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To advance our understanding on the factors that affect photochemical pollution at different elevations in mountainous areas, concurrent systematic field measurements were conducted at a mountain site and at an urban site at the foot of the mountain in Hong Kong. The mixing ratios of air pollutants were greater at the foot of the mountain (i. e. Tsuen Wan urban site, TW) than near the summit (i. e. Tai Mao Shan mountain site, TMS), expect for ozone. In total, only 1 O3 episode day was observed at TW, whereas 21 O3 episode days were observed at TMS. The discrepancy of O3 at the two sites was attributed to the mixed effects of No titration, vertical meteorological conditions, regional transport and mesoscale circulations. The lower No levels at TMS and the smaller differences of "oxidant" Ox (O3 + NO2) than O3 between the two sites suggested that variations of O3 at the two sites were partly attributed to different degree of NO titration. In addition, analysis of vertical structure of meteorological variables revealed that the inversion layer at the range of altitudes of 500 ~ 1000 m might be another factor that caused the high O3 levels at TMS. Furthermore, analyses of the wind fields and the levels of air pollutants in different air flows indicated that high O3 concentrations at TMS were somewhat influenced by regional air masses from the highly polluted Pearl River Delta (PRD) region. In particular, the analysis of diurnal profiles and correlations of gaseous pollutants suggested influence of mesoscale circulations which is confirmed using the Master Chemical Mechanism moving box model (Mbox) and the Weather Research and Forecasting (WRF) model. By investigating the correlations of observed O3 and Nox*, as well as the ratios of VOC/Nox, it is concluded that photochemical O3 formation was VOC-sensitive or both Nox and VOC-sensitive at TMS, while it was VOC sensitive at TW.