Study on spatio-temporal variability of dust aerosol and its impact on atmospheric variables in and around Bangladesh

Abstract

Acknowledgment

All praises are due to the Almighty God who has enabled me to complete this thesis for the M.Sc degree.

I would like to express my gratitude and appreciation to my supervisor Dr. Nasreen Akter, Professor, Department of Physics, Bangladesh University of Engineering and Technology (BUET) for her guidance throughout the entire period of research works and during preparation of the manuscript of this thesis.

I am grateful to Professor Fahima Khanam, Head, Department of Physics, Bangladesh University of Engineering and Technology (BUET) to support and to allow me to do this research. I am very much thankful to Dr. Md. Rafi Uddin, Professor, Department of Physics (BUET) for his kind cooperation for improving my research related computer skill and valuable suggestions about my research.

I would also like to thank A.T.M. Shafiul Azam, Assistant Professor, Department of Physics, Bangladesh University of Engineering and Technology (BUET), for his valuable teaching efforts and suggestions that helped me quite a great extent in writing of this thesis.

I would like to thank Atmospheric Laboratory, Department of Physics (BUET) to provide the facilities for the study. I express my cordial thanks to my friends and all well-wishers especially each and every members of Atmospheric Lab. (BUET), including the entire faculty members and stuff of BUET for their help and suggestions during the study period.

Lastly but not the least, I express my profound gratitude to my beloved parents and my siblings and all relatives for their inspiration, encouragement and endless love to complete my study. The Author

ABSTRACT

Aerosols are liquid or solid particles suspended in the air which is measured by aerosol optical depth (AOD) have influence on climate change, rain formation, and weather forecasting. In this study, variability of dust aerosol in and around Bangladesh (20°N-27.5ºN, 85°E- 95ºE) is analyzed for 10 years from 2003 to 2012. For this purpose, 3-hourly Monitoring Atmospheric Composition & Climate (MACC) reanalysis data collected from European Centre for Medium-Range Weather Forecasts (ECMWF) dataset were used. In the study period of ten years the average value of dust aerosol is found 0.147 AOD which is 42.44% of total aerosols (all aerosols including dust) present in the study area. Monthly variation of dust aerosol shows a maximum value of 0.246 AOD in May which gradually decreases with month’s progress and minimum value is attained in September with the value of 0.07 AOD. In seasonal scale, the concentration of dust aerosol is noticed the values of 0.144, 0.217, 0.119 and 0.106 AOD in winter, pre-monsoon, monsoon and post-monsoon, respectively. Dust aerosol is found 43.32% higher in pre-monsoon than all other seasons. The contribution of dust aerosol in the total aerosols is found 47.84%, 49.45%, 32.02% and 39.68% in winter, pre-monsoon, monsoon and post monsoon seasons, respectively. Yearly variation exhibits the increasing trend for the total dust concentration and the increasing rate of dust aerosol is 4.4%. The coefficient of determinant (R2) for the yearly increasing trend is calculated 0.5. In yearly increasing rate, winter season mainly contributes the total increasing annual rate (5.6%) of dust aerosol. In the month of February dust aerosol increases more (5.4%) than the other months. Dust aerosol is mainly blown by the wind flow at 700 hPa from the arid and semi-arid region. The relation of dust aerosol with wind speed at 700 hPa is found significant with the correlation coefficient (r) of 0.76 and 0.64 in seasonal and monthly scale, respectively. Precipitation is another reason for the existence of dust aerosol in the atmosphere. The monthly and seasonal average of dust aerosol and precipitation are found negatively correlated with the value -0.3 However, the separate correlation for wet period (pre-monsoon and monsoon) is found more significant with the value of -0.52 and for dry period (winter and post monsoon) it is -0.68. The result indicates that the rainy washout or deposition of dust particles by the precipitation which further verified by the relation with runoff (E-P) parameter. The significant correlations (0.64 and 0.53) are found for dry and wet periods. Dust aerosol may have direct effect on the mid-level (500 hPa) and upper-level (300 hPa) temperature with the significant r values of -0.51 and -0.62, respectively. Dust aerosol has indirect effect on relative humidity which shows that the low-level high relative humidity at 1000 hPa is decreased by increasing dust aerosol with correlation value of -0.51. As dust aerosol comes from the western side of the study area so the west area has 19.32% more dust aerosol than the east area. However, dust aerosol characteristics are found almost same between these two areas.