The final result is summarized in Table 3. Obviously, the accuracy of multi-directional method is better than the single-directional method, except on May 11 (party cloudy). The error is reduced from 33% to 9%. The result reveals improvement in accuracy derived from multi-directional method and the optimal number decision

Table 3. The comparison between the estimated optical depths estimated by the single & multi direction structure function methods. (* : reference date).

Date	Sunphotometer measurements	Single-directional		Multi-directional
		Retrieved	Error(%)	retrieved	Error(%)
1998/04/24	0.339	0.386	14	0.348	3
1998/05/11	0.647	0.394	39	0.374	43
1998/06/27	0.299	0.347	16	0.311	4
1998/07/02	0.209	0.277	33	0.228	9
1998/07/30	0.315	0.356	13	0.345	9
1998/08/21*	0.078	--	--	--	--

Conclusions
The results strongly suggest that including multi-direction and optimal number in structure function method will greatly improve the accuracy of derived aerosol optical depth. In the further study, the relationship of test area sizes and distance values are necessary to be investigate in detail. In order to verify the accuracy of optical depth estimation, the accuracy and method of deriving optical depth from sunphotometer observation is also an important task.

Acknowledgements
This work has been supported by grant from the National Science Council, ROC under NSC86-2612-E-008-007.

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