: Aerosols (0305); KEYWORDS: aerosols, radiative forcing, climate, chemical composition

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 tau (p lambda) 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 degreesS; 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 tau (p) at the shorter (visible) wavelengths (lambda < 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 tau (p) is seen in the central Arabian Sea, which is attributed to air trajectory effects. Angstrom parameters, deduced from optical depth spectra, reveal a high value of alpha (similar to0.9) for north of the ITCZ, while for the south alpha 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 similar to 1000 to 2000 kin 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.

Aerosol black carbon over Arabian Sea during intermonsoon and summer monsoon seasons

Extensive, collocated measurements of the mass concentrations of composite and black carbon (BC) aerosols were made over coastal Arabian Sea, adjoining Indian Peninsula, for the first time during the inter-monsoon and summer monsoon periods, of 2003, as part of Arabian Sea Monsoon Experiment (ARMEX). Results showed that the diurnal variations are weak in March, and vanish completely by May/June, associated with the change in the synoptic circulations. The concentration of BC (and its share to total aerosol mass) decreases continuously, from similar to700 ng m(-3) (2.5%) in March to similar to104 ng m(-3) (0.5%) by June. Consequently, the net atmospheric forcing (heating) efficiency decreases from similar to70 W m(-2) (for reported winter conditions) to similar to30 W m(-2) for inter-monsoon and to similar to15 W m(-2) for summer monsoon seasons. These will have implications on regional climate forcing.

Size Distribution of Coastal Aerosols: Effects of Local Sources and Sinks

Abstract Using aerosol optical depth as a function of wavelength obtained from ground-based multiwavelength radiometer observations, columnar size-distribution functions of aerosols have been derived. It has been found that the nature of the derived size-distribution function is strongly dependent on season. The derived size-distribution functions are discussed in term of seasonally dependent natural aerosol sources and sinks.

Wintertime spatial characteristics of boundary layer aerosols over peninsular India

During an intense field campaign for generating a spatial composite of aerosol characteristics over peninsular India, collocated measurements of the mass concentration and size distribution of near-surface aerosols were made onboard instrumented vehicles along the road network during the dry, winter season (February-March) of 2004. The study regions covered coastal, industrial, urban, village, remote, semiarid, and vegetated forestlands. The results showed (1) comparatively high aerosol (mass) concentrations (exceeding 50 μ g m(-3)), in general, along the coastal regions (east and west) and adjacent to urban locations, and (2) reduced mass concentration ( 50% of the total) of coarse-mode aerosols (>1 μ m). The spatial composite of accumulation-mode share to the total aerosol mass concentration agreed very well with the monthly mean spatial composite of aerosol fine-mode fraction for February 2004, deduced from Moderate-Resolution Imaging Spectroradiometer data for the study region, while a point by point comparison yielded a linear association with a slope of 1.09 and correlation coefficient of 0.79 for 76 independent data pairs. Pockets of enhanced aerosol concentration were observed around the industrialized and urban centers along the coast as well as inland. Aerosol size distributions were parameterized using a power law. Spatial variation of the retrieved aerosol size index shows relatively high values (>4) along the coast compared to interior continental regions except at a few locations. Urban locations showed steeper size spectra than the remote locations.