P. C. S. Devara

Aerosol Chemistry over a High Altitude Station at Northeastern Himalayas, India

Background There is an urgent need for an improved understanding of the sources, distributions and properties of atmospheric aerosol in order to control the atmospheric pollution over northeastern Himalayas where rising anthropogenic interferences from rapid urbanization and development is becoming an increasing concern. Methodology/Principal Findings An extensive aerosol sampling program was conducted in Darjeeling (altitude ∼2200 meter above sea level (masl), latitude 27°01′N and longitude 88°15′E), a high altitude station in northeastern Himalayas, during January–December 2005. Samples were collected using a respirable dust sampler and a fine dust sampler simultaneously. Ion chromatograph was used to analyze the water soluble ionic species of aerosol. The average concentrations of fine and coarse mode aerosol were found to be 29.5±20.8 µg m−3 and 19.6±11.1 µg m−3 respectively. Fine mode aerosol dominated during dry seasons and coarse mode aerosol dominated during monsoon. Nitrate existed as NH4NO3 in fine mode aerosol during winter and as NaNO3 in coarse mode aerosol during monsoon. Gas phase photochemical oxidation of SO2 during premonsoon and aqueous phase oxidation during winter and postmonsoon were the major pathways for the formation of SO4 2− in the atmosphere. Long range transport of dust aerosol from arid regions of western India was observed during premonsoon. The acidity of fine mode aerosol was higher in dry seasons compared to monsoon whereas the coarse mode acidity was higher in monsoon compared to dry seasons. Biomass burning, vehicular emissions and dust particles were the major types of aerosol from local and continental regions whereas sea salt particles were the major types of aerosol from marine source regions. Conclusions/Significance The year-long data presented in this paper provide substantial improvements to the heretofore poor knowledge regarding aerosol chemistry over northeastern Himalayas, and should be useful to policy makers in making control strategies.

Effect of dust and anthropogenic aerosols on columnar aerosol optical properties over Darjeeling (2200 m asl), eastern Himalayas, India.

Background The loading of atmospheric particulate matter (aerosol) in the eastern Himalaya is mainly regulated by the locally generated anthropogenic aerosols from the biomass burning and by the aerosols transported from the distance sources. These different types of aerosol loading not only affect the aerosol chemistry but also produce consequent signature on the radiative properties of aerosol. Methodology/Principal Findings An extensive study has been made to study the seasonal variations in aerosol components of fine and coarse mode aerosols and black carbon along with the simultaneous measurements of aerosol optical depth on clear sky days over Darjeeling, a high altitude station (2200 masl) at eastern Himalayas during the year 2008. We observed a heavy loading of fine mode dust component (Ca2+) during pre-monsoon (Apr – May) which was higher by 162% than its annual mean whereas during winter (Dec – Feb), the loading of anthropogenic aerosol components mainly from biomass burning (fine mode SO4 2− and black carbon) were higher (76% for black carbon and 96% for fine mode SO4 2−) from their annual means. These high increases in dust aerosols during pre-monsoon and anthropogenic aerosols during winter enhanced the aerosol optical depth by 25 and 40%, respectively. We observed that for every 1% increase in anthropogenic aerosols, AOD increased by 0.55% during winter whereas for every 1% increase in dust aerosols, AOD increased by 0.46% during pre-monsoon. Conclusion/Significance The natural dust transport process (during pre-monsoon) plays as important a role in the radiation effects as the anthropogenic biomass burning (during winter) and their differential effects (rate of increase of the AOD with that of the aerosol concentration) are also very similar. This should be taken into account in proper modeling of the atmospheric environment over eastern Himalayas.

Study of aerosol optical depth, ozone, and precipitable water vapour content over Sinhagad, a high-altitude station in the Western Ghats

This article reports the results of a study related to variations in columnar aerosol optical depth (AOD), total column ozone (TCO), and precipitable water content (PWC) over a high-altitude station, Sinhagad (18° 21′ N, 73° 45′ E, 1450 m above mean sea level (AMSL)), employing a microprocessor-based total ozone portable spectrometer, MICROTOPS-II, comprising both a sun photometer and ozonometer, during November 2009–April 2010. The aerosol optical depth at 500 nm (AOD500 nm) portrayed seasonal variation with higher values (0.39) in summer and lower values (0.15) in winter. The TCO and PWC also exhibited lower values in winter and started increasing by the pre-monsoon season. The Ångström wavelength exponent, α, was found to be high (1.79) during February, indicating the relative dominance of accumulation-mode particles. During the summer season, the lower value (0.94) of the Ångström wavelength exponent indicates the relative dominance of coarse-mode particles. The ground-based observations from MICROTOPS-II revealed good correlation with satellite observations of the Moderate Resolution Imaging Spectroradiometer (MODIS) and Ozone Monitoring Instrument (OMI). The observed short-wave solar flux at the bottom of the atmosphere decreased due to aerosol extinction and was found to be 19 and 78 W m–2 for the winter and pre-monsoon seasons, respectively. This implies that greater concentrations of accumulation-mode particles – which are due to local anthropogenic sources – affected the down-welling radiation than those from natural sources – which are due to long-range transport processes – over the experimental location.

Lidar measurements of aerosol column content in an urban nocturnal boundary layer

Abstract Lidar aerosol measurements made at Pune (lat. 18°32′N, long. 73°51′E, 559 m AMSL), India, a tropical urban station, during the nine-year period from October 1986 to September 1995 have been used to study the temporal variations in the aerosol-loading in the nocturnal boundary layer. There is a long-term increasing trend in the aerosol column content in the 50–1100 m layer that has been attributed to be due to the increasing anthropogenic activity around the lidar site. The seasonal variations in aerosol content show a maximum in the pre-monsoon month of May and a minimum in the SW monsoon month of July. The percentage contribution of the 50–200 m layer to the total loading in the 50–1100 m layer is about 41% which points to the predominant surface source of particulate matter. On a seasonal scale, there is a decrease of about 36% in the aerosol content from premonsoon (March-May) to monsoon (June-September) season and this decrease on year-to-year basis is directly related to the amount of rainfall received at the ground at Pune during the monsoon season. This effect has not changed appreciably over the years whatever be the increase in the aerosol loading. The temporal evolution of aerosol content in the nighttime showed a rapid decrease soon after sunset and a slower rate of decrease in the midnight hours. The study also showed that surface relative humidity and winds influence the temporal variations in the aerosol column content.

ON THE STABLE STRATIFICATION OF THE NOCTURNAL LOWER TROPOSPHERE INFERRED FROM LIDAR OBSERVATIONS OVER PUNE, INDIA

The nocturnal structure of the lower troposphere is studied using aerosol profile data (50–2800 m AGL) obtained with a bistatic, continuous wave, Argon ion lidar system during October 1986–September 1989 at Pune (18°32′ N, 73°51′ E, 559m AMSL), India. The top of the nocturnal groundbased inversion is taken as the height above ground where the negative vertical gradient in aerosol concentration first reaches a maximum. During the post-sunset period over this station, this height is as low as 160m and frequently lies around 550m. Greater heights are observed in pre-monsoon months and smaller ones during the southwest monsoon season. Positive vertical gradients in aerosol concentration, indicative of stable/elevated layers, appear frequently around 750m. Temporal variations of aerosol concentration gradients in two adjacent air layers, 920–1000m and 100–1100m, provide evidence that stability increases downward in the early night hours.

Aerosol characteristics during the coolest June month over New Delhi, northern India

June 2008, which is also the transition month between two major seasons for Indo-Gangetic Basin (IGB), has been identified the coolest June over New Delhi during the past century, showing mean temperature of 31.6 ± 1.7°C, which was found to be ∼2°C less than its climatological mean (33.9°C). Aerosol optical properties for this month and thus obtained physical parameters have been studied using data from the CIMEL sun/sky radiometer, installed in New Delhi under the Aerosol Robotic Network (AERONET) programme. Results reveal bimodal aerosol volume size distribution. The monthly mean values for aerosol optical depth (AOD) at 500 nm (0.96 ± 0.31) and Ångström exponent at the wavelength pair of 440–870 nm (0.79 ± 0.42) show significant lower values whereas single scattering albedo at 675 nm shows a significantly larger value (0.94 ± 0.04) compared with previous measurements over the station. Results suggest dominance of scattering-type particles such as water-soluble aerosols from anthropogenic sources and dust aerosols from natural sources with higher relative humidity over the station. Radiative forcing caused due to the aerosols for the month of June 2008, which have been computed using the radiative-transfer model, informs low forcing at the top of atmosphere (TOA, +14 W m−2) as well as at surface (−33 W m−2). The resultant atmospheric forcing (+47 W m−2) indicates warming effect that caused heating of lower atmosphere at the rate of 0.89 K day−1.

Aerosol characterization: comparison between measured and modelled surface radiative forcing over Bay of Bengal

Under the aegis of Indian Space Research Organisations ongoing climate change research programme, Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) was undertaken from 27 December 2008 to 30 January 2009 over the Bay of Bengal (BoB). Since measurements of this sort have not been made near the eastern-most coastline of BoB, a cruise was planned to have observations along the boundaries of the BoB. Here we report the columnar aerosol optical thickness (AOT), total water vapour (TWV), total ozone column (TOC) and aerosol direct surface forcing calculations by using a pair of handheld Microtops II (Ozonometer and Sunphotometer) and a gimbal-mounted short-wave (SW) pyranometer, with high temporal resolution (5 minutes). The presence of high aerosol loading and absorbing aerosols over the coastal and Andaman regions resulted in a diurnal mean aerosol surface radiative forcings of −24 and −35 W m−2, respectively. These results are in good agreement with radiative transfer model estimations.

Lidar profiling of aerosols and clouds for regional climate and pollution research

The space-time variability of aerosol inhomogeneities provides unique information on atmospheric behavior needed for climate and environmental research and operational programs. An additional indirect forcing from aerosols results from their involvement in nucleation and growth of cloud droplets, reducing droplet size and thereby potentially influencing cloud albedo. These studies have particular significance over tropics where the convective and dynamical processes associated with high-altitude thunderstorms greatly affect the vertical distributions of aerosols and pre-cursor gases. As the anthropogenic share of the total aerosol loading is quite substantial over many parts of the world, it is essential to monitor the aerosol features systematically over longer time scales. Such observations are very important for understanding the coupling processes that exist between physico-chemical, radiative, dynamical and biological phenomena in the Earths environment, and provide valuable input information for modeling and simulation studies of climate and air quality. The multi-year aerosol number density data acquired during October 1986-September 2000 with a computer-controlled lidar at the Indian Institute of Tropical Meteorology (IITM), Pune, an urban station in India have been utilized to investigate (i) climate variability, (ii) cloud macro-physical parameters and (iii) environmental pollution. The results reveal a long-term trend in aerosol loading, single and multiple layer clouds with low cloud-base during the south-west monsoon months, and high pollution potential during winter late evenings. The trends in aerosol loading and air quality are found to be changing from year to year depending upon meteorological parameters (precipitation in particular). Some of these parameters have also been compared with co-located complementary facilities such as solar radiometers. In order to enlarge the scope of these studies, a dual polarization micro pulse lidar (DPMPL) has been installed at IITM recently to investigate the cloud composition, and aerosol-cloud-climate interactions. The initial results obtained from this state-of-the-art lidar system showed interesting features in the time evolution of nocturnal (stable) boundary layer which have strong bearing on air pollution potential over the experimental station. The complete details of the lidar systems used in the above studies together with discussion of salient results are presented in this paper.

Optical remote sensing of atmospheric total ozone with radiometers

A high-spectral resolution radiometer and a Volz sunphotometer have been in operation to monitor solar irradiance in the visible region 0.4 micrometers 0.7 micrometers , and at five discrete wavelengths covering the visible and near IR regions, respectively, since November 1993 at the Indian Institute of Tropical Meteorology, Pune, India. Using the spectroradiometer-derived spectral variation of total optical depth in the Chappuis-band, ozone content in the atmospheric column was estimated by following the multiple regression method. Total ozone content in the atmosphere was estimated by following the multiple regression method. Total ozone content in the atmosphere was also determined by using the differential absorption of solar radiation at two wavelengths in and around the Chappuis-band from the sunphotometer observations. The observations carried out on about 200 cloud-free days spread over the period from February 1993 to May 1996 were used in the study. A comparison of total ozone, thus retrieved by the Chappuis- band method differed from the Dobson spectrophotometer measurements by about +/- 20 percent on the most stable days. These differences can be attributed to a combination of large aerosol optical depths, diurnal variation of aerosol optical depth, the deviation from the assumed power law relationship coefficients. The ozone optical depths inferred experimentally from the Chappuis-band method have been used to determine more accurate aerosol optical depths as compared to those routinely to those routinely obtained by using model ozone vertical profiles.

An Optical Scintillometer for Simultaneous Measurements of Atmospheric C2n and Winds

Laser scintillations produced due to the inhomogeneities present in the atmosphere can be used for measuring refractive index structure parameter (Cn 2) as well as wind velocity. The present paper deals with the description of an optical scintillometer developed at the Indian Institute of Tropical Meteorology (IITM), Pune, India for simultaneous measurements of path-averaged Cn 2 and crosswinds. Different evaluation techniques and their relative merits for wind estimation from optical scintillometer observations are discussed. The results show good agreement between the scintillometer evaluated winds and the winds measured simultaneously by using propeller anemometer at the same location. In addition, this paper explains the effect of Cn 2 and wind fluctuations along the pathlength on the derived wind information from scintillometer observations.