IASTA 2010 Proceedings
+ Invited Talks
+ SESSION A - AEROSOL CHARACTERIZATION
A-O-1 Aerosol Effect on Precipitation...
A-O-2 Sunshine Duration Trend...
A-O-3 A Model Study of the...
A-O-4 Small Ion Concentration...
A-O-5 Vertical Distribution of...
A-O-6 Aerosol transport and...
A-O-7 Aerosol and Cloud...
A-O-8 Effects of Rain Drop-...
A-O-9 On the Association...
A-O-10 Evaluation of the Seasonal...
A-P-1 A Study on Distribution...
A-P-2 A Study on Optical...
A-P-3 The Relationship between...
A-P-4 Influence of Tropical...
A-P-5 Vertical Aerosol Profiles...
A-P-6 Dissimilarities in Maximum...
A-P-7 Measurement of the Atmospheric...
A-P-8 Assessment of Solid and...
A-P-9 Seasonal Variations in PM10...
A-P-10 Variation of Aerosol Optical...
A-P-11 Effect of Dust and Rain...
A-P-12 Aerosol Properties of the...
A-P-13 A Study of the Aerosol...
A-P-14 Retrieval of Background...
A-P-15 Influence of Charged Dust...
A-P-16 Number Density Characteristics...
A-P-17 Temporal and Spectral...
A-P-18 Numerical Estimation of the...
A-P-19 Lidar Measurements of Vertical...
A-P-20 Aircraft Observations of Cloud...
A-P-21 Aerosol Characteristics at High...
A-P-22 Seasonal Variability in Aerosol...
A-P-23 Spatial and Temporal Variability...
A-P-24 Airborne Measurements of Micron-...
A-P-25 Wintertime Vertical Profiles of...
A-P-26 Relationship between Pre-monsoon...
A-P-27 Seasonal Variation of Aerosol...
A-P-28 Influence of Aerosols on near...
A-P-29 Association between Stratosphere...
A-P-30 Response of Surface Ozone...
A-P-31 Aerosol Source Characteristics...
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+ SESSION B - AEROSOL REMOTE SENSING-I
+ SESSION C- RADIOACTIVE AEROSOL, HEALTH EFFECTS
+ SESSION D AEROSOL CAMPAIGNS / SPECIAL EVENTS
+ SESSION E AEROSOL REMOTE SENSING
+ SESSION F CARBONACEOUS AEROSOLS
+ SESSION G NANO PARTICLES SYNTHESIS
+ SESSION H AEROSOL CHARACTERIZATION II
+ SESSIONS I INDOOR AEROSOLS
IASTA-1C

IASTA-2010

A–P–21

Aerosol Characteristics at High Altitude Western Himalayan Region: Observational Results from Skyradiometer

Shantikumar Singh Ningombam, S. P. Bagare, Neeharika Sinha and Rajendra B. Singh

Indian Institute of Astrophysics, Koramangala Block II, Bangalore 560034, India

ABSTRACT : Attenuation of solar radiation due to scattering and absorption by aerosols, which is referred to as turbidity of the atmosphere, is a function of both number and size distribution. Aerosol concentration and type are highly heterogeneous in space and time, depending upon the characteristics of the sources, lower and upper air synoptic meteorological conditions as well as the topography of the region. Aerosols are confined mainly at the boundary layer and their number concentrations depend on various factors such as topography, meteorological parameters, annual and diurnal cycles and the presence of local and transport sources. Study of aerosols has many applications in the aspect on geosphere and biosphere and it plays a significant role in climate change phenomenon through direct and indirect effects.

In astronomical terms, a good astronomical site can be characterized by various factors on meteorological parameters, low atmospheric turbulence, minimal cloud coverage, high atmospheric transparency i.e. minimal aerosol concentration. Recently, the Indian Institute of Astrophysics, Bangalore has installed a Skyradiometer (POM-01L, Prede, Ltd., Japan) at the Indian Astronomical Observatory (IAO), Hanle (32047', N and 78058' E, 4500 m, amsl), Ladakh, as a part of astronomical site survey program for searching a suitable site for the proposed National Large Solar Telescope (NLST). The instrument consists of an automatic sun tracker, a spectral scanning radiometer, with the sensors of rain and sun for the purpose of self guiding and observation system. It retrieved the aerosol optical properties such as aerosol optical depth (AOD), single scattering albedo, phase functions and volume size distribution from the sun-sky radiance measurement at 400, 500, 675,870 and 1020 nm.

It is found that AOD present at Hale is very low throughout the year. The annual averaged AOD value obtained at Hanle is about 0.050 +/- 0.001 at 500 nm during October 2007 to October 2008. The measured AOD values at Hanle is comparable to those reported at other high altitude location such as Nam Co (4720 m amsl), located in central Tibetan Plateau. The nature of aerosol size distribution shows bi-modality [in Fig.1 & Fig. 2(a)], which suggests the presence of both coarse and fine mode aerosols at Hanle. However, on few occasions during summer month, the size distribution shows tri-model signature [in Fig. 2 (b)]. Such features are common during summer season due to the dominance of coarse mode aerosol sizes. The presence of coarse mode aerosol is responsible due to wind driven desert dust aerosols at the site. There are indications for transport of African desert aerosols towards the observing site from study of back trajectory analysis of Hybrid Single Particle Langrangian Integrated Trajectory (HYSPLIT) model. The prevailing wind direction at Hanle is predominantly south westerly with the yearly

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averaged wind speed of ~ 5 m/s. However, during the case of windy day, it went up gradually with the maximum of 15-20 m/s from after noon and it went down in the evening. The details of these results are discussed in the paper.

Figure 1 Figure 2

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A–P-22

Seasonal Variability in Aerosol Parameters over Pune, an Urban Site in the Western-Ghats

Sumit Kumar and P.C.S. Devara

Indian Institute of Tropical Meteorology, Dr. Homi Bhabha Road, Pune-8, India

Introduction

Studying climate system and its changes in both space and time are important because changing environmental conditions will affect people around the world. Due to heterogeneous nature, aerosols have been recognized as the most important atmospheric constituents, which can be used as indicators of regional air pollution and constitutes unequivocal impact on global climate. Aerosol can directly affect climate change by scattering and absorption of solar and other radiations, and also indirectly affect the cloud microphysics, chemistry and dynamics. While tropospheric aerosols associate with air quality and cause adverse health effects, stratospheric aerosols often contribute to global air pollution. These effects strongly depend on the physico-chemical and optical properties of aerosol particles. For estimating more accurately the abundance and sources of aerosols and better understanding of aerosol affects global climate, various active and passive measurements have been far improved in recent years.

Equipment, Data and Analysis Procedures

A ground-based polarimetric CIMEL Sun-sky radiometer has been deployed in October 2004 and put into regular operation at the Indian Institute of Tropical Meteorology (IITM) Pune, India with an objective to characterize aerosols in the Western Ghat region. This instrument is described in detail in the literature (Holben et al., 1998). The automatic tracking polarized Sun and sky scanning radiometer makes direct Sun measurements with a 1.2° full field-of-view every 15 min at 440, 675, 870, 940, and 1020 nm (nominal wavelengths), in addition to three polarized channels at 870 nm. The direct Sun measurements take ~ 8 seconds to scan all 8 wavelengths, with a motor driven filter wheel positioning each filter in front of the detector. These solar extinction measurements are then used to compute aerosol optical depth at each wavelength except for the 940-nm channel, which is used to retrieve total columnar (or precipitable) water vapor in centimeters. The spectral aerosol optical depth data have been screened for clouds following the methodology developed by Smirnov et al. (2000).

The CIMEL sky radiance measurements in the almucantar geometry (fixed elevation angle equal to solar elevation and a full 360° azimuthal sweep) at 440, 675, 870, and 1020 nm (nominal wavelengths) in conjunction with the direct Sun measured ta at these same wavelengths were used to retrieve optical equivalent aerosol size distributions and refractive indices. Using this microphysical information the spectral dependence of single scattering albedo (wo) is calculated. The algorithm of Dubovik and King (2000) was utilized in these

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retrievals. The optical properties of the aerosols over Pune, which is one of the rapidly growing cities, show strong seasonal and inter-annual variations. Also, its meteorological climatology is quite peculiar, affected by seasonally varying air-masses originating from land and oceanic regions. It remains mostly under cloud cover during the monsoon months and receive good amount of rain, whereas during other seasons the dry weather prevails over the experimental station.

Results and Discussions

Figure 1 shows the monthly average of AOD at 440 nm and Angstrom exponent (a) at 440-870 nm for 2004–2007. Both these parameters show inter-annual and seasonal

Figure 1. Monthly average AOD at 440 nm wavelength and Angstrom exponent over Pune during Oct-2004– 2007, with the vertical bars representing ±1 standard deviation

variations. The seasonal variations in AOD were discussed in detail by Devara and Sumit (2009). It is clear from the figure that AOD shows an increasing trend and Angstrom exponent shows decreasing trend,

indicating abundance of coarse-mode particle concentration, which is consistent with the results reported over Pune by Rohini (2008).

The parameter, effective radius (Reff) is quite representative of the optical properties of coarse-mode particles, whereas for fine-mode particles, volume weighted mean radius (RV) is more appropriate parameter (Tanre et al., 2001). The monthly mean Reff (for coarse-mode) and RV (for fine-mode) are shown in

Fig. 2. Reff and RV are found to be higher during post-monsoon season.

Both show decreasing trend from post-monsoon to pre-monsoon

Figure 2. Monthly mean effective radius (for coarse mode) and volume weighted mean radius (for fine mode) of aerosol size distribution observed over Pune

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season. The increase in RV is attributed to the hygroscopic growth of the fine-mode particles in the presence of higher relative humidity.

However, the hygroscopic growth of the fine particles is more pre-dominant during post-monsoon and winter seasons, when the temperature condition is more favorable. Another probability for this observation could be coagulation of particles. RV is observed to be low during the pre-monsoon season indicating the relatively higher contribution of the coarse-mode particles to the observed AOD as compared to that of fine-mode particles.

The increase and decrease in Reff during post-monsoon and pre-monsoon seasons respectively, are interesting. Although no significant coarse-mode particle loading takes place during post-monsoon and the hygroscopic growth of these particles are unlikely, the effective radius of coarse-mode particles may increase if the fine-mode particles get attached to the surface of the coarse-mode particles. Further, the nature of the particles can be inferred from the single scattering albedo (SSA). SSA at 440 and 1020 nm wavelengths shows significant differences in their absolute values (Figure 3) with maximum difference during the months of winter and post-monsoon seasons. The variations in SSA values suggest the dominance of scattering type particles during pre-monsoon and post-monsoon whereas absorbing type during winter season. The difference between SSA440 nm and

SSA1020nm is positive for all seasons, however it is greater for post-monsoon and winter months, which indicates the presence of anthropogenic pollutants during these months.

Figure 3. Monthly average of SSA at 440 nm and 1020 nm with the vertical bars representing ±1standard deviation observed over Pune

Acknowledgement

The authors wish to acknowledge M. G. Manoj for the help rendered in archiving the data. Thanks are also due to Director, IITM for the infrastructure support. One of the authors (Sumit) wishes to acknowledge the Institute for the SRF.

References

Devara, P.C.S. and Sumit, K. (2009): Local aerosol climatology from Cimel Sun sky radiometer and its synergism with in-situ observations over Pune, a tropical urban site in India, Proc. The Second International Conference of Aerosol Science and Global Change, August 16-21, 2009, Hangzhou, China.

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Dubovik, O. and King, M.D., (2000): A flexible inversion algorithm for retrieval of aerosol optical properties from Sun and sky radiance measurements. J. Geophys. Res., 105, 20673-20696.

Holben, B.N., and Co-authors (1998): AERONET – A federated instrument network and data archive for aerosol characterization. Remote Sens. Environ., 66, 1–16.

Rohini, L.B. (2008): Aerosol Characterization from Satellite and Ground-based Measurements, Ph.D. Thesis, University of Pune, Pune.

Smirnov, A., and Co-authors (2000): Cloud screening and quality control algorithms for the AERONET database. Remote Sens. Environ., 73, 337-349.

Tanre, D., and Co-authors (2001): Climatology of dust aerosol size distribution and optical properties derived from remotely sensed data in the solar spectrum. J. Geophys. Res., 106, 18205–18217.

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A–P–23

Spatial and Temporal Variability in Aerosol Radiative Forcing over Different Environments in India

Sumita Kedia* and S. Ramachandran

Physical Research Laboratory, Navrangpura, Ahmedabad, India-380009

sumita@prl.res.in ram@prl.res.in

Atmospheric aerosols from both natural and anthropogenic sources affect the Earth atmosphere radiation budget directly by scattering and absorbing the incoming solar radiation, and indirectly by modifying the cloud radiative properties through altering the cloud microphysical properties. Due to very short lifetimes and widely distributed sources, aerosols are still the largest source of uncertainty in prediction of climate change [IPCC, 2007]. The potential for aerosol forcing of climate can vary according to regional differences in aerosol columnar concentration as well as its chemical composition [Eldering et al., 2002]. It is also well known that different aerosols interact with radiation in different ways; for example black carbon is highly absorbing and has a warming effect while sulfate is highly scattering and exhibit cooling effect in the atmosphere. Thus, knowledge of aerosol chemical composition is important to determine the scattering and absorption characteristics of aerosols.

Indian subcontinent is one of the most densely populated areas in the world. This region has become one of the potential sources for many kinds of natural and anthropogenic aerosols such as mineral dust, soot, nitrate, sulfate, and organic particles due to rapidly growing industrialization and expanding urbanization in recent years. In the present study we have analyzed the variation in the shortwave aerosol radiative forcing estimated using measured aerosol optical physical and chemical properties over different locations (Ahmedabad, Gurushikhar, Kanpur, and Gandhi College) in India (Figure 1).

Figure 1. Map showing Ahmedabad, Gurushikhar (Mount Abu), Kanpur, and Gandhi College

Ahmedabad is an urban, industrialized region situated in the western India; while Gurushikhar located in the same longitudinal belt as Ahmedabad is a high altitude remote location. Kanpur and Gandhi College are situated in the most polluted and densely populated Indo- Gangetic plain. Shortwave aerosol radiative forcing are estimated over all these locations to get further understanding on the spatial and

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temporal variability of aerosol radiative forcings over different environments.

Measurement of aerosol optical depths (AODs) at six different wavelength bands centered around 0.4, 0.5, 0.65, 0.75, 0.875, and 1.02 μm were conducted over Ahmedabad and Gurushikhar, Mount Abu using a hand held sun photometer during 2006-2008. AODs are also obtained over Kanpur and Gandhi College for the same time period from AERONET (Aerosol Robotic Network) locations using ground based multi wavelength sun photometer in the wavelength range of 0.38-1.02 μm.

Estimation of aerosol radiative forcings are performed using radiative transfer model, SBDART [Ricchiazzi et al., 1998], making use of the measured aerosol optical, physical (mass) and chemical (sulfate, dust, black carbon, and sea salt) characteristics over all the study locations during the study period. The principal input parameters required for calculating aerosol radiative forcing are aerosol optical depth, single scattering albedo (SSA) and asymmetry parameter (g). OPAC [Hess et al., 1998] model has been used to estimate these input parameters for entire shortwave range (0.25-4.0 μm) making use of all the measured aerosol properties [Kedia et al., 2009].

Over Ahmedabad, aerosol scattering and absorption coefficients measured using Nephelometer and Aethalometer are used to calculate the single scattering albedo (SSA) which is the second important parameter next to AOD to influence the aerosol radiative forcing. Aerosol optical depth, single scattering albedo measured over Ahmedabad are further utilized to constrain the output of OPAC and AOD, SSA, g are obtained for entire shortwave region.

Figure 2. Monthly mean clear sky shortwave aerosol radiative forcing at the top of the atmosphere, surface, and atmosphere over Ahmedabad during 2008

Figure 2 shows the monthly mean aerosol shortwave radiative forcing estimated at Ahmedabad during 2008 at the top of the atmosphere (TOA), surface, and atmosphere. Over Ahmedabad, the shortwave aerosol radiative forcing at the surface is found to be maximum (-50 Wm-2) during October-December, while the minimum surface forcing (-23 Wm-2) is observed during July. A large variability is observed in the monthly mean forcing values at all the atmospheric levels over Ahmedabad. The forcing values are found to be highest during winter which is caused due to highest BC mass concentration during this season. The magnitude and the sign of TOA show seasonal dependence. The TOA is found to be negative during April-September and it becomes positive during October- March. Results showed that surface and atmospheric forcing during pre-monsoon and monsoon are 40-60% lower than those obtained during winter and post-monsoon.

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The monthly mean shortwave aerosol radiative forcings estimated over Gurushikhar during 2007 are shown in Figure 3. Note that due to the absence of the aerosol optical measurements over Gurushikhar owing to clouds, aerosol radiative forcing could not be estimated during monsoon. Large variability in the forcing is observed at all the atmospheric levels over Gurushikhar indicating changing aerosol properties in the atmosphere. The forcing is highest during premonsoon season which can be attributed to the presence of dust aerosols over Gurushikhar. The forcings over Gurushikhar for all the months are about a factor of four less when compared to Ahmedabad (Figure 2).

Figure 3. Monthly mean clear sky shortwave aerosol radiative forcing at the top of the atmosphere, surface, and atmosphere over Gurushikhar during 2007

Similarly, shortwave clear sky aerosol radiative forcings have been estimated over Kanpur and Gandhi College. Results obtained on the spatial and temporal variability in aerosol properties over different environments in India will be presented and discussed.

References

Eldering, A., et al. (2002), Aerosol optical properties during INDOEX based on measured aerosol particle size and composition, J. Geophys. Res., 107, doi:10.1029/2001JD001572.

Hess, M., P. Koepke, and I. Schult (1998), Optical properties of aerosols and clouds: The software package OPAC, Bull. Am. Meteorol. Soc., 79, 831-844.

Intergovernmental Panel on Climate Change (IPCC) (2007), Summary for policymakers, in Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, edited by S. Solomon et al., pp. 1 – 18, Cambridge Univ. Press, Cambridge, U. K.

Ricchiazzi, P., S. Yang, C. Gautier and D. Sowle (1998), SBDART: A research and teaching software for plane- parallel radiative transfer in the Earth’s atmosphere, Bull. Am. Meteorol. Soc., 79, 2101-2114.

Kedia, S., S. Ramachandran, Ashwini Kumar, and M. M. Sarin (2009), Spatiotemporal gradients in aerosol radiative forcing and heating rate over marine regions derived incorporating optical, physical and chemical properties, J. Geophys. Res., In press.

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A–P–24

Airborne Measurements of Micron-sized Aerosols over Eastern India

V Gopalakrishnan1, Jai Devi2, P. Murugavel1 and S.N. Tripati2

1Indian Institute of Tropical Meteorology, Pune

2Indian Institute of Technology, Kanpur

Introduction

Role of atmospheric aerosols in influencing various atmospheric processes such as climate change, cloud formation, etc has been much emphasized in recent year. Various processes ranging from large-scale atmospheric circulation to local convective motion transport the aerosols produced at one location to distant places, and cause them to disperse into the atmosphere. For example, observations made during INDOEX effectively demonstrate pollutants are transported even to southern hemisphere from continents of northern hemisphere. (Ramanathan et al., 2001; Lelieveld et al., 2001). Further, the vertical stability of the atmosphere strongly influences the vertical distribution of aerosols over a region. Murugavel et.al (2005) has studied the variability of aerosol concentration within the boundary layer and has found sharp gradients of aerosols just above the top of the boundary layer. There are numerous reports (O’Dowd, 2002,; O’Dowd et al., 1996; Murugavel et al., 2008) that show new particles formed over the coastal region are carried into the atmosphere for upto 1 km altitude by favourable atmospheric conditions. Here, we present the results of airborne measurements of concentrations and size-distribution of micron-sized aerosols made at different levels ranging from 3500 ft to 7000 ft over eastern India and Bay of Bengal during September 12, 2008 as a part of pre-CTCZ campaign.

Instrumentation

Measurements of the number concentration and size distribution of aerosol particles in the size range of 0.5 to 20 μm are made by using an Aerodynamic Particle Sizer (of TSI,.USA make) installed onboard a Beachcraft aircraft. The air sample is taken through an air inlet fixed at the belly of the aircraft and is located ~ 2 m away from the nose of the aircraft. The inlet is smoothly bent to face the airflow. The system is kept close to the inlet to minimize the diffusion losses of aerosol particles inside the tube. Teflon tube is used to draw the air into the APS to keep the loss of particles due to collision with the walls of the tube at minimum.

It takes about 30 sec to complete one distribution and a minimum of at least 20 samples are collected at each level. The APS requires a total airflow rate of 5 lpm and aerosol flow rate of 4 lpm. The flow rates are adjusted at each level to keep the airflow within the error limit. However, as the airflow decreases with altitude, a small pump is used to suck air at higher altitudes to maintain the required the flow.

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Sampling of air by aircraft is prone to various errors and losses, which may be introduced by change in airflow rate due to changes in pressure and temperature with altitude, diffusion losses, loss due to bends in inlet tube and that due to turbulent flow. However, such an error is limited less than 10-12% for particles < 1 μm diameter and < 5percentage for particles of diameter > 1 μm. To minimize diffusion losses, sampling was done at a distance of only about 2 m from the air inlet. The diffusion losses are less than 2% for particles of 0.5 μm diameter (Baron and Willeke, 2001). As there are no sharp or right- angled bends Tin inlet tube as wells as the telfon tube used for sampling, errors that may have introduced due to such bends are negligible.

Measurements

Measurements were made when the aircraft flew from Kanpur to Bhubaneswar on September 12, 2008. The aircraft initially made a constant level flight at 3500 ft and then climbed to 5500 ft. As the flight was made in late morning hours, some cumulus clouds started developing at the level of flight or just above the level of flight. Occasionally, the aircraft passed through small clouds. There was moderate rain when the aircraft approached Bhubaneswar. No sampling was done during rainy period. The atmosphere, as seen from the radio-sonde flight (not shown) made at Ranchi and Bhubaneswar, was stable upto 2 km and humidity was more than 80% upto 2 km.

a) Observations over Land

The aerosol number concentrations obtained during the flight are plotted in the Figure 1. As seen from figure, the concentrations at 3500 ft level are intially very high and reduces drastically as we move away from industrially polluted Kanpur to less populated and densely vegetaged Jharkhand. The concentrations at 5500 ft level is almost constant at around 500 cm-3. As already stated, the atmosphere over the cruise path is highly stratified and chances of any vertical transport of aerosols from the ground level are less. Eventhen, we observe higher concentrations of aerosols over places close to industrialised zones. Under such stratified conditions, advection of airmass can play dominant role in vertical distribution of aerosols. The 5-days back trajectory analysis shows that the airmass originated from Bay of Bengal arrives the point of observation after travelling through

Figure 1. Total number concentration aerosols along Figure 2. Size Distribution of aerosols observed at
the flight track at different levels 14000 ft altitude at different time intervals

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Figure 3. Total number concentration aerosols over Bay of Bengal at 4000 and 7000 ft altitude

AEROSOLS & CLOUDS : CLIMATE CHANGE PERSPECTIVES

land mass for 2-days and has less travel history. Hence, the higher concentrations of aerosols measured intially close to Kanpur can be attributed to advection of aerosols from ocean.

However, when the point of observation shift to further east, the airmass originates from Arabian Sea and reaches the Pobservation point after long history of travel through Bay of Bengal and parts of East India. The airmass may lose aerosols of larger size and/ or the size may be modified during their long travel. Further, the flight track is over will vegetated parts of Jharkhand and Orissa and comparatively less polluted. Also there are no vertical transport from ground as the atmosphere is stratified. This may explain the lower concentrations of aersols measured at 5500 ft level.

The size distribution of aerosols follows the similar pattern of logarythnic distribution at these two levels during different period of the cruise (Figure 2). Except when the observations are made close to industrial places, the distributions are similar and follow one another in the size range upto about 1.5 μm. The increase in concentrations seen in the higher size range of more than 2 μm can be attributed to penetration of aircraft through clouds.

b) Observations over Bay of Bengal

Measurements are made over Bay of Bengal at two levels - 4000ft and 7000 ft a short duration of 15 minutes each. As it takes only 20 sec for one sampling cycles we could get about 40 samples at each level. As shown in Figure 3, concentrations of aerosols the increases about three fold at the coastline at 4000 ft level. Thereafter, as we go into the ocean the concentration decreases gradually. No such variation in concentration of aerosols is observed at 7000 ft level. The increase in concentration at the coastline is in confirmity with the observations of Murugavel et al. (2008) who also

observed such increase at coastline at 1 km altitude over Bay of Bengal. They have attributed this increase to vertical transport of newly formed particles in coastal zones. Such formation of new particles due to DMS emission over marine regions has been proposed by O’Dowd (2002). Our observations reinforces such formation of particles and their transport upto about 1 km.

Acknowledgements

The authors are grateful to Department of Science and Technology, Govt. of India for research grant under Continental Tropical Convergence Zone (CTCZ) programme. They also thank National Remote Sensing Agency (NRSA), Hyderabad and ISRO for providing the aircraft.

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References:

Lelieveld J. et al, 2001, “The India Ocean Experiment: wide spread air pollution from south and south east Asia”. Science, 291, pp. 1031-1035.

Murugavel, P, V Gopalakrishnan and A K Kamra, 2005, Airborne Measurements of the Size Distribution of Submicron Aerosols over the Arabian Sea during ARMEX – Phase I, Mausam,56, 301-314.

Murugavel, P, V Gopalakrishnan and A K Kamra, 2008, Airborne measurements of submicron aerosols across the coastline at Bhubaneswar during ICARB, J. Ear. Syst. Sci., 117, 273-280.

O’Dowd , C. D., 2002, On the spatial extent and evolution of coastal aerosol plumes, J. Geophys. Res., 107, 8105.

O’Dowd, C D, M H Smith, J A Lowe, B M Davison, C N Hewitt and R M Harrison, 1996, New particle formation in marine environment, Proc. 14 Intl. Conf. on Nucleation and Atmospheric aerosols: (Eds)Kulmala, M and P Wangner, 925-929.

Ramanathan, V., et al., 2001. “Indian Ocean Experiment: An integrated analysis of the climate forcing and effects of the great Indo-Asian haze”, J. Geophy. Res, 106, pp. 28371 – 28398.

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A–P–25

Wintertime Vertical Profiles of Aerosol Optical Properties over West Coast of India

V. Sreekanth(1), S. Suresh Babu(1), K. Krishna Moorthy(1)

and S. K. Satheesh(2)

1Space Physics Laboratory,Vikram Sarabhai Space Centre,Trivandrum – 695 022, India

2Centre for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore - 560 012, India

ABSTRACT : Altitude profiles of several aerosol optical properties including Single Scattering Albedo (SSA), derived from the simultaneous in-situ airborne measurements of aerosol absorption and scattering coefficients, off the west coast of India over the Arabian Sea, is presented. The absorption and scattering coefficients decreased with altitude, while the vertical structure differed significantly. Consequently, the derived SSA, with a surface value of 0.94, decreased with altitude, illustrating increasing relative dominance of aerosol absorption over scattering at higher altitudes. Altitude profile of SSA, when examined in conjunction with that of hemispheric backscatter fraction, revealed that the continental influence on the aerosols has been increased with altitude, rather than the marine environment. The results are examined with those reported during earlier campaigns and the implications are discussed.

Introduction

Recent studies during Integrated Campaign for Aerosol gases and Radiation Budget (ICARB-2006) on the vertical distribution of aerosols over India and its coastal regions during pre-monsoon have revealed several interesting findings which have far reaching implications, such as (i) elevated aerosol layers above the MABL, (ii) trapping of aerosols in the convectively stable layers of the lower atmosphere which are sandwiched between convectively unstable regions, (iii) substantial fraction of aerosol abundance above the reflecting low-level clouds, (iv) aerosol induced elevated warming and its meridional gradient (Satheesh et al., 2008, 2009). As a sequel to ICARB-2006 (for details see Moorthy et al., 2008), a winter campaign under ICARB (W_ICARB) has been carried out during

December 2008 – January 2009. This campaign, which also consisted of three segments (viz., the land segment, air segment and the ocean segment) like its predecessor, was aimed at examining the consistency and seasonal distinctiveness of the elevated aerosol layers. The present study report the high- resolution altitudinal profiles of aerosol optical parameters, in the range 0 – 3000 m AGL, deduced from concurrent airborne measurements of absorption and scattering coefficients off the west coast (Mangalore being the base station) of India using a dual channel Aethalometer (Magee Scientific, Figure 1. Flight track during altitudinal profiling

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Model No. AE-20) and an integrating Nephelometer (TSI Inc., Model No. 3563) respectively.

The ground projection of the aircraft flight track during the altitudinal profiling is shown in Fig. 1.

Results

The altitude profiles of aerosol absorption coefficient, (sabs at 880 nm) and total scattering coefficient, (stsc at 550 nm) in the range 0 - 3000 m are shown in Fig. 2. The surface level values of sabs and stsc are ~5.6 Mm-1 (1Mm = 106 m) and ~140 Mm-1 respectively, representing a relatively less aerosol laden environment over MNG, which was bounded by Arabian Sea to its west and the Western Ghats to its east. Broadly, the shapes of the two profiles are similar where both the parameters decreased with altitude except that (i) there is a peak in sabs at ~500 m AGL (surface value being much lower), which is not seen in stsc and (ii) the shallower altitude gradient of sabs compared to that of stsc, suggesting perhaps a slower decrement in the concentration of absorbing aerosols with altitude compared to that of its counterpart, the scattering component.

The wavelength exponent values of the scattering coefficient (which determines the dominance of the accumulation more aerosols) are almost steady with altitude upto 2000 m with a mean value of ~1.6 and then slightly dropped off, while the values of hemispheric backscatter fraction increased steadily with altitude. The columnar mean values of both the above parameters (at 550 nm) are 1.53±0.12 and 0.11±0.007 respectively characterizing a relatively accumulation mode dominant

aerosol system over the study region. The      
columnar mean value of the asymmetry      
factor (at 550 nm) was 0.64±0.02. In the      
present study, SSA with a surface value of      
~0.94, is found to decrease with altitude,      
showing relative higher dominance of the      
absorbing aerosols at higher altitudes. This      
might be due to the reason that the      
absorbing aerosols are mostly of sub-micron      
size range and are capable of lofting to      
higher altitudes; higher life time and      
amenable to long range transport, when      
compared to their counterpart scattering Figure 2. Altitude profiles of aerosol absorption
aerosols, which is further confirmed by the coefficient (sabs, first panel) and scattering coefficient
parameterization of both the profiles.   (stsc, second panel) off west coast    

References

Satheesh, S. K., K. Krishna Moorthy, S. Suresh Babu, V. Vinoj, and C. B. S. Dutt (2008), Climate implications of large warming by elevated aerosol over India, Geophys. Res. Lett., 35, L19809, doi:10.1029/2008GL034944.

Satheesh, S. K., K. K. Moorthy, S. S. Babu, V. Vinoj, V. S. Nair, S. N. Beegum, C. B. S. Dutt, D. P. Alappattu, and P. K. Kunhikrishnan (2009), Vertical structure and horizontal gradients of aerosol extinction coefficients over coastal India inferred from airborne lidar measurements during the Integrated Campaign for Aerosol, Gases and Radiation Budget (ICARB) field campaign, J. Geophys. Res., 114, D05204, doi:10.1029/ 2008JD011033.

Moorthy, K. K., S. K. Satheesh, S. S. Babu, and C.B.S. Dutt (2008), Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB): An Overview, J. Earth System Sci., 117, S1, 243-262.

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A–P–26

Relationship between Pre-monsoon Aerosol Index and all- India Summer Monsoon Rainfall

A. A. Munot, S. D. Patil and B. Preethi

Indian Institute of Tropical Meteorology, Pune-411008

Introduction

As India is an agricultural country its economy is closely linked with the performance of the summer monsoon rainfall which gives 75-90 % of the annual rainfall. Timely and well distributed rainfall in the season generate good amount of food production whereas erratic behaviour of the monsoon has an adverse effect on it. Hence prediction of monsoon rainfall is of immense importance. It has been suggested by recent studies (Patra et al. 2005, Gautam et al., 2009,) that aerosol solar absorption over the Indian region plays significant role in modulating the monsoon circulation and rainfall distribution. In view of this, an attempt has been made to find the relationship between pre-monsoon aerosol over Indian region and monsoon rainfall and identify the aerosol parameter which could be useful in forecasting the monsoon rainfall.

Data

In this study we have used daily TOMS Aerosol Index (AI) data over the Indian region during the period 2005-2009 for pre-monsoon season. We have also used all-India summer monsoon rainfall (AISMR) data for the period 2005 -2009.

Result and discussion

From daily aerosol index we have prepared monthly aerosol index data for the months March, April and May as well as for MAM season as a whole for the above mentioned period. We worked out the correlation coefficients between AI and AISMR for March, April and May as well as for MAM season. The spatial patterns of the CCs are shown in figure 1. It is seen from figure 1 that CCs are consistently significant (negative and significant at 5 % level) over the Bay of Bengal region. Thus, pre-monsoon aerosol index over the Bay of Bengal region is found to be an important parameter influencing the subsequent monsoon rainfall. To support this relationship we have also plotted the anomalies in AI during deficient and excess rainfall years. During these five years, there is one deficient year of 2009 and one excess year of 2007. Spatial pattern of AI anomalies during 2007 (excess) year and during 2009 (deficient) are shown in figure 2 and figure 3 respectively. It is found that anomalies in AI are negative during 2007 over Bay of Bengal region and are positive during 2009, supporting the negative relationship between AI during MAM over Bay of Bengal and AISMR. We have further worked out the average AI over the Bay of Bengal region (15° -20° N, 85°-95° E), BOB , over which CCs are significant. It is observed that CC between AI over BOB and AISMR for MAM is -0.98 ( note : data points are only 5) significant at 1 % level of significance. The AI anomalies over BOB

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Figure 1. Spatial patterns of Correlation coefficients between AISMR and AI

Figure 2. Spatial patterns of Aerosol Index anomalies during 2007 (excess year)

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Figure 3. Spatial patterns of Aerosol Index anomalies during 2009 (drought year)

during MAM season are -0.14 during excess year of 2007 and 0.32 during one of the severe drought years of 2009.

Summary and conclusion

Using only five years data we have found that there is strong negative relationship between pre-monsoon aerosol index and forthcoming monsoon rainfall. Strong aerosol loading over the Bay of Bengal region during pre-monsoon months leads to weak monsoon and visa-versa.

Acknowledgment

Authors are thankful to Prof. B. N. Goswami, Director, Indian Institute of Tropical Meteorology, Pune and Dr. N. Singh, Scientist F, Indian Institute of Tropical Meteorology, Pune for the facilities.

References

Patra et al. 2005, Atmos. Chem. Phys. Discuss., 5, 2879-2895

Gautam et al., 2009, Ann. Geophys., 27, 3691-3703

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A–P–27

Seasonal Variation of Aerosol Radiative Effects over the East-Coast of India

S. N. Bhanja1 and S. Verma1

1Department of Civil Engineering, Indian Institute of Technology,Kharagpur

West Bengal, India

ABSTRACT : Atmospheric aerosols consisting of a mixture of light-absorbing and light-scattering components, contribute to atmospheric solar heating and surface cooling. However, there is a large uncertainty in the estimation of net aerosol forcing effect (Ramanathan et al., 2007). Since aerosols exhibit large spatial and temporal variation with their concentrations peaking close to major source regions, intensive spatial and temporal variation studies are required on a regional scale to properly understand the optical and microphysical properties of aerosol.

Keywords : Aerosols, Aerosol optical depth, Radiative forcing, Forcing efficiency.

In the present study, aerosol optical properties and estimated aerosol direct clear-sky shortwave radiative forcing are evaluated over the two cities situated on the eastern coast of India, namely, Kharagpur (22.30N, 87.30E), a small town, and Kolkata (22.60N, 88.40E), an urban industrial location during the different seasonal periods. The aerosol optical depth (AOD) measured at Kolkata using Microtops II sun photometer showed diurnal variation (Figure 1) with a higher value in afternoon compared to morning and evening. In afternoon due to the convective turbulence processes the existing atmospheric particles and lighter aerosol particles generated by anthropogenic activity are well mixed (Devara et al., 1996) compared to morning and evening, and hence lead to higher AOD during afternoon. Measurements of AOD showed values over Kolkata (-0.71 ± 0.20 at 500 nm) higher by a factor of ~1.5 compared to AOD (500 nm) at Kharagpur (0.57 ± 0.03) during post monsoon season (November). The direct aerosol radiative forcing calculated using a discrete ordinate radiative transfer model (SBDART) (Ricchiazzi et al., 1998) showed the seasonal variation in clear-sky surface forcing over Kharagpur (Figure 2). The clear-sky aerosol radiative forcing over Kharagpur at the surface is larger in winter months (-40.79 ± 4.60 to -66.11 ± 8.10 w m-2) and smaller in summer months (-23.70 to -38.06 w m-2). The clear-sky aerosol radiative forcing at top-of-atmosphere (TOA) also showed larger values in winter months (-12.11 ± 2.20 to -24.47 ± 3.30 w m-2) and smaller in summer months (0.20 to -0.65 w m-2). The surface forcing efficiency varies from -53.85 to -84.35 w m-2 throughout the year. In winter the range is -77.02 to -84.35 w m-2 and in summer it is -53.84 to -55.97 w m-2 indicating the higher potential of winter-time aerosol to climate impact compared to the other seasons. The clear sky aerosol direct radiative forcing over Kolkata at surface (-27.20 to -81.80 w m-2 with an average of -56.54 ± 16.90 w m-2), and TOA (-8.80 to -29.20 w m-2 with an average of -18.70 ± 6.71 w m-2) calculated from measured AOD data in November is higher than Kharagpur (-40.11 ± 4.15 at surface; -12.63 ± 2.64 at TOA) showing higher extent of pollution over Kolkata with a higher potential to climate impact.

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Figure 1. Diurnal variation of AOD over Kolkata in November

Figure 2. Seasonal variation of clear-sky aerosol radiative forcing over Kharagpur

References

1.Devara P. C. S., G. Pandithurai, P. E. Raj and S. Sharma. Investigations of aerosol optical depth variations using spectroradiometer at an urban station, Pune, India. J. Aerosol Sci., 27:621-632, 1996.

2.Ramanathan V., M. V. Ramana, G. Roberts, D. Kim, C. Corrigan, C. Chung and D. Winker. Warming trends in Asia amplified by brown cloud solar absorption. Nature, 448: 575-579, 2007.

3.Ricchiazzi P., S. Yang, C. Gautier and D. Sowle. SBDART: A Research and Teaching Software Tool for Plane-Parallel Radiative Transfer in the Earth’s Atmosphere. Bull. Am. Meteorol. Soc., 79:2101-2114, 1998.

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A–P–28

Influence of Aerosols on near Surface Ozone Mixing Ratio at a Tropical Coastal Environment

Liji Mary David, Prabha R Nair and Girach Imran Asatar

Space Physics Laboratory, Vikram Sarabhai Space Centre,

Trivandrum 695 022, India

Introduction

Studies on the influence of aerosols on tropospheric photolysis rates and chemical budget of various trace gases like ozone, NOx, etc are of great relevance in assessing the climatic impacts of these gases. The photochemistry of troposphere depends of the concentration of trace gases and aerosols along with meteorological conditions and availability of solar flux. The aerosols present in the atmosphere interact with the incoming solar radiation and alter the photochemical production rates of oxidants in the atmosphere. However, knowledge on the impact of aerosols on tropospheric oxidant cycle is limited. This is mainly due to the large spatial variabilities in the physical and chemical nature of aerosols and trace gases. Most of the inference on the inter-dependence of aerosols and trace gases are based on model based analysis. Tropospheric ozone- a green house gas and pollutant- is a major source of OH radical which oxidizes several other pollutant gases in the atmosphere and removes them. It is known that atmospheric aerosols play a significant role in altering the tropospheric ozone concentration. The photochemical production/destruction of ozone depends on the aerosol loading, its size distribution and optical characteristics. Through several experimental studies as well as model simulations, it is observed that presence of aerosols like soot can cause ozone loss (Disselkamp et al., 2000). In the lower troposphere, ozone formation is found to be highly sensitive to urban aerosols (He and Carmichael, 1999). This paper presents the results of the investigations on the inter-dependence of near-surface ozone and aerosol number density at the tropical coastal site Trivandrum (8.55ºN, 77ºE).

Experimental details and data

Near-surface ozone measurements are carried out by using a UV Photometric Ozone analyzer (Model 49C, Thermo Electron Corporation, USA) which works on the principle of absorption of UV light by the ozone molecules at the wavelength of 254 nm. Calibration of the instrument is done using the built-in ozonator (ozone generator) and zero air generator. The instrument has a lower detection limit of 1.0 ppb.

An aerosol spectrometer (model 1.108 of GRIMM, Germany) is used to measure the number density of near-surface aerosols at regular intervals of time. This instrument is an optical particle counter operating at 15 size channels with cut-off diameters 0.3, 0.4, 0.5, 0.65, 0.8, 1, 1.6, 2, 3, 4, 5, 7.5, 10, 15 and 20 μm and provides size-segregated number density (http:/www.grimm-aerosol.com). The built-in software generates the aerosol number size spectrum averaged for a fixed time period. In addition to this, an automatic

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weather station (AWS) operating at the experimental location records the meteorological parameters viz temperature, relative humidity (RH), rainfall, wind speed and direction every 5 minutes.

Results and discussions

Diurnal variation of near-surface ozone and aerosol number density

Figure 1 shows the typical diurnal variation of near-surface ozone and total aerosol number density (with aerodynamic dia>0.3 m) which are opposite in nature. Near- surface ozone which remains low during night and early morning shows increase around 0800 hrs IST attaining a broad peak which lasts till late night. The daytime peak is attributed to the photochemical reactions involving solar flux and precursor gases of ozone like NOx, CH4, CO etc. Nighttime decrease is due to titration of ozone by NOx. On the other hand the diurnal variation of aerosol number density decreases ~0800-0900 hrs (about the same time as increase of ozone occurs) and remains low till late night. The diurnal pattern is found to be closely associated with the atmospheric boundary layer parameters and sea breeze/land breeze (SB/LB) activities characteristic of the coastal site. During night, boundary layer height increases ~3 times its daytime value thus enabling the vertical mixing of aerosols to higher altitudes and consequently reducing the near surface concentration. The high ozone mixing ratio established due to photochemical activity continues even after sunset and till the onset of LB. However, the diurnal variation of aerosol number density is also size-dependent. Small particles (<1 m) shows a pattern similar to that of total number density (which of course is dominated by small particles) shown in the figure.

Figure 1. Diurnal variation of ozone and total aerosol number density

But as particle size increases the daytime decrease becomes less prominent. In fact the larger particles show increase during daytime. In this context it is to be noted that during daytime SB prevails over the site bringing in sea salt aerosols whose sizes lie in the large particle regime. However, no one-to-one dependence is observed in the diurnal patterns of ozone and aerosol concentration.

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Dependence of ozone on aerosol number density

With a view to examine the dependence of ozone on aerosol number density further, the daytime mixing ratios of ozone and the size resolved aerosol number density are averaged on daily basis and their variations studied. Ozone shows an increase with increase in small particle concentration (size<1 m). However, ozone mixing ratio is rather insensitive to changes in large particle concentration or occasionally shows inverse dependence. Studies carried out elsewhere have indicated negative as well as positive correlation between near-surface ozone and aerosol concentration depending on physical and chemical nature of aerosols (Bonasoni et al., 2004; Zhang et al., 1994). An examination of the month-to-month variation of near-surface ozone along with that of size-resolved number density revealed the dependence of ozone on aerosol size. It is observed that while ozone shows positive correlation with small particle concentration, it shows decrease with increase in large particles. Role of dust aerosols, which are mostly in coarse mode, in producing ozone loss is established (Bonasoni et al., 2004; Dentener et al., 1996). The large surface area provided by these particle forms the sites where heterogeneous chemistry becomes active leading to destruction of ozone.

References

Bonasoni, P., P. Cristofanelli, F. Calzolari, U. Bonafè, F. Evangelisti, A. Stohl, R. van Dingenen, T. Colombo, and Y. Balkanski (2004), Aerosol-ozone correlations during dust transport episodes, Atmos. Chem. Phys. Discuss., 4, 2055-2088.

Dentener, F. J., G. R. Carmichael, Y. Zhang, J. Lelieveld, and P. J. Crutzen (1996), Role of mineral aerosol as a reactive surface in the global troposphere, J. Geophys. Res., 101(D17), 22,869-22,889.

Disselkamp, R. S., M. A. Carpenter, J. P. Cowin, C. M. Berkowitz, E. G. Chapman, R. A. Zaveri, and N. S. Laulainen (2000), Ozone loss in soot aerosols, J. Geophys. Res., 105(D8), 9767-9771.

He, S., and G. R. Carmichael (1999), Sensitivity of photolysis rates and ozone production in the troposphere to aerosol properties, J. Geophys. Res., 104(D21), 26,307-26,324.

Zhang, Y., Y. Sunwoo, V. Kotamarthi, and G. R. Carmichael (1994), Photochemical oxidant processes in the presence of dust: An evaluation of the impact of dust on particulate nitrate and ozone formation, J. Appl. Met., 33, 813-824.

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A–P–29

Association between Stratosphere Aerosols and Total Ozone

Indira Sudhir Joshi

Indian Institute of Tropical Meteorology

Dr. HomiBhabha Road, Pashan, Pune-411008

E-mail : indira@tropmet.res.in

ABSTRACT : Large volcanic eruptions affect climate by injecting sulfate aerosol into the stratosphere, which in turn reduces the solar energy reaching the earth’s surface. The sulfate aerosols produced by large eruptions also lead to depletion of stratospheric ozone through a series of chemical processes. Reactions of sulfate particles with chlorine and nitrogen species, favors destruction of ozone Prather, (1992). Keeping the above in view a study has been undertaken to examine the effect of volcanic aerosols on total ozone.

Data and Analysis

For the above analysis volcanic eruptions having Volcanic Explosivity Index (VEIe = 3) data are collected for the 15-year period (1979-1994) from Gregg, et.al., (1997) (Eruption date = eruption associated with the SO2 emissions measured by the TOMS satellite instrument). Monthly means of total ozone data are collected for all the 15-year period for Arosa, Alma-Ata (high-latitude stations), New Delhi, Srinagar, Varanasi, Poona, Kodaikanal and singpore Middle and low-latitude stations from, Data Books, “Ozone Data for the World” publications from Canada. Total 33 volcanic eruptions are recorded during 1979-1994. Volcanic eruptions data are bifurcated latitude vise, high, middle and low-latitude volcanic eruptions. High-latitude volcanic eruptions are 11, mid-latitude volcanic eruptions are 3 and low-latitude volcanic eruptions are 19 in number. By using

Figure 1. Plots of total ozone values preceding and following the Ke Day(0) for the (19 )low-latitude volcanic eruptions

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Superposed epoch analysis, i.e. volcanic eruption date as key day and 2days preceding and 2days following the key date, ozone data are analyzed. In this study two months preceding the volcanic eruption data, two months following the volcanic eruption date, ozone data are analyzed. Superposed epoch analysis of ozone values for the high, middle and low-latitude volcanic eruptions are plotted and are shown in the figures 1-3.

Figure 2. Plots of total ozone values preceding and following the Ke Day(0) for the (3) mid-latitude volcanic eruptions

Figure 3. Plots of total ozone values preceding and following the Ke Day(0) for the (11) high-latitude volcanic eruptions

Results

From Fig.1 it is noticed, in low-latitude volcanic eruptions, decrease in total ozone is observed after two months, following the volcanic eruptions in all the stations.

From Fig.2 it is noticed ,in mid-latitude volcanic eruptions, except in Arosa and Alma- Ata, decrease in total ozone is observed following the volcanic eruptions. In Arosa and Alma-Ata an increase in total ozone is observed following the volcanic eruptions.

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From Fig.3 it is seen, that, in high-latitude volcanic eruptions except in Srinagar following the volcanic eruption, decrease in total ozone in all other stations is noticed. In Srinagar an increase in total ozone following the volcanic eruption is noticed

Conclusions

It is observed from the above study, that, aerosols induced into the stratosphere from volcanic eruptions whose VEI is e = 3 causes depletion in total ozone amount through a series of chemical processes, depending upon the nearness of the latitude where the volcano is situated.

Acknowledgements

I am thankful to Dr.P.C.S.Devera,head PM & A.Division for his constant encouragement in preparing the manuscript

References

Gregg ,J.S.Bluths, William I.Rose,Ian E.Sprod, and Arlin J .Krueger.,1997, The Journal of Geology, Vol.105, PP.671-683

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A–P–30

Response of Surface Ozone to Tropical Atmospheric Disturbances: A Case Study

A. L. Londhe, M. J. Kartha and P. Seetaramayya

Indian Institute of Tropical Meteorology, Pune (India)

Introduction

Ozone is a secondary pollutant produced photo chemically in the troposphere by reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOC).

Ozone production is most conducive on cloudless, warm and dry days with light winds. The meteorological conditions which are conducive to ozone formation include high solar radiation on cloudless days, light wind speed, low mixing boundary layer height and low relative humidity. Ozone can be removed from the atmosphere by wet and/or dry deposition. Ozone in contact with the surface can be deposited or absorbed on to vegetation like trees and other plants; also ozone can be scavenged by precipitation and can be transformed in to the aqueous phase with other atmospheric constituents such as hydrogen peroxide. Muralidharan et al., (1989) studied the surface ozone variation associated with rainfall and noticed that the low ozone is associated with day time rainfall. The surface ozone observations over the Indian region suggested that the ozone minimum is found during south-west (SW) monsoon season (Lal et al., 2000; Nazeer Ahammed et al., 2006; Londhe et al., 2008). Some states of India such as in Karnataka, Maharashtra, Gujarat etc. get occasional rainfall due to thunder showers in this period and also due to some weather disturbances associated with NE-monsoon in the Bay of Bengal (Moolay and Shukla, 1989) and move west/north-westward towards the sub-continent. Indeed the present study region of Pune is also come under the grip of these weather disturbances. At the time of our observations two such cyclones, Khai Muk, Nisha and a depression have

Figure 1. Map showing experimental locations in Maharashtra State of India

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formed in the Bay of Bengal during the months November-December 2008. The surface ozone variations are severely influenced by these cyclones and depressions over Pune. These variations in surface O3 are presented here. In this study we discuss some plausible evidence and concurrent reasons for the variations in O3.

Data and Methodology

The surface ozone data have been collected simultaneously at an interval of one minute at Pune (18.32o N, 73.55ï E, 559 m ASL) and Khadakwasala (located 25 km away towards south-west of Pune) for the period 1 November-31 December 2008 part of the experimental period of two years. This data have been utilized for the present study. The experimental sites are shown in figure 1. The star in the figure denotes the stations Pune and Khadakwasala. The daily mean ozone for 24 hours, the day time mean ozone for 12 hours (0700 to 1800) and the maximum ozone on the day have been derived from the observed minute data. The working principle and other details of the electrochemical ozone sensor (KI solution) have been discussed briefly in the past (Sreedharan and Tiwari 1971; Londhe et al., 2008). A comparison of the ozone sensor (KI solution) with UV ozone sensor (O3 42M, Environment S. A.) is in good agreement within ± 5%.

Results and Discussions

The strong weather disturbances like cyclones and depressions occur in Bay of Bengal in the months of October to December to enhance the cloud cover with some occasional thunder showers over Pune. Such cloudy conditions are prevailed in November-December 2008. The results of the observed ozone concentration associated with these cloudy and bad weather conditions are summarized below.

Figure 2. Daily surface ozone for the period 1 November to 31 December 2008

The daily mean ozone (24 hours), the day time mean ozone (12 hours, 0700 to 1800 hrs), the maximum and the minimum ozone on each day for the period from 1 November to 31 December 2008 at Pune and Khadakwasala have been shown in figure 2 (a, b). It is seen from these figures that the ozone is of the lower order at Pune (Fig. 2a) from 18 November onward compared to the former period. Though the ozone is seen to fluctuate during the period prior to 18 November, the over all ozone values are almost doubled than those of the later period. A minimum ozone value of 10 ppb was recorded on 29 and

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30 November 2008. The ozone values observed on the days from 18 November to 20 December 2008 are generally below 20 ppb; however the ozone values on the days prior to 18 November are more than 30 ppb. Similar features were also observed over the rural station Khadakwasala (Fig. 2b).

Figure 3. Satellite cloud pictures on different dates during period of study ( Pune)

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The days, 18, 29, and 30 November and 15 December are of typical cloudy conditions. The central line/axis of the maximum cloud zone on the above days is more or less oriented in the direction from north-east to south-west (Visual). This line seems to have either passed through the Pune city or slightly graced the city (Fig. 3 b, c, d, and f) and seems to have origin at north, but inclined to large scale systems which formed in the south Bay of Bengal and traveled to the Arabian Sea. Indeed the surface wind speed, wind direction and the cloud motion are mainly controlled by the evolving isobaric pattern of a moving low pressure system. The wind direction tends to change according to the orientation of the isobars, keeping the low pressure centre (about 90o) towards the left of it according to “Buys Ballot’s Law” in the northern hemisphere. Accordingly, centers of these low pressure systems originated in the Bay of Bengal and moved towards the northwest in their course must have finally reached over the east central Arabian Sea off the Maharashtra coast line and dissipated there subsequently. The effect of these moving systems caused distinct type of cloudiness over the Pune region. On 18 November and 15 December, the cloudy conditions are more or less similar (Fig. 3 b, f). That is, the clouds are diffused and almost high/medium type of clouds. Whereas on 29 and 30 November, the clouds are low cloud type which are associated with the approaching cyclonic system (Fig. 3 c, d). Because of these distinct cloud conditions the weather (temperature, relative humidity and wind field) also distinctly varied over the station Pune and hence, the consequent inheritance in the ozone production. The effect of these clouds on the surface ozone at Pune is inherent.

Conclusions

In the months of November and December 2008, the two tropical cyclones Khai Muk, Nisha and a depression have caused cloudiness over Pune and the surrounding region. These cloudy sky conditions are observed to have affected the photochemical production of the ozone over Pune and Khadakwasala (Fig. 2 a, b). The conclusions are as follows.

(i)The lower ozone over Pune region during 18 November to 20 December 2008 shows a strong dependence of photochemical ozone production on disturbed weather conditions forced by the tropical cyclones and the depression.

(ii)The photochemical ozone production varies with solar radiation reaching at surface depending upon cloud cover and type of clouds. On partly cloudy sky with high

clouds, the transfer of solar radiation improves at the surface and conducive for O3 photochemical mechanism and hence, high O3 values (Fig. 3b & 3f). On cloudy sky with low clouds, the transfer of solar radiation cutoff at the surface and redundant to O3 production and hence relatively low ozone (Fig. 3 c, d).

(iii)A strong dip in the ozone of the order of 10 – 20 ppb is observed during the period of observations, the highest ozone depletion (20 ppb) confined to heavy rainfall due to the above low pressure system’s effect on 29 November, 2008.

Acknowledgements

The authors are grateful to Prof. B. N. Goswami, Director, IITM, Pune for his encouragement in the study. We acknowledge NERC Satellite Receiving Station, U. K. for utilizing their satellite cloud pictures on website. This work has been supported by the Department of Science & Technology, New Delhi (India).

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References

Lal, S., Naja, M., Subbaraya, B.H., 2000. Atmos. Environ. 34, 2713-2724.

Londhe, A.L., Jadhav, D.B., Buchunde, P.S., Kartha, M.J., 2008. Current Science. 95, 1724-1729. Mooley, D.A., Shukla, J., 1989. Mausam. 40, 137–152.

Muralidharan, V., Mohan Kumar, G., Sampath, S., 1989. Pure Appl. Geophys.130, 47-55. Nazeer Ahammed, Y., Reddy, R.R., Ramagopal, K., Nara Sinhulu, K., Babu Basha, D., Siva Shankara Reddy, L., Rao, T.V.R., 2006. Atmos. Res. 80, 151-164.

Sreedharan, C.R., Tiwari, V.S., 1971. J. Phys. E. Sci. Instrum. 4, 706-707.

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A–P–31

Aerosol Source Characteristics over Different Environments

Kapil Dev Sindhu and S. K. Satheesh

Centre for atmospheric and Oceanic Sciences (CAOS)

Indian Institute of Science, Bangalore-560012

E mail : kapil@caos.iisc.ernet.in

Extremely fine liquid droplets or solid particles, those remain suspended in the air, are known as aerosols. They are produced by natural sources and anthropogenic activities.

Several types of aerosols produced by different processes are present in the atmosphere and every type of aerosol species exhibit different types of physical and chemical properties. Even though making up only a small fraction of atmospheric mass aerosols are capable of altering Earth’s climate by scattering and absorbing incoming solar radiation and absorbing outgoing radiation (IPCC 2007). Adding to the complexity, they can act as cloud condensation nuclei and modify cloud properties. Aerosols can be transported up to long distances and therefore, it is extremely important to understand the relative contributions of different sources (natural and anthropogenic) to local air mass. The source apportionment is required to find out the relative contributions of different sources. Quantitatively identifying the relative contributions of different source types to any location is referred to as source apportionment. We have carried out source apportionment using backward air parcel trajectories by applying k-means method of clustering and obtained various aerosol terms corresponding to each cluster. There are two possibilities for verification of chosen number of clusters and these are the following: 1) for less spread trajectories patterns, choose number of points independently and 2) for much spread trajectories, where a large number of clusters are possible, we use RMSD method for verifying the number of clusters (Dorling et al., 1992). This method is focused on the distances within one cluster only. Here, we have selected three island sites and one site in the middle of Saharan desert for this study. Detail about time period and sites chosen for study are given in Table (I). Ouagadougou is situated in state Burkina Faso in Northern Africa. It represents to the continental site. Ascension Island is in Atlantic Ocean, Male in Indian Ocean and Midway Island in Pacific Ocean, all these three sites represent the maritime environment. Aerosol optical and radiative properties data sets are taken by AERONET sunphotometers. Corrected fire pixel count data is obtained from MODIS level-3 data sets from period January, 2001 to December, 2003 (figure 1).

Table I. Time period and sites chosen for study

Sl. No. Region   Time Period
           
1 Ouagadougou 1 January, 2001   31 December, 2001  
2 Ascension Island 1 January, 2003   31 December, 2003
3 Male 1 February, 2001   31 December, 2001
4 Midway Island 1 May, 2005   31 April, 2006
             

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Figure 1

Figure 2

Figure 3

We have calculated 8- days backward 24-hourly trajectories using web version of HYSPLIT model at 0000 UTC using the national weather service’s National Centers for Environmental Prediction (NCEP) model data available in NOAA’s Air Resources Laboratory (ARL) archives for each site. We have taken backward trajectory data at altitude of 500 meter for continental site (Ouagadougou) while for, all maritime sites (Ascension Island, Male and Midway Island), the altitude is chosen as 1000 meter. This is because island locations are several hundred kilometers from the mainland and aerosols at lower altitudes (close to the surface) may not travel long distances over ocean. All trajectories data can be assumed as a set of latitude-longitude points. Full year trajectory data show a very complex pattern of transportation of air parcels from different sources and it does not give a clear picture of individual sources. Each individual trajectory represent to the different source of origin as well as different paths. So with the help of cluster analysis, we have grouped the trajectories into clusters, representing the different air masses

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transported from different emission sources. Air trajectories data are from HYSPLIT model. Un-clustered air parcel trajectories corresponding to Ouagadougou are shown in figure 3 where as figure 4 shows the clustered 8-day backward air parcel trajectories. Each cluster represents a group of several trajectories having same source of air mass. Different colors show to different clusters (figure 4). We found eight, three, six and three clusters for Ouagadougou (middle of Sahara), Ascension Island, Male and Midway Island (island sites) respectively. We also found the relative frequency of occurrence (RFO) value for each cluster for each location (figure 2). Various aerosol properties correspond to each cluster are also estimated. We found the influence of soot particles over natural (dust and sea-salt) aerosols and hence high radiative forcing values are observed even over remote regions such as Ouagadougou and Ascension Island. For Male, air mass transported from desert regions shows high absorption indicating the presence of contaminated dust (by anthropogenic aerosols) while air mass transported from south east countries, are non- absorbing type in nature. For Midway Island, all clusters showed low AOD values, high SSA values and hence low radiative forcing values. Our study demonstrates the role of aerosols transported from the main land in influencing the aerosol environment even over remote marine regions.

Figure 4

References

Dorling, S.R., et al. “Cluster Analysis: “A Technique for Estimating the Synoptic Meteorological Controls on Air and Precipitation Chemistry—Method and Applications.” Atmospheric Environment, Vol. 26A, No. 14, pp. 2575-2581, 1992.

IPCC, 2007. Climate change 2007: the physical science basis. In: Solomon, S., et al., (Eds.) Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge and New York, 2007.

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