First simultaneous measurements of peroxyacetyl nitrate (PAN) and ozone at Nam Co in the central Tibetan Plateau: impacts from the PBL evolution and transport processes

Abstract. Both peroxyacetyl nitrate (PAN) and ozone (O₃) are key photochemical products in the atmosphere. Most of the previous in situ observations of both gases have been made in polluted regions and at low-altitude sites. Here we present the first simultaneous measurements of PAN and O₃ at Nam Co (NMC; 30°46′ N, 90°57′ E, 4745 m a.s.l.), a remote site in the central Tibetan Plateau (TP). The observations were made during summer periods in 2011 and 2012. The PAN levels averaged 0.36 ppb (range: 0.11–0.76 ppb) and 0.44 ppb (range: 0.21–0.99 ppb) during 17–24 August 2011 and 15 May to 13 July 2012, respectively. The O₃ level varied from 27.9 to 96.4 ppb, with an average of 60.0 ppb. Profound diurnal cycles of PAN and O₃ were observed with minimum values around 05:00 LT, steep rises in the early morning, and broader platforms of high values during 09:00–20:00 LT. The evolution of the planetary boundary layer (PBL) played a key role in shaping the diurnal patterns of both gases, particularly the rapid increases of PAN and O₃ in the early morning. Air entrainment from the free troposphere into the PBL seemed to cause the early-morning increase and be a key factor for sustaining the daytime high concentrations of both gases. The days with higher daytime PBL (about 3 km) showed stronger diurnal variations in both gases and were mainly distributed in the drier pre-monsoon period, while those with shallower daytime PBL (about 2 km) showed minor diurnal variations and were mainly distributed in the humid monsoon period. Episodes of higher PAN levels were occasionally observed at NMC. These PAN episodes were caused either by rapid downward transport of air masses from the middle/upper troposphere or by long-range transport of PAN plumes from north India, north Pakistan, and Nepal. The maximum PAN level in the downward transport cases ranged from 0.5 to 0.7 ppb. In the long-range transport case, the PAN level varied in the range of 0.3–1.0 ppb, with an average of 0.6 ppb. This long-range transport process influenced most of the western and central TP region for about a week in early June 2012. Our results suggest that polluted air masses from South Asia can significantly enhance the PAN level over the TP. As PAN acts as a reservoir of NO_x, the impacts of pollution transport from South Asia on tropospheric photochemistry over the TP region deserve further studies.