Peroxyacetyl nitrate (PAN) is produced by photochemical oxidation reactions with abundant NOx and volatile organic compounds (VOCs); therefore, it is considered as a photochemical pollution indicator. In this study, PAN, O3, and their precursors were measured at three heights (5.4, 23, and 40.5 m) on a 41-m tower in Taehwa Research Forest (TRF) near the Seoul Metropolitan Area (SMA) from August 25 to September 9, 2011. The PAN was determined every 2 minutes using gas chromatography with luminol chemiluminescence detection (GC-LCD). All reactive gases were measured for 15 minutes at each height. The mean and maximum PAN concentrations were 0.3 and 3.1 ppbv, respectively. The mean and maximum O3 concentrations were 13.1 and 79.8 ppbv, respectively. The average NOx concentration was 6.57 ppbv. At the TRF, PAN and O3 concentrations were well correlated (r = 0.8) and greatly elevated when the air mass was affected by urban outflows from the SMA, which was clearly demonstrated by an increase in NO2. These high NO2 concentrations were observed along with a shift in wind direction at 17:00 (KST) and resulted in the maximum observed values of PAN and O3 in the present study. In addition, the concentration enhancement was more pronounced for PAN and at heights above the canopy. These results highlight PAN as a robust tracer indicating urban impacts at peri-urban forest sites.