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 Seoul Metropolitan Areas (SMA) from August 25th to September 9th, 2011. PAN was determined every 2 minutes using a 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 SMA, which was clearly demonstrated by an NO2 increase. These high NO2 concentrations were observed along with a wind direction shift 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 result highlight PAN as a robust tracer indicating urban impacts at peri-urban forest sites.