Peroxyacetyl nitrate and ozone enhancement at taehwa research forest under the influence of Seoul metropolitan area

Peroxyacetyl nitrate (PAN) is produced by photochemical oxidation reactions with abundant NO_x 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 NO_x 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.