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

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 NOx, the impacts of pollution transport from South Asia on tropospheric photochemistry over the TP region deserve further studies.

Measurement report: Fast photochemical production of peroxyacetyl nitrate (PAN) over the rural North China Plain during cold-season haze events

“…Compared with recent studies in China (Table S2), the observed PAN concentration during autumn at the SDZ site is generally lower than PAN levels over the urban NCP (Zhang et al, 2017;Liu et al, 2018) and in southwestern China but comparable to those in the suburban NCP region (Qiu et al, 2019a;Zhang et al, 2019). In addition, the PAN level at the SDZ site is remarkably higher than those obtained from the southern coastal region (Zhu et al, 2018;Zeng et al, 2019a;Hu et al, 2020) and Tibet (Xu et al, 2018), implying a severe photochemical pollution level over the NCP on a regional scale. We find two pollution events (10/20 and 10/25-10/26) occurring at the SDZ site with hourly PAN concentrations in excess of 3 ppb (Figure 2a).…”

Quantifying variability, source, and transport of CO in the urban areas over the Himalayas and Tibetan Plateau

“…Due to its unique feature for interactions among the atmosphere, biosphere, hydrosphere, and cryosphere, the HTP is referred to as an important indicator of regional and global climate change (Pu et al, 2007;Yao et al, 2012;Zhang et al, 2015). The HTP stores a large amount of ice masses on the planet and provides the headwater of many Asian rivers which contribute water resources to over 1.4 billion people, and it thus is referred to as the "Water Tower of Asia" (Xu et al, 2008;Immerzeel et al, 2010;Gao et al, 2019;Kang et al, 2019). The glaciers and snowmelt over the HTP can potentially modify the regional hydrology, contribute to global sea-level rise, and trigger natural hazards which may threaten the health and wealth of many populations (Singh and Bengtsson, 2004;Barnett et al, 2005;Immerzeel et al, 2010;Kaser et al, 2010;Bolch et al, 2012;Yao et al, 2012;Gao et al, 2019).…”

“…Convective transport around the HTP areas has also been verified by satellite observations, chemical transport model simulations, flask sampling analyses, and in situ measurements of some key atmospheric constituents. These atmospheric constituents include carbon monoxide (CO) (Park et al, 2007(Park et al, , 2008, methane (CH 4 ) (Xiong et al, 2009), hydrogen cyanide (HCN) (Randel et al, 2010), polyacrylonitrile (PAN) (Zhang et al, 2009;Ungermann et al, 2016;Xu et al, 2018), ozone (O 3 ) (Yin et al, 2017;Xu et al, 2018), and aerosol (Cong et al, 2007(Cong et al, , 2009(Cong et al, , 2013He et al, 2014;Zhang et al, 2015;Zhu et al, 2019;Li et al, 2021;Gul et al, 2021;Thind et al, 2021). Furthermore, urbanization, industrialization, land use, and infrastructure construction over the HTP have expanded rapidly in recent years, which could also emit air pollutants into the atmosphere (Ran et al, 2014;.…”

Seasonal characteristics of atmospheric peroxyacetyl nitrate (PAN) in a coastal city of Southeast China: Explanatory factors and photochemical effects

“…Recently, negative and positive impacts of PAN photochemistry on O3 production were captured under the low and high NOx conditions, respectively. However, the PAN formation and its influencing mechanism on O3 production are still complex and unclear (Hu et al, 2020;Zhang et al 2019;Xu et al, 2018). Long-term field measurements and model simulations could help to verify the mechanisms under various pollution scenarios and environmental conditions.…”