Spatial variations in aerosol characteristics and regional radiative forcing over India: Measurements and modeling of 2004 road campaign experiment
Introduction
Aerosols exert large radiative imbalance over regional scales both by scattering and absorption of radiation and also by modifying cloud properties such as cloud cover and cloud albedo (IPCC, 2001). The aerosol radiative forcing could be an order of magnitude larger than compared to well-mixed greenhouse gases over polluted regions, such as that reported for the Indo-Asian haze (Ramanathan et al., 2001). Similar ‘hot spots’ exist around the globe, for example the Saharan dust plume that spreads over most of the sub-tropical Atlantic, plumes from Indonesia, Brazil and southern Africa which consist mostly of biomass burning products etc. The challenging job is to estimate quantitatively the radiative forcing over these regions and incorporate them in global climate models to study the impact on regional and global climate scenario. In the recent past there have been several field campaigns, most of them are conducted over oceans to understand the impact of anthropogenically produced aerosols from nearby landmasses on the marine environment. TARFOX (Russell et al., 1999), ACE-I (Bates et al., 1998), ACE-II (Raes et al., 2000), INDOEX (Ramanathan et al., 2001), ACE-Asia (Huebert et al., 2003) are a few examples. Characteristics of aerosols found over ocean surrounding the peninsular India have also been studied extensively and their impact on surface reaching and atmosphere leaving solar radiation flux have been reported (e.g. Jayaraman et al., 1998; Satheesh and Ramanathan, 2000; Rajeev et al., 2001; Ramanathan et al., 2001; Coakley et al., 2002; Satheesh 2002; Ramachandran and Jayaraman, 2003; Ganguly et al., 2005a, Ganguly et al., 2005b). Realizing the need to have detailed information on the characteristics of aerosols, as well as on the amount of various trace gases found over the Indian main land, a field campaign was specially designed under the Indian Space Research Organization-Geosphere Biosphere Program (ISRO-GBP). The field experiment, conducted in Feb 2004, was participated by more than a dozen experimental groups belonging to different Institutes and Universities in India specialized in making measurements on aerosols, radiation and trace gases. We report here the important findings on the aerosol physical and optical properties obtained between Ahmedabad (23°2′N, 72°33′E) and Hyderabad (17°28′N, 78°27′E) from a variety of different but complementary techniques employed during the campaign and their estimated impact on aerosol radiative forcing.
Section snippets
The experiment
Aerosol measurements were carried out from a specially designed vehicle in which a variety of equipments such as the Micro Pulse Lidar (MPL), nephelometer, athelometer, Quartz Crystal Microbalance (QCM), Grimm Particle size analyzer etc. were mounted. The campaign started on 7 Feb 2004 from Ahmedabad and observations were made on daily basis conducted at selected locations (herein after referred to as stations) roughly at an interval of about 120 km, en route to Hyderabad, in a total stretch of
Meteorological conditions
Fig. 2 shows the average values of surface air temperature and relative humidity (RH) for all the stations obtained for the daytime period when aerosol measurements were made. Surface air temperature is in general high, above 30 °C in the northern region (station above 19°N) than compared to southern stations where the values are less than 30 °C. Similarly, the northern region is found very dry with RH less than 20% whereas in the southern region the RH value is slightly high, in the range of
Column aerosol optical depth
AOD observations are carried out using a hand-held multichannel sun-photometer (Microtops-II, Solar Light Co., USA) at six wavelength bands centered around 380, 440, 500, 675, 870 and 1020 nm. The sun-photometer works in the principle of measuring the surface-reaching solar radiation intensity at the specified wavelength bands and converts to optical depth by knowing the corresponding intensities at the top of the atmosphere (TOA). The TOA intensity values are periodically obtained using Langley
Summary and conclusion
A detailed study on the aerosol properties has been carried out in central India during Feb 2004 as part of the ISRO-GBP land campaign and the results are used to estimate the aerosol radiative forcing over the region. Measurements were made at 18 locations from 6 to 29 Feb, between Ahmedabad and Hyderabad, including a 6 days inter-comparison experiment at Shadnagar, 80 km south of Hyderabad. Results of the inter-comparison experiments are reported elsewhere. Based on the results obtained from a
Acknowledgments
The experiment was funded under the ISRO-Geosphere Biosphere Program. The authors would like to thank the ISRO-GBP office staff for their involvement in organizing the campaign.
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