Aerosol Optical Thickness Measurements on Tsunami Day at a Continental Station , Mysore

Daily atmospheric measurements of spectral Aerosol Optical Thickness (AOT), precipitable water vapor (W), and irradiance were being carried out (ISRO-RESPOND Project) at Mysore, India (12.3N), a continental station, on the day a massive tsunami struck India’s coast, December 26, 2004. A comparison was made of that day’s data with two other days: December 25, 2004, the day before tsunami and December 27, 2004, the day after. Compared to the values of AOT on December 25, 2004 and December 27, 2004, AOTs on December 26, 2004 were very low throughout the day. Values were almost one-third the magnitude of the other two days. The values measured on December 26, 2004 are unique because no other day in this month showed such low values. Comparison of the results with some other location is not possible at this point, because there appears to be no other study of this kind to support the observed results. Possible conclusions are drawn based on the available information.


INTRODUCTION
gas, usually air" (2003).The sources and sinks of atmospheric aerosols are so varied and distributed over the globe that their physical and optical effects show distinct variation with geographic locations (Muralikrishnan, 1993).
Study of the optical and physical properties of aerosols is important for assessment of their effect on climate.Tropospheric aerosol particles have a short lifetime; as a result their properties vary from one region to another over time.In the presence of any atmospheric perturbations, the variation becomes much Tiny suspended particles in the atmosphere ranging in size from 10 -3 µm to 10 2 µm are k n o w n a s a t mo s p h e r i c a e r o s o l s .T h e Encyclopedia Britannica defines aerosol as "a system of liquid or solid particles uniformly distributed in a finely divided state through a more significant.Because of this kind of variability in aerosol properties, it is difficult to assess aerosol climatology.For a full assessment of aerosol characteristics, such measurements have to be performed frequently in locations with different aerosol types and in varying meteorological conditions (Kaufman et al., 1994).In this paper Aerosol Optical Thickness (AOT) characteristics on the tsunami day are discussed.
The scans can be conveniently viewed on the LCD (Ganesh et al., 2008).

CALIBRATION
The instrument was calibrated at regular intervals.Table 1 shows the extent of deviation of calibration constants from the factory set values.The degradation of the filters or the drift in the calibration values was found to be marginal.The calibration was carried out atop Sri Chamundeshwari Hill which is at about 300 m from the ground level using the standard Langley technique (Ganesh et al., 2008).

EXPERIMENTAL
Initially, several MICROTOPS II settings are made with the help of a GPS (Global Positioning System) receiver.These include universal date and time, geographic coordinates, altitude and atmospheric pressure of the measurement site.For the observations, the MICROTOPS II is mounted on a tripod in order to minimize the sun targeting error (Fig. 1).
On the days of clear sky, that is when there are no clouds, the measurements are made.

METEOROLOGICAL & PHYSICAL FEATURES OF MYSORE
Mysore is a tropical (12.

Observations of AOT on Tsunami Day
In Mysore, about 500 km away from the east coast, routine AOT measurements were being recorded with a MICROTOPS II Sun  (Iqbal, 1983).This was also evident from the "very blue" appearance of the sky on that day.The studies made by Kim et al. (2005), Glantz et al. (2006), Ritweej et al. (2007) supports the fact that aerosol properties change with respect to changes in wind speed and direction.

Day
Large difference between the AOT values of forenoon (τ FN ) and afternoon (τ AN ) is said to be the asymmetry of AOT (Ganesh et al., 2008).The extent of asymmetry varies with the site, as well as with the wavelength.For a quantitative study, asymmetry factor ηλ is defined through the expression:

CONCLUSIONS
The results presented in this paper are the fruit of routine work on atmospheric measurements for the month of December 2004.By making use of the available information the following conclusions are drawn to the best of our knowledge.The AOTs recorded on the tsunami day December photometer MICROTOPS II developed by Solar Light Company, USA (2003) is used.The instrument is equipped with five accurately aligned optical collimators, with a full field view of 2.5°.Internal baffles are also integrated into the device to eliminate internal reflections.Each channel is fitted with a (09:30 -17:30 hrs, IST) at 15-30 minute intervals.Each set of data contains five values of spectral AOT and one value of water vapor.Experimental observations are carried out on individual days at different sites.Care has been taken during the collection of data so as to avoid strong seasonal effects, such as drizzle and hazy sky.
Photometer throughout December 2004.The measurements showed rather interesting results on December 26, 2004, the day that tsunami struck India's coast.A comparison of values of AOT with the days before and after(December 25 and December 27, 2004)    shows that AOTs onDecember 26, 2004    were very low throughout the day; almost
were very low compared to all other days in the month of December 2004.A plot of Angstrom's, parameters α versus β showed a linear behavior.The slope of this graph which is a measure of the ratio of change in small to large particle concentration, is large on December 26, 2004.This indicates the dominance of finer aerosols compared to the coarse ones.The value of intercept greater than three is an indication of Rayleigh scattering to a larger extent.These results may be attributed to drift in air caused by the change in wind direction on tsunami day from northeasterly to easterly.This study emphasizes the importance of aerosol observation during the days involving atmospheric perturbations due to natural events.

Table 1 .
Comparison of calibration constants.