Ozone dry deposition above a tropical forest in the dry season in northern Thailand

In order to investigate the ozone dry deposition above a tropical forest in Southeast Asia, the field experiments were performed in a flat deciduous (teak) forest in Mea Moh located in northern Thailand, in the dry season, from January to April 2002. The experimental period included both the leafless period and transitional period between lush and leafless. Ozone fluxes were obtained by coupling a UV photometric ozone monitor with an ultrasonic anemometer on the basis of the gradient method. Ozone concentrations exceeded 60 ppb on average in the afternoons during the experimental period. The ozone fluxes peak around noon. The daytime level of the fluxes in this study is relatively high compared with the fluxes from previous experiments over a tropical forest or a leafless deciduous forest in other regions. This high daytime flux level is mainly caused by the high concentrations in the dry season in Mae Moh. Median deposition velocities of ozone were 0.32 cm s−1 in daytime (6:00–17:00) and 0.04 cm s−1 in nighttime (18:00–5:00), respectively. The deposition velocities as well as the fluxes peaks around noon. Observed surface resistances decreased in daytime. The surface resistances were found to be affected by aerodynamic process rather than stomatal process during the time from 10:00 to 2:00. Weselyʹs parameterization to infer surface resistance was examined to consider its applicability in the region. The observed surface resistance in the dry season is closer to the inferred surface resistance using the input resistances of transitional spring than that using the input resistances of leafless seasons. This indicates that the parameterization of non-stomatal resistances needs modification above the teak forest in the dry season taking into account the influence of aerodynamic process and the specific conditions in northern Thailand.