Thesis (M.Sc.)--Chulalongkorn University, 2013

Tropospheric ozone (O3) has been measured at The Observatory for Atmospheric Research at Phimai by UV-photometric continuous ozone monitor during September 2007 – August 2009 in order to investigate the seasonal behaviors and relations between O3 and factors comprising of carbon monoxide (CO), relative humidity (RH), and direct radiation. Seasonal variations of O3 show dry season maximum in local summer (March to April) and wet season minimum (June to October). The highest and lowest daily O3 concentrations were 63.77 and 9.00 ppb in 5 March 2008 and 19 June 2009, respectively. The maximum and minimum of O3 monthly average were 45.7 ppb in January 2009 and 15.8 ppb in July 2009. O3 diurnal variations increase during day time and decrease during nighttime. Higher diurnal cycles were found in dry season, while, lower diurnal cycles were found in wet season. In order to investigate the effect of long-range transport, air masses were categorized by the backward trajectory analysis on HYSPLT program into four types, Northeast Continental (NE-C), Northeast/East Continental and Marine (NEE-CM), Northeast/East/South Marine (NEES-M), and Southeast Marine (SW-M). Highest O3 concentration at 39.3 ppb found in NE-C during local winter (November – February) due to long-range transport of O3 precursors from North Asia. NEE-CM occurred in the same period as NE-C but with lower O3 than NE-C by influence of cleaner air in Pacific. NEES-M prevailed during local summer (March – April), O3 was found lower than continental originated air masses (NE-C and NEE-CM). However, the strong O3 photochemical production was found in NEES-M due to heavy biomass burning in Southeast Asia continent. The lowest O3 of 22.1 ppb was found with air mass originated in Indian Ocean, SW-M, due to long-range transport of clean marine air mass. Correlation coefficients (R2) between O3 and CO show highest positive relation in NEES-M and lowest in SW-M. R2 between O3 and RH show highest negative relation in marine originated, NEES-M and SW-M. R2 between O3 and direct radiation was found very low or no significant correlation.